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International A Level Biology Edexcel
5. Energy Flow, Ecosystems & the Environment
CONTENTS
Photosynthesis
5.1 Photosynthesis: Overview
5.2 The Role of ATP in Photosynthesis
5.3 Photosynthesis: Light-Dependent Stage
5.4 Photosynthesis: Light-Independent Stage
5.5 Chloroplasts
5.6 Absorption & Action Spectra
5.7 Separation of Photosynthetic Pigments with Chromatography
5.8 Core Practical 10: Rate of Photosynthesis
Ecosystems
5.9 Ecological Productivity
5.10 Energy & Biomass Transfers
5.11 Ecology: Key Terms
5.12 Factors Affecting Populations
5.13 Niche
5.14 Core Practical 11: Quadrat & Transect Study
5.15 Ecological Succession
Environmental Biology
5.16 Evidence for Climate Change
5.17 Anthropogenic Climate Change
5.18 Carbon Cycle & Environmental Management
5.19 Models for Predicting Climate Change
5.20 How Climate Change Affects Species
5.21 The Effect of Temperature on Enzyme Reactions
5.22 Core Practical 12: Effect of Temperature on Development
5.23 Mutation, Natural Selection & Evolution
5.24 Isolation & Speciation
5.25 Contentious Issues in Environmental Science
5.26 Examples of Sustainable Conservation
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Photosynthesis

5.1 Photosynthesis: Overview
Photosynthesis: Overview
is a series of chemical reactions that occurs in producers such as
plants and algae
Producers are also known as autotrophs; organisms that make their own
organic compounds
Photosynthesis converts light energy into chemical energy which is then stored in
the biomass of producers
The light energy is used to split strong bonds in water molecules (H2O), releasing
hydrogen and oxygen
Oxygen is released into the atmosphere as a waste product
Hydrogen is combined with carbon dioxide to produce glucose
Chemical energy is stored within the bonds in glucose molecules; glucose
can therefore function as a fuel for respiration
It can be said that hydrogen is stored in glucose molecules
Photosynthesis
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Photosynthesis requires energy from light to split water molecules. The resulting
hydrogen combines with carbon dioxide and is stored in glucose, which fuels
respiration. Oxygen is released as a waste product.
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5.2 The Role of ATP in Photosynthesis
ATP as an Energy Carrier in Photosynthesis
All organisms require a constant supply of energy to maintain their cells and stay
alive
This energy is required e.g.
For building new molecules from the products of digestion during anabolic
reactions
To move substances across cell membranes in active transport or to move
substances within cells
For muscle contraction
In the conduction of nerve impulses
In all known forms of life the molecule adenosine triphosphate, or ATP, is used to
transfer and supply energy within cells
ATP is therefore known as the universal energy currency
ATP diffuses within cells to where it is needed
ATP is a type of nucleic acid and is structurally very similar to the nucleotides that
make up DNA and RNA
It is a phosphorylated nucleotide
A nucleotide consists of a nitrogenous base, a sugar, and a single
phosphate group
ATP contains three phosphate groups, hence triphosphate
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ATP contains adenine, a ribose sugar, and three phosphates molecules. Removal of
one phosphate creates ADP, and removal of two phosphates creates AMP.
ATP is produced by the addition of inorganic phosphate (Pi), a type of phosphate
group, to adenosine diphosphate, or ADP
ADP + Pi → ATP
ADP contains two phosphate groups, hence diphosphate
ATP can be produced when the passage of electrons along a series of proteins
known as the electron transport chain releases energy for the phosphorylation of
ADP
This process occurs in the mitochondria during respiration and in chloroplasts
during photosynthesis
In photosynthesis the energy originally gained by the electrons in this
process comes from light, so this method of ATP production is known as
photophosphorylation
Photo = light
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The hydrolysis, or breakdown, of ATP releases an inorganic phosphate as well as a
small amount of energy which can be used by the cell
ATP → ADP + Pi
The removal of a phosphate group is known as dephosphorylation
The hydrolysis of ATP is catalysed by the enzyme ATPase
The ADP and inorganic phosphate produced by the hydrolysis of ATP can be
recycled to make more ATP
ADP + Pi → ATP
ATP is formed during respiration and can be hydrolysed to release energy for
processes such as active transport, muscle contraction, and building new molecules
(anabolic reactions). ATP can then be regenerated from ADP and phosphate.
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5.3 Photosynthesis: Light-Dependent Stage
Photosynthesis: Light-Dependent Stage
Photosynthesis takes place in two distinct stages
The light-dependent reactions, which rely on light directly
The light-independent reactions, which do not use light directly, though do
rely on the products of the light-dependent reactions
Both these sets of reactions take place within the chloroplast
The light-dependent reactions take place across the thylakoid membrane
The light-independent reactions take place in the stroma
Light energy in the light-dependent reactions enables the splitting of water
molecules in a reaction known as photolysis
Photolysis of one molecule of water, or H2O, produces
2 hydrogen ions (2H+), also known as protons
2 electrons (2e-)
One atom of oxygen (O)
The hydrogen ions and electrons are used during the light-dependent
reactions while the oxygen is given off as a waste product
During the light-dependent reactions light energy is converted into chemical
energy in the form of ATP and reduced NADP
NADP is a type of molecule called a coenzyme; its role is to transfer hydrogen
from one molecule to another
When NADP gains hydrogen it is reduced, and can be known as either reduced
NADP or NADPH
Remember that
Reduction is gain of electrons, gain of hydrogen, or loss of oxygen
Oxidation is loss of electrons, loss of hydrogen, or gain of oxygen
Reduced NADP can reduce other molecules by giving away hydrogen
NADP can oxidise other molecules by receiving hydrogen
The useful products of the light-dependent reactions, ATP and NADPH, are
transferred to the light-independent reactions within the chloroplast
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The products of the light-dependent reaction are ATP, NADPH, and oxygen. Oxygen is
given off as a waste product while ATP and NADPH pass to the light-independent
reactions. The ADP and NADP produced during the light-independent reaction can
pass back to the light-dependent reactions to allow more ATP and NADPH to be
produced.
Production of ATP and NADPH
ATP and NADPH are produced during the light-dependent reactions as a result of a
series of events that occur on the thylakoid membrane known as
photophosphorylation
Photo = light
Phosphorylation = the addition of phosphate; in this case to ADP to form ATP
Two types of photophosphorylation take place
Non-cyclic photophosphorylation
This produces both ATP and NADPH
Cyclic photophosphorylation
This produces ATP only
Both cyclic and non-cyclic photophosphorylation involve
A series of membrane proteins which together make up the electron
transport chain
Electrons pass from one protein to another along the electron transport
chain, releasing energy as they do so
Chemiosmosis
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The energy released as electrons pass down the electron transport chain
is used to produce ATP
Non-cyclic photophosphorylation
hits photosystem II in the thylakoid membrane
It is slightly confusing that photosystem II comes first in this sequence; the
numbers simply reflect the order in which the photosystems were discovered
Two electrons gain energy and are said to be excited to a higher energy level
The excited electrons leave the photosystem and pass to the first protein in the
Light energy
electron transport chain
As the excited electrons leave photosystem II they
are replaced by electrons from the photolysis of water
The electrons pass down the chain of electron carriers known as an electron
transport chain
Energy is released as the electrons pass down the electron transport chain which
enables chemiosmosis to occur
H­+
­ ions are actively pumped from a low concentration in the stroma to a
high concentration in the thylakoid space, generating a concentration gradient
across the thylakoid membrane
H­+­ ions diffuse back across the thylakoid membrane into the stroma via ATP
synthase enzymes embedded in the membrane
The movement of H­+­ ions causes the ATP synthase enzyme to catalyse the
production of ATP
At the end of the electron transport chain the electrons from photosystem II are
passed to photosystem I
Light energy also hits photosystem I, exciting another pair of electrons which leave
the photosystem
The excited electrons from photosystem I also pass along an electron transport
chain
These electrons combine with hydrogen ions from the photolysis of water and
the coenzyme NADP to form reduced NADP
H+ + 2e- + NADP+ → NADPH
The reduced NADP and the ATP pass to the light-independent reactions
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Non-cyclic photophosphorylation involves photosystems I and II and produces both
ATP and NADPH
Cyclic photophosphorylation
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hits photosystem I
Electrons are excited to a higher energy level and leave the photosystem
The excited electrons pass along the electron transport chain, releasing energy as
they do so
The energy released as the electrons pass down the electron transport chain
provides energy to drive the process of chemiosmosis
H­+
­ ions are actively pumped from a low concentration in the stroma to a
high concentration in the thylakoid space, generating a concentration gradient
across the thylakoid membrane
H­+­ ions diffuse back across the thylakoid membrane into the stroma via ATP
synthase enzymes embedded in the membrane
The movement of H­+­ ions cause the ATP synthase enzyme to catalyse the
Light
production of ATP
At the end of the electron transport chain the electrons rejoin photosystem I in a
complete cycle; hence the term cyclic photophosphorylation
The ATP produced enters the light-independent reaction
Cyclic photophosphorylation involves Photosystem I and produces ATP
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5.4 Photosynthesis: Light-Independent Stage
Fixation of Carbon from Carbon Dioxide
The light-independent reactions of photosynthesis are sometimes referred to as
the Calvin cycle
The reactions eventually allow for the production of complex organic molecules
such as
Starch for storage
Sucrose for transport
Cellulose for making cell walls
The light-independent reactions do not require energy from light but do require
ATP and reduced NADP from the light-dependent reactions
There are three main steps within the light-independent reactions
1. Carbon dioxide is combined with ribulose bisphosphate (RuBP), a 5-carbon
(5C) compound; this yields two molecules of glycerate 3-phosphate (GP), a 3carbon (3C) compound
2. GP is reduced to glyceraldehyde 3-phosphate (GALP), another 3C
compound, in a reaction involving reduced NADP and ATP
3. RuBP is regenerated from GALP in reactions that use ATP
Carbon dioxide and RuBP are combined
Carbon dioxide combines with a 5C sugar known as RuBP in a reaction catalysed
by the enzyme rubisco
The resulting 6-carbon (6C) compound is unstable and splits in two
This results in two molecules of a 3C compound known as glycerate 3phosphate (GP)
The carbon dioxide has been ‘fixed’, meaning that it has been removed from the
external environment and become part of a molecule inside the plant cell
Reduction of glycerate 3-phosphate
and hydrogen from reduced NADP, both produced during the
light-dependent reactions, are used to reduce the two 3C molecules of GP to two
3C molecules known as GALP
Some of the carbons in GALP go towards the production of useful organic
molecules such as glucose, while the rest remain in the Calvin cycle to allow the
Energy from ATP
regeneration of RuBP
Two molecules of GALP contain six carbon atoms, five of which are needed to
regenerate RuBP; this means that for every turn through the Calvin cycle only
one sixth of a molecule of glucose is produced
Glucose is a 6-carbon molecule, so six turns of the Calvin cycle are required
to produce one molecule of glucose
Regeneration of ribulose bisphosphate
Five sixths of the GALP molecules are used to regenerate RuBP
This process requires ATP
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The Calvin cycle produces glucose and other important biological molecules
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Products of the Light-Dependent Stage
Intermediate molecules of the Calvin cycle, such as glyceraldehyde 3-phosphate
(GALP), are used to produce various other biological molecules needed by plants,
such as:
Hexose sugars e.g. glucose
Glucose can enter the respiration reactions during which ATP is produced
Hexose sugars can be converted into other hexose sugars e.g. glucose can
be converted to sucrose for transport in the phloem
Hexose sugars can be joined to make polysaccharides such
as starch and cellulose
Glycerol can be used for building lipid molecules such as triglycerides and
phospholipids
Fatty acids which form the tails of lipid molecules such as triglycerides and
phospholipids
Nucleic acids form the basis of DNA and RNA
Phosphates from the soil are combined with the molecules of the Calvin
cycle to produce nucleic acids
Acetyl coenzyme A is important coenzyme in respiration
Amino acids which can be used in protein synthesis for building polypeptides
Nitrates from the soil need to be combined with the molecules of the
Calvin cycle for amino acids to be produced
Many of the molecules produced are used to build new plant biomass; these
molecules are passed on to consumers when plant tissue is eaten
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The products of photosynthesis include amino acids, polysaccarides, lipids and
nucleic acids
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5.5 Chloroplasts

Chloroplasts: Structure & Function
Chloroplasts are the organelles in plant cells where photosynthesis occurs
Each chloroplast is surrounded by a double-membrane known as the chloroplast
envelope
Each of the envelope membranes is a phospholipid bilayer
Chloroplasts are filled with a cytoplasm-like fluid known as the stroma
The stroma contains enzymes and sugars, as well as ribosomes and
chloroplast DNA
If the chloroplast has been photosynthesising there may be starch
grains or lipid droplets in the stroma
A separate system of membranes is found in the stroma
This membrane system consists of a series of flattened fluid-filled sacs known
as thylakoids, each surrounded by a thylakoid membrane
Thylakoids stack up to form structures known as grana (singular granum)
Grana are connected by membranous channels called lamellae (singular
lamella), which ensure the stacks of sacs are connected but distanced from
each other
Several components that are essential for photosynthesis are embedded in the
thylakoid membranes, including:
ATP synthase enzymes
Proteins called photosystems
that contain photosynthetic pigments such as
chlorophyll a, chlorophyll b, and carotene
Chloroplasts are the site of photosynthesis
Chloroplast structure is related to function
Chloroplast envelope
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The double membrane encloses the chloroplast, keeping all of the
components needed for photosynthesis close to each other
The transport proteins present in the inner membrane control the flow of
molecules between the stroma and cytoplasm
Stroma
The gel-like fluid contains enzymes that catalyse the reactions of
photosynthesis
DNA
The chloroplast DNA contains genes that code for some of the proteins used
in photosynthesis
Ribosomes
Ribosomes enable the translation of proteins coded by the chloroplast DNA
Thylakoid membrane
There is a space between the two thylakoid membranes known as the
thylakoid space, in which conditions can differ from the stroma e.g. a proton
gradient can be established between the thylakoid space and the stroma
The space has a very small volume so a proton gradient can develop
very quickly
Grana
The grana create a large surface area, maximising the number of
photosystems and allowing maximum light absorption
Grana also provide more membrane area for proteins such as electron carriers
and ATP synthase enzymes, which together enable the production of ATP
Photosystems
There are two types of photosystems; photosystem I and photosystem II,
containing different combinations of photosynthetic pigments such as
chlorophyll a, chlorophyll b, and carotene
Each photosystem absorbs light of a different wavelength, maximising light
absorption e.g. photosystem I absorbs light at a wavelength of 700 nm while
photosystem II absorbs light at a wavelength of 680 nm
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5.6 Absorption & Action Spectra
Absorption & Action Spectra
Chloroplasts contain several different photosynthetic pigments within
photosystems embedded in the thylakoid membranes
Different pigments absorb light of different wavelengths
Chlorophylls absorb wavelengths in the blue-violet and red regions of the
light spectrum, reflecting green light and appearing green in colour
Carotenoids absorb wavelengths of light mainly in the blue-violet region of
the spectrum, reflecting yellow and orange light
Carotenoids often remain in leaves after the breakdown of chlorophyll in
the autumn, giving some leaves their yellow, orange, and red autumn
colours
Examples of Photosynthetic pigments Table
The amount of light at different wavelengths absorbed by a particular pigment
gives that pigment's absorption spectrum (plural spectra)
Because each type of pigment absorbs light at different wavelengths the
absorption spectrum of each pigment is different
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Different photosynthetic pigments absorb light of different wavelengths, giving
different absorption spectra
A plant's rate of photosynthesis varies depending on the wavelengths of light
available
The changing rate of photosynthesis at different wavelengths is known as
an action spectrum
Action spectra are very closely correlated to the absorption spectra of the different
pigments
Having several different pigments with different absorption spectra allows
plants to photosynthesise under a wider variety of light wavelengths; this
extends the action spectra of plants and maximises rates of photosynthesis
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Plant action spectra are closely related to the absorption spectra of the different
photosynthetic pigments
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5.7 Separation of Photosynthetic Pigments with Chromatography
Separation of Photosynthetic Pigments with Chromatography
Chloroplasts contain several different photosynthetic
pigments within photosystems embedded in their thylakoid membranes
Photosynthetic pigments absorb different wavelengths of light, so are different in
colour
The colour of a pigment is due to the wavelengths of light reflected by that
pigment, e.g. chlorophylls absorb light at the red and blue ends of the visible
spectrum and reflect light in the green part of the spectrum, so appear as
green pigments
Chromatography can be used to separate and identify chloroplast pigments that
have been extracted from a leaf
Chromatography
Chromatography is a technique that is used to separate mixtures
Different components within a mixture travel through materials at different
speeds due to their size or charge
This causes different components to separate
An Rf value can be calculated for each component of the mixture on the basis
of its rate of movement
Two of the most common techniques for separating photosynthetic pigments are
Paper chromatography
The mixture of pigments is passed through paper made of cellulose
Thin-layer chromatography (TLC )
The mixture of pigments is passed through a thin layer of an adsorbent,
e.g. silica gel
The pigments travel faster than through paper, so they separate more
distinctly
Apparatus
Leaf sample
Dropping pipette
Acetone
Pestle and mortar
Filter paper or TLC paper
Pencil
Ruler
Capillary tube
Beaker or boiling tube
Chromatography solvent
Method
1. Draw a straight line in pencil approximately 1cm above the bottom of the paper
being used, and use the pencil to draw a dot in the middle of the line; this marks
where you will place the leaf sample
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as the ink will separate into pigments within the experiment
and obscure the results
2. Cut a section of leaf and place it in a mortar
It is important to choose a healthy leaf that has been in direct sunlight so you
can be sure it contains many active photosystems
3. Add 20 drops of acetone and use the pestle to grind up the leaf sample and
release the pigments
Acetone is an organic solvent and therefore fats, such as the phospholipid
membranes in plant cells, dissolve in it
Acetone and mechanical pressure are used to break down the cell, chloroplast
and thylakoid membranes to release the pigments
4. Extract some of the pigment using a capillary tube and spot it onto the dot in the
centre of the pencil line you have drawn
5. Suspend the paper over a beaker containing a small amount of chromatography
solvent; the end of the paper closest to the pigment extract needs to touch the
chromatography solvent, but the level of the solvent should be below the pencil
line at this stage
The solvent will move up the paper
The pigment mixture will be dissolved in the solvent and carried with the
solvent as it moves
6. Leave the paper suspended in the solvent until the solvent has almost reached the
top of the paper
7. Remove the paper from the solvent and draw a pencil line marking the level of the
solvent on the paper
The solvent may continue moving after the paper is removed from it, so it is
important to draw a pencil line immediately
The pigments should have separated out and there should be different spots
on the paper at different heights above the pencil line; these are the separate
pigments
8. Calculate the Rf value for each pigment spot
Do not use a pen
Rf value = distance travelled by pigment ÷ distance travelled by the solvent
Always measure to the centre of each spot of pigment
Results
Chromatography can be used to separate and identify chloroplast pigments that
have been extracted from a leaf as each pigment will have a unique Rf value
The Rf value is a measure of how far a dissolved pigment travel
Larger, less soluble molecules will travel more slowly and therefore have
a smaller Rf value
Smaller, more soluble molecules will travel faster and therefore have a larger
Rf value
Although specific Rf values depend on the solvent that is being used, in general
Carotenoids have the highest Rf values, usually close to 1
Chlorophyll b has a much lower Rf value
Chlorophyll a has an Rf value somewhere between those of carotenoids and
chlorophyll b
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Chromatography can be used to separate photosynthetic pigments, which can then be
identified by their Rf values.
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5.8 Core Practical 10: Rate of Photosynthesis

Rate of Photosynthesis
For photosynthesis to occur the following are required
The presence of photosynthetic pigments
A supply of carbon dioxide
A supply of water
Light energy
A suitable temperature
If there is a shortage of any of these factors, photosynthesis cannot occur at
its maximum possible rate
The main external factors that affect the rate of photosynthesis are
Light intensity and wavelength
Carbon dioxide concentration
Temperature
These are known as limiting factors of photosynthesis
If any one of these factors is below the optimum level for the plant, its
photosynthesis will be reduced, even if the other two factors are at the
rate of
optimum
level
Note that although a lack of water can reduce the rate of photosynthesis, water
shortages usually affect other processes in the plant before affecting
photosynthesis and water is therefore not one of the main limiting factors
Light intensity
The rate of photosynthesis increases as light intensity increases
The greater the light intensity, the more energy supplied to the plant and
therefore the faster the light-dependent stage of photosynthesis can occur
This produces more ATP and reduced NADP for the Calvin cycle, which can
then also occur at a greater rate
During this stage, light intensity is said to be a limiting factor of
photosynthesis
At some point, if light intensity continues to increase, the relationship above will
no longer apply and the rate of photosynthesis will reach a plateau
At this point light intensity is no longer a limiting factor of photosynthesis;
another factor is limiting the rate
E.g. temperature being too low or too high, or not enough carbon dioxide
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Light intensity can be a limiting factor for photosynthesis
Carbon dioxide concentration
The rate of photosynthesis increases as carbon dioxide concentration increases
Carbon dioxide is one of the raw materials required for photosynthesis
It is required for the light-independent stage of photosynthesis; CO2 is
combined with the five-carbon compound ribulose bisphosphate (RuBP)
during carbon fixation
This means the more carbon dioxide that is present, the faster this step of
the Calvin cycle can occur and the faster the overall rate of photosynthesis
This trend will continue until some other factor, e.g. temperature or light intensity,
prevents the rate from increasing further
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Carbon dioxide concentration can be a limiting factor for photosynthesis
Temperature
As temperature increases the rate of photosynthesis increases up to a point,
after which the rate decreases
This is because many of the reactions of photosynthesis are catalysed by
enzymes
At low temperatures enzyme-catalysed reactions occur slowly due to a
lack of kinetic energy and therefore few collisions between enzyme and
substrate molecules
At high temperatures enzymes denature
The Calvin cycle is affected by temperature, e.g. rubisco catalyses the reaction
between CO2 and the five-carbon compound ribulose bisphosphate
As long as there is enough light to produce ATP and NADPH in the lightdependent reaction, increasing temperature up to an optimum
temperature will increase the rate of the light-independent reactions and
therefore the rate of photosynthesis
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Temperature can be a limiting factor for photosynthesis
Investigating the rate of photosynthesis
Investigations to determine the effects of light intensity, light wavelength, carbon
dioxide concentration, and temperature on the rate of photosynthesis can be
carried out using aquatic plants such as Elodea or Cabomba
Aquatic plants are especially useful for investigating the rate of
photosynthesis because the waste oxygen gas produced can be easily
collected and measured underwater
The effect of these limiting factors on the rate of photosynthesis can be
investigated in the following ways
Light intensity
Change the distance of a light source from a plant
Light wavelength
Change a colour filter placed over a light source illuminating a plant
Carbon dioxide concentration
Add different quantities of sodium hydrogencarbonate (NaHCO3) to the
water surrounding a plant; this dissolves to produce CO2
Temperature
Place boiling tubes containing submerged plants in water baths of
different temperatures
Whilst changing one of these factors during an investigation, ensure that other
factors remain constant
E.g. when investigating the effect of light intensity on the rate of
photosynthesis, a glass tank should be placed in between the lamp and the
boiling tube containing the pondweed to absorb heat from the lamp; this
prevents the solution surrounding the plant from changing temperature
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Investigating the effect of light intensity on the rate of photosynthesis
Apparatus
Distilled water
Boiling tube
Beaker
Lamp
Aquatic plant, algae or algal beads
Metre ruler
Sodium hydrogen carbonate solution
Thermometer
Boiling tube bung and delivery tube
Measuring syringe
Method
1. Optional: ensure the water is well aerated before use by bubbling air through it
This will ensure that any oxygen gas given off by the plant during the
investigation forms bubbles instead of dissolving
2. Ensure the plant has been well illuminated before use
This will ensure that the plant contains all the enzymes required for
photosynthesis and that any changes of rate are due to changes in light
intensity
3. Set up the apparatus in a darkened room with a 10 cm distance between the lamp
and the boiling tube
This means that no external light sources will affect the rate of photosynthesis
4. Cut the stem of the pondweed cleanly just before placing into the boiling tube
This enables bubbles of oxygen to form from the cut plant stem
5. Submerge the sample of pondweed in sodium hydrogen carbonate solution
This ensures that the pondweed has a controlled supply of carbon dioxide
6. Measure the volume of gas collected in the gas syringe over a set period of
time, e.g. 1 minute; repeat this measurement e.g. 3 times
7. Change the independent variable by moving the light source 10 cm further away
from the plant and repeat step 6
8. Repeat step 7 several more times, moving the lamp 10 cm further away from the
plant each time
9. Record the results in a table and plot a graph of volume of oxygen produced per
minute against the distance from the lamp
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The effect of light intensity on the rate of photosynthesis can be investigated by
measuring the volume of oxygen produced as a lamp is moved further away from an
aquatic plant
Results
The closer the lamp, the higher the light intensity, therefore the volume of oxygen
produced should increase as the light intensity is increased
At a point the volume of oxygen produced will stop increasing even if the light is
moved closer
This is the point at which light intensity stops being the limiting factor and,
e.g. temperature or concentration of carbon dioxide becomes limiting
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Ecosystems

5.9 Ecological Productivity
Ecological Productivity
During photosynthesis organisms such as plants convert light energy into
chemical energy stored in biological molecules
Organisms that do this are known as producers
The rate at which producers convert light energy into chemical energy is known as
primary productivity
Gross primary productivity, or GPP, can be defined as the rate at which chemical
energy is converted into carbohydrates during photosynthesis
Net primary productivity, or NPP, is the GPP minus plant respiratory losses
Of the total energy stored in glucose during photosynthesis, 90 % will be
released from glucose to create ATP for the plant during respiration
90 % of the energy originally converted by the plant will therefore not be
stored as new plant biomass and will not be available to be passed on to
herbivores, also known as primary consumers
The NPP can therefore be defined as the rate at which energy is stored in plant
biomass
NPP is important because it represents the energy that is available to
organisms at higher trophic levels in the ecosystem, such as primary
consumers and decomposers
Net primary productivity can be calculated using the equation
NPP = GPP - R
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Net primary productivity, or NPP, is the rate at which energy is stored in plant
biomass and made available to primary consumers.
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Calculation of Ecological Productivity
Net primary productivity
can be calculated using the equation
NPP = GPP - R
Where
NPP = net primary productivity; the rate at which light energy is stored in
plant biomass
GPP = gross primary productivity; the rate at which light energy is converted
into carbohydrates during photosynthesis
R = respiratory losses; the carbohydrates used up by the plant that are not
stored in plant biomass
NPP is expressed in units of energy per unit area or volume per unit time e.g.
Using area: J m–2 yr-1 (joules per square metre per year)
Using volume: J m–3 yr-1 (joules per cubic metre per year)
Volume would be used when calculating NPP in aquatic habitats
Example
 Worked
The grass in a meadow habitat converts light energy into carbohydrates at a
rate of
17 500 kJ m-2 yr-1. The grass releases 14 000 kJ m-2 yr-1 of that energy
during respiration. Calculate the net primary productivity of the grass in the
meadow habitat.
Step 1: Work out which numbers correspond to which parts of the equation
The meadow grass converts 17 500 kJ m-2 yr-1 into carbohydrates; this is its GPP
The meadow grass releases 14 000 kJ m-2 yr-1 of that energy in respiration; this is
R
Step 2: Substitute numbers into the equation
NPP = GPP - R
NPP = 17 500 - 14 000
Step 3: Complete calculation
17 500 - 14 000 = 3 500
NPP = 3 500 kJ m-2 yr-1
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Tip
 Exam
The worked example above uses the equation in its basic form, but you may
also be expected to rearrange the equation e.g. to calculate GPP or R
If a question provides you with the NPP and R and asks you to calculate
GPP, you will need to use the equation
GPP = NPP + R
If a question provides you with the NPP and the GPP and asks you to
calculate R, you will need to use the equation
R = GPP - NPP
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5.10 Energy & Biomass Transfers
Energy & Biomass Transfer Calculations
Transfer of energy through a food chain
During photosynthesis organisms such as plants convert light
energy into chemical energy stored in biological molecules
Organisms that do this are known as producers
The chemical energy stored in plant biomass is passed to primary
consumers when the plant is ingested
Primary consumers are animals that eat plant material; they can be herbivores
or omnivores
The primary consumer digests the plant tissues and uses the stored chemical
energy either to fuel respiration or to build up biomass; this latter means that
the stored chemical energy is transferred to the tissues of the primary consumer
When the primary consumer is ingested by a secondary consumer the stored
chemical energy passes to the secondary consumer, and so on up the food chain
When an organism dies, the chemical energy stored in its tissues passes
to decomposers such as bacteria and fungi
In a food chain the arrows represent the transfer of energy from one trophic level
to the next by the process of feeding
The term trophic level refers to the stage in a food chain
The light energy converted into chemical energy by producers is passed up the food
chain by the process of feeding
Energy losses in a food chain
The transfer of energy in a food chain is not 100 % efficient; energy is lost to the
environment at every trophic level
A large proportion of the sun's energy is not available to producers for building
biomass
Light passes through leaves or is reflected away
Light hits non-photosynthetic parts of the plant e.g. bark or flowers
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Light is a mixture of wavelengths and only certain wavelengths are absorbed
in photosynthesis
Plants release energy during respiration, some of which is lost to the
environment in the form of heat
When a consumer ingests another organism, not all the chemical energy in the
consumer's food is transferred to the consumer's biomass
Only around 10 % of the energy is available to the consumer
to store in their
tissues
This is because around 90 % of the energy is lost to the environment
Around 90 % of the energy is lost to the environment because
Not every part of the food organism is eaten, e.g. the roots and woody parts
of plants or the bones of animals, meaning that the stored energy in these
uneaten tissues is lost to the environment
Consumers are not able to digest all of the food they ingest, e.g. cellulose in
plants or the fur of animals, so some is egested as faeces; the chemical
energy in this undigested food is also lost to the environment
Energy is lost to the environment in the form of heat when consumers respire
Energy is lost to the environment when organisms excrete the waste products
of metabolism e.g. urea in urine
The energy that is left after these losses is available to the consumer to fuel
their life functions, including being stored in biomass during growth
The rate at which energy is converted into biomass in the body of a consumer
is known as net productivity
Note that this is slightly different to the rate at which energy is converted
into biomass in producers, which is known as net primary productivity
The energy losses at each trophic level act to limit the maximum length of food
chains; organisms at higher trophic levels need to consume more food to gain
enough energy to survive
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Energy is lost to the environment at every trophic level of a food chain
Calculating the efficiency of energy transfer

The efficiency of energy transfer in a food chain can be calculated using the
following equation
Energy efficiency = (net productivity ÷ energy received) × 100
Note that net productivity can be calculated by subtracting energy losses from
energy received
Efficiency of energy transfer is given as a percentage
Example
 Worked
A wheat farmer decides to use biological control against insect pests that
are eating her wheat crop. The farmer introduces a species of toad. By
eating the insect pests the toads ingest 10 000 kJ m-2 yr-1 of energy. The
toads lose 7 000 kJ m-2 yr-1 of this energy as heat from respiration and 2
000 kJ m-2 yr-1 of energy in faeces and urine. Calculate the efficiency of
energy transfer from the insects to the toads.
Step 1: Calculate the net productivity of the toads
Toad energy received= 10 000 kJ m-2 yr-1
Toad energy losses = 7 000 + 2 000 = 9 000 kJ m-2 yr-1
Toad net productivity = 10 000 - 9 000 = 1 000
Toad net productivity = 1 000 kJ m-2 yr-1
Step 2: Substitute values into the equation
Energy efficiency = (net productivity
Energy efficiency = (1 000
÷
energy received) x 100
÷ 10
000)
×
100
÷ 10
000)
×
100
Step 3: Complete calculation
Energy efficiency = (1 000
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Energy efficiency = 10 %
Calculating the efficiency of biomass transfer
It is also possible to calculate the efficiency of biomass transfer from one trophic
level to the next in a food chain
In order to calculate this scientists need to know the biomass of the organisms
concerned
Dry biomass is used, as the amount of water stored in tissues can vary
Dry biomass can be measured by drying a sample of the organism in an oven
at a low heat and weighing the sample at regular intervals until the mass
becomes constant; this will be the dry biomass
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The organism needs to be dead for this process to be carried out, so an
estimate is often used
Depending on the transfer being studied, it may then be necessary to multiply
up the sample to take into account the size of the area or number of
organisms being studied
The efficiency of biomass transfer can be calculated using the following equation
Efficiency of biomass transfer = (biomass transferred ÷ biomass intake) × 100
Biomass transferred refers to biomass that has passed to the higher trophic level
while biomass intake refers to biomass of the lower trophic level that has been
consumed
Efficiency of biomass transfer is given as a percentage
Example
 Worked
A blackberry bush with a dry mass of 35 kg is fed upon by aphids with a
collective dry mass of 4.1 kg. Calculate the percentage efficiency of
biomass transfer in this step of the food chain.
Step 1: Ensure both units are the same
In this case, both are expressed in kg so the units do not need to be converted
Step 2: Substitute the values into the equation
Efficiency of biomass transfer = (biomass transferred
Efficiency of biomass transfer = (4.1
÷
biomass intake) × 100
÷
35)
×
100
÷
35)
×
100
Step 3: Complete calculation
Efficiency of biomass transfer = (4.1
Efficiency of biomass transfer = 11.7 %
Tip
 Exam
It is worth bearing in mind that the biomass of an organism is effectively a
measure of how much chemical energy is stored within it, so a calculation
of the efficiency of biomass transfer essentially provides the same
information as a calculation of the efficiency of energy transfer.
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5.11 Ecology: Key Terms

Ecology: Key Terms
Habitat
Species are adapted, or well-suited, to life in a particular habitat
A habitat can be defined as the place where an organism lives
A habitat can be large, e.g. a desert, or small, e.g. an individual tree
Small habitats are sometimes referred to as microhabitats
Some species are habitat specialists, meaning that they can only survive in a
very specific type of habitat, while others are generalists and can survive in a
range of habitats
Generalists are more likely to be able to invade and take over a new
habitat; such species are known as invasive species
Humans sometime release new species into a habitat, either
accidentally or on purpose; these species can disrupt the normal
species interactions in a habitat and cause serious problems
Population
When a species is found in a habitat, that habitat is said to support a population
A population can be defined as all of the individuals of one species living in a
habitat
The size of a population can be measured; this is the abundance of a species
in a habitat
The exact location of a population within a habitat is a species' distribution
within that habitat
Community
Species do not exist by themselves in their own isolated environment;
they interact with other species, forming communities
A community can be defined as multiple populations living and interacting in the
same area
For example, a garden pond community is made up of populations of fish,
frogs, newts, pond snails, damselflies and dragonflies and their larvae,
pondweed, water lilies, and all other populations living in the pond
Ecosystem
Communities interact with the non-living components of the habitat they live in,
forming ecosystems
An ecosystem can be defined as a community and its interactions with the nonliving parts of its habitat
There is a flow of energy within an ecosystem and nutrients within it are
recycled
There are both biotic and abiotic components within an ecosystem
Ecosystems vary greatly in size and scale
A small pond in a back garden and the open ocean could both be
described as ecosystems
Ecosystems vary in complexity
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A desert is a relatively simple ecosystem
A tropical rainforest is a very complex ecosystem
No ecosystem is completely self-contained as organisms from one ecosystem
can often move to another
E.g. birds and aquatic animals are able to migrate long distances to feed
from multiple ecosystems
Individual members of a species together in a habitat form a population, populations
interact within a community, and communities interact with each other and with nonliving components of their habitat to form an ecosystem.
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5.12 Factors Affecting Populations
Factors Affecting Populations
The abundance and distribution of a species within a habitat are determined by a
combination of biotic and abiotic factors
Biotic factors are living factors that influence populations within their community;
biotic factors come about as a result of the activity of other organisms e.g.
Predation
Food availability
Intraspecific competition, arising when individuals of the same species
compete for resources
Interspecific competition, arising when individuals of different species
compete for resources
Cooperation between organisms
Parasitism
Disease
The Impact of Biotic Factors on a Community Table
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are non-living factors that influence populations within their
community e.g.
Light intensity and wavelength
Temperature
Turbidity, or cloudiness, of water
Humidity
Soil or water pH
Soil or water salinity
Soil composition
Oxygen or Carbon dioxide concentration
Abiotic factors
The impact of Abiotic Factors on a Community Table
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5.13 Niche

The Concept of Ecological Niche
The place where a species lives is known as its habitat
Species will occupy a specific niche within a habitat
The term niche can be defined as the role of a species within its habitat
The role of a species includes
What it eats
Which other species depend on it for food
What time of day a species is active
Exactly where in a habitat a species lives
Exactly where in a habitat a species feeds
No two species can fill the same niche within a habitat; if this ever happens the
two species will be in direct competition with each other for resources, and one of
the two species will out-compete the other, causing it to die out in that particular
habitat
It can sometimes seem as though species are occupying the same niche, but
there will still be subtle differences in their role; e.g. they might feed at
different times of day, or have different food sources
The niche filled by a species determines its abundance within a habitat
The term abundance can be defined as the number of individuals of a
particular species living in a habitat
If two species occupy a similar niche within a habitat they will be competing
with each other, so their populations will be smaller, and their abundance
will therefore be lower
The niche filled by a species determines its distribution
The term distribution can be defined as where a species lives
Species can only survive in habitats to which they are well adapted; if they are
not well suited to a habitat's biotic and abiotic factors then they will move to a
more suitable habitat and their distribution will change
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Feeding location is an example of a feature that may differ between niches.
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5.14 Core Practical 11: Quadrat & Transect Study
Quadrat & Transect Study
Finding out about the abundance and distribution of species can be achieved by
counting all of the organisms present in a habitat
This is possible for areas that are very small or where the species are very large
For larger and more complex habitats it is not possible to find, identify, and count
every organism that is present
When this is the case sampling can be used to make an estimate for the total
species numbers
Sampling
Sampling is a method of investigating the abundance and distribution of species
and populations
There are two different types of sampling
In
Random
Systematic
random sampling
the positions of the sampling points are selected at random
This method avoids bias by the person that is carrying out the sampling
Bias can affect the results e.g.
A student might choose to carry out samples in a particular location
because it looks interesting, and this might give the impression that the
habitat contains more species than it really does
In systematic sampling the positions of the sampling points are located at fixed
intervals throughout the sampling site
This avoids accidentally missing out sections of habitat due to chance
Systematic sampling allows researchers to investigate the effect of the
presence of certain environmental features on species distribution e.g. by
taking samples along a line that extends away from an environmental feature
such as a river
A line of this type is known as a transect
When a sampling area is reasonably uniform then random sampling is the best
choice
Random sample sites can be selected by
Laying out a grid over the area to be studied
Generating random number co-ordinates
Placing sample sites in the grid squares that match the random number coordinates
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Random sampling involves selecting sample sites at random while systematic
sampling involves placing sample sites at regular intervals.
Determining distribution and abundance
The distribution and abundance of a species in an area can be assessed using
different practical methods:
Frame and point quadrats
Line and belt transects
Frame quadrats
A frame quadrat is a square frame that is placed within the area to be studied to
provide a sample
Quadrats are used to study the distribution of sessile organisms
Quadrats can be different sizes depending on the species being studied
A 1 m² quadrat can be used to study small organisms such as herbaceous
plants in a grassland or limpets on a rocky shore
A 400 m² quadrat can be used to study large organisms such as trees
Quadrats like this will usually be marked out with string rather than a
frame!
Frame quadrats can be placed in a habitat randomly, e.g. using random coordinates, or systematically, e.g. along a transect
A frame quadrat can be used to measure abundance and distribution
Scientists can record different types of data from a frame quadrat depending on
the aim of a study and the species involved
Presence or absence of a species
Species frequency; how many individuals are in the quadrat
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Species abundance;
measured on a scale called the ACFOR scale on which
species are recorded as being abundant, common, frequent, occasional, rare,
or none
Percentage cover; the percentage of the quadrat covered by a species
Quadrats can be divided up into smaller squares to allow percentage cover
to be assessed more easily
Abundance in a frame quadrat can be assessed by measuring percentage cover
Point quadrats
A point quadrat is a vertical frame with holes across the top through which pins
are lowered
This is useful in areas with dense plant cover as the ground may be difficult to
study using a frame quadrat
Point quadrats can be placed in a habitat randomly, e.g. using random coordinates, or systematically, e.g. along a transect
When a lowered pin touches a species, that species is recorded as being present
If several species are touching the pin then all of those species are recorded
Point quadrats can be used to measure abundance in the following ways
The number of individuals of a species present
Each individual that touches a pin is recorded
Percentage cover of a species
The number of pins touched by a species is divided by the total number of
pins used
Most point quadrats have ten pins, so if all the pins are used then a
species touching one pin counts as 10 percent cover
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Point quadrats can be used to measure the the number of individuals of a species or
percentage cover
Transects
are lines laid out across a site that can be used to measure abundance
and distribution across a habitat
Transects are useful for determining how species abundance and distribution
might change along a gradient e.g. at increasing distances from a field
margin or perpendicular to the water's edge on a rocky shore
To carry out a transect, a tape measure is laid out along the gradient of interest,
and samples are taken along the line
There are different ways of carrying out transect studies
Transects
Continuous line transect
Every species touching
Interrupted line transect
the tape measure is recorded
Species touching the line at regular intervals, e.g. every metre, are
recorded
Continuous belt transect
Frame quadrats are placed end-to-end along the line
Interrupted belt transect
Frame or point quadrats are placed at regular intervals, e.g. every metre,
along the line
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Interrupted line and belt transects can be used to measure abundance and
distribution of species along a line across a habitat
Measuring abiotic factors
The distribution and abundance of species in a habitat are affected by abiotic
factors
When investigating the impact of an abiotic factors on species abundance and
distribution it is important to measure the relevant abiotic factors at the sample
sites
It is only necessary to record relevant abiotic factors
A study may only be interested in one particular abiotic factor
Some abiotic factors may not be relevant in certain habitats e.g. water
turbidity (cloudiness) will not be relevant in a woodland habitat
Abiotic factors can be measured using specialised equipment and techniques
Measuring Abiotic Factors Table
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
Representing results
The results of an investigation into the distribution and abundance of organisms
can be represented visually using a type of graph known as a kite diagram
Kite diagrams can show both distribution and abundance
The distribution of a species along a transect can be shown by its position
along a central horizontal line in each section of a kite diagram
Each section represents a different species
The distance along the transect is given on the x-axis, to which the
horizontal line is parallel
The abundance of a species can be shown by the width of the 'kite' around the
central horizontal line
The shape is referred to as a kite because it extends an equal distance on
each side of the central horizontal line
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Additional sections can be added to a kite diagram to show the changes in abiotic
factors at different points along a transect e.g. the height above sea level or the
pH of soil
Kite diagrams can be used to provide a visual representation of both abundance and
distribution of species, as well as changes to abiotic factors such as elevation
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Tip
 Exam
You could be asked to describe or design an investigation that could be
used to measure the effect of a specific abiotic factor on species abundance
or distribution, so make sure that you know the circumstances in which each
sampling technique would be used, and how to use it.
Remember that when describing a practical you should always consider:
How you will change the independent variable
In this context you might be measuring a change in the independent
variable, or abiotic factor, rather than causing the change yourself
Note that this might not be relevant if you have just been asked to
measure the abundance of a species in one habitat
How you will measure the dependent variable
How you will ensure that your results are valid
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5.15 Ecological Succession
Ecological Succession
Ecosystems are dynamic, meaning that they are constantly changing
The process of ecosystem change over time is known as succession
During succession, the biotic and abiotic conditions change
Primary succession is the process that occurs when newly formed or newly
exposed land is inhabited by an increasing number of species
Newly formed land can be created by e.g.
The magma from erupting volcanoes cooling and forming new
rock surfaces or new rocky islands in the sea
Newly exposed land can form by e.g.
A landslide that exposes bare rock
A glacier that retreats to reveal bare rock
The arrival of organisms on bare land is known as colonisation, and the bare land
is said to be colonised
Primary succession occurs in a series of stages
can occur on any type of bare land, including sand dunes at
the edge of the ocean, and on exposed rock
Primary succession on bare rock involves the following stages
Seeds and spores that are carried by the wind land on exposed rock and
begin to grow
The first species to colonise the new land, often mosses and lichens, are
known as pioneer species
Pioneer species can germinate easily and withstand harsh conditions
such as low nutrient and water availability
As pioneer species die and decompose, the dead organic matter forms soil
Seeds of small plants and grasses land on this soil and begin to grow
The plants at this early stage of succession are adapted to survive
in shallow, nutrient-poor soils
The roots of these small plants form a network that helps to hold the soil in
place and prevent it from being washed away
As these small plants die and decompose, the soil becomes deeper and
more nutrient-rich
Larger plants and shrubs, as well as small trees can now begin to grow
These larger plants and small trees also require more water, which can be
stored in deeper soils
Over time the soil becomes sufficiently deep, contains enough nutrients, and
can hold enough water to support the growth of large trees
The final species to colonise the new land become the dominant species of
the now complex ecosystem
The final community formed, containing all the different plant and animal
species that have now colonised the land, is known as the climax community
The type of climax community that forms depends on the location of the
original bare land; in the tropics the climax community would be a rain
forest, while in temperate regions it might be deciduous woodland
Primary succession
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A climax community is not always the most biodiverse stage of
succession, but it is a stable community
Primary succession is the process of ecosystem change over time, beginning with
newly formed or newly exposed land
Succession changes the biotic and abiotic conditions
At each stage in succession the newly arriving species change the local
environment so that it becomes more suitable for other species that have not yet
colonised the new land e.g.
Pioneer species such as lichens help to slowly break apart the top surface of
bare rock; this fragmented rock, along with the dead organic matter left
behind when the lichens die and are broken down, forms a basic soil
Species such as grasses grow roots that stabilise the soil, enabling it to hold
more moisture and nutrients
Often the new colonising species then change the environment in such a way that
it becomes less suitable for the previous species e.g.
Lichens cannot grow on soil so they disappear from the ecosystem once soil
begins to form; the new species change the environment in such a way that it
becomes less suitable for the lichens
Pioneer species may not be found in a climax community as they will
be out-competed for light and other resources by the species that arrive
during the later stages of succession
Pioneer species are well adapted for harsh conditions but are often poor
competitors
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As soil deepens and trees are able to grow, they may block out the light to
shrubs and other smaller plant, out-competing them and causing them to die
Secondary succession
There is also a type of succession called secondary succession which takes place
on previously occupied land e.g. after a wild fire or deforestation
Secondary succession is very similar to primary succession except that soil is
already present so the process begins at a later stage
Humans can prevent succession
Human activities often prevent or interrupt the process of succession
This stops a climax community from developing e.g.
Regular mowing prevents woody plants from establishing themselves
in a
lawn
The grazing activity of livestock such as sheep and cattle prevent new plants
from establishing
Climax communities that develop as a result of human intervention are known as a
plagioclimax; these communities are stable but would not have occurred without
human intervention, e.g. heathland
Tip
 Exam
You could be presented with an example of succession other than the one
provided here e.g. succession on a sand dune. As long as you understand
the principles of the stages of succession you should be able to apply your
knowledge to any example that an exam question might throw at you.
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Environmental Biology

5.16 Evidence for Climate Change
Evidence for Climate Change
The term climate refers to the weather conditions in a region over a long period
of time i.e. several decades
When the weather conditions in a region change significantly over a long period of
time, this is climate change
The term climate change is most often used today to refer to global
warming that is occurring as a result of human activities
Scientists have long hypothesised that
Climate change in the form of global warming is currently taking place
Human activities that increase the concentrations of greenhouse gases in the
atmosphere are responsible for climate change
Several different types of evidence can be used to support these hypotheses
Records of atmospheric carbon dioxide levels
Records of average global temperatures
Records of changing plant communities gained from sampling of pollen
grains preserved in peat over time
Records of tree growth gained by analysing the rings in the trunks of trees;
known as dendrochronology
Atmospheric carbon dioxide
Atmospheric carbon dioxide levels have fluctuated throughout Earth's history due
to events such as volcanic eruptions and the weathering of limestone rocks
Scientists know this from having analysed the gas composition of bubbles
formed in ancient ice cores
Ice is deposited as water freezes over time, so the deeper into the ice you
go, the older it is
Since the industrial revolution, however, atmospheric carbon dioxide levels
have risen to their highest in Earth's history
Prior to the industrial revolution, the highest atmospheric carbon dioxide
concentration was around 300 parts per million (ppm), and it is currently
above 400 ppm
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Atmospheric carbon dioxide levels have fluctuated throughout earth's history, but
recent increases have been faster and greater than ever before
Data show a correlation between changing atmospheric carbon dioxide levels and
temperature over thousands of years
Note that carbon dioxide in the atmosphere is not thought to be the only
factor affecting climate; it is known that events such as solar winds and sun
spots can affect the climate on Earth, but scientists think that the effects of
such events are small in comparison to that of atmospheric carbon dioxide
Correlation does not equal causation, but together with what scientists know
about carbon dioxide as a greenhouse gas, this is strong evidence that carbon
dioxide released by human activities since the industrial revolution is causing
increasing global temperatures
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There is a correlation between atmospheric carbon dioxide concentrations and
average antarctic temperatures over time
Average global temperatures
Thermometers can be used to measure air temperature, and thermometer records
from different places around the world over extended periods of time can be put
together to show average global temperature change over time
Records from the mid-1800s show an overall trend of increasing average global
temperatures
There are some short time periods within this window during which
temperatures have declined, but the overall trend is upwards
The time period since the mid-1800s corresponds with the time during which
humans have been burning fossil fuels and therefore releasing carbon dioxide
into the atmosphere
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Average global temperature records show some temperature fluctuations but an
overall trend of increasing temperatures over time
Pollen grains preserved in peat bogs
Under waterlogged and acidic conditions partly decomposed dead plant matter
accumulates and becomes compacted under its own weight over time; this
compacted, partially decomposed plant matter forms peat
The place where peat accumulates is known as a peat bog, or peatland
Peat builds up in layers, meaning that layers of peat at the top of a bog are
recently formed and the peat become older as you dig down into a bog
Peat cores can be taken from a bog and the layers can be analysed to assess the
pollen grains that have become trapped in the peat
Pollen grains from peat samples can be observed under a microscope, and
because the pollen grains of each plant species are unique to that plant, the plant
species that were growing around the bog at different points in time can be
identified
Different plant species grow under different climatic conditions, so the plants
present at different times can be used a measure of the climate at that time
E.g. an increase in the number of plant species that grow better in warmer
climates combined with a decrease in the number of plant species that grow
better in cooler climates indicates a gradual warming of the climate
Dendrochronology
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Tree trunks grow in diameter each growing season as they produce more vascular
tissue
This vascular tissue grows in a ring around the outside of the trunk
Light coloured rings are produced by fast growth during warmer spring and
summer months and dark coloured rings form as a result of slow autumn
growth, meaning that one light ring and one dark ring together represent a full
year's growth in a tree
Trees grow faster when conditions are warmer, so the rings that form during
warm years are wider than the rings that form during cool years
Analysis of the width of tree rings can provide a measure of climate during each
year of growth
Taking cores from the trunks of older trees can provide samples that go back
over hundreds of years
Dendrochronology uses the growth in a tree trunk each year as a measure of climate
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5.17 Anthropogenic Climate Change
Anthropogenic Climate Change
When radiation from the sun hits the earth, it is radiated back from the earth's
surface
A greenhouse gas is a gas that absorbs this re-radiated radiation, trapping it in
the earth's atmosphere so that it is not lost to space
Greenhouse gases in the atmosphere have a similar effect to the glass in
a greenhouse, hence the term greenhouse gas, and their effect being known
as the greenhouse effect
The greenhouse effect is important to ensure that Earth is warm enough for life; if
it were not for the insulating effect of greenhouse gases, Earth would see similar
dramatic temperature fluctuations to its neighbouring planets
Temperatures on Mars range between 20°C and −153°C
There are many greenhouse gases including
Carbon dioxide
Methane
It is thought that increasing levels of carbon dioxide and methane are entering the
atmosphere as a result of human activities, leading to increased rates of
atmospheric warming
The atmospheric warming, and therefore the changing climate, for which
humans are thought to be responsible is known as anthropogenic climate
change
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Greenhouse gases absorb radiation re-emitted from the earth's surface, trapping it in
the atmosphere
Carbon Dioxide
Atmospheric carbon dioxide levels have fluctuated throughout Earth's history due
to events such as volcanic eruptions and the weathering of limestone rocks
Since the industrial revolution, however, atmospheric carbon dioxide levels
have risen to their highest in Earth's history
The industrial revolution began in the late 1700s when the combustion of fossil
fuels to power factories, transport, and homes became commonplace
Fossil fuel combustion releases carbon dioxide
A clear correlation can be seen between increasing levels of carbon dioxide since
the industrial revolution and increasing global temperatures, providing evidence
for the role of human activities in causing global warming
Note that a correlation alone is not enough to prove causation, but this
evidence can be taken alongside what we know about greenhouse gases and
other evidence to provide a growing body of proof
In addition to the burning of fossil fuels, carbon dioxide is also released into the
atmosphere when natural stores of carbon are damaged or destroyed by human
activities
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These carbon stores are known as carbon sinks
Carbon sinks include trees, soils, peat bogs, and the oceans
Deforestation, soil degradation, peat harvesting, and ocean warming all
contribute to the addition of carbon dioxide to the atmosphere
Methane
Methane (CH4)
is a simple hydrocarbon
It is present as a gas in the atmosphere, and underground, and is the main
component of natural gas fossil fuel
Methane can be produced by naturally occurring processes in some types of
bacteria, but levels have risen significantly in the last 150 years due to human
activities
Methane can be produced by several human activities
Methane is released from the guts of ruminant mammals such as cattle
While this is clearly not a direct human activity(!) the intensive farming of
such animals has greatly increased their contribution to atmospheric
methane
Landfill sites release methane when organic matter such as food waste
decomposes
Extraction of fossil fuels from underground releases methane
Anaerobic bacteria in waterlogged rice paddy fields release methane
In addition to the list above, the warming of the poles that results from global
warming also leads to the release of methane from natural stores such as
permafrost
Permafrost is ground that remains frozen all year round
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Atmospheric concentrations of both carbon dioxide and methane have increased
since the industrial revolution due to human activities. Note that ppm = parts per
million and ppb = parts per billion.
Example
 Worked
The graph below shows changes in average global temperatures and
atmospheric carbon dioxide concentrations since the year 1000; describe
the data and explain what can be concluded from the graph
Descriptions of data must include
that don’t fit with the trend
any trends, as well as any sections of data
Descriptions of data should also include numbers to support the description
Be careful that any conclusions reached are taken directly from the data and
do not go beyond what the data show
Step 1: Describe the data
Atmospheric carbon dioxide levels and average global temperatures have both
increased since the year 1000
Atmospheric carbon dioxide concentration has increased from around 280 ppm to
around 380 ppm
Average global temperatures have increased from around 13.8 °C to around 14.4 °
C
Average global temperatures have fluctuated, showing periods of decrease e.g.
during the 1400s and periods of increase e.g. during the early 1700s
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Atmospheric carbon dioxide levels and average global temperatures were both
reasonably constant between the years 1000 and the mid-1800s/1900, and then
both show a steep increase between 1900 and 2000
Step 2: State what can be concluded
There is a correlation between atmospheric carbon dioxide concentration and
average global temperature
Both atmospheric carbon dioxide concentration and average global temperature
increase from the industrial revolution onward
Average global temperatures fluctuate at times when atmospheric carbon dioxide
concentrations are relatively constant
Note that you cannot conclude a causal relationship from this data alone; in fact
the fluctuations in temperature when carbon dioxide levels are constant suggest
that there are other factors involved
Tip
 Exam
Note that the greenhouse effect, global warming and climate change are
terms that are often used interchangeably, but in fact they have slightly
different meanings:
refers to the rise in global temperatures mainly due to
the increasing concentrations of greenhouse gases in the atmosphere.
Climate change refers to the increasing changes in the measures of
climate over a long period of time – including precipitation,
temperature, and wind patterns. These are often a consequence of
global warming.
The greenhouse effect is a naturally occurring event, constantly
occurring due to the atmosphere and sunlight.
Global warming
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5.18 Carbon Cycle & Environmental Management
The Carbon Cycle
The atmospheric carbon dioxide that contributes to the greenhouse effect is part
of the global carbon cycle
The term carbon cycle refers to the many processes by which carbon is transferred
and stored, e.g.
Carbon is found in the biomass of living organisms e.g.
in carbohydrates and proteins
Carbon is transferred when one organism consumes and digests another
Carbon is found in the atmosphere as carbon dioxide and in the oceans as
e.g. hydrogen carbonate ions
The carbon cycle
The following events occur during the carbon cycle
Carbon is present in the atmosphere in the form of carbon dioxide
Carbon dioxide is removed from the atmosphere by producers during
photosynthesis
Producers incorporate carbon into their biomass in the form of
carbohydrates and other biological molecules
Carbon is transferred to and between consumers as a result of feeding
Carbon is transferred back into the atmosphere by both plants and animals as
a result of respiration
Respiration releases carbon dioxide as a product
Carbon dioxide can also be removed from the atmosphere by dissolving in the
oceans
Dissolved carbon can be taken in by marine plants when they
photosynthesise or by other marine organisms as they build calcium
carbonate exoskeletons
When living organisms die their tissues are broken down by decomposers
such as bacteria and fungi
When these organisms respire, they too release carbon dioxide back into
the atmosphere
Any living tissue that is not fully decomposed can go towards the formation of
peat or fossil fuels over millions of years; carbon can be stored in these sinks
for long periods
The combustion of peat and fossil fuels releases carbon dioxide back into the
atmosphere
The combustion of biomass such as wood also returns carbon to the
atmosphere
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The carbon cycle includes the locations in which carbon is stored, shown here as
'carbon pools', and the processes by which it is transferred, shown here as 'carbon
fluxes'
Carbon Cycle & Environmental Management
A good understanding of the carbon cycle is essential in the fight against global
warming
It is possible to see the points at which carbon enters the atmosphere;
reducing the carbon transfer at these points will prevent further increases in
atmospheric carbon dioxide e.g.
Reducing the combustion of fossil fuels
Reducing the combustion of biomass
Reducing disturbance of carbon pools such as soils and peat bogs
We can also see the points at which carbon is removed from the atmosphere;
increasing the transfers here could help to reduce the greenhouse effect e.g.
Increasing rates of photosynthesis by planting trees
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5.19 Models for Predicting Climate Change
Models for Predicting Climate Change
It is possible to use existing data relating to global warming to make predictions
about global temperatures in the future
Using data in this way is known as extrapolating from data
Extrapolated data can be used to produce models that show how the climate
may change in the future
Global warming predictions can be used to
Plan for the future e.g.
Building flood defences
Funding scientific research into climate change technologies
Encourage people to change their activities e.g.
Reduce the burning of fossil fuels
Increase the use of renewable energy sources such as solar and wind
energy
Reduce meat consumption
The Intergovernmental Panel on Climate Change, or IPCC, is a group of climate
scientists around the world that has used existing data to extrapolate how global
temperatures might change in the future under different human activity scenarios
e.g.
If humans manage to immediately begin reducing fossil fuel use, global
temperature change could be limited to around 2°C hotter than pre-industrial
times
If humans do nothing to change their fossil fuel use, global temperature
increase may exceed 4°C
The IPCC data can be added to other computer models on climate change to see
how different parts of the world might be affected under the different scenarios
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Future predictions of temperature change can be modelled on a range of scenarios
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Limitations of Climate Change Prediction Models
There are limitations to models based on extrapolated data
The IPCC has produced models based on several emissions scenarios, and we
do not know which of these scenarios is most likely
I.e. we don't know how successful humans will be at cutting greenhouse
gas emissions
We do not know whether future technologies will be successful at removing
greenhouse gases from the atmosphere e.g. carbon capture technologies may
or may not be effective
It is unknown exactly how atmospheric gas concentrations might affect global
temperatures
Global climate patterns are complex and therefore predictions are difficult
It is possible that a certain tipping point in global temperatures could
lead to a sudden acceleration in global warming e.g. permafrost melting
may cause a sudden increase in atmospheric methane
Permafrost is ground that is frozen all year round
We don't know exactly how factors other than human activities may affect
climate in the future e.g. a volcanic eruption could increase ash in the
atmosphere, reflecting radiation back into space and cooling the earth
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5.20 How Climate Change Affects Species
How Climate Change Affects Species
absorb infrared radiation emitted by the earth, causing the
atmosphere to warm
The higher the concentration of greenhouse gases in the atmosphere, the more
infrared radiation is absorbed , and the warmer the atmosphere will become
Increased atmospheric warming has had, and will have, multiple impacts on
climate patterns, e.g.
Weather events becoming more extreme e.g. hotter, longer, heatwaves, and
more violent storms
Changes to ocean currents leading to altered local climates e.g. the Gulf
stream that currently brings warm water to the west coast of the UK might
change direction, causing parts of the UK's climate to cool
Warmer air can hold more moisture, leading to changes in patterns of
rainfall; more, heavier rainfall in some places could lead to reduced rainfall in
other locations
Greenhouse gases
for some of these changes in climate patterns can already be seen in
many parts of the world
Warming climates cause animals to move towards the poles or to higher
Evidence
altitudes
A concern is that these species may not be able to compete with, or may
even out-compete, the species already present in these habitats, with
either result leading to decreased biodiversity
Some species, such as plant species, may not be able to move or change
their distribution fast enough to adapt to changing temperatures and
may become extinct as a result
Water availability in some habitats is changing
Changes to rainfall patterns can be devastating to species that rely on
seasonal rains for their survival e.g. some desert plants rely on rains that
may come only once a year and climate change may mean that such
seasonal events occur less frequently or stop altogether
Some species may no longer be able to survive in their habitat due to a
lack of rainfall; such species may migrate to a new habitat or may become
extinct
Seasonal cycles are changing e.g.
Plant species are producing flowers earlier in the year
Animals are producing young earlier in the year
Bird migratory patterns may lose their synchronisation with their habitats,
leading to a change in migration patterns
E.g. earlier plant growth leads to alterations in invertebrate life cycles,
meaning that when a bird species arrives for its summer migration
their usual food source is not available
Polar ice and glaciers are retreating ; it is thought that there may soon be no
summer ice in the arctic if rates of warming there continue
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The loss of glacier ice from mountain ranges may affect the water supplies
of many people and surrounding wildlife
Sea levels have been rising faster in recent years, putting many more people
and animals at risk of being flooded out of their homes
Sea levels are rising due to the expansion of warmer water and due to
melting polar ice
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5.21 The Effect of Temperature on Enzyme Reactions
The Effect of Temperature on Enzyme Reactions
Changing air temperature can have a significant impact on the metabolism of
living organisms due to the effect of temperature on enzyme activity
Enzymes have a specific optimum temperature
This is the temperature at which they catalyse a reaction at the maximum rate
Lower temperatures either prevent reactions from proceeding or slow them down
Molecules move relatively slowly as they have less kinetic energy
Less kinetic energy results in a lower frequency of successful
collisions between substrate molecules and the active sites of the enzymes
which leads to less frequent enzyme-substrate complex formation
Substrates and enzymes also collide with less energy, making it less likely for
bonds to be formed or broken
Higher temperatures cause reactions to speed up
Molecules move more quickly as they have more kinetic energy
Increased kinetic energy results in a higher frequency of successful
collisions between substrate molecules and the active sites of the enzymes
which leads to more frequent enzyme-substrate complex formation
Substrates and enzymes also collide with more energy, making it more likely
for bonds to be formed or broken
Denaturation
If temperatures continue to increase past a certain point, the rate at which an
enzyme catalyses a reaction drops sharply as the enzymes begin to denature
The increased kinetic energy and vibration of an enzyme puts a strain on its
bonds, eventually causing the weaker hydrogen and ionic bonds that hold the
enzyme molecule in its precise shape to start to break
The breaking of bonds causes the tertiary structure of the enzyme to change
The active site is permanently damaged and its shape is no
longer complementary to the substrate, preventing
the substrate from binding
Denaturation has occurred if the substrate can no longer bind
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At high temperatures enzymes can denature
The rate of an enzyme catalysed reaction is affected by temperature. Note that 35 °C
is not the optimum temperature for all enzyme-controlled reactions.
Calculating the temperature coefficient
The temperature coefficient, represented by Q10, calculates the increase in rate of
reaction when the temperature is increased by 10 ° C
Q10 can be calculated using the following equation
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Q 10 = rate at higher temperature ÷ rate at lower temperature
A Q10 value of 2 indicates that the reaction rate doubles with an increase in
temperature of 10 ° C, while a value of 3 indicates that it trebles with every 10 °C
increase
Example
 Worked
In an enzyme catalysed reaction the rate of reaction can measured by
recording the volume of product produced per unit time at different
temperatures.
At 30 ° C 3.5 cm3 s-1 of product was recorded and at 40 °C 6.8 cm3 s-1 was
recorded. Calculate Q10 for this reaction.
Step 1: Write out the relevant equation
Q10 = rate at higher temperature
÷
rate at lower temperature
Step 2: Substitute numbers into the equation
Q10 = 6.8
÷ 3.5
Step 3: Complete calculation
Q10 = 1.94
This value is close to 2, indicating that the rate of reaction has almost doubled
Enzyme activity and living organisms
Changes to enzyme activity that result from changing global temperatures can
affect living organisms
Some chemical reactions take place faster at higher temperatures
Photosynthesis is essential for converting carbon dioxide into carbohydrates,
the process which produces food for producers and other organisms higher
up the food chain; it relies on the function of proteins in the electron transport
chain and that of enzymes such as rubisco
E.g. blue-green algae, also known as cyanobacteria, photosynthesise at a
higher rate in warmer water due to increased enzyme activity; this
increases the formation of potentially harmful algal blooms
Some chemical reactions are slowed down at higher temperatures
At high temperatures plants carry out a reaction called photorespiration at a
faster rate; this reaction uses the enzyme rubisco and so slows down
photosynthesis
This can reduce crop yields
as temperatures rise
Some fish eggs have been shown to develop more slowly at higher
temperatures
Many species' successful egg development is dependent on temperature,
with impacts such as
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Extreme temperature fluctuations can reduce hatching rates in some
invertebrates
The sex of the young inside the egg of some species is determined by
temperature, so increasing temperatures can affect the sex ratios in a species
E.g. in alligators
Species may have to change their distribution in response to changing
temperatures in order to survive
Species may migrate to higher altitudes or further from the equator to find
cooler temperatures
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5.22 Core Practical 12: Effect of Temperature on Development
Effect of Temperature on Development
Evidence from the natural world indicates that global warming affects the
development of living organisms
It is possible to investigate the effect of temperature change on development in the
laboratory
This enables the elimination of other factors that might influence
development e.g. light intensity or food availability
It is possible to investigate the effect of temperature on
Seedling growth rate
Rate of hatching in brine shrimps
Temperature and seedling growth rate
1. Plant seeds of the same plant variety in several pots or trays of compost
Ensure that all seeds are kept in identical conditions at this stage
2. Allow the seeds to germinate and produce some initial days of growth
3. Measure the initial height of every seedling
4. Place each pot or tray into an incubator at a different temperature for the same
amount of time e.g. 5 days
During this time ensure that all factors other than temperature are kept the
same e.g. soil moisture, soil pH, light intensity
5. After the allotted time remove the seedlings from the incubators and record the
final height of every seedling
6. Use the measurements and the following formula to calculate the average growth
rate of seedlings in each incubator per day
average growth rate = average change in seedling height for incubator ÷ days of
incubation
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Seedling growth rate increases as temperature increases up to 25 °C, after which
growth rate decreases. This could be due to cellular enzymes denaturing at high
temperatures.
Temperature and brine shrimp hatching
1. Place an equal number, e.g. 40, of brine shrimp eggs into a series of water baths
at different temperatures
Water baths should contain non-chlorinated water with 2 g of salt added per
100 cm3
A magnifying lens may be needed to count the eggs
A wet piece of paper can be used to pick up and transfer the eggs to the water
bath
Ensure that all factors other than temperature are kept the same between
water baths e.g. age of shrimp eggs, water pH, water volume, dissolved
oxygen concentration
2. Observe and record the number of brine shrimps that hatch at set time intervals
e.g. every 12 hours
A bright lamp can be used to illuminate the water bath and count the
hatchlings
3. Use the number of eggs hatched to calculate the hatching rate per hour
hatching rate = number of hatched shrimp eggs ÷ hours in water bath
Note that brine shrimps are living organisms and so welfare considerations
should be taken into account when using them for experimental purposes
Hatched shrimps should be returned to a suitable environment that replicates
their natural habitat at the end of the experiment
Any handling and transfer of hatched shrimps should be carried out gently
and quickly
Dangerously high temperatures should not be used
Any bright lamp used for observation should be switched off when not in use
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Shrimp hatch rate increases as temperature increases up to a temperature of 25 °C,
after which hatch rate decreases
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5.23 Mutation, Natural Selection & Evolution
Mutation, Natural Selection & Evolution
Species do not stay the same over time; the species that we see around us today
have developed over millions of years
This process of species change is known as evolution
Evolution can be defined as changes in the heritable characteristics of organisms
over generations
Heritable characteristics
the next generation
are those that can be inherited by, or passed on to,
Changes in characteristics that are not inherited, e.g. a plant having its
leaves eaten, do not lead to evolution
Heritable characteristics are determined by the alleles of genes that are
present in an individual
Alleles may change as a result of random mutation, causing them to become
more or less advantageous
Heritable characteristics that are advantageous are more likely to be passed on to
offspring, leading to a gradual change in a species over time
This is the process of natural selection
Natural Selection
Natural selection can be defined as the process by which organisms that are
better adapted to their environment survive, reproduce, and pass on their
advantageous alleles, causing advantageous characteristics to increase in
frequency within a population
Natural selection involves the following stages
Variation exists between individuals in a population
Natural selection can only take place if variation is present
Variation results from small differences in DNA base sequences between
individual organisms within a population
Sources of variation include
Mutation
Meiosis
Random fertilisation during sexual reproduction
In any habitat there are environmental factors that affect survival chances
E.g. predation, competition for food, and disease
Environmental factors that influence survival chances are said to act
as selection pressures
In any population, due to the variation present, some individuals will
have characteristics that make them better adapted for survival in the face of
any selection pressures
This is sometimes described as 'survival of the fittest'
Individuals that are well adapted and survive into adulthood are more likely
to find a mate and reproduce, producing many offspring
Individuals that are less well adapted do not survive long into adulthood
are likely to reproduce less often than those that survive for longer, so
producing fewer offspring
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These individuals may not reach adulthood and so do not get the
chance to reproduce at all
This means that they are more likely to pass on the alleles that code for
these advantageous characteristics to their offspring
Note that non-heritable characteristics are not passed on to offspring
Non-heritable characteristics are those acquired during the
lifetime of an organism e.g. gaining weight after eating lots of nuts
and berries in autumn, or being injured by a predator
The number of individuals in a population with a particular favourable
characteristic will increase over time; the characteristic is said to increase in
frequency
Eventually this favourable characteristic will become the most common of its
kind in the population; the population can be said to have adapted to its
environment by the process of natural selection
While favourable characteristics increase in frequency by natural
selection, unfavourable characteristics decrease in frequency by the
same process
Individuals with unfavourable characteristics are less likely to survive,
reproduce, and pass on the alleles for their characteristics, so
unfavourable characteristics are eventually lost from the population
An example of natural selection in rabbits
Variation in fur colour exists within a rabbit population
One allele code for brown fur and another for white
fur
Rabbits have natural predators, such as foxes, which act as a selection pressure
The brown rabbits are more likely to survive and reproduce due to having more
effective camouflage
When the brown rabbits reproduce, they pass on their alleles to their offspring
The frequency of brown fur alleles in the population will increase
Over many generations, the frequency of brown fur will increase and the
frequency of white fur will decrease
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Selection pressures acting on a rabbit population for one generation; predation by
foxes causes the frequency of brown fur in rabbits to increase and the frequency of
white fur in rabbits to decrease
Tip
 Exam
Remember that evolution occurs as a result of natural selection, a process
that acts on randomly occurring variation; it does not occur as a direct,
purposeful response to an environment. Avoid any statements that imply
that evolution occurs 'so that' an organism can survive in its environment.
Instead, it is correct to say that evolution occurs by natural selection as a
result of random variation in populations.
You should be able to apply the process of natural selection to any scenario
that you are presented with in an exam, as with the rabbit example above.
Remember the following essential stages
1. Variation is present in a population
2. Selection pressures affect a population
3. Those with advantageous alleles are more likely to survive and
reproduce
4. Advantageous alleles are passed to offspring
5. Advantageous alleles become more frequent in the population
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5.24 Isolation & Speciation
Isolation & Speciation
The theory of evolution states that species do not stay the same, but change over
time; this can lead to the process of speciation
Speciation can be defined as the development of new species from pre-existing
species over time
In order for speciation to occur two populations of the same species must be
isolated from each other in some way
When this happens, there can no longer be an exchange of genes between the
two populations
The exchange of genes is sometimes known as gene flow
Isolation of populations may occur as a result of
Geographical isolation
This leads to a type of speciation known as allopatric speciation
Random mutations that prevent them from interbreeding with each other
This leads to a type of speciation known as sympatric speciation
Populations that are isolated from each other may face different selection
pressures in their environment e.g. different predators or sources of food
The different environmental conditions for the two populations might mean
that different alleles are advantageous, so different alleles are more likely to be
passed on and become more frequent in each population; this is the process
of natural selection
The allele frequencies in the two populations change over time
Note that a process known as genetic drift can also affect allele frequencies
Over time the two populations may begin to
differ physiologically, behaviourally and morphologically to such an extent that
they can no longer interbreed to produce fertile offspring; speciation has occurred
Allopatric speciation
Allopatric speciation occurs as a result of geographical isolation
It is the most common type of speciation
Allopatric speciation occurs when populations of a species
become separated from each other by geographical barriers
The barrier could be natural e.g. a body of water or a mountain range
It can also be man-made e.g. a motorway
This creates two populations of the same species between which no gene flow is
taking place
Allele frequencies in the gene pools of the two populations may change in different
ways due to
Different selection pressures acting on them
The accumulation of random changes resulting from genetic drift
Changing allele frequencies will lead to changes in the phenotypes of the two
populations
If enough allele frequency differences arise between the two populations, then they
will eventually no longer be able to breed with each other and produce fertile
offspring , and can be said to be separate species
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E.g. allopatric speciation in trees
A population of trees exists in a mountainous habitat
A new mountain range forms that divides the species into two populations
The geographical barrier prevents the two populations from interbreeding so
there is no gene flow between them
The two populations experience different environments, so
different alleles become advantageous
Different alleles are therefore more likely to be passed on in each population
Different alleles become more frequent in each population
Over thousands of years the divided populations form two distinct species that
can no longer interbreed to produce fertile offspring
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The geographical barrier of a mountain range can lead to allopatric speciation in trees
Sympatric speciation
Sympatric speciation takes place with no geographical barrier
Isolation instead occurs when random changes in the alleles and
therefore phenotypes of some individuals in a population prevent them from
successfully breeding with other individuals in the population
Examples of phenotype changes that can lead to isolation include
Seasonal changes
Some individuals in a population may develop
different mating or flowering seasons to the rest of the population i.e
their reproductive timings no longer match up
Mechanical changes
Some individuals in a population may develop changes in
their genitalia that prevent them from mating successfully with
individuals of the opposite sex i.e. their reproductive body parts no
longer match up
Behavioural changes
Some individuals in a population may develop changes in their courtship
behaviours meaning they can no longer attract individuals of the opposite
sex for mating i.e. their methods of attracting a mate are no longer
effective
The populations may still live in the same habitat but they are isolated from each
other in the sense that they do not interbreed
The lack of gene flow between the two populations means that allele frequencies
in the gene pools of the two populations may change in different ways
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Changing allele frequencies will lead to changes in the phenotypes of the two
populations
If enough allele frequency differences arise between the two populations, then they
will eventually no longer be able to breed with each other and produce fertile
offspring , and can be said to be separate species
E.g. sympatric speciation in fruit flies
A population of fruit flies exists in a laboratory
A random allele change resulting from mutation divides the species into two
populations
The allele changes leads to a change in phenotype e.g. food preference
The difference in phenotype prevents the two populations from interbreeding so
there is no gene flow between them
Different alleles are therefore passed on in each population
This could be due to difference in selection pressure e.g. certain enzymes are
advantageous for the digestion of different foods or due to genetic drift; the
random passing on of different alleles
Different alleles become more frequent in each population
Over time the divided populations form two distinct species that can no longer
interbreed to produce fertile offspring
Isolation mechanisms other than geographical isolation can also lead to speciation
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Tip
 Exam
Note that you need to be able to apply the principles of natural selection to
the process of speciation; the difference here is that natural selection will be
acting differently on two isolated populations;
1. Variation is present
2. Selection pressures act on a population
These may be different between two isolated populations
3. Advantageous alleles provide some individuals with increase survival
and reproduction chances
Advantageous alleles may be different between two isolated
populations
4. Advantageous alleles are passed on
5. Allele frequencies change
Different advantageous alleles will accumulate in the two isolated
populations until they become so different that they can no longer
interbreed
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5.25 Contentious Issues in Environmental Science
Contentious Issues in Environmental Science
It can be said that there is a consensus, i.e. everyone agrees, among the scientific
community that
Increasing concentrations of greenhouse gases cause global warming
Human activities are the direct cause of increasing greenhouse gas
concentrations
Despite this there are some individuals, even among the scientific community, who
do not believe that the correlation seen between humans burning fossil fuels and
global warming is a causal relationship
These individuals claim that global warming is caused by factors other than
human activities
Evaluating the data
It is important to evaluate any statement that is made about the causes of climate
change in the light of scientific evidence
Consider how good the evidence is
Does a statement address all of the evidence,
or only part of it?
E.g. there may be some years when global temperatures go down, but
there is strong evidence for an overall upward trend
Is the data reliable?
Does the data come from several independent studies i.e. is there plenty
of evidence?
Does statistical analysis show that findings are statistically significant?
Find out whether the statement comes from a trustworthy, unbiased
source e.g.
An individual working for an oil company or a particular government is
likely to be biased because they have a financial or political interest in
the outcome of a study
Several countries wrote to the United Nations in 2021 to ask that
urgent recommendations against burning fossil fuels were toned
down; all of these countries had economies that depended on the use
of fossil fuels
An individual who campaigns passionately for conservation may be biased
because they strongly believe that humans are causing climate change
and they have an emotional stake in the outcome of a study
An individual who works for a renewable energy company may be biased
because they are of the opinion that their technologies are better than
fossil fuel technologies as well as having a financial interest in the
outcome of a study
Evaluating Claims that Human Activities are not the Cause of Cimate Change Table
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When claims are made about the causes of climate change, it is important to
evaluate these claims while bearing the following factors in mind
There is a great deal of scientific evidence that has been tested and checked
by other scientists that supports the hypothesis that humans burning fossil
fuels causes climate change; this increases the likelihood that further claims
of this nature are correct
Climate is highly complex, so scientists need to be careful not to state that
one factor alone has led to a specific event
Climate can be affected by any number of factors in any given year; it is
important to look at all of the data
Climate and weather experts in the media are often asked about
whether one particular extreme weather event is due to climate
change; they always say that it is wrong to draw conclusions from
one event, while also pointing to that event's place in a trend of
increasingly extreme weather
Climate change is not expected to be linear in effect; scientists expect
that there may be a tipping point beyond which changes happen faster
This makes it very difficult to make predictions about exact future
climate conditions
People may have a personal interest; some are especially passionate about
the environment, while others depend financially on fossil fuels
It is important that we are aware of the personal biases of those making
claims about the causes of climate change
If predictions about global warming are correct, then the potential impacts on the
future of Earth are huge
As scientists, it is our responsibility to be aware of the important factors
surrounding this debate so that we can help other to assess evidence thoroughly
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5.26 Examples of Sustainable Conservation
Examples of Sustainable Conservation
The use of extrapolation to model climate change shows that the consequences of
global warming could be extremely serious for humans and global biodiversity if
we continue with current human activities
If we want to limit the consequences of global warming it is essential that we act
quickly to reduce carbon emissions and increase the rate at which carbon is
removed from the atmosphere
Our knowledge of the carbon cycle tells us that we can do these things in several
ways
Reducing carbon emissions
Carbon emissions can be reduced by limiting the rate at which fossil fuels are
burned
This is challenging as so many of our daily activities depend on the burning
of fossil fuels to release energy
Transport of people and goods
Electricity generation
Food production
Two current ways of reducing carbon emissions are
Burning biofuels instead of fossil fuels
The use of other renewable energy resources
Biofuels
These fuels are made from recently living plant biomass such as sugar cane
Biofuels can be burned in the same way as fossil fuels, releasing carbon dioxide
as they burn
Arguments in support of biofuel use include
Biofuels are often cheaper than oil
It is argued that biofuels are 'carbon neutral' meaning that they only release
carbon that was recently removed from the atmosphere when the plants were
alive
They do not release carbon that has been stored away for millions of years
as with fossil fuels
Biofuels are a renewable source of energy, i.e. they can be regrown quickly
There are several arguments against the use of biofuels
They do still release carbon dioxide into the atmosphere
The vast amounts of land required to grow biofuels could otherwise have
been used for food production
Creating land for biofuel growth often involves the loss of other types of
habitats e.g. rainforest; this is bad for biodiversity
Cutting down mature trees to create land for biofuel growth reduces the
removal of carbon from the atmosphere by photosynthesis
Other renewable sources
For example wind, solar, geothermal, and tidal energy
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Arguments in favour of such renewable resources include
These kinds of technologies are advancing quickly and are becoming cheaper
and more efficient to use
No carbon dioxide is released when these technologies are used to generate
electricity
The current disadvantage of such renewable resources is that no single source is
perfect e.g.
Geothermal energy can only be used when there is volcanic activity close to
the earth's surface
Solar energy depends on sunshine hours
Wind energy depends on wind speeds and some conservationists are
concerned about the impact of wind turbines on birds and bats
Some are also concerned about the visual impact of wind turbines on the
landscape
Tidal energy can only be generated near the coast
Comparing Energy Resources Table
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Increasing carbon removal
There is much excitement over the future potential of new carbon capture
technologies, but the reality is that the technology to remove vast quantities of
carbon from the atmosphere and store it away does not yet exist
We need to rely on existing methods of carbon removal, and the main existing
mechanism is photosynthesis
Humans can increase the global rates of photosynthesis by
Stopping the destruction of forests by deforestation
Planting trees, also known as reforestation
If trees are allowed to grow to maturity, they can store huge amounts of
carbon in their biomass
Some countries around the world have shown that it is possible to restore
lost areas of forest by carrying out reforestation, e.g. Costa Rica now
plants seven times more trees than it cuts down
This kind of achievement requires huge government inputs in the
form of benefits to landowners
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