Unit 1
Communication, Homeostasis and Energy

Which process evolved first on Earth –
aerobic respiration or photosynthesis?
 Give reasons for your answer
 You are going to present your answer with
your reasons to the rest of the class!!

Plants and animals rely on aerobic
respiration
 Requires oxygen
 Oxygen is a by-product of photosynthesis
 Until photosynthesis evolved there was no
free oxygen in the atmosphere

Photosynthesis evolved first!!
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Where in plants does photosynthesis
take place?
What are the raw materials needed
for photosynthesis?
What is the energy source for
photosynthesis?
Draw a flow diagram showing how
energy from sunlight is used to
produce muscle contractions in your
arm.
Define the terms autotroph and
heterotroph.
 State that light energy is used during
photosynthesis to produce complex organic
molecules.
 Explain how respiration in plants and
animals depends upon the products of
photosynthesis.
 State that, in plants, photosynthesis is a twostage process taking place in chloroplasts.
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Photosynthesis transfers light energy
into the chemical potential energy of
organic molecules.
Photosynthesis releases oxygen from
water, so all aerobes depend on
photosynthesis for their respiration.

Autotroph
 an organism that uses an external energy source
and inorganic molecules to make complex
organic molecules.
▪ Chemoautotroph
▪ Photoautotroph

Heterotroph
 Organism that needs to take in complex organic
molecules which act as a source of energy and
as usable carbon compounds.

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In order to maintain life, organisms
need a source of energy.
In most organisms this is provided by
the oxidation of organic molecules.
Autotrophic nutrition
 Synthesise organic materials from
inorganic sources e.g. photosynthesis

Heterotrophic nutrition
 Obtained in organic form

Photosynthesis (p/s) is the fixation of
carbon dioxide and its reduction to
carbohydrate, using hydrogen from
water

Word equation for photosynthesis
Light energy
Carbon dioxide + watercarbohydrate + oxygen
chlorophyll

Overall Chemical Equation
nCO2 +

Light energy
nH20  (CH2O)n + nO2
chlorophyll
Balanced Equation for hexose sugars
6CO2
Light energy
+ 6H2O  C6H12O6 + 6O2
chlorophyll

Testing a leaf for starch

What are the requirements for
photosynthesis
 Light
 Chlorophyll
 carbon dioxide

Factors limiting photosynthesis
 chlorophyll (enzymes)
 carbon dioxide
 Light
 Water
 Temperature

Photosynthesis is a 2 stage process
 Light dependent reactions
▪ thylakoid membranes
 Light independent reactions
▪ Stroma

Explain, with the aid of diagrams and
electron micrographs, how the
structure of chloroplasts enables them
to carry out their functions.
x
y
z

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3 – 10μm diameter
Envelope of 2 phospholipid
membranes
Stroma = fluid interior
Thylakoids are series of flattened sacs,
which form stacks (grana) in places

Grana
 Provides a LSA to hold pigments, electron
carriers, and enzymes for light dependent
reactions.
 Photosystems arranged in funnel like
structure in thylakoid
 Membrane of grana holds ATPsynthase
(chemiosmosis)

Stroma
 Site of light independent reactions
(carbon fixation)
 Contains sugars, organic acids and
enzymes for Calvin cycle
 Store starch grains
 Loop DNA – codes for chloroplast proteins

Lamellae
 Do not contain chlorophyll
 Form a network between the grana



Define the term photosynthetic
pigment.
Explain the importance of
photosynthetic pigments in
photosynthesis.
State that the light-dependent stage
takes place in thylakoid membranes
and that the light-independent stage
takes place in the stroma.

The fate of light which strikes the leaf

The fate of light which strikes the leaf

Light shining on leaf (100%)
 12% light reflected
 83% light absorbed, but only 4% of this is
used in photosynthesis
 5% of light transmitted

These values will be affected by
 the amount of chloroplasts in the leaf
 how shiny the leaf is
 how thick the leaf is

Features of light that make it important
 spectral quality (colour)
 intensity (brightness)
 duration (time)

Visible light has a wavelength between
400nm and 700nm
Leaves contain a variety of photosynthetic
pigments, of which chlorophyll is the most
obvious.
 It is these pigments which absorb light
energy.
 There are two different groups of pigments

 chlorophylls – chlorophyll a, chlorophyll b
 Carotenoids – xanthophyll, carotene

Different photosynthetic pigments absorb
different wavelengths.

Absorption Spectrum
 A graph of absorbance of different wavelengths
of light by a pigment

Action Spectrum
 A graph of the rate of photosynthesis at different
wavelengths of light.
Chlorophylls absorb red and blue violet
regions of light, and reflect green
 Carotenoids absorb the blue-violet region
of the spectrum.
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Outline how light energy is converted
to chemical energy (ATP and
reduced NADP) in the lightdependent stage.
Explain the role of water in the lightdependent stage.

Evidence for this comes from
experiments with isotopes of oxygen.
 Plants provided with C18O2 combine the
atoms into carbohydrates
 Plants provided with H218O release the 18O
atoms as oxygen gas

All the oxygen released by
photosynthesis comes from water.
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In p/s the light energy absorbed by
the p/s pigments is converted to
chemical energy.
The absorbed light energy excites
electrons in pigment molecules.
In functioning photosystems this is the
energy which drives the process of
photosynthesis.

There are two categories of p/s
pigment
 Primary pigments
▪ chlorophyll a
 Accessory pigments
▪ chlorophyll a, chlorophyll b and carotenoids


Water is split in a reaction called photolysis,
These reactions provide the energy to:
 Synthesis ATP from ADP and Pi
(photophosphorylation)
 Transfer H+ and e- to NADP to form reduced
NADP
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Photophosphorylation can be cyclic
or non cyclic depending on the
pattern of electron flow in one or both
photo systems
Cyclic photophosphorylation
 PSI

Non cyclic photophosphorylation
 PSII & PSI
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Pigments are arranged into light
harvesting clusters called
photosystems
light energy absorbed by pigments is
passed to the primary pigment, which
acts as a reaction centre.

Photosystem I
 Arranged around chlorophyll a molecule
with an absorption peak at 700 nm.
 Reaction centre P700

Photosystem II
 Chlorophyll a molecule with absorption
peak at 680nm
 Reaction centre P680
Electron
acceptor
2e-
Light energy
absorbed by
Chlorophyll a
ADP + Pi
ATP
PSI
involves only photosystem I, which has a
chlorophyll a with a reaction centre P 700.
 An electron from the molecule is excited
to a higher energy level.
 It is captured by an electron acceptor, and
then is passed back to one of the
chlorophyll a P700 molecules.
 This happens due to a chain of electron
carriers .
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The whole process releases energy to
make ATP from ADP and inorganic
phosphate.
This ATP will then be used in the light –
independent reaction.
Electron
Acceptor B
Electron
Acceptor A
NADP
2e-
Light
energy
PSII
2eLight
energy
NADPH
+ H+
PSI
H2O ½O2 + 2e- + 2H+
involves both photosystems.
Both absorb light and the electrons which
are excited leave the reaction centres of
P680 and P700 of the chlorophyll a
molecules.
 Electron acceptors pass the electrons along
chains of electron carriers.
 The P700 of the photosystem I absorbs
electrons from photosystem II.
 Replacement electrons from the photolysis
of water go to photosystem II.
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The electrons lose energy passing
along the electron chain and this goes
towards synthesising ATP.
The photolysis of water releases two
protons/H+s
H+ combine with electrons from
photosystem I and NADP to give
reduced NADP (NADPH + H+)
H2O 
2H+
+
2e-
+
½O2

Oxygen is released as a waste product

The H+ and e- are transferred to NADP to give
reduced NADP
2H+ + 2e- + NADP  reduced NADP

The reduced NADP then passes onto the light
independent reactions

Complete the diagram of
Photophosphorylation

Describe the structure of a chloroplast
and then give an account of the role
played by chlorophyll in
photosynthesis. Refer to action and
absorption spectra in your answer.
 Write in bullet points and include a
diagram.
(a)
 blue and red light used in photosynthesis;
 (light of) wavelength 420 – 450 nm, gives
high rate / AW;
 (light of) wavelength 650 – 690 nm, gives
high rate / AW;
 (light of) wavelength of 500 – 650 nm /
green light, less effective / reflected;
 sharp / AW, drop after 680 – 690 nm;
(b)
 (i)
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chlorophyll a;
chlorophyll b;
carotenoids / carotene;
xanthophylls;
phaeophytin;
(ii)
 absorb/ trap/ capture / harvest, light / transfer energy /
transfer electrons;

(iii)
 granum/ thylakoid (membrane) / lamella / quantasome;
The light independent reaction
involves the fixation of carbon
dioxide, and it takes place in the
stroma of the chloroplast.
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Outline how the products of the lightdependent stage are used in the lightindependent stage (Calvin cycle) to
produce triose phosphate (TP).
Explain the role of carbon dioxide in
the light-independent stage (Calvin
cycle).
State that TP can be used to make
carbohydrates, lipids and amino
acids.
State that most TP is recycled to RuBP.

The stages are:
 Carbon dioxide is linked with a molecule of
ribulose bisphosphate (RuBP), which is a 5 carbon
sugar, using the enzyme ribulose bisphosphate
carboxylase.
 A highly unstable 6C structure is formed which
immediately splits into 2 molecules of the 3
carbon compound glycerate-3-phosphate (GP).
 GP is converted into triose phosphate (3C) with
the addition of hydrogen from reduced NADP
and energy from ATP

Triose phosphate has two purposes
within the cell
 Synthesis of molecules
▪ Synthesis of hexose sugars, starch and cellulose
▪ Synthesis of amino acids
 5/6 are used in the conversion to RuBP so
that more CO2 can be taken up
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Specialist carbohydrates
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glucose
fructose
Sucrose
Cellulose
Lipids
Amino acids and proteins
Nucleic acids
Growth factors, vitamins, hormones,
pigments
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How can the rate of photosynthesis be
measured?
Which Environmental factors could
limit the rate of photosynthesis?

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Discuss limiting factors in
photosynthesis, with reference to
carbon dioxide concentration, light
intensity and temperature.
Describe how to investigate
experimentally the factors that affect
the rate of photosynthesis.

Raw materials
 Water
 Carbon dioxide


Energy in the form of sunlight
Light independent stage requires a
relatively high temperature
 The light-dependent reactions are not
directly affected by temperature, why is
this?

In the light-dependent stage what is
water a source of?
 Hydrogen ions used in chemiosmosis
 Hydrogen ions accepted by NADP
 Electrons to replace lost by oxidised
chlorophyll
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If any of these factors are in short
supply, it can limit the rate at which
photosynthesis takes place
The factor in the shortest supply is
known as the limiting factor.
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Light drives the light-dependent
reactions
More light, more photosynthesis
At a point where increasing light
intensity has no effect on the rate of
photosynthesis, light is no longer the
limiting factor
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Carbon dioxide in air is about 0.04%
Carbon dioxide is needed for the
Calvin cycle
If a plant is given extra CO2 they will
photosynthesis faster
Over which part
of this curve is
carbon dioxide
the limiting
factor for
photosynthesis?
 Suggest why the
curve flattens
out at high
levels of CO2.

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Temperature affects the kinetic
energy of molecules
 Higher the temperature, the faster the
molecules move
 More collisions
 Rate of reaction increases

At temperatures that are too high,
enzyme molecules denature and the
rate of reaction slows down.

Describe the effect on the rate of
photosynthesis, and on levels of GP,
RuBP and TP, of changing carbon
dioxide concentration, light intensity
and temperature.

The Calvin cycle depends on the products
from the light-dependent reactions.

Explain why the Calvin cycle stops running
when there is no light and the TP is used up.

Make a copy of this diagram and add another line
to show what you would expect to happen to the
levels of RuBP during this 8 minute period.

What effect would you expect a rise
or a fall in temperature to have on the
relative levels of GP, TP and RuBP?
 When answering this assume that the
temperature does not go high enough to
denature the enzymes.
 Explain your reasoning.

If CO2 is in short supply
 Less for RuBP to react with
 Less GP
 Less TP
 Initial accumulation of RuBP