Energy transfer in organisms
Autotroph
Synthesize their own food
Captures energy from sun and converts it to chemical energy
Able to use simple inorganic materials as starting materials for synthesis of
complex organic compounds using either:
O Light energy (photoautotroph)
O Chemical energy (chemoautotroph)
Photosynthesis
Heterotroph
Cannot synthesize own food
Captures energy from the autotrophs
and converts it to chemical energy
Respiration
Photosynthesis
Process by which organisms use carbon dioxide and water to manufacture food
Using energy supplied by light that is absorbed by organisms and converted to chemical energy
General equation: (light)
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Water
O Via vessels running from the root, through stems, to leaves
O Xylem: water vessel
O Phloem: nutrient vessel
O Cohesion-tension theory of water flow
o Water molecules evaporate from leaf to surroundings via openings (stomata) through transpiration
o Other water molecules from xylem replace those evaporated
o Water molecule chain (leaf veins to roots) is pulled up by evaporation (cohesion of water molecules
to each other and adhesion to xylem wall through hydrogen bonds create water chain)
o As water retreats up the xylem, water pressure in the xylem in the roots decreases
o Osmosis: water molecules travel from a region of higher water potential to lower water potential
o Water enters the vascular cylinder of the root, replenishing the bottom of the water chain
Carbon dioxide
O Via openings on leaf (stomata)
O Diffusion of CO2: concentration of carbon dioxide in leaf must be low, so that it will enter from air due to conc
grad
O Opening and closing of stomata
Open
Close
1. The sun rises, and light intensity rises
1. Sun sets, light intensity drops
2. Photosynthesis begins in guard cells
2. Photosynthesis does not occur
3. Glucose formed, water potential in guard cells
3. Glucose will be converted to starch or used up in
drops
other processes (eg respiration)
4. Water enters guard cells via osmosis
4. Water potential in guard cell increases
5. Volume of guard cells increase, become turgid
5. Water leaves guard cell, becomes flaccid
6. Due to uneven thickness of cell wall, guard cell will
6. Guard cell shrinks, and straightens
7.
1.
2.
3.
4.
5.
6.
7.
8.
9.
curve outwards
Stoma opens
High light intensity, high humidity
Proton pump drives protons (H+) from guard cells
Electrical potential of cell decreases
Potassium ions are pumped into guard cells
through active diffusion
Increases osmotic pressure in the guard cell
Water enters cell through osmosis
Increases cell volume and turgor pressure
Rings of cellulose microfibrils prevent the width of
guard cells from swelling, allows extra turgor
pressure to elongate guard cells
Guard cells lengthen
7.
Stoma closes
1.
2.
Roots experience water shortage
Abscisic acid is released, which binds to certain
receptors in the guard cells’ plasma membranes
Raises the pH of cytosol of the cell
Increase the concentration of free Ca2+ in the
cytosol (due to influx from outside cell, and the
release of calcium ions from internal stores)
Chloride and inorganic ions exit cells
Loss of K+ in cells
Reduce osmotic pressure
Cell flaccid
Stoma closes
3.
4.
5.
6.
7.
8.
9.
Leaf
Part of the leaf
Cuticle
Upper epidermis
Palisade
Chloroplasts
Spongy
Characteristic
Transparent and thin
1 cell thick
O Lies just below upper
epidermis
O High layer of cells closely
packed with chloroplasts
O Numerous
O Located near periphery of cell
O Phototactic
O
Oval in shape
How it helps!
Prevents water loss and focuses sunlight
Thin: allows light to pass through
Allows maximum absorption of sunlight
O
O
Maximum absorption of sunlight
Facilitates gaseous exchange with intercellular air
spaces
O Move within cell towards light
Allow efficient diffusion of carbon dioxide
mesophyll
Vascular bundle
O Loosely packed with air spaces
Connects leaf to rest of plant
Stomata
Guard cell opens and closes
O
O
O
Transport water for photosynthesis
Removes glucose
Provides support to keep leaf up, so that leaf blade
is held at right angles to incident light
Xylem:
O Transports water and mineral salts to leaf cells
O Prevents leaf from wilting
O 1% of water is used for light reactions of
photosynthesis
Phloem:
O Transports products of photosynthesis away from
leaf (sugars, amino acids)
O Allow passage of air into plant
O Prevents excessive water loss
O Closes at night, when respiration occurs: CO2 is
produced and stored in leaf so that in the morning,
there is a ready supply of CO2 for photosynthesis
Adaptations
O Large surface area
o Allows maximum absorption of sunlight
O Thin
o Carbon dioxide only needs to diffuse across short distance to reach mesophyll cells
O More stomata on lower epidermis than upper epidermis
o Allows entry of carbon dioxide into leaf, and still minimizes water loss from the plant
o Upper epidermis directly exposed to sun and a lot of water will be lost
O Guard cell
Factors affecting photosynthesis
O Carbon dioxide concentration
O Water
O Light intensity
O Chlorophyll levels
Light intensity
O At low light intensities, rate of photosynthesis increases linearly with
increasing light intensity
O Very high light intensities, chlorophyll may be damaged, decreasing
rate of photosynthesis
O Plants living under such conditions are usually protected by thick
cuticles
Compensation point
O Light intensity at which rate of photosynthesis = rate of respiration
O All carbon dioxide produced during respiration is used for photosynthesis
O All oxygen produced during photosynthesis is used for respiration
O No net gaseous exchange between plant and environment
O Reached at low light intensities
Below compensation point (dark)
Respiration
No photosynthesis
Intake: oxygen
Release: carbon dioxide
Compensation point
Rate of respiration =
rate of photosynthesis
Intake = release
No gaseous exchange with
surroundings
Above compensation point (bright)
Rate of respiration < rate of
photosynthesis
Intake: carbon dioxide
Release: oxygen
Light wavelength
O Peak at 470 and 650 nm (red + blue/violet light)
O Absorption spectrum and action spectrum
o Chlorophylls A and B absorb red and blue/violet light
o Carotene and Xanthophyll absorb only blue/violet light
o Different photosynthetic pigments effectively increase the range of wavelengths from which plants
can obtain energy
o Action spectrum similar to absorption spectrum, indicating that those pigments are responsible for
light absorption for photosynthesis
Carbon dioxide concentration
O CO2 required for dark reactions
O Rate of photosynthesis can be increased by increasing the carbon dioxide concentration
O Short term optimum: 0.5%
O Long term optimum: 0.1%
Temperature
O Reactions of photosynthesis are catalysed by enzymes, whose activity is greatly affected by temperature
O Rate would double every 10°C increase (same as enzyme activity) until optimum temp
O Higher than optimum temp: enzymes will denature, rate would decrease
Water
O If plant has low water content, it would close stomata in response to wilting
O Prevents carbon dioxide from entering plant for photosynthesis
O Deficiency in water will decrease rate of photosynthesis
Oxygen concentration
O High concentration of oxygen will inhibit photosynthesis (decrease rate)
O Oxygen would compete with carbon dioxide for active site in RuBP carboxylase
Chlorophyll concentration
O Not normally a limiting factor
O Decrease in chlorophyll levels will decrease rate of photosynthesis, and cause leaves to turn yellow
O Reasons for decrease in chlorophyll levels:
o Disease
o Ageing
o Nitrogen and magnesium deficiency
o Lack of light
Limiting factors
O Rate of biochemical process
o (which consists of a series of reactions) is limited by the slowest reaction in the series
o (which is affected by several factors) is limited by the factor that is nearest minimum value
O
O
O
O
O
O
O
At A, light is the only limiting factor. Light
saturation occurs at B, C, D, where an
increase in light intensity will not increase the
rate of photosynthesis. This means that
another factor that affects photosynthesis is
obstructing the rate of photosynthesis from
increasing.
At C, an increase in light intensity would not
cause rate of photosynthesis to increase. This
is because of light saturation. There is excess
light, and another factor has “too little” and unable to keep up with the increasing light intensity. This other
factor is carbon dioxide concentration 0.04%. at C, carbon dioxide concentration is too little and hence,
despite increase in light intensity, photosynthesis would not speed up.
Compare:
B and C: a higher concentration of carbon dioxide would increase the rate of photosynthesis
B and D: a higher concentration of carbon dioxide would increase the rate of photosynthesis
However, if D is changed to be at B,
Despite differences in carbon dioxide concentration, the rate of photosynthesis would not increase. This
means that even though the carbon dioxide concentration increases, another factor is obstructing
photosynthesis.
Different conditions (eg 0.4% and 0.04%)
o Likened to advancing right on the x-axis, increase in one factor > would the rate increase?
o If yes, then that is the limiting factor.
o If no, there is another limiting factor.
Cellular
2 stages:
O Light-dependent stage (light reaction)
O Light-independent stage (dark reaction
Light-dependent stage (light reaction)
O Occurs in thylakoid membrane (chlorophyll)
O Sets of integral protein
O Where light energy is converted to chemical
energy to be used in the dark reactions
O Produce ATP and NADPH for dark reactions
Formula:
(light and chlorophyll)
12 H2O + 12NADP + 18ADP + 18Pi
6 O2 + 12 NADPH + 18 ATP
Photosystem 2: Light Harvesting Complex
O Light-harvesting system acts like funnel
O When an accessory pigment in the light harvesting system absorbs light energy, its energy level increases and
gets ‘excited’
O The ‘excited’ accessory pigment molecule transfers its energy to neighbouring accessory pigment molecules,
until it reaches the reaction centre
O At the reaction centre, energy is absorbed by special chlorophyll A molecule
O An electron of chlorophyll A molecule is boosted to a very high energy level and displaced
Chlorophyll A > Chlorophyll A+ + electron
O However, light harvesting complex cannot afford to keep losing electrons
O Water is split by manganese complex to produce electrons to replace those that are released
O 2H2O
4e++ O2 + 4 H+
O2 = 1st product of photosynthesis
Photosystem 2: Electron Transport Chain
O The electron that was displaced is
transferred to an electron acceptor
Y which passes it on to a chain of
electron carriers
O ETC is used to pump hydrogen as
well
O Hydrogen (H+) will be attracted to
the electron and the momentum of
electron passing through will cause
the hydrogen to pass through the
chain, and into the thylakoid space
O Transport of electrons down the
chain provide energy for active
transport of hydrogen ions from stroma, across the thylakoid membrane and into thylakoid space
O High concentration of hydrogen ions in thylakoid space, must be diffused out
O ATP synthase (enzyme to create ATP; integral protein in thylakoid membrane)
O Active site only ready to catalyse reaction of ADP and Pi when there’s hydrogen
O Hydrogen concentration gradient drives ATP synthase
O Everytime 1 hydrogen ion leaves via ATP synthase, 1 ADP is released
Photosystem 1: Light Harvesting Complex
O Electron from ETC is fitted into light harvesting system
O Electrons get excited from the light energy again, and releases one electron from the reaction centre (this time,
electron is replaced by electron from photosystem 2)
Photosystem 1: Electron Transport Chain
O Electron passes through chain of electron carriers again, and hydrogen ions are pumped into the thylakoid
space through active transport again
O
Hydrogen ions in thylakoid space and the electron at the end of the ETC creates the active site to reduce
NADP+ to NADPH
NADP+ + 2H+ + 2e
NADPH + H+
Light-independent stage
O Occurs in the stroma
O Fixes CO2 to produce glucose
O 12NADPH + 18ATP + 6CO2
Glucose + 12NADP+ + 18ADP + 18Pi + O2
Caboxylation
1. Ribulose biphosphate caboxylase (enzyme) binds carbon dioxide (input of photosynthesis) with a five-carbon
compound (ribulose biphosphate RuBP) to form an unstable 6-carbon compound
2. 6-carbon compound breaks down to form 2 molecules of 3-carbon compound called glycerate-3-phosphate
(3pGA)
Reduction
1. 3pGA is reduced to glyceraldehydes-3-phosphate (G3P)
O NADPH > NADP + H+
O ATP > ADP + Pi
O The hydrogen needed for this reduction comes from NADPH and the energy comes from the breaking
down of ATP
2. 2 G3P molecules combine to form a six-carbon sugar
Regeneration
1. The rest of the G3P molecules enter a series of reactions driven by ATP to regenerate RuBP, to ensure
continued fixation of carbon dioxide
Photosynthesis . Respiration
Photosynthesis
Produces sugars for energy
Energy stored
Occurs only in cells with chloroplasts
Oxygen produced
Water used
Carbon dioxide used
Light required
Respiration
Burns sugars for energy
Energy released
Occurs in most cells
Oxygen used
Water produced
Carbon dioxide produced
Occurs in dark and light
ATP production
Photophosphorylation (photosynthesis)
In thylakoids of chloroplasts
Energy source is electrons excited by light
NADP is electron acceptor
Chlorophyll is necessary
Most ATP produced is used in light-independent
reaction
Oxidative phosphorylation (respiration)
Mitochondria membrane
Energy source is transfer of electrons during oxidation reactions
NAD is electron acceptor
No chlorophyll in mitochondria
ATP used in wide variety of reactions