F214 – Photosynthesis
1. Define autotroph and heterotroph.
Autotroph = organisms that uses light or chemical energy and inorganic molecules
(co2/h2o) to synthesise complex organic molecules
Heterotroph = organisms that ingest and digest complex organic molecules, releasing
the chemical potential energy stored in them.
2. State that light energy is used during photosynthesis to produce complex
organic molecules
3. Explain how respiration in plants and animals depends upon the products of
photosynthesis
Photosynthesis is the conversion of light energy into chemical energy. Both autotrophs
and heterotrophs use the energy from complex organic molecules (made during
photosynthesis) for both anaerobic and aerobic respiration.
6CO2 + 6H2O (+ light energy)  C6H12O6 + 6O2
e.g. Glucose (C6H12O6) is used in anaerobic respiration. Oxygen was first released by
photosynthesis and is used for aerobic respiration.
4. State that in plants photosynthesis is a two-stage process, taking place in
chloroplasts
5. Explain, with the aid of diagrams and electron micrographs, how the
structure of chloroplasts enables them to carry out their functions.
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The inner membrane contains transport proteins that control the entry and exit of
substances between the cytoplasm and the stroma.
The granum provides a surface area for
photosynthetic pigments, electron carriers,
and ATP synthase, all involved in the Lightdependent reaction.
The photosynthetic pigments are arranged
into photosystems to allow for maximum
absorption of light energy.
Proteins embedded in the grana hold the
photosystems in place
The stroma contains enzymes needed to
catalyse the reactions in the lightindependent stage.
The stroma surrounds the granum, so the
products of the light-dependent reaction,
needed in the light- independent reaction, can
readily pass into the stroma.
Chloroplasts can make proteins they need for photosynthesis using the genetic
instructions on their chloroplast DNA, and the chloroplast ribosomes to assemble
the proteins.
F214 – Photosynthesis
6. Define the term photosynthetic pigment.
Molecules that absorb light energy. Each pigment absorbs a range of wavelengths and
has its own distinct peak of absorption. Other wavelengths are reflected.
7. Explain the importance of photosynthetic pigments in photosynthesis.
Many different pigments act together to trap and absorb as much light energy as
possible. Light hitting chlorophyll pigments causes a pair of electrons associated with
magnesium to become excited, which starts the process of photophosphorylation.
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The primary pigment reaction centre is a molecule of the pigment chlorophyll a
There is two forms chlorophyll a: P-680 and P-700, each appearing yellow green and
absorbing red light at different wavelengths.
P680 is found in photosystem 2, whilst P700 is found in photosystem 1.
Chlorophyll b absorbs at 500nm and appears blue-green
Accessory pigments e.g. carotenoids reflect yellow and orange lights but absorb blue
(those that are not absorbed well by chlorophylls in order to pass the energy to
chlorophyll a)
They are not directly involved in the LD reaction
8. State that the light-dependent stage takes place in thylakoid membranes
and that the light-independent stage takes place in the stroma
9. Outline how light energy is converted to chemical energy (ATP and reduced
NADP) in the light-dependent stage (reference should be made to cyclic and
non-cyclic photophosphorylation, but no biochemical detail is required)
F214 – Photosynthesis
(1) Photophosphorylation
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When a light photon hits a chlorophyll molecule, the energy of the photon is
transferred to two electrons and they become excited.
These electrons are captures by electron carriers and they become reduced
whilst acting as oxidising agents.
They are then passed through a series of electron carriers embedded in the
thylakoid membranes.
Energy is released as the electrons flow along the chain of electron carriers; this
energy is used to pump protons across the thylakoid membrane from the stroma
into the thylakoid space.
Protons begin to accumulate in the thylakoid space, which builds up a proton
gradient across the thylakoid membrane. This causes the protons to flow down
their gradient, through ATP-synthase enzymes from the thylakoid space back
into the stroma. This flow of protons is known as chemiosmosis.
Chemiosmosis produces a force that joins ADP and Pi to produce ATP. The
kinetic energy from the flow of protons, that was first stimulated by light energy, is
converted to chemical energy in the ATP molecules.
There are two types of photophosphorylation – cyclic and non-cyclic.
(2) Cyclic Photophosphorylation
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Only involves photosystem I.
Light strikes and the excited electrons are passed to an electron acceptor, before
being passed back to the chlorophyll molecule from which they were lost.
This generates enough energy to allow ATP synthesis from ADP and Pi.
There is no generation of reduced NADP and there is no photolysis of water.
The ATP produced may be used in the light-independent reaction, or it may be
used in guard cells to bring in potassium ions. This lowers the water potential of
the guard cells, causing water to flow into them via osmosis, which then causes
the stomata to open.
(3) Non-Cyclic Photophosphorylation
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Involves both photosystems
Light striking photosystem II excites a pair of electrons, which leave the
chlorophyll molecule. Electrons pass along carriers and release energy to
synthesise ATP
Light striking photosystem I also excites a pair of electrons to leave, which join
with protons and NADP (a coenzyme) to form reduced NADP
Electrons from PSII replace electrons lost in PSI and electrons made in
photolysis replace electrons lost in PSII.
Protons made in photolysis take part in chemiosmosis before being captured by
NADP in the stroma for use in the light-independent stage.
F214 – Photosynthesis
10. Outline the role of water in the light dependent stage.
In the presence of light and an enzyme, photosystem II splits water into H+ ions,
electrons and oxygen – this is photolysis.
2H2O  4H+ + 4e- +O2
The H+ ions (protons) are used in chemiosmosis to produce ATP, then are accepted by
NADP to form NADP reduced for reduction of CO2 in the light-independent stage
The electrons are used to replace those lost by the oxidised chlorophyll in PSII.
The oxygen can be used by the plant for aerobic respiration, but most diffuses out
though the stomata into the air.
10. Outline how the products of the light-dependent stage are used in the lightindependent stage (Calvin cycle) to produce triose phosphate (TP)
(reference should be made to ribulose bisphosphate (RuBP), ribulose
bisphosphate carboxylase (rubisco) and glycerate 3-phosphate (GP), but no
other biochemical detail is required)
The Calvin Cycle:
1) CO2 diffuses into the leaf through open stomata.
2) Once CO2 has reached the stroma of the chloroplast, it combines with a 5C compound
known as Ribulose Bisphosphate (RuBP) - rubisco catalyses this reaction.
3) RuBP becomes carboxylated (combined with CO2, contains a carboxyl group).
4) Product of reaction = 2 molecules of 3C compound triose phosphate (TP). The carbon
dioxide has now been fixed.
5) TP is then reduced and phosphorylated to produce another 3C compound, glycerate3-phosphate (GP). Reduced NADP and ATP from the LD stage are used in this process.
6) 5/6 molecules of GP are recycled by phosphorylation (+ATP from the LD stage) to
regenerate 3 molecules of RuBP (5C).
F214 – Photosynthesis
11. Explain the role of CO2 in the light-independent stage.
Carbon dioxide is a source of carbon, which is used to make complex organic molecules.
These molecules act as energy stores or sources of energy, for all carbon-based life on
this planet. The fixation of carbon dioxide leads to the formation of triose phosphate
(TP), which then leads to the formation of glycerate-3-phosphate (GP). TP and GP can be
used to synthesis complex organic molecules.
12. State that TP (and GP) can be used to make carbohydrates, lipids and amino
acids
13. State that most TP is recycled to RuBP
14. 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.
Carbon Dioxide Concentration:
 More CO2 = More CO2 becomes fixed, so more TP and therefore more GP is
produced.
 However, the number of stomata open for gaseous exchange leads to
transpiration and may lead to the plant wilting. This happens if the plant’s water
uptake from the soil cannot exceed the transpiration rate. A stress response is
activated, following the release of a plant growth regulator (abscisic acid), which
causes the stomata to close.
 Decreases the CO2 uptake = reduces the rate of photosynthesis.
 If CO2 levels drop too low then the CO2 acceptor (RuBP) will accumulate.
 If carbon dioxide is not available for the RuBP to combine with, then levels of GP
and TP will fall.
Light Intensity (increase):
 More light to excite more electrons, therefore more photophosphorylation.
 More photophosphorylation = more reduced NADP and ATP produced; therefore
more GP converted to TP.
 More RuBP generated from TP, therefore more carbon dioxide fixation.
Light Intensity (decrease):
 Less light available to excite electrons, therefore less photophosphorylation.
 GP will not be changed to TP = GP accumulates and TP levels fall.
 So less RuBP regenerated and less carbon dioxide is fixed.
Temperature:
 Little effect on light dependent as not dependent on enzymes (except for
photolysis of water).
 Light-independent = series of biochemical steps, each catalysed by a specific
enzyme, so a change in temperature can have a great effect.
 Above 25°C, photorespiration exceeds photosynthesis, as the oxygenase activity
of Rubisco increases more than the carboxylase activity.
F214 – Photosynthesis
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Therefore ATP and reduced NADP from the light-dependent reaction are
dissipated and wasted, reducing the overall rate of photosynthesis
High temps may also denature proteins
Or high temp= high water loss - this may lead to stomata closure stress
response, and the reduction of photosynthesis due to less CO2
15. Discuss the limiting factors of photosynthesis, with reference to CO2
concentration, light intensity and temperature.
Temperature, light intensity and carbon dioxide concentration are factors that can affect
the rate of photosynthesis. At any point in time, one of the three factors will be in low
supply (compared to the other two) and limit the rate of photosynthesis. Thus, the factor
in lowest supply is called the limiting factor. Changing the limiting factor increases or
decreases the rate of photosynthesis but changing the other two factors has no effect.
Carbon Dioxide:
 Increasing the level of carbon dioxide will increase the rate of photosynthesis.
 At low CO2 concentrations, ATP and NADPH are made at a faster rate than CO2
can be fixed. Low CO2 is a common limiting factor in many habitats.
 Growers can increase the amounts of CO2 in their greenhouses by burning
methane or oil -fired heaters Light Intensity:
 Increasing the light intensity will increase the rate of photosynthesis
 At low light intensities, fewer water molecules are split resulting in a shortage of
ATP and NADPH - this slows down the rate at which CO2 can be fixed.
Temperature:
 Increasing the temperature will increase the rate of photosynthesis at first, as it
will speed up the rate of enzyme activity in the Calvin cycle (rubisco etc.)
 However, a high temperature can cause the enzymes involved in the Calvin cycle
to denature, therefore reducing the rate of photosynthesis, as the Calvin cycle
will cease.
16. Describe how to investigate experimentally the factors that affect the rate
of photosynthesis.
Gas, given off by the plant over a known period of time, collects in
the flared end of the capillary tube. By pulling the syringe the
collected gas moves to the scale and the length of the bubble can be
multiplied by πr² to calculate the volume of gas collected. This
experiment can be done in different light intensities, so calculations
of volume of gas collected over time can be compared and the effect
of changing light intensities on the rate of photosynthesis can be
measured.
Hydrogencarbonate indicator solution is
sensitive to small changes in pH. The change
in absorption divided by the time taken can
give an indication of the rate of uptake of
carbon dioxide by a piece of aquatic plant.