5.1 Photosynthesis Flashcards
What is the role of chlorophyll?
In the light-dependent reaction, chlorophyll is oxidised (loses electrons) when it absorbs light, a process called photoionisation.
Some of the energy from the electrons released during photoionisation is conserved in the production of ATP and reduced NADP.
Oxidised cholorophyll then accepts electrons from water, which causes water to dissociate (photolysis).
How is ATP produced in the chloroplast?
- Light energy excites (raises energy level) of electrons in chlorophyll.
- Electrons enter the electron transfer chain (a series of carrier proteins in the thylakoid membrane).
- As the electrons pass along the ETC (in a series of redox reactions), they lose energy.
- The energy released is used by the proteins to pump protons into the thylakoid lumen, generating a proton gradient.
- The protons pass (by facilitated diffusion) through ATP synthase, driving the phosphorylation of ADP (chemiosmotic theory).
How is reduced NADP produced in the chloroplast?
NADP accepts electrons from chlorophyll (in photosysytem I)
What does photolysis of water produce?
protons, electrons and oxygen.
The light-independent reaction (the Calvin cycle):
Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco.
Reduced NADP from the light-dependent reaction is used to reduce GP to triose phosphate (a simple sugar). The hydrolysis of ATP, also from the light-dependent reaction, provides the additional energy for this reaction.
Most of the triose phosphate is used to regenerate RuBP in the Calvin cycle. Some of the triose phosphate is converted to useful organic substances.
LIMITING FACTORS 1
There is always RuBP, GP and triose phosphate in the stroma of the chloroplast, but their relative concentrations change with changing light intensity. Describe and explain how.
In bright light:
Plenty of ATP and reduced NADP are produced in the light-dependent reaction, so GP is quickly converted into triose phosphate, so there will be more RuBP and more triose phosphate, and less GP.
In dim light:-
A low light intensity would mean that less reduced NADP and ATP are produced in the light-dependent reaction, so less is available to convert GP into triose phosphate, so GP accumulates. Therefore triose phosphate is not made, so RuBP is not regenerated, so there will be more GP and less RuBP and triose phosphate.
LIMITING FACTORS 2
There is always RuBP, GP and triose phosphate in the stroma of the chloroplast, but their relative concentrations change with changing CO₂ concentration. Describe and explain how.
Increasing CO₂ concentrations will increase the rate of photosynthesis but this is not always the case. If either light intensity or temperature are low, then a high carbon dioxide concentration will not increase the rate of photosynthesis.
High CO₂ concentration:-
Higher rate of carbon fixation so RuBP quickly combines with CO₂. Less RuBP, more GP and triose phosphate
Low CO₂ concentration:-
Less carbon fixation and so RuBP is not converted to GP.
RuBP accumulates
Less GP and triose phosphate
LIMITING FACTORS 3
There is always RuBP, GP and triose phosphate in the stroma of the chloroplast, but their relative concentrations change with changing temperature. Describe and explain how.
Temperature has a direct affect upon the enzymes in the light-independent stage as they needed optimum conditions to function efficiently. At very high temperatures, the enzymes can become denatured and at very low temperatures the enzymes may not function with due to a lack of kinetic energy.
High temperatures may also increase water loss leading to a stress response in which stomata close. This limits the availability of carbon dioxide and so effects rate of photosynthesis.
