P1 Photosynthesis

Question 1

Where does the light dependant reaction take place?

Answer 1

Thylakoid membrane (between the thylakoid space and the stroma)

Question 2

What is chemiosmosis? (LDR)

Answer 2

Proteins diffuse down an electrochemical gradient from the thylakoid space to the stroma through ATP synthase.

Question 3

What is photophosphorylation? (LDR)

Answer 3

Conversion of ADP and an organic phosphate (Pi) into ATP using energy from the diffusion of protons in chemiosmosis.

Question 4

How is the proton gradient maintained in the thylakoid? (LDR)

Answer 4

Protons are actively transported from the stroma to the thylakoid space.

Question 5

What is photoionisation? (LDR)

Answer 5

There are protein complexes containing chlorophyll in the thylakoid membrane, chlorophyll absorbs light and transfers the light energy to a pair of electrons in chlorophyll. These electrons become ‘excited’ and leave the chlorophyll, leaving the chlorophyll positively charged.

Question 6

Describe the electron transport chain in the thylakoid membrane. (LDR)

Answer 6

The excited electrons from photoionisation are passed along a series of molecules/protein complexes. Electrons move through the ETC by oxidation-reduction reactions, which release energy and allow protons to be actively transported from the stroma to the thylakoid space.

Question 7

What happens when electrons reach the final molecule of the electron transport chain? (LDR)

Answer 7

Electrons from the final molecules react with NADP and a proton forming NADPH.

Question 8

What is photolysis? (LDR)

Answer 8

In order for the ETC to continue, chlorophyll needs a supply of electrons.
When light hits the leaf, it splits water (H2O –> 2H+ + 2e- + 0.5 O2) and electrons formed in this reaction move into the chlorophyll. Oxygen diffuses out of the plant, or is used in respiration, and protons help the proton gradient by remaining in the thylakoid space.

Question 9

Summary of the light dependant reaction.

Answer 9

1. Light hits the leaf and is absorbed by chlorophyll, which is transferred to a pair of electrons, leaving them in an excited state. These electrons are propelled out of the chlorophyll.
2. To replace these electrons light splits water into electrons, protons and oxygen (photolysis).
3. The electrons that left chlorophyll move along a set of protein complexes and molecules in the membrane, by a series of oxidation-reduction reactions (the electron transport chain). These reactions release energy which is transferred to a protein complex, enabling it to actively transport protons from the stroma to the thylakoid space to maintain the proton gradient.
4. Electrons at the end of the ETC react with NADP and a proton, forming NADPH.
5. Protons in the thylakoid space diffuse into the stroma via ATP synthase (chemiosmosis), supplying ATP synthase with energy, which it uses to catalyse the reaction of ADP and an inorganic phosphate to form ATP.

Question 10

Compare cyclic and non-cyclic photophosphorylation.

Answer 10

• Cyclic: when photosystem I absorbs light, light energy is transferred to a single electron, exciting it out of chlorophyll, this electron moves to the final molecule in the ETC, but it doesn’t react with NADP. This molecule carries the electron back in the ETC, where it travels through the ETC, taking part in reactions that release energy, enabling the protein complex to actively transport protons. This electron travels back to PSI, but as the electron has replaced itself, photolysis doesn’t need to take place.
• Non-cyclic: when PSII absorbs light, electrons at the end of the ETC react with NADP and a proton.
• PSI and PSII absorb light at different wavelengths, so which type takes place depends on the wavelength of light. This maximises the rate of photosynthesis.
• Cyclic photophosphorylation is useful when there is enough NADPH, but not enough ATP.

Question 11

How are chloroplasts adapted for photosynthesis?

Answer 11

1. Thylakoid membrane has a large surface area, maximising the amount of ATP and NADPH that can be made.
2. Proteins in the grana hold the chlorophyll in a way that the maximum amount of light can be absorbed at one time.
3. Thylakoid membraanes contain ATP synthase for efficient ATP production. The membranes are also selectively permeable, allowing them to establish and maintain a proton gradient.
4. Chloroplasts contain DNA and ribosomes, meaning proteins involves in the light dependant reaction can be easily and quickly produced.

Question 12

What is a photosystem?

Answer 12

A cluster of pigments (including chlorophyll) that absorb light.

Question 13

Where does the light independent reaction take place?

Answer 13

Stroma

Question 14

As well as CO2, which two products does the light independent reaction require from the light dependant reaction?

Answer 14

ATP and NADPH

Question 15

What is the first reaction in the light independent reaction/calvin cycle?

Answer 15

• CO2 reacts with ribulose bisphosphate/RuBP (5C) to form 2 glycerate-3-phosphate/GP (3C) molecules.
• In this reaction CO2 becomes fixed.
• This reaction is catalysed by the enzyme rubisco.

Question 16

What is the second reaction in the light independent reaction/calvin cycle?

Answer 16

• 2 glycerate-3-phosphate molecules are reduced to form 2 triose phosphate/TP (3C) molecules.
• The hydrogen needed for this reaction is supplied by NADPH (which travels to the stroma after the LDR), forming NADP.
• This reaction requires energy, supplied by ATP. ATP breaks down into ADP and an inorganic phosphate group (Pi).
• NADP, ADP and Pi return to the thylakoid membrane for the LDR.

Question 17

What does TP produce? (LIR)

Answer 17

• 80% of triose phosphate molecules are converted back to ribulose bisphosphate, to ensure the Calvin cycle continues. ATP is required for this reaction, converting to ADP and an inorganic phosphate.
• The remaining 20% of TP molecules form glucose (used in respiration), amino acids (used to make proteins) and lipids (energy storage, eg. glycerol forms triglycerides).

Question 18

How does light limit the rate of photosynthesis?

Answer 18

• Increasing light increases the rate of photosynthesis because light is used to excite electrons in photoionisation and to split water in photolysis. Increasing light intensity increases the rate of these reactions, meaning the ETC proceeds at a greater rate, so NADPH and ATP are produced at a greater rate.
• Therefore more NADPH and ATP travel to the stroma, so the LIR happens at a greater rate.
• However after a certain point, if temperature and CO2 remain constant, one of these becomes the limiting factor for the rate of photosynthesis.

Question 19

How does carbon dioxide limit the rate of photosynthesis?

Answer 19

• Increasing CO2 increases the rate of photosynthesis as CO2 is used in the conversion of ribulose bisphosphate to glycerate-3-phosphate in the LIR. If more CO2 is available the rate of this reaction increases, so the rate of the whole LIR increases so more organic substances are produced.
• However after a certain point, if temperature and light remain constant, one of these factors becomes limiting instead.

Question 20

How does temperature limit the rate of photosynthesis?

Answer 20

• As temperature increases, the rate of photosynthesis initially increases because increasing temperature increases the rate of reaction of enzymes in the LIR, such as rubisco, therefore increasing the rate of the LIR.
• However if the temperature is too high, enzymes can denature, decreasing the rate of the LIR.
• Very high temperature can also damage chlorophyll containing proteins in the thylakoid membrane, preventing the ETC from functioning correctly and decreasing the rate of the LDR.