5.1 Photosynthesis

Question 1

What are the two main stages of photosynthesis?

Answer 1

• Light-dependent reaction
• Light-independent reaction

Question 2

What is the balanced photosynthesis equation?

Answer 2

6CO2 + 6H2O → 6O2 + C6H12O6

Question 3

What is another name for the light-independent reaction?

Answer 3

The Calvin cycle

Question 4

What is the first step of the light dependent reaction?

Answer 4

Photoionisation - light is absorbed by chlorophyll in the photosystems which excites electrons there which are then released.

Question 5

What three reactions are driven by energy from the photoionisation of chlorophyll?

Answer 5

• Photophosphorylation
• Photolysis
• Reduction

Question 6

Explain how ATP is produced during the light-dependent reaction and give the name of the processes involved

Answer 6

• Light energy excites electrons in chlorophyll causing them to move to a higher energy level and start moving along the electron transport chain. This is the photoionisation of chlorophyll
• As the electrons move down the ETC, they lose energy.
• This energy pumps protons from the stroma into the thylakoids. The protons are being transported against their concentration gradient and this requires energy.
• As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane because the concentration of protons inside the thylakoids is greater than in the stroma.
• The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme which releases energy that can convert ADP and inorganic phosphate to ATP. This is photophosphorylation.
• This process of the protons moving out of the thylakoid across its membrane is called chemiosmosis.

Question 7

Explain how reduced NADP is produced during the light-dependent reaction and give the name of the processes involved

Answer 7

When light energy is absorbed, high-energy electrons are released.
Some electrons are transferred directly to NADP to reduce it and make NADPH

Question 8

What is photolysis and why is it necessary?

Answer 8

Electrons can be replaced by photolysis.
In this process, light energy splits water into protons, electrons and oxygen.
The electrons can then replace those released when light is absorbed.

Question 9

Cyclic vs non-cyclic photophosphorylation

Answer 9

In cyclic photophosphorylation:
- ATP is produced.
- No reduced NADP is produced.
- Electrons are continuously recycled.
- Photolysis does not take place.

In non-cyclic photophosphorylation:
- ATP and reduced NADP are produced.
- Electrons in PSII are replaced by photolysis.

Question 10

Where does the light-independent reaction take place?

Answer 10

The stroma of the chloroplast

Question 11

Describe the Calvin cycle/light-independent reaction

Answer 11

When CO2 diffuses into the leaves through stomata, it combines with ribulose bisphosphate (RuBP) to give an unstable 6-carbon compound. This is catalysed by an enzyme, rubisco.
The unstable 6-carbon molecule splits into two molecules of glycerate 3-phosphate (GP).
The combination of CO2 with RuBP is described as carbon fixation.
The two molecules of GP are then reduced to triose phosphates (TP) with the energy from two molecules of ATP and protons from two molecules of reduced NADP.
The ATP and reduced NADP are provided by the light-dependent reaction and are recycled after they have been used.
One out of every six TP molecules is converted into hexose sugars but most continue in the Calvin cycle.
Five out of every six TP molecules are used to regenerate RuBP.
This reaction is driven by one molecule of ATP which is then recycled in the light-dependent reaction.
The formation of RuBP from TP allows the Calvin cycle to continue.

Question 12

What are the four key factors that affect the rate of photosynthesis?

Answer 12

• Light intensity
• Temperature
• Carbon dioxide concentration
• Chlorophyll concentration