Ocr A2 Biology - Limiting Factors In Photosynthesis

Front 1

State the optimal conditions of photosynthesis.

Back 1

• High light intensity of a certain wavelength
• Temperature around 25 degrees celsius
• Carbon dioxide at 0.4%

Front 2

Explain how light intensity affects the rate of photosynthesis.

Back 2

• Light is needed to provide the energy for the light-dependant reaction - the higher the light intensity, the more energy it provides.
• Only certain wavelengths are used for photosynthesis. The photosynthetic pigments chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight.
• Green light is reflected, which is why plants look green.

Front 3

Explain how temperature affects the rate of photosynthesis.

Back 3

• Photosynthesis involves enzymes (e.g. ATP synthase, RuBisCO).
• If the temperature falls below 10C, the enzyme becomes inactive.
• If the temperature is more than 45C, they may start to denature.

Front 4

State four things that occur in plants at high temperatures that would negatively affect photosynthesis.

Back 4

• Stomata close to avoid losing too much water. This causes photosynthesis to slow down as less CO2 enters the leaf when the stomata are closed.
• The thylakoid membranes may be damaged. This could reduce the light-dependant stage reactions by reducing the number os sites available for electron transfer.
• The membranes around the chloroplasts could be damaged, which could cause enzymes important in the Calvin cycle to be released into the cell. This would reduce the rate of the light-independent stage reactions.
• Chlorophyll could be damaged. This would reduce the amount of pigment that can absorb light energy, which would reduce the rate of the light-dependant stage reactions.

Front 5

What is the percentage of carbon dioxide in the air?

Back 5

0.04%

Front 6

What happens if the percentage of carbon dioxide goes higher than 0.4%?

Back 6

The stomata will start to close.

Front 7

What is the point B on this graph called? What does it mean?

Back 7

• The saturation point.
  
• Where a factor is no longer limiting the reaction - something else has to become the limiting factor.

Front 8

What is the limiting factor in this graph?

Back 8

Temperature as both these graphs level off when light intensity is no longer the limiting factor. The 25C graph levels off at a point higher than the 15C one, showing that 15C temperature must be a limiting factor.

Front 9

What is the limiting factor for this graph?

Back 9

The CO2 concentration as the graph levels off when light intensity is no longer the limiting factor and the temperatures are the same.

Front 10

How can water stress affect photosynthesis?

Back 10

When there is little water, the stomata will close to conserve water. This does not allow a lot of CO2 to enter the leaf for the Calvin cycle, slowing photosynthesis down.

Front 11

How does light intensity affect the levels of GP, RuBP and TP?

Back 11

• In low light intensities, the products of the light-dependant stage (reduced NADP and ATP) will be in short supply.
• This means the conversion of GP to TP and RuBP is slow.
• So the levels of GP will rise (as it is still being made) and the levels of TP and RuBP will fall (as they are being used up by making GP).

Front 12

How does temperature affect the levels of GP, RuBP and TP?

Back 12

• All the reactions in the Calvin cycle are catalysed by enzymes.
• At low temperatures, all of the reactions will be slower as the enzymes work more slowly.
• This results in the levels of RuBP, GP and TP falling.
• RuBP, GP and TP will be affected in the same way at very high temperatures because the enzymes will start to denature.

Front 13

How does carbon dioxide levels affect the levels of GP, RuBP and TP?

Back 13

• At low CO2 concentrations, conversion of RuBP to GP is slow (as there's less CO2 to combine with RuBP to make GP).
• This results in the levels of RuBP continuing to rise (as it is still being made) and the levels of GP and TP falling (as they are used up to make RuBP).