• This is where increasing the factor after this point makes no difference because something else has become the limiting factor, i.e., a graph levels off here.

1. Light Intensity

• Lights needed to give energy for the light-dependent reaction.
• The higher the intensity, the more energy provided.
• Only certain wavelengths of light are used for photosynthesis.
• The photosynthetic pigments (chlorophyll a, chlorophyll b and carotene) only absorb red and blue light (green is reflected, which is why plants look green).

2. Temperature

• Photosynthesis involves enzymes (e.g. ATP synthase, RuBisCO).
• If temperature falls below 10°C, the enzymes become inactive, and they denature at temperature above 45°C.
• High temperature has en effect on:
  (a) Stomata - They close at high temps to avoid losing too much water. This slows down photosynthesis because less CO2 enters the leaf when the stomata are closed.
  (b) Thylakoid membranes - They may be damaged, reducing the rate of the light-dependent stage by reducing the number of sites available for electron transfer.
  (c) Chloroplasts – The membranes around them could be damaged, possibly causing enzymes important in the cycle to be released into the cell. This would lessen the rate of the light-independent stage.
  (d) Chlorophyll – It could be damaged, reducing the amount of pigment that can absorb light energy, reducing the rate of the light-dependent reactions.

3. Carbon Dioxide (CO2)

• Makes up 0.04% of the gasses in the atmosphere.
• Increasing this to 0.4% gives a higher rate of photosynthesis.
• Any higher than this and the stomata will close.

4. Water Stress

• When plants do not have enough water, their stomata will close to save the little water that they have.
• Thus, less CO2 will enter the leaf for the Calvin cycle.
• This slows down photosynthesis.

When it comes to A2/A-level Biology, can you explain why that light intensity, temperature and CO2 concentration are affecting the levels of GP, RuBP, and TP in the Calvin cycle? 

Light intensity

• When it is low, the products of the light-dependent reaction will be in short supply (ATP and NADPH).
• The conversion of GP into TP and RuBP will also be slow.
• The level of GP will rise (still being made) and levels of TP and RuBP will fall (as they’re being used to make GP).

Temperature

• The reactions in the Calvin cycle are catalysed by enzymes (e.g. RuBisCO).
• At low temperatures, all the reactions will be slower as the enzymes work more slowly.
• The levels of RuBP, GP, and TP will fall eventually.
• They are all affected in the same way at high temperatures because the enzymes will start to denature.

Carbon dioxide Concentration

• When it is low, conversion of RuBP to GP is slow (there’s less CO2 to combine with RuBP to make GP).
• The level of RuBP will rise (still being made) and levels of GP and TP will fall (used to make RuBP).

References:
1. https://biology-igcse.weebly.com/limiting-factors-in-photosynthesis.html

That's the end of the topic!

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