16. RuBisCO and its role in photosynthesis in C3, C4 and CAM plants

What is the role of RuBisCO in photosynthesis?

The first stage of the Calvin cycle (carbon fixation) involves attaching a carbon from a CO2 to a 5-carbon molecule called RuBP.

The resultant 6 carbon molecule then immediately splits to produce two 3-carbon molecules (3-PGA).

These steps are catalysed by the enzyme RuBisCO.

What is photorespiration and why does it occur?

The process of RuBisCo using O2 instead of CO2 is called photorespiration.

As well as having an active site that can bind CO2 as the substrate, RuBisCO can also bind O2 as the substrate.

When O2 binds to RuBisCO it disrupts the Calvin cycle, reducing photosynthesis efficiency.

O2 is a competitive inhibitor of CO2 for the enzyme RuBisCO.

What are the two main factors photorespiration is affected by?

Environmental temperature and substrate concentration

How does environmental temperature affect photorespiration?

At low temps, CO2 is more likely to bind to RuBisCO than O2.

As temperatures increase, more O2 and less CO2 will bind to RuBisCO, leading to an increase in photorespiration and a decrease in photosynthetic efficiency (lowered glucose production).

How does substrate concentration affect photorespiration?

When light in available, the stomata of plants are usually open, allowing CO2 to enter and O2 and H20 vapour to diffuse out.

If stomata close, O2 levels will increase inside the mesophyll cells due to photosynthesis and CO2 levels will reduce, leading to an increase in photorespiration and a decrease in photosynthetic efficiency (lowered glucose production).

Plants are classified by the type of photosynthesis they undergo - what are these classifications?

C3 plants, C4 plants, and CAM plants.

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