Photorespiration and the factors that limit photosynthesis

An enzyme with two jobs

RuBisCO is a long name. It stands for ribulose bisphosphate carboxylase-oxygenase. The last word is the key. The active site of RuBisCO can bind carbon dioxide, and it can also bind oxygen. Which one wins depends on how much of each is nearby.

• When there is plenty of carbon dioxide, RuBisCO acts as a carboxylase. The Calvin cycle runs.
• When carbon dioxide is low and oxygen is high, RuBisCO acts as an oxygenase.

Photorespiration

When RuBisCO binds oxygen, RuBP is oxidised. It gives one molecule of PGA and one molecule of a 2-carbon compound, phosphoglycolate. The PGA goes back into the cycle. The 2-carbon compound does not.

This is photorespiration. It involves three organelles: the chloroplast, the peroxisome and the mitochondrion. In this pathway:

• no sugar is made,
• no ATP and no NADPH are made,
• carbon dioxide is released, and ATP is used.

It is a wasteful process. It tends to happen on hot, dry days, when stomata close, so carbon dioxide falls and oxygen rises in the leaf.

C4 plants lack photorespiration. They concentrate carbon dioxide round RuBisCO in the bundle sheath cells, so oxygen has little chance to bind.

Factors that affect photosynthesis

The rate of photosynthesis depends on both the plant and the world around it. Blackman gave a law in 1905, the law of limiting factors:

If a process depends on more than one factor, its rate is set by the factor that is nearest to its minimum value.

Think of a chain. It is as strong as its weakest link. The factor that is in shortest supply is the limiting factor. Changing it changes the rate. Changing the others does not.

• Light. At low light, the rate rises in proportion to the light. The rise stops, and the rate saturates, at about 10 percent of full sunlight. Beyond that more light brings no gain. Very bright light can even break down chlorophyll and cut the rate.
• Carbon dioxide. This is the major limiting factor. The air holds only $0.03$ to $0.04$ percent. A C4 plant saturates at about $360\ \mu\text{L/L}$. A C3 plant keeps responding until about $450\ \mu\text{L/L}$. Growers of tomato and bell pepper in greenhouses add carbon dioxide to the air and get a higher yield.
• Temperature. The sugar-making steps are controlled by enzymes, so they are sensitive to temperature. C4 plants have a higher optimum than C3 plants.
• Water. Lack of water closes the stomata, so less carbon dioxide gets in. The leaves also wilt and have less surface.

You should find that with $5$ percent light and $300\ \mu\text{L/L}$ of carbon dioxide, the rate is $50$, and light is the limit. Raise the light to $15$ percent, and the rate climbs to $75$ and stops. Now carbon dioxide is the limit. Only more carbon dioxide raises the rate.

Try it in Python

The program uses the same small model to name the limiting factor in four sets of conditions.

You should see that light limits the first case, and carbon dioxide limits the others. With $500\ \mu\text{L/L}$ the rate rises to $100$, because the light has now become the limit.

Practise with new numbers

Common mistakes

• Thinking that photorespiration makes ATP. It makes none, and it uses some.
• Saying that RuBisCO binds only carbon dioxide. It also binds oxygen.
• Saying that C4 plants photorespire strongly. They lack it.
• Thinking that more light always helps. After about $10$ percent of full sunlight, the rate is limited by something else.
• Naming the factor that is most plentiful as limiting. The limiting factor is the one nearest its minimum.

Remember these

• RuBisCO is a carboxylase and an oxygenase. Low carbon dioxide and high oxygen give photorespiration.
• Photorespiration makes no sugar, no ATP and no NADPH, and it releases carbon dioxide.
• Blackman: the factor nearest its minimum sets the rate.
• Light saturates near $10$ percent of full sunlight. C4 saturates near $360\ \mu\text{L/L}$ carbon dioxide and C3 near $450\ \mu\text{L/L}$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. When does RuBisCO act as an oxygenase?

2. Name three things that photorespiration does not make.

3. Why do C4 plants lack photorespiration?

4. State Blackman's law of limiting factors.

5. Why does a plant in dry soil photosynthesise slowly even in good light?