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Chapter 8 · Part 9 of 10
  1. Chapter overview
  2. 1What photosynthesis does
  3. 2Chloroplasts and the two stages
  4. 3Light and how pigments absorb it
  5. 4The pigments plants use
  6. 5How photosystem II captures light
  7. 6Linear electron flow from water to NADPH
  8. 7How chloroplasts make ATP
  9. 8The Calvin cycle
  10. 9Photorespiration
  11. 10How C₄ and CAM plants avoid photorespiration

Photorespiration

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In the Calvin cycle, the enzyme rubisco fixes carbon by attaching CO₂ to RuBP, a five-carbon sugar. But rubisco sometimes attaches O₂ instead, and on hot, dry days that side reaction can undo a large share of a plant’s photosynthesis.

Rubisco’s two reactions

In most plants, rubisco is the only enzyme that fixes carbon, and the first product it makes is 3PG, a molecule with three carbons. These are called C₃ plants. About 85% of the plant species on Earth are C₃ plants, including wheat and oak trees.

Rubisco’s active site, the pocket where it binds the molecules it acts on, can also attach O₂ to RuBP. The reaction with CO₂ is strongly favored, by more than tenfold. But when CO₂ is scarce and O₂ is plentiful, O₂ gets attached more and more often.

The two reactions give different results:

Rubisco attaches… Products What follows
CO₂ to RuBP Two 3PG Carbon is fixed, and the Calvin cycle continues
O₂ to RuBP One 3PG plus phosphoglycolate, a two-carbon molecule Photorespiration, which releases CO₂

Phosphoglycolate has its phosphate removed, becoming glycolate, which leaves the chloroplast. Over several steps in peroxisomes (small organelles that carry out oxidation reactions) and mitochondria, glycolate is oxidized, producing an organic molecule and CO₂.

This pathway, which uses O₂ and releases CO₂, is photorespiration. It’s considered wasteful: carbon the plant has already captured goes back into the air as CO₂, and that limits the plant’s growth.

Hot, dry days make it worse

Photorespiration is most likely when CO₂ inside the leaf is low and O₂ is high. Hot, dry weather creates exactly those conditions:

  1. To save water, the plant closes its stomata, the pores in the leaf surface.
  2. With the stomata shut, CO₂ from the air can’t get in, and the O₂ made by the light reactions can’t get out.
  3. Inside the leaf, CO₂ falls and O₂ builds up.
  4. Rubisco attaches O₂ to RuBP more often, so photorespiration increases.

When C₃ plants face hot, dry conditions, photorespiration can reverse as much as 25–50% of what their photosynthesis accomplishes.

Could photorespiration help the plant?

No one is sure why plants carry out photorespiration. One hypothesis is that it works as a safety valve. When the stomata stay shut on a hot, dry day, O₂ builds up inside the leaf, and the excess may give rise to harmful, highly reactive forms of oxygen, such as free radicals. By using up O₂ and releasing CO₂, photorespiration might keep those molecules from damaging the plant.

Check yourselfWhy does photorespiration increase on hot, dry days?

To save water, the plant closes its stomata. CO₂ can no longer get in, and O₂ from the light reactions builds up. With CO₂ low and O₂ high, rubisco attaches O₂ to RuBP more often, starting photorespiration, which releases CO₂ the plant had already fixed.

Practice questions

  1. 1 of 2

    Why is photorespiration considered wasteful for a plant?

  2. 2 of 2

    On a hot, dry afternoon, a wheat plant closes its stomata to save water. How does this affect photorespiration in its leaves?