Photorespiration occurs because the enzyme rubisco, crucial in the Calvin cycle, sometimes binds to oxygen instead of carbon dioxide. This misstep initiates a wasteful process that consumes energy and carbon without producing sugars.
Understanding the Role of Rubisco
Rubisco's primary function is to catalyze the reaction between carbon dioxide and ribulose-1,5-bisphosphate (RuBP), which is the first step in the Calvin cycle for producing sugars. However, rubisco isn't perfectly specific to carbon dioxide and sometimes interacts with oxygen instead.
Why Rubisco Binds to Oxygen
- Structural Similarity: Oxygen and carbon dioxide molecules are structurally similar, making it possible for rubisco to bind to either.
- Competition: When oxygen concentration is higher than carbon dioxide, as can happen in hot, dry conditions with closed stomata, the likelihood of rubisco binding to oxygen increases.
Photorespiration Pathway
When rubisco binds to oxygen, it initiates photorespiration. Here’s a breakdown:
- Oxygen Binding: Rubisco catalyzes a reaction between RuBP and oxygen, resulting in one molecule of 3-phosphoglycerate (a Calvin cycle intermediate) and one molecule of 2-phosphoglycolate.
- 2-Phosphoglycolate Conversion: The 2-phosphoglycolate is converted to other molecules in the chloroplast, peroxisome, and mitochondrion.
- Carbon Dioxide Release: This process eventually releases some carbon dioxide.
The Problem with Photorespiration
Photorespiration is considered wasteful because:
- It consumes energy in the form of ATP and NADPH without producing sugars.
- It reduces the efficiency of photosynthesis by lowering carbon fixation.
- It can undo some of the work of the Calvin cycle by releasing carbon dioxide previously fixed.
Environmental Factors
Conditions that favor photorespiration:
- High Temperatures: Increase the rate of oxygen binding to rubisco and reduce the solubility of carbon dioxide.
- Low Carbon Dioxide: Happens when stomata are closed during hot, dry conditions to prevent water loss, leading to an increased oxygen to carbon dioxide ratio in leaves.
Comparison of Photosynthesis and Photorespiration
| Feature | Photosynthesis | Photorespiration |
|---|---|---|
| Key Enzyme | Rubisco | Rubisco |
| Primary Reactant | Carbon dioxide | Oxygen |
| End Product | Sugars (glucose) | No sugars, release of carbon dioxide |
| Energy Input | Light energy used | ATP and NADPH consumed |
| Benefit | Produces food | No apparent benefit for plants |
In summary, photorespiration occurs due to rubisco's ability to bind oxygen, a reaction that is favored when oxygen concentrations in the leaf are high relative to carbon dioxide.
