Question

Describe how photorespiration lowers photosynthetic output for plants.

Short answer

Photorespiration replaces carbon dioxide in the Calvin cycle with oxygen. As a result, sugar is not created since carbon is not fixed, and the process uses more oxygen. As a result, photorespiration reduces photosynthetic output.

Step-by-step solution

Meaning of photorespiration

Photorespiration is the process where plants use oxygen and releases carbon dioxide in the presence of sunlight. Chloroplasts, mitochondria, and peroxisomes are involved in photorespiration.

The process is initiated in chloroplasts, but mitochondria and peroxisomes are responsible for releasing carbon dioxide.

Chemistry of photorespiration

Photorespiration is a process that occurs where the concentration of oxygen is high. In conditions of high oxygen, RuBisCo (the enzyme that catalyzes the first step of the Calvin cycle) functions as oxygenase and binds oxygen.

With less carbon dioxide, the concentration of oxygen increases, and RuBP combines with oxygen to form phosphoglycerate and phosphoglycolate. Phosphoglycolate leaves the chloroplasts and enters mitochondria and chloroplasts.

These organelles then release carbon dioxide from the compound with the utilization of ATP.

Effect of photorespiration on photosynthetic output

In photorespiration, carbon is oxidized to release carbon dioxide. O₂ is utilized in chloroplasts and peroxisomes, and ATP is also utilized.

Photorespiration leads to the loss of the fixed carbon as CO₂, which is otherwise used in the Calvin cycle to synthesize sugar. It affects the ability of plants to fix carbon and its efficiency in synthesizing sugar.

It also utilizes oxygen which plants release through photosynthesis. Thus, photorespiration is considered a waste process as it decreases the photosynthetic output for plants.