Difference Between Photosynthesis and Photorespiration

The key difference between photosynthesis and photorespiration is that the photosynthesis is the process by which photoautotrophs, mainly green plants, algae and cyanobacteria, generate carbohydrates and oxygen from carbon dioxide and water using the energy in sunlight while photorespiration is a side reaction in which the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted.

Photosynthesis and photorespiration are two processes that occur in plants. However, photosynthesis is an important process while photorespiration is a wasteful process. Some of the energy and fixed carbon are wasted by the photorespiration by the enzyme called RuBP oxygenase-carboxylase. Hence photorespiration can be referred to as a process that reduces the efficiency of photosynthesis in plants.

CONTENTS

1. Overview and Key Difference
2. What is Photosynthesis
3. What is Photorespiration
4. Similarities Between Photosynthesis and Photorespiration
5. Side by Side Comparison – Photosynthesis vs Photorespiration in Tabular Form
6. Summary

What is Photosynthesis?

Photosynthesis is the process by which green plants, cyanobacteria and algae convert light energy into chemical energy. This is a unique process occurring only in photoautotrophs. These organisms capture energy from sunlight and convert it into chemical energy in the form of carbohydrates, which can be utilized by organisms as their foods. Photosynthetic organisms do not depend on other organisms for their food unlike humans and other heterotrophs. They produce their own food by photosynthesis and provide others with what they produced. In plants, photosynthesis takes place in the organelle called chloroplast.

The pigments called chlorophylls to involve in capturing light. Plant leaves are the main sites of photosynthesis in plants. Leaf structure supports efficient photosynthesis by minimizing the water loss and by doing the efficient gas exchange via stomata. Using several steps, carbon dioxide and water convert into glucose and oxygen using the energy in the sunlight. There are two major reactions in photosynthesis. They are a light-dependent reaction (light reaction) and light independent reaction (dark reaction or Calvin cycle). The light reaction produces ATP and NADPH while dark reaction produces sugar molecules utilizing the products of the light reaction. Calvin cycle proceeds through three major steps namely carboxylation, reduction and regeneration. As byproduct oxygen is released to the atmosphere and this is the molecular oxygen that what we breathe. When organisms need energy, they break these sugar molecules mainly glucose and produce ATP (energy molecules) for their cellular processes.

What is Photorespiration?

Photorespiration is a side reaction of the Calvin cycle. During the Calvin cycle, one major enzyme called RuBP oxygenase – carboxylase (rubisco) converts RuBP into phosphoglyceraldehyde by incorporating carbon dioxide. It is the normal process of producing glucose molecule. However, this enzyme has an ability to incorporate oxygen instead of carbon dioxide. That means Rubisco has the ability to use oxygen as its substrate instead of carbon dioxide. When this occurs, it initiates the above-called process photorespiration. Photorespiration actually wastes energy and some of the fixed carbon.

Furthermore, it decreases the synthesis of sugar molecules (it reduces the number of sugar molecules that can be produced by the normal Calvin cycle). Photorespiration favours by several conditions such as low carbon dioxide: oxygen ratio, high temperature, etc. When temperature increases, rubisco enzyme has a higher affinity towards oxygen than carbon dioxide. Hence, the plants which grow under hot and dry conditions undergo photorespiration more than the plants grown in other areas. However, plats show different adaptations and mechanisms to minimize photorespiration and loss of energy. One example is C4 plants.

What are the Similarities Between Photosynthesis and Photorespiration?

• Both occur in plants.
• Sunlight is involved with photosynthesis and photorespiration.
• Both utilize enzymes.
• They occur within cellular organelles.

What is the Difference Between Photosynthesis and Photorespiration?

Photosynthesis and photorespiration are two processes occur in plants. Photosynthesis produces foods while photorespiration wastes products of photosynthesis. Some plants that grow under hot and dry conditions undergo photorespiration more. Hence, plants try to minimize photorespiration by adopting several mechanisms.

The below infographic presents more details on the difference between photosynthesis and photorespiration in tabular form.

Summary – Photosynthesis vs Photorespiration

Photosynthesis fixes carbon dioxide while photorespiration wastes some of the fixed carbon by photosynthesis. Both processes are enzyme-driven. During the photosynthesis, light energy converts into chemical energy by photoautotrophs. It needs sunlight, green colour pigments, carbon dioxide and water. On the other hand, photorespiration works as a side reaction of the Calvin cycle in which rubisco enzyme converts RuBP into PGA and PG by incorporating O₂ instead of CO₂. Photorespiration occurs under high temperature and low carbon dioxide concentrations. This is the difference between photosynthesis and photorespiration.

Reference:

1.Boundless. “Boundless Biology.” Lumen, Open SUNY Textbooks. Available here  
2.“Photorespiration.” Khan Academy, Khan Academy. Available here  

Image Courtesy:

1.”10187194256″ by Zappys Technology Solutions (CC BY 2.0) via Flickr
2.”Photorespiration allgemein” By Yikrazuul (talk) – Own work, (CC BY-SA 3.0) via Commons Wikimedia