Difference Between Photosynthesis and Photorespiration

The main difference between photosynthesis and photorespiration is that the photosynthesis occurs when RuBisCO enzyme reacts with carbon dioxide while the photorespiration occurs when RuBisCO enzyme reacts with oxygen. Furthermore, photorespiration reduces the efficiency of photosynthesis.

Photosynthesis and photorespiration are two processes that occur during the production of energy using sunlight in plants. RuBisCO is the censurable enzyme for switching between two processes.

Key Areas Covered

1. What is Photosynthesis
     – Definition, Process, Importance
2. What is Photorespiration
     – Definition, Process, Importance
3. What are the Similarities Between Photosynthesis and Photorespiration
     – Outline of Common Features
4. What is the Difference Between Photosynthesis and Photorespiration
     – Comparison of Key Differences

Key Terms

Carbon Dioxide, Dark Reaction, Light Reaction, Photorespiration, Photosynthesis, RuBisCO

What is Photosynthesis

Photosynthesis is the process that produces glucose starting from carbon dioxide and water by using the energy from the sunlight. Photosynthetic pigments such as chlorophyll, carotenoids, and phycobilins trap the energy of the sunlight. In plants and algae, these pigments are concentrated into chloroplasts. Oxygen is released as a by-product of photosynthesis. Photosynthesis is one of the key processes that occur on earth, converting light energy into chemical energy. The glucose produced from the process can be used to produce ATP in another process called cellular respiration.   

The process of photosynthesis can be divided into two: light reaction and dark reaction.

Light Reaction

Light reaction occurs on the thylakoid membrane of grana, the stacks of thylakoids embedded in the stroma of a chloroplast. Photosynthetic pigments are organized into photocenters on the thylakoid membrane. Photosystem II absorbs light energy and transport into photocenters, allowing the production of high energy electrons. These high energy electrons move into the photosystem I through the cytochrome b6f complex. They further move through a series of ferredoxin carriers, producing NADPH. The electron deficiency that occurs in the photosystems is filled by splitting water molecules in a process called photolysis. The resultant hydrogen ions are used in the production of ATP.

Figure 1: Light Reaction

Dark Reaction

Light reaction is followed by the dark reaction. Here, NADPH and ATP produced by the light reaction are used to produce glucose from carbon dioxide and water. Dark reaction, which occurs through the C3 cycle, is also called the Calvin cycle and it occurs in the stroma of the chloroplast without the use of light. Fixation of carbon occurs in the Calvin cycle with the use of the enzyme, RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), which fixes a carbon atom from carbon dioxide into RuBP (ribulose 1,5-bisphosphate), producing 3-phosphoglycerate. Some of the 3-phosphoglycerate molecules reduce to form glucose while rest is recycled to produce RuBP. In addition to glucose, 18 ATP and 12 NADPH are also produced during the Calvin cycle.

The dark reaction, which occurs through the C4 cycle, is called the Hatch–Slack pathway in which carbon dioxide is fixed first into PEP and then into RuBP.

What is Photorespiration

Photorespiration is the inhibition of the Calvin cycle in the presence of excess oxygen. It leads to the loss of already-fixed carbon dioxide; hence, photorespiration decreases the sugar synthesis and wastes the energy of the cell. RuBisCO’s ability to bind with oxygen is responsible for photorespiration. Hence, in the presence of oxygen, RuBisCO adds oxygen  to the RuBP in the Calvin cycle instead of carbon dioxide. Two molecules are produced in this reaction: 3-PGA, which is an intermediate of the Calvin cycle, and phosphoglycolate, which cannot enter the Calvin cycle. On that account, photorespiration steals or removes carbons from the Calvin cycle. Furthermore, plants use a series of reactions to recover phosphoglycolate, which steals the energy of the cell as well. Therefore, photorespiration is considered as an inefficient method of producing energy.

Figure 2: Photorespiration and Calvin Cycle

The C4 cycle eliminates this problem with the double fixation of carbon dioxide. It fixes carbon dioxide into PEP (phosphoenolpyruvate) by PEP carboxylase, producing oxaloacetate in mesophyll cells. PEP carboxylase has a higher affinity towards carbon dioxide and a low affinity towards oxygen. Then, oxaloacetate is converted into malate and is transported to the bundle-sheath cells. Malate dissociates into carbon dioxide and pyruvate inside the bundle sheath cells, increasing the carbon dioxide concentration inside the cell. In the presence of high carbon dioxide concentration, RuBisCO does not bind with oxygen.

Similarities Between Photosynthesis and Photorespiration

• Photosynthesis and photorespiration are two processes which occur during the production of glucose in plants.
• They undergo light reaction.
• Both processes get the use of RuBisCO enzyme.

Difference Between Photosynthesis and Photorespiration

Definition

Photosynthesis refers to the process by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water while photorespiration refers to a respiratory process by which plants take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.

Carbon Dioxide/Oxygen

Photosynthesis predominantly occurs in the presence of carbon dioxide while photorespiration predominantly occurs in the presence of oxygen. This is one main difference between photosynthesis and photorespiration.

Influence of Light

Dark reaction of photosynthesis occurs in the absence of light, at night while photorespiration occurs in the presence of light, during the day.

Type of Plants

Photosynthesis predominantly occurs in C4 plants while photorespiration predominantly occurs in C3 plants.

RuBisCO Activity

RuBisCO produces 3-PGA from RuBP in photosynthesis while RuBisCO produces 3-PGA and phosphoglycolate from RuBP in photorespiration.

Carbon Fixation

Photosynthesis is the main process of carbon fixation in plants while photorespiration wastes some of the already fixed carbon.

Energy Fixation

Photosynthesis is the main process of energy fixation in plants while photorespiration wastes some of the energy produced by the cell.

Efficiency

Another important difference between photosynthesis and photorespiration is the efficiency of producing glucose. Photosynthesis is an efficient process of producing glucose while photorespiration is a less efficient process of producing glucose.

Conclusion

Photosynthesis is the process involved in the production of glucose from carbon dioxide and water by using the energy from the sunlight. During photosynthesis, the enzyme, RuBisCo binds with carbon dioxide, adding it to RuBP. However, photorespiration is an alternative process of photosynthesis in which the RuBisCO enzyme binds to oxygen in the low concentrations of carbon dioxide. Furthermore, photorespiration is a less efficient process since it wastes both already fixed carbon and energy. Thus, one important difference between photosynthesis and photorespiration is the efficiency of producing glucose.

Reference:

1. Farabee, M J. “PHOTOSYNTHESIS.” PHOTOSYNTHESIS, Available Here
2. “Photorespiration.” Khan Academy, Khan Academy, Available Here

Image Courtesy:

1. “Photosynthesis light reaction diagram” By BlueRidgeKitties (CC BY 2.0) via flickr
2. “Simplified photorespiration diagram” By Rachel Purdon – Own work (CC BY-SA 3.0) via Commons Wikimedia  

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