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Photorespiration, C2 Cycle, Factors Affecting Photorespiration, Difference Between Dark Respiration and Photorespiration, Practice Problems and FAQs

Photorespiration, C2 Cycle, Factors Affecting Photorespiration, Difference Between Dark Respiration and Photorespiration, Practice Problems and FAQs

We all know that photosynthesis is the process by which the plants produce their own food. We all depend on the plants directly or indirectly for our food. But, do you know there is a process present in plants that reduces the efficiency of photosynthesis? It is the process of photorespiration. 

Do you have any idea how this process takes place in plants? Well, we know that carbon dioxide is fixed as carbohydrates in plants in the presence of sunlight and chlorophyll. In case of photorespiration, this carbon is oxidised (oxygen is added to the carbon). In photorespiration neither ATP is produced nor carbohydrates (sugars) are formed.

Fig: Photorespiration

It is also the process which creates a difference between C3 and C4 plants. In which plants do you think photorespiration takes place? It takes place in C3 plants. But why does it occur in C3 and not in C4 plants? It is due to the oxygenase activity of an enzyme called RuBisCO in C3 plants. It is the most abundant enzyme in the world. Both CO2 and O2 compete for the same active site of the enzyme. But RuBisCO has more affinity towards CO2, when the CO2:O2 ratio is the same. Let’s understand the process of photorespiration in detail in this article. 

Table of contents

  • Photorespiration
  • Factors affecting binding of RuBisCO with its substrates
  • Comparison of respiration and photorespiration
  • Crucial steps in photorespiration
  • Photorespiration: a wasteful process
  • C2 Cycle
  • Balance sheet - Number of carbons lost or recovered
  • Advantages and disadvantages of photorespiration
  • Difference between dark respiration and photorespiration
  • Factors affecting photorespiration
  • Practice Problems
  • FAQs

Photorespiration

Photorespiration is the process of uptake of oxygen and production of carbon dioxide in light by the photosynthetic tissues. It results in the loss of fixed carbon as CO2 in plants when they are exposed to light. Here the food is oxidised and releases carbon dioxide, but no energy is generated. When there is a high concentration of oxygen, photorespiration occurs normally in plants.

Factors affecting binding of RuBisCO with its substrates

RuBisCO (Ribulose bisphosphate oxygenase) has both carboxylase and oxygenase activities. 

Low temperature

At low temperature, the CO2:O2 ratio will be high. RuBisCO behaves as carboxylase and it enters the Calvin or C3 cycle.

High temperature

At high temperatures, the CO2:O2 ratio will be low. RuBisCO behaves as an oxygenase and enters the photorespiration or C2 cycle.

Fig: Dual nature of RuBisCO

Fig: Dual nature of RuBisCO

Comparison of respiration and photorespiration

Respiration and photorespiration takes place in plants.

Respiration 

In the respiration process, O2 is used for burning of food which results in the release of energy and CO2. It occurs independent of sunlight, hence called dark respiration.

Fig: Photorespiration

Fig: Cellular respiration

Photorespiration

It begins in the chloroplast of plant cells. In this process, O2 is consumed and CO2 is released in the presence of sunlight. It is dependent on light, hence called photorespiration. 

Fig: Photorespiration

Fig: Photorespiration

Crucial steps in photorespiration

The process of photorespiration occurs in a series of steps as follows:

  • RuBP reacts with O2 and it forms 3-PGA (phosphoglyceric acid) and 2-Phosphoglycolate or 2-PG.

Fig: Formation of 2-PG

Fig: Formation of 2-PG

  • 3-PGA can enter the Calvin cycle. 2-PG is a 2-carbon molecule which cannot be used to make the plant biomass.

Fig: Entry of 3-PGA to Calvin cycle

Fig: Entry of 3-PGA to Calvin cycle

  • RuBP is a plant’s reserve material and is formed due to the fixation of CO2.
  • Burning of RuBP to form 2-PG results in the loss of two carbon atoms from the photosynthetic cycle. 

Photorespiration - A wasteful process

When CO2 is in the surrounding area, RuBisCO helps in carbon fixation and biomass is produced. When O2 is in the surrounding area, RuBisCO performs oxygenase activity and there is a loss of 2 carbons from the photosynthetic cycle. So there will be no carbon fixation and no RuBP formation. Hence photorespiration is a wasteful process.

6 RuBP combines with 6O2 to form 6 molecules of 3-PGA and 6 molecules of 2-PG. So there is no gain of fixed carbon atoms but there is a loss of fixed carbon atoms in RuBP. As 2-PG is a 2-carbon compound, it cannot go in the Calvin cycle; there is a net loss of 12 carbon atoms. Plants try to rescue the 2 carbon atoms in 2-PG molecule, through the C2 cycle. 

Fig: Photorespiration

Fig: Photorespiration

C2 Cycle

The oxidative photosynthetic carbon cycle, or C2 cycle, is the metabolic pathway for photorespiration, in which sugars are oxidised to CO2 in the light. In the C2 cycle, the reactions of the pathway take place in the following three cell organelles:

  • Chloroplast
  • Peroxisome
  • Mitochondria 

Chloroplast

First product of the C2 cycle is formed in the chloroplast which is 2-phosphoglycolate, a two-carbon molecule. It recovers fixed carbon lost during photorespiration.

Fig: Photorespiration reactions in chloroplast

Fig: Photorespiration reactions in chloroplast

Phosphoglycolate then loses a phosphate group to form glycolate.

Fig: Formation of glycolate in chloroplast

Fig: Formation of glycolate in chloroplast

Peroxisome

Glycolate then enters inside the peroxisomes. Here glycolate is then converted to glyoxylate by using O2 and releases hydrogen peroxide. Glyoxylate is further converted to glycine (an amino acid).

Fig: Formation of Glycine in peroxisome

Fig: Formation of Glycine in peroxisome

Mitochondria

Glycine enters inside the mitochondria. One more glycine from the repetition of the cycle also reaches mitochondria. Two molecules of glycine interact here and form CO2, ammonia and serine.

Fig: Photorespiration reaction in mitochondria

Fig: Photorespiration reaction in mitochondria

Movement of serine from mitochondria to peroxisome

Serine ( three carbon amino acid) then moves to peroxisome and forms glycerate.

Fig: Reactions from mitochondria to peroxisome

Fig: Reactions from mitochondria to peroxisome

Movement of the glycerate from peroxisomes to chloroplast

Glycerate then moves to chloroplast. Here it forms 3-phosphoglycerate or 3-PGA by using ATP. 3-PGA enters the Calvin cycle.

Fig: C2 cycle 

Fig: C2 cycle 

Balance sheet - Number of carbons are lost or recovered

The plant has a pathway to rescue the lost carbon atoms. Six RuBP combines with six O2 to form six molecules of 3-PGA and six molecules of 2-PG. Two C2 cycles are needed to form serine and CO2. Hence, 12 RuBP and 12 O2 form 12 molecules of 3-PGA and 12 molecules of 2-PG. 12 molecules of 2-PG molecules form 6 serine and 6 CO2 molecules. 6 serine molecules go ahead in the cycle to form 6 molecules of 3-PGA molecules. Now (12+6) = 18 molecules of 3-PGA molecules can enter the Calvin cycle. Hence, 18 atoms of carbon have been recovered and 6 atoms of carbon are lost which is equal to 10% of carbon lost.

Fig: Loss and gain of carbon in photorespiration and C2 cycle

Fig: Loss and gain of carbon in photorespiration and C2 cycle

Advantages and disadvantages of photorespiration

The following are the advantages and disadvantages of photorespiration.

Advantages

The advantages of photorespiration are as follows:

  • It removes toxic metabolic intermediates.
  • It shields plants from the harmful effects of too much light.

Disadvantages

The disadvantages of photorespiration are as follows:

  • It causes the reduction in the efficiency of photosynthesis.
  • It does not produce energy intermediates ATP and NADPH2.

Difference between dark respiration and photorespiration

Dark respiration (Normal respiration)

Photorespiration

It takes place in all living cells

It takes place in photosynthetic cells

It occurs independent of light

It can take place only in the presence of light

Carbon dioxide and water are considered as the end products

Carbon dioxide and phosphoglycerate are the end products

It occurs in cytoplasm and mitochondria

It occurs in the chloroplast, mitochondria and peroxisome

It is not a wasteful process

It is a wasteful process

It produces energy

Energy is not produced

Substrates include glucose, fats, proteins etc

Substrate include RuBP

Its substrates include glucose, fats, proteins, and organic acids

Its substrate is RUBP

End products are not recycled

Both the end products are utilised in photosynthesis

Hydrogen peroxide is not produced

Toxic hydrogen peroxide is produced

It is normally not affected by the change in oxygen concentration

It increases with increase in oxygen concentration

Factors affecting photorespiration

Photorespiration is affected by the following factors:

Temperature

Photorespiration increases with increase in temperature. The rising temperature decreases the solubility of CO2 more rapidly than O2. Hence there is availability of more O2 and hence the rate of photorespiration increases.

Intensity of light

Photorespiration increases with increase in the intensity of light as it is a light dependent reaction.

Concentration of oxygen

Photorespiration increases with increase in concentration of oxygen. In this condition, oxygen competes with carbon dioxide for the active site of the enzyme RuBisCo for the oxidation of RUBP to phosphoglycolic acid. 

Age of leaf

Photorespiration increases with the increase in age of leaves as the older leaves are less efficient in photosynthesis compared to younger leaves.

Photorespiration in primitive Earth

In primitive Earth, oxygen was absent and the atmosphere was reducing, hence photorespiration was absent. Now there is a high percentage of oxygen in the atmosphere in comparison to carbon dioxide, hence plants must have evolved some mechanism to increase their productivity and reduce their losses.

Fig: Primitive Earth

Fig: Primitive Earth

Practice Problems

Q1. Choose the correct set of statements regarding photorespiration?

A. It takes place only in the chloroplast of plant cells.
B. It generates energy in the form of ATP.
C. Phosphoglycerate is the end product.
D. Occurs by oxygenation of RuBP.

a. III and IV
b. II and III
c. I and II
d. I and III

Solution: When plants are exposed to light, photorespiration takes place. This process occurs due to RuBisCO enzyme’s oxygenase activity. This oxygenase activity fixes oxygen rather than carbon dioxide in the Calvin cycle's first reaction. Instead of being converted to two molecules of phosphoglyceric acid (PGA), RuBP binds to O2 to form one molecule of PGA and one molecule of phosphoglycolate. There is no sugar synthesis or synthesis of ATP in the photorespiratory pathway. But the release of CO2 occurs as a result of the use of ATP. This pathway takes place in the chloroplast, peroxisomes, and mitochondria of cells. Hence, option a is correct.

Q2. Which of the following shows the correct sequence of intermediates produced during photorespiration?

A. Glycine → Serine → Glycolate → Glyoxylate → Phosphoglycolate
B. Phosphoglycolate → Glycolate → Glyoxylate → Glycine → Serine
C. Serine → Glycine → Phosphoglycerate → Glyoxylate → Glycolate
D. Glycolate → Phosphoglycolate → Glyoxylate → Glycine → Serine

Solution: In chloroplast, the first product of the C2 cycle called 2 phosphoglycolate, a two-carbon molecule, is produced. The phosphate group in phosphoglycolate is then lost, and glycolate is formed. Using O2 and hydrogen peroxide, glycolate is converted to glyoxylate in the peroxisome. Glyoxylate is then transformed into glycine (an amino acid). The amino acid glycine enters the mitochondria. One more glycine reaches mitochondria as a result of the repetition of the cycle. CO2, ammonia, and serine are formed when two glycine molecules interact. The sequence of intermediates produced in the pathway is as follows:

Phosphoglycolate → Glycolate → Glyoxylate →Glycine → Serine. 

Hence, option b is correct.

Q3. Explain how temperature plays an important role in photorespiration?
Solution: The photosynthetic enzyme RuBisCO has a high affinity for oxygen when exposed to high temperatures and light. Through the process of photorespiration, oxygen can attach to RuBisCO instead of carbon dioxide which slows down the rate of photosynthesis.

Q4. How is photorespiration beneficial to plants?
Solution: The oxidative photosynthetic carbon cycle (or C2 cycle) is the metabolic mechanism that is responsible for photosynthetic oxygen intake and photorespiration, which is the light-dependent synthesis of carbon dioxide. It eliminates harmful metabolic intermediates. By dispersing excess excitation energy, it protects the plant from photooxidative damage.

FAQs

Question 1. Why does photorespiration occur in C3 plants and not in C4 plants?
Answer: RuBisCO attaches to oxygen molecules in C3 plants, causing the process to diverge from the normal metabolic route. When RuBP and oxygen molecules come together, one molecule of phosphoglycerate and phosphoglycolate are formed. This process is termed photorespiration. In C4 plants, the photorespiration does not take place. This is due to the fact that they have a mechanism that raises the CO2 concentration at the enzyme's site. This occurs when the mesophyll C4 acid in bundle sheath cells is broken down to release CO2, resulting in an increase in intercellular CO2 concentration.

Question 2. Who discovered the process of photorespiration?
Answer: Photorespiration is the loss of fixed carbon as CO2 in plants when they are exposed to light. Dicker and Tio (1959) found it when they noticed that the rate of respiration of green leaves is substantially higher in the light than in the dark.

Question 3. Why is the process of photorespiration also referred to as the C2 cycle?
Answer: The oxidative photosynthetic carbon cycle, or C2 cycle, is the metabolic mechanism for photorespiration, in which carbohydrates are converted to CO2 in the light. Photorespiration is also known as the C2 cycle because the first major product is phosphoglycolate, a two-carbon molecule that is later transformed to glycolate.

Question 4. What is the first stable product produced in photorespiration?
Answer: Photorespiration is a respiratory mechanism in which higher plants absorb oxygen from the air and release carbon dioxide. The manufacture of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the leaf cell produce the initial product, two-carbon phosphoglycolate. 

Related Topics

Calvin cycle, Practice Problems and FAQs

The C4 pathway, Practice Problems and FAQs

Dual nature of RuBisCO and CAM pathway, Practice Problems and FAQs, 

Cyclic photophosphorylation, Practice Problems and FAQs 

Kranz Anatomy, Practice Problems and FAQs 

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