Photorespiration In C3 And C4 plants: Overview, Examples, Materials

What Is Photorespiration?

Photorespiration is a process whereby the enzyme RuBP carboxylase/oxygenase of chloroplasts of plant cells oxygenates RuBP (ribulose bisphosphate) instead of carboxylating it, as in photosynthesis. One molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate are formed as by-products. In sharp contrast to photosynthesis, which produces glucose and oxygen, photorespiration consumes energy and releases fixed carbon in the form of CO₂, this makes it a wasteful pathway.

Photorespiration generally occurs when the concentration of oxygen gas is high and that of carbon dioxide is low, such as on hot and dry days when plants close their stomata to prevent the loss of water. This closes the stomata and limits the intake of CO₂ while increasing the level of O₂ inside the leaf, thus favouring oxygenation activity by RuBisCO. This is more common in C3 plants because they lack the mechanism required to concentrate CO2 around RuBisCo.

Why Is Photorespiration Wasteful?

Photorespiration lowers photosynthetic efficiency since it consumes ATP and NADPH that would otherwise be involved in carbon fixation and releases CO₂ that could have been fixed in the Calvin cycle. This lowered energy and carbon supply brings down the general productivity and growth of the plant, hence making it less competitive compared to plants which can suppress photorespiration.

Overview Of Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria transform the light energy from the sun into chemical energy, stored as glucose. There are two major stages through which photosynthesis takes place: the light-dependent one in which light energy is absorbed and converted in the thylakoid membranes of chloroplasts into ATP and NADPH, and the Calvin cycle, otherwise called the light-independent reactions, in which carbon dioxide is fixed into glucose in chloroplasts' stroma using the above-mentioned ATP and NADPH.

Photorespiration In C3 Plants

Photorespiration is especially important in C3 plants. They get their name from the fact that the first stable compound formed from carbon fixation is a three-carbon molecule, 3-PGA.

Description Of C3 Plants

• Most common types of plants (e.g., wheat, rice, soybean)

• Fix carbon directly through the Calvin cycle.

• Lacking specialised anatomy to minimise photorespiration.

Mechanism Of Photorespiration In C3 Plants

• Rubisco oxygenates RuBP instead of carboxylating it.

• Produces 3-PGA and 2-phosphoglycolate

• 2-phosphoglycolate is regenerated by a suite of reactions that require the consumption of ATP and the release of CO₂

Impact On Productivity

• Reduces net carbon fixed

• Low overall photosynthetic efficiency.

• Low growth and yield of the plant.

Photorespiration In C4 Plants

C4 plants have evolved a way to minimise photorespiration, which makes them more productive than most other plants in certain environments.

Description Of C4 Plants

• Examples include maize, sugarcane and sorghum.

• Leaves have a distinctive anatomy with mesophyll and bundle sheath cells.

• Have a two-step carbon fixation.

Mechanism Of CO₂ Fixation (C4 Pathway)

• PEP carboxylase fixes CO₂ in mesophyll cells

• First formed four-carbon compound—oxaloacetate

• Oxaloacetate transported to bundle sheath cells

• CO₂ release and refixation by Rubisco in the Calvin cycle.

How C4 Plants Avoid Photorespiration

• PEP carboxylase binds CO₂ more tightly and does not bind O₂

• High concentration of CO₂around Rubisco, inhibiting its oxygenation activity.

• The spatial separation of initial CO₂ fixation and the Calvin cycle.

Advantages Of C4 Plants Over High-Temperature Environments

• More productive under high light intensity and temperatures

• High water-use efficiency because stomata do not open widely

• Higher productivity and yield in the tropics and subtropics

Comparative Analysis – C3 vs C4 Plants

The differences between C3 and C4 plants explain their efficiencies and adaptations to different environments.

Photorespiration In C3 And C4 Plants NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Mechanism of Photorespiration in C3 and C4 Plants

• C3 vs C4 Plants

Practice Questions for NEET

Q1. The substrate for the process of photorespiration is

1. Glycerate

2. Phosphoglycolate

3. Glycine

4. Phosphoglycerate

Correct answer: 2) Phosphoglycolate

Explanation:

Photorespiration is a metabolic process that occurs in plants, particularly under conditions of high oxygen and low carbon dioxide concentrations. The process begins in the chloroplasts, where phosphoglycolate is formed as a byproduct of RuBisCO's oxygenation reaction. Phosphoglycolate is then converted into glycolate, which is transported to the peroxisomes, where it undergoes further metabolism. Finally, the pathway involves the mitochondria, where specific reactions take place to recycle intermediates. Thus, the photorespiration process requires the coordinated activity of chloroplasts, peroxisomes, and mitochondria, highlighting its complexity and energy-consuming nature.

Hence, the correct answer is option 2) Phospoglycolate.

Q2. The sequence of organelles in which photorespiration occurs is

1. Mitochondria - peroxisome - chloroplast

2. Chloroplast - peroxisome - mitochondria

3. Peroxisome - Chloroplast - mitochondria

4. Peroxisome -mitochondria - chloroplast

Correct answer: 2) Chloroplast - peroxisome - mitochondria

Explanation:

Photorespiration is a process that occurs in plants, primarily when the enzyme RuBisCO fixes oxygen instead of carbon dioxide during the Calvin cycle, leading to the formation of phosphoglycolate as a substrate. This process can be wasteful for the plant, as it reduces the efficiency of photosynthesis. Photorespiration involves three key organelles: the chloroplasts, where the initial reaction occurs; the peroxisomes, where part of the process is completed and toxic by-products like hydrogen peroxide are neutralized; and the mitochondria, where some of the carbon from phosphoglycolate is used to regenerate useful compounds. These organelles work together in a complex cycle to reduce the harmful effects of photorespiration, though the process still results in energy loss for the plant.

Hence, the correct answer is option 2) Chloroplast - peroxisome - mitochondria.

Q3. Photorespiration shows formation of

1. Sugar but not ATP

2. ATP but not sugar

3. Both ATP and sugar

4. Neither ATP nor sugar

Correct answer: 4) Neither ATP nor sugar

Explanation:

Photorespiration is the light-dependent process of oxygenation of ribulose biphosphate (RuBP) and the release of carbon dioxide by the photosynthetic organs of a plant. In the light, photosynthetic organs typically absorb CO2 and release O2, the opposite of what happens normally. At high temperatures, RuBP carboxylase acts as an oxygenase and oxidizes ribulose 1, 5-biphosphate to form a 3-carbon phosphoglyceric acid and a 2-carbon phosphoglycolate rather than fixing carbon dioxide (C3 cycle).

Photorespiration doesn't generate power or produce energy. Instead, it uses energy. Additionally, it reverses the effects of photosynthesis. Fixed CO2 is lost by 25%. Photorespiration is therefore a rather wasteful process. This only occurs with C3 plants. Photorespiration has been a concern for C4 plants, but not anymore.

Hence, the correct answer is option 4) Neither ATP nor sugar.

Also Read:

• MCQs on Photorespiration

• Factors Affecting Photosynthesis

• Cyclic Photophosphorylation