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.

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.

Differences Between C3 And C4 Photosynthesis

C3 photosynthesis is the most common pathway: In this case, CO2 is directly fixed by Rubisco in the Calvin cycle and forms a three-carbon compound, 3-PGA. On the other hand, the processes of C4 photosynthesis include a set of reactions wherein, firstly, CO2 is fixed into a four-carbon compound called oxaloacetate in the mesophyll cells. Then, diffusion of this compound in the bundle-sheath cells releases CO2 and refixes it with Rubisco, effectively concentrating CO2 around Rubisco and minimizing photorespiration.

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 Of Photorespiration On Productivity Of C3 Plants

• 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.

Photorespiration Mechanism In C4 Plants

• 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 CO₂ around 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.

Differences In Leaf Anatomy Between C3 And C4 Plants

• C3 plants: Uniform mesophyll cells, all with chloroplasts

• C4 plants: Specialised bundle-sheath cells with packed chloroplasts, tightly ensheathing the vascular bundles.

Enzymatic Differences: Rubisco vs. PEP Carboxylase

• C3 plants: Only Rubisco involved in CO₂ fixation

• C4 plants: Initial fixation by PEP carboxylase followed by Rubisco in a high-CO₂ environment.

Comparative Efficiency In Various Environmental Conditions

• C3 plants: More productive in cooler, wetter climates

• C4 plants: superior in hot and dry conditions due to minimised photorespiration and efficient use of water.

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