Careers360 Logo
Photorespiration In C3 And C4 plants: Overview, Examples, Materials

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

Edited By Irshad Anwar | Updated on Aug 26, 2024 06:39 PM IST

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.

NEET Highest Scoring Chapters & Topics
Know Most Scoring Concepts in NEET 2024 Based on Previous Year Analysis
Know More

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.

Recommended Video On 'Photorespiration In C3 And C4 Plants'


Frequently Asked Questions (FAQs)

1. What is photorespiration, and why does it take place?

Photorespiration is a process in which, on account of oxygenation by the enzyme RuBP, energy and carbon dioxide get wasted. It mainly takes place in plants of the C3 category under high oxygen and low carbon-di-oxide conditions.

2. How do plants of the C4 category minimise photorespiration?

C4 plants reduce the levels of photorespiration by taking up a two-step photosynthetic process to ensure high CO2 concentration around the Rubisco enzyme, excluding the interaction of oxygen with it.

3. What are the main differences between C3 and C4 plants?

C3 plants fix CO2 directly through the Calvin cycle, while C4 plants use an additional step to concentrate CO2, making them more efficient in hot, dry environments.

4. Why is photorespiration considered inefficient for plants?

Photorespiration is regarded as inefficient because it results in the loss of reduced carbon and energy, hence reducing the overall photosynthetic efficiency of the plant.

5. How does climate change impact photorespiration?

Climate change can elevate photorespiration due to increasing temperatures and variable CO2. This can lower crop yields.

Articles

Back to top