Question

How does photosynthesis in \(\mathrm{C}_{4}\) plants differ from the process in \(\mathrm{C}_{3}\) plants?

Short answer

\text{\text{C}_4} plants have an extra step involving PEP carboxylase and are more efficient in high light and temperature conditions compared to \text{\text{C}_3} plants, which only use Rubisco.

Step-by-step solution

Define Photosynthesis

Photosynthesis is the process by which green plants use sunlight to synthesize foods with the help of chlorophyll. This process generally involves the intake of carbon dioxide and water, with the release of oxygen as a byproduct.

Identify Photosynthesis in \(\text{C}_3\) Plants

In \(\text{C}_3\) plants, the first product of carbon fixation is a 3-carbon molecule called 3-phosphoglycerate (3-PGA). The Calvin cycle, occurring in the chloroplasts, is the main pathway through which \(\text{C}_3\) photosynthesis operates.

Describe Photosynthesis in \(\text{C}_4\) Plants

In \(\text{C}_4\) plants, the first product of carbon fixation is a 4-carbon molecule called oxaloacetate. These plants use an additional set of reactions known as the Hatch-Slack pathway before the Calvin cycle. This occurs primarily in mesophyll cells and bundle-sheath cells.

Compare the Carbon Fixation Pathways

\(\text{C}_3\) plants utilize the enzyme Rubisco for carbon fixation, which directly leads to the formation of 3-PGA. In contrast, \(\text{C}_4\) plants initially use the enzyme PEP carboxylase to fix carbon into a 4-carbon compound, which is later transported to bundle-sheath cells where the Calvin cycle occurs.

Adaptations to Different Environments

\(\text{C}_4\) plants are typically more efficient than \(\text{C}_3\) plants under conditions of high light intensity, high temperatures, and limited nitrogen or \(\text{CO}_2\) concentrations because the Hatch-Slack pathway reduces photorespiration. \(\text{C}_3\) plants are more common in cooler, wetter environments.

Key concepts

Photosynthesis

Photosynthesis is a critical process carried out by green plants, algae, and cyanobacteria.
These organisms capture light energy to synthesize organic compounds like glucose from carbon dioxide (CO₂) and water (H₂O).
During this process, oxygen (O₂) is released as a byproduct.
This occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
Photosynthesis not only fuels the growth and metabolism of plants but also produces the oxygen we breathe.

C3 Plants

C3 plants are the most common type of plants found worldwide.
The term 'C3' refers to the three-carbon compound, 3-phosphoglycerate (3-PGA), formed during the first step of the Calvin cycle.
These plants use a direct carbon fixation pathway where the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) captures CO₂.
This process mainly occurs in the mesophyll cells of the leaves.
C3 plants are efficient in cooler, wetter environments but are less efficient in hot, arid conditions due to photorespiration.

C4 Plants

C4 plants have evolved an additional carbon fixation pathway known as the Hatch-Slack pathway to overcome the limitations of photorespiration.
This pathway initially forms a four-carbon compound, oxaloacetate, hence the name 'C4'.
These plants utilize the enzyme PEP carboxylase for the initial fixation of CO₂ into oxaloacetate in the mesophyll cells.
This compound is then shuttled to the bundle-sheath cells where it releases CO₂ for the Calvin cycle.
C4 plants are more efficient in high light intensity, high temperature, and low CO₂ conditions, making them well-suited to hot and dry environments.

Carbon Fixation

Carbon fixation is the process by which inorganic carbon (CO₂) is converted into organic compounds by living organisms.
In C3 plants, carbon fixation occurs solely via the Calvin cycle using the enzyme Rubisco.
However, in C4 plants, an additional step is added before the Calvin cycle.
Initially, CO₂ is fixed into a four-carbon compound using PEP carboxylase, which is later broken down to release CO₂ in the bundle-sheath cells.
This dual-carbon fixation mechanism significantly reduces photorespiration in C4 plants, enhancing their efficiency in stressful environmental conditions.

Calvin Cycle

The Calvin cycle, also known as the light-independent reactions, is the set of biochemical reactions that take place in the chloroplasts of photosynthetic organisms.
This cycle uses ATP and NADPH produced during the light-dependent reactions to fix carbon dioxide into organic molecules.
The Calvin cycle consists of three main stages: carbon fixation (where CO₂ is attached to a five-carbon molecule, RuBP, to form a six-carbon compound), reduction (where ATP and NADPH are used to convert 3-PGA into G3P), and regeneration (where RuBP is regenerated for the cycle to continue).
In C3 plants, this entire cycle takes place in the mesophyll cells, while in C4 plants, it occurs in the bundle-sheath cells.

Hatch-Slack Pathway

The Hatch-Slack pathway is an additional set of reactions found in C4 plants that precedes the Calvin cycle.
It was discovered by M.D. Hatch and C.R. Slack and involves the initial fixation of CO₂ into a four-carbon compound.
This pathway starts in the mesophyll cells where CO₂ is fixed by PEP carboxylase into oxaloacetate, which is then converted into malate or aspartate.
These compounds are transported to the bundle-sheath cells and broken down to release CO₂, which then enters the Calvin cycle.
This process minimizes photorespiration and maximizes photosynthetic efficiency under hot and dry conditions.