Plants use two different photosynthetic routes, the C₃ and C₄ pathways, to fix carbon dioxide and make sugars. Their early handling of CO2 and the following stages of the Calvin cycle are where they diverge most. In the Calvin cycle, C₃ plants, such as rice and wheat, immediately fix CO2 into a three-carbon molecule (3-PGA). Prior to being moved to bundle sheath cells, where the Calvin cycle takes place, CO2 is first fixed into a four-carbon molecule (oxaloacetate) in mesophyll cells in C₄ plants like corn and sugarcane. This separation reduces photorespiration, a wasteful process that, in some cases, happens in C₃ plants.
Carbon dioxide (CO2) enters the leaf of C₃ plants and is immediately fixed by the enzyme RuBisCO into 3-phosphoglycerate (3-PGA), a three-carbon molecule found in the chloroplasts of mesophyll cells. This process is known as the C₃ Pathway (Calvin Cycle).
Location: The chloroplasts of mesophyll cells are where the whole C₃ pathway, which includes carbon fixation and the Calvin cycle, takes place.
Benefits: Cool, humid conditions are ideal for C₃ photosynthesis.
Cons: RuBisCO interacts with oxygen rather than CO2 during photorespiration, which lowers photosynthetic efficiency in C₃ plants, particularly in hot, dry weather.
C₄ Pathway (Hatch-Slack Pathway):
- Mechanism: C₄ plants have a two-step process. First, CO2 is fixed into a four-carbon compound (oxaloacetate) in mesophyll cells by the enzyme PEP carboxylase. This oxaloacetate is then converted to malate, which is transported to bundle sheath cells. Inside the bundle sheath cells, malate is broken down, releasing CO2, which then enters the Calvin cycle.
- Location: C₄ photosynthesis involves two types of cells: mesophyll cells for initial CO2 fixation and bundle sheath cells for the Calvin cycle.
- Advantages: The C₄ pathway minimizes photorespiration because the initial CO2 fixation in mesophyll cells concentrates CO2 around RuBisCO in bundle sheath cells, reducing oxygen’s ability to bind.
- Examples: C₄ plants are commonly found in tropical and subtropical regions with high temperatures and intense sunlight, like sugarcane, corn, and sorghum.
All green plants, photosynthetic bacteria, and other autotrophs use the biological process of photosynthesis to transform light energy into chemical energy. This method uses sunlight to synthesize glucose from carbon dioxide and water. Additionally, as a byproduct of photosynthesis, oxygen gas is released into the atmosphere.
The following is the balanced chemical equation for the process of photosynthesis:
6CO2 + 6H2O —> C6H12O6 + 6O2
The best energy source is sunlight. During the process of photosynthesis, plants employ this light energy to prepare chemical energy. There are two stages to the entire photosynthesis process: the photochemical phase and the biosynthetic phase.
The first step in preparing ATP and NADPH for the biosynthetic phase is the photochemical phase.
Sunlight is the ultimate source of energy. Plants use this light energy to prepare chemical energy during the process of photosynthesis. The whole process of photosynthesis takes place in two phases- photochemical phase and biosynthetic phase.
The photochemical phase is the initial stage where ATP and NADPH for the biosynthetic phase are prepared. In the biosynthetic phase, the end product – glucose is produced. Let us focus more on pathways in biosynthetic phase.
During the biosynthetic phase, carbon dioxide and water combine to give carbohydrates i.e. sugar molecules. This reaction of carbon dioxide is termed as carbon fixation. Different plants follow different pathways for carbon fixation.
Based on the first product formed during carbon fixation there are two pathways: the C3 pathway and C4 pathway.
The Pathway of Photosynthesis
C3 Pathway (Calvin Cycle)
The majority of plants produce 3-carbon acid called 3-phosphoglyceric acid (PGA) as a first product during carbon dioxide fixation. Such a pathway is known as the C3 pathway which is also called the Calvin cycle.
Calvin Cycle occurs in three steps:
- carboxylation
- reduction
- regeneration
In the first step, the two molecules of 3-phosphoglyceric acid (PGA) are produced with the help of the enzyme called RuBP carboxylase. Later in the second and third steps, the ATP and NADPH phosphorylate the 3-PGA and ultimately produces glucose. Then the cycle restarts again by regeneration of RuBP.
Beans, Rice, Wheat, and Potatoes are an example of plants that follow the C3 pathway.
C4 Pathway (Hatch and Slack Pathway)
Every photosynthetic plant follows Calvin cycle, but in some plants, there is a primary stage to the Calvin Cycle known as C4 pathway. Plants in tropical desert regions commonly follow the C4 pathway. Here, a 4-carbon compound called oxaloacetic acid (OAA) is the first product by carbon fixation. Such plants are special and have certain adaptations as well.
The C4 pathway initiates with a molecule called phosphoenolpyruvate (PEP) which is a 3-carbon molecule. This is the primary CO2 acceptor and the carboxylation takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule called oxaloacetic acid (OAA).
Eventually, it is converted into another 4-carbon compound known as malic acid. Later, they are transferred from mesophyll cells to bundle sheath cells. Here, OAA is broken down to yield carbon dioxide and a 3-C molecule.
The CO2 thus formed, is utilized in the Calvin cycle, whereas 3-C molecule is transferred back to mesophyll cells for regeneration of PEP.
Corn, sugarcane and some shrubs are examples of plants that follow the C4 pathway. Calvin pathway is a common pathway in both C3 plants and C4 plants, but it takes place only in the mesophyll cells of the C3 Plants but not in the C4 Plants.
