[Biology Class Notes] on C3 and C4 Pathways Pdf

Plants use photosynthesis to convert light energy into chemical energy. In this process, plants produce glucose by using atmospheric carbon dioxide and water in the presence of sunlight. As a result, oxygen is released into the atmosphere as a by-product.

 

The photosynthesis process is divided into two broad phases, photochemical and biosynthetic. C3 and C4 pathways, the topic of this article, are two ways of biosynthetic process. In a biosynthetic process, plants use water and carbon dioxide to produce carbohydrates. 

 

C3 Pathway

Also known as the Calvin cycle, Melvin Kelvin first discovered this process. The majority of the plants on the earth use this process. The first stable product produced by this process is three compound carbon. Hence, the name C3 pathway.

 

Moreover, this cycle occurs in three steps, these are –

  1. Carboxylation: This step fixates CO2 into a steady organic intermediate. Here CO2 is used to carboxylate RuBP. RuBisCO enzyme catalyses this process.

  2. Reduction: This procedure leads to the creation of glucose. It involves two molecules of NADPH for reduction and two molecules of ATP for phosphorylation.

  3. Regeneration: Regeneration of CO2 acceptor molecule is necessary for a C3 pathway to continue its operations.

 

C4 Pathway

The C4 pathway of photosynthesis occurs before the C3 pathway. Even though the C3 cycle is found in every plant, the C4 pathway is primarily noticeable in plants of the tropical region.

 

Furthermore, this process is also known as the Hatch and Slack pathways. The first stable product of this process is a four-carbon compound, hence the name.

 

Moreover, there are two C4 cycle steps that every plant follow; these are –

  1. At first, when CO2 enters a mesophyll cell, phosphoenolpyruvate (PEP) carboxylase enzymes add it to the three-carbon PEP. It produces a four-carbon compound called oxaloacetate. This substance then turns into an organic acid called malate.

  2. Subsequently, malate is conveyed to the bundle sheath cells deep into the leaf where the oxygen concentration is low. The malate is disintegrated, which releases a molecule of CO2. This CO2 enters the Calvin cycle or C3 pathway afterwards, where the rubisco enzyme transforms it into sugar.

 

Furthermore, the C4 cycle in plants is useful in hot and arid conditions, as plants close their stomata to save water.

 

Before moving forward, here are some C4 cycle notes for a quick recap –

  1. C4 cycle is not seen in every plant.

  2. C3 and C4 pathways are not the same. They have different purposes and characteristics.

  3. The C4 cycle is primarily seen in plants in the tropical region.

  4. Moreover, this process helps plants to conserve water.

  5. Examples of C4 pathway plants are crabgrass, corn, sugarcane, etc.

 

Difference between C3 and C4 Pathway

C3 Pathway

C4 Pathway

Photosynthetic activities are exclusively found in mesophyll cells, which are found on the leaf’s surface.

Mesophyll and bundle sheath cells both have photosynthetic functions.

C3 thrives in cool, moist environments.

C4 thrives in tropical and arid climates.

Kranz anatomy is not seen in the leaves of these plants.

Kranz anatomy may be seen in the leaves of several plants.

In temperate areas, C3 plants are common.

Tropical areas are home to C4 plants.

3-carbon acid: phosphoglyceric acid, is the first stable intermediate product.

A 4 carbon acid: oxaloacetate, is the first stable intermediate product.

Only granal chloroplasts are seen in C3 plants.

The chloroplasts of C4 plants are both granal and agranal types.

Carbon dioxide fixing is a one-time event.

The process of carbon dioxide fixing takes place twice.

In mesophyll cells, all of the steps of the dark reaction take place.

The first steps are completed in mesophyll cells, and the second and third steps are completed in bundle-sheath cells.

In comparison to C4 plants, the optimal temperature for photosynthesis is lower.

In comparison to C3 plants, the optimal temperature for photosynthesis is higher.

The earliest and most prevalent kind of photosynthesis is C3.

C4 photosynthesis is a relatively new phenomenon, having emerged following C3 photosynthesis.

Carbon dioxide fixing is a time-consuming process.

Carbon dioxide fixing is a more rapid process.

Photorespiration is not suppressed.

Photorespiration is suppressed.

When the stomata are open, photosynthesis takes place.

Even when the stomata are closed, photosynthesis takes place.

C3 plants account for 95% of all green plants.

C4 plants make up 5% of all green plants.

Rice, wheat, oats, cotton, tobacco, sugar beets, and spinach are some examples of C3 pathways.

Maize, sugarcane, and sorghum are some examples of C4 pathways.

Some key differences between these two pathways are –

  1. First stable compound

The first compound produced in a C3 cycle is a 3-carbon substance named 3-phosphoglyceric acid. On the other hand, in C4 pathways, the primary stable compound is a 4-carbon compound known as oxaloacetate acid.

  1. Presence in plants

C3 cycle is found in every plant. But the C4 pathway diagram is found primarily in plants of the tropical region.

  1. Fundamental carbon dioxide acceptor

In the case of C3, it is Ribulose Bi Phosphate (RUBP). For C4, it is phosphoenolpyruvate (PEP).

  1. Carboxylase enzyme

In C4 plants it is PEP carboxylase and rubisco. However, in the case of C3, it is only rubisco.

  1. Carbon fixation

Only one carbon fixation occurs in a C3 cycle. In the C4 cycle, double carbon fixation occurs.

  1. Photorespiration 

The photorespiration rate in C3 is high. In the case of C4, photorespiration is absent.

 

C3 and C4 pathways are two essential steps in every photosynthesis process. For more biology study materials like this, you can download the app or go through the study materials available on our website. Additionally, you can attend all the live classes with our subject experts and students from across the country.

 

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Questions

1. The Calvin pathway occurs in which of the following:

  1. Only C3

  2. Only C4

  3. All the photosynthesis plants

  4. Only in C2 plants

 

Ans: c. All the photosynthesis plants.

 

2. CO2 fixation happens in the chloroplast of____ during the C4 pathway.

  1. Guard cells

  2. Mesophyll cells

  3. Bundle sheath cells

  4. Spongy parenchyma

 

Ans: b. Mesophyll cells.

 

3. The _______ separates the C 4 and C 3 pathways of CAM plants.

  1. Bundle sheath

  2. Mesophyll and bundle sheath cells

  3. Mesophyll and bundle sheath chloroplast

  4. Time

 

Ans: d. Time

 

4. The C4 pathway is used by plants that have evolved to dry tropical regions. Is this true or false?

 

Ans: The statement is true. Plants that use the C4 pathway have unique adaptations that enable them to withstand extreme environmental conditions. Tolerance of high temperatures, responsiveness to high light intensities, and lack of photorespiration are examples of such adaptations.

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