C4 And CAM pathway: Meaning, Process In C3 C4 And CAM Plants

Overview Of Photosynthesis

Photosynthesis is the biological process through which green plants and other autotrophs synthesize glucose through the conversion of light energy to chemical energy. It is a fundamental biological process through which photosynthesis in bacteria manufactures glucose using only carbon dioxide and water, with the gaseous release of molecular oxygen.

In this, plants use sunlight as the ultimate energy source in driving this important process, which has two major phases: a photochemical phase and a biosynthetic phase. The former is involved in the production of ATP and NADPH through light, while the latter is concerned with the biosynthesis of glucose through carbon fixation.

Introduction To Alternative Photosynthetic Pathways

The Calvin cycle, or the C3 pathway, is the most widespread carbon fixation pathway in plants. It is a 3-carbon acid, and the first stable product is the 3-phosphoglyceric acid (PGA). Some of the common C3 plants are beans, rice, wheat, and potatoes, which grow in temperate regions. The Calvin cycle has three major steps:

• Carboxylation: In carboxylation, the enzyme RuBisCo catalyzes the reaction of carbon dioxide with RuBP to yield two molecules of PGA.

• Reduction: The ATP and NADPH produced in the light-dependent reactions are used in the phosphorylation and reduction of PGA to form glyceraldehyde-3-phosphate (G3P).

• Regeneration: A regeneration step in which some of the G3P molecules are used in generating more molecules of the acceptor molecule by combining with RuBP, whereas the rest of the G3P molecules are used to generate glucose and other carbohydrates.

C4 Pathway (Hatch And Slack Pathway)

The C4 pathway is also known as the Hatch and Slack pathway. A photorespiratory escape mechanism is a rescue pathway developed by some plants on a normal mode of distribution and operation found in their tropical and arid regions. In the process, the first stable product of carbon fixation is a 4-carbon compound, oxaloacetic acid (OAA).

The main C4 pathway employs a 3-carbon molecule- phosphoenolpyruvate (PEP) as the primary CO2 receptor molecule. This pathway is of the biphasic type and takes place through the subsequent steps. Examples of C4 plants are corn, sugarcane, and some bushes, favouring high light intensity and temperatures.

Steps of C4 Pathway

• Carboxylation: The reaction of PEP with CO2 resulting from PEP carboxylase gives rise to oxaloacetic acid (OAA).

• Reduction: OAA is further decarboxylated to produce malic acid or aspartic acid and then transferred to the bundle sheath cells from the mesophyll cells.

• Decarboxylation: In the bundle sheath cells, malic acid undergoes decarboxylation to release carbon dioxide, which is subsequently released in the Calvin cycle. Then, the 3-carbon molecule is diffused back to the mesophyll cells to regenerate PEP.

Kranz Anatomy

In leaves of C4 plants, each vascular bundle or rib is enclosed with a bundle sheath composed of larger parenchymatous cells. The bundle sheath cells have larger chloroplasts, lacking inter grana lamellae and starch grains, while the mesophyll cells have small chloroplasts with grana. The anatomy is especially suited to increase carbon fixation efficiency, so it is called Kranz anatomy from the German for "wreath".

CAM Pathway (Crassulacean Acid Metabolism)

The Crassulacean Acid Metabolism (CAM) is among the most unique photosynthetic pathways. It is a characteristic very unique and common among several specially adapted plants which gives excellent performance in dry and semi-arid habitats. Such a pathway confers to the plants to effectively capture and utilise carbon dioxide and still conserve water efficiently. Carbon fixation takes place in CAM plants in a different manner from either C3 or C4 plants.

Steps of CAM Pathway

In the CAM pathway, carbon dioxide is fixed at night rather than during the day. These adaptations greatly avoid the loss of water due to transpiration, especially when the atmosphere is hot and dry. The major steps are:

Step 1 – Nocturnal CO₂ Fixation (Night)

In CAM plants, the stoma opens at night when it becomes cold and humid. Carbon dioxide enters through the opened stomata. This carbon dioxide is fixed into a 4-carbon compound, normally malate, which is stored in vacuoles and used at night. Thus, carbon dioxide is saved in a form that can be used both day and night.

Step 2 – Daytime Decarboxylation (Day)

During the day, after stomata have been closed to prevent the loss of water, the stored malate is again re-converted for use as carbon dioxide. This CO2 results in the production of sugar during the Calvin cycle.

Difference Between C4 And CAM Pathways

Both of these pathways have some very important differences:

Significance of C4 and CAM Pathways

The significance of C4 and CAM pathways:

• It reduces the losses caused by photorespiration.

• It enables plants to conserve water.

• It maintains high rates of photosynthetic processes under stress.

• It is important for crop productivity in the areas with high temperatures.

C4 and CAM Pathways NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Mechanism of CAM and C4 pathways

• Difference between C4 and CAM pathways

Practice Questions for NEET

Q1.The plant family that does not show CAM pathway is

1. Crassulaceae

2. Euphorbiacea

3. Cactaceace

4. Gramineae

Correct answer: 4) Gramineae

Explanation:

CAMs are the most adapted xerophytic conditions, with plants from the Crassulaceae, Euphorbiaceae, and Cactaceae families. The stomata of these plants open at night, and the carbon dioxide absorbed is fixed by them in the form of malic acid within the vacuoles. At daytime, the fixed carbon is released and utilized for the C3 cycle, along with photosynthesis. Thus, CAM plants conserve water in arid conditions and can survive in such habitats.

Hence, the correct answer is option 4) Gramineae.

Q2. CAM helps the plants in:

1. Conserving water

2. Secondary growth

3. Disease resistance

4. Reproduction

Correct answer: 1) Conserving water

Explanation:

Crassulacean acid metabolism (CAM cycle) -

• In this pathway, stomata open at night to reduce transpiration and fix CO2 to malic acid which gets stored in vacuoles.

• CAM plants are succulents in the habit and have the slowest photosynthetic rate.

Hence, the correct answer is option 1)Conserving water.

Q3. Which of the following statements is true about the CAM pathway?

1. The CAM pathway is a type of photosynthesis found only in desert plants.

2. The CAM pathway is an adaptation to low CO2 levels in the atmosphere.

3. The CAM pathway involves the storage of CO2 as malic acid during the night and its release during the day

4. The CAM pathway is more efficient than the C3 and C4 pathways in terms of CO2 fixation.

Correct answer: 3) The CAM pathway involves the storage of CO2 as malic acid during the night and its release during the day

Explanation:

The CAM pathway involves the storage of CO2 as malic acid during the night and its release during the day is a true statement. The CAM pathway is a type of photosynthesis found in many plants, including those in arid environments. It involves the storage of CO2 as malic acid during the night when the stomata are open and its release during the day when the stomata are closed, allowing for reduced water loss through transpiration. While the CAM pathway is efficient in terms of water use, it is generally less efficient in terms of CO2 fixation than the C4 pathway and is not typically found in plants adapted to high CO2 levels.

Hence, the correct answer is option 3) The CAM pathway involves the storage of CO2 as malic acid during the night and its release during the day.

Also Read:

• MCQs on CAM Plants

• Difference between C3 and C4 Pathway

• Cyclic Photophosphorylation