Difference Between C3, C4 and CAM Pathway: Steps, Differences and FAQ

What Is Photosynthesis?

Photosynthesis is the process through which green plants, algae, and some bacteria transform the energy obtained from sunlight into a storable form of chemical energy in the form of glucose. The process involves the fixation of carbon dioxide from the atmosphere, which transforms organic compounds. Carbon fixation can occur through one of three main pathways, namely, C3, C4 and CAM pathways.

What Is Carbon Fixation?

Carbon fixation is the first step of photosynthesis where inorganic carbon in the form of CO2 gets converted into organic compounds. The process is essential in making sugars, products, plants and, indirectly, other living organisms required for sources of energy.

The C3 Pathway (Calvin Cycle)

Another name for the C3 pathway is the Calvin Cycle, is the most common form of carbon fixation. It takes place in most plants that are mainly found in temperate climates.

Overview

Some of the key features of the C3 pathway include that the first stable product formed from carbon fixation is 3-phosphoglyceric acid, which is a 3-carbon compound. The process happens in the stroma of chloroplasts. The stroma of chloroplasts is the fluid-filled space inside chloroplasts in which the Calvin Cycle occurs.

Steps Of The C3 Pathway

1. CO2 is fixed by the enzyme RuBisCO as the result of the reaction between CO2 and the RuBP (ribulose bisphosphate), which produces PGA.

2. The PGA that is produced is reduced into G3P, a 3-carbon sugar, glyceraldehyde-3- phosphate, with the use of ATP and NADPH that is generated during light-dependent photosynthesis reactions.

3. The G3P molecules are used in regenerating the RuBP so that it can pick up more CO2. The cycle turns six times to produce 1 molecule of glucose, and a total of 6 CO2, 18 ATP, and 12 NADPH are consumed.

Examples Of C3 Plants

Some of the common examples of C4 plants are:

1. Beans

2. Spinach

3. Sunflower

4. Rice

5. Cotton

The C4 Pathway (Hatch–Slack Pathway)

C4 pathway is named for the number of carbons in the first organic compound produced in it, but it is also known as the Hatch-Slack pathway. It is an adaptation by plants to be able to effectively fix carbon dioxide in hot and dry environments.

Overview

The primary product is a 4 -carbon molecule, oxaloacetic acid. The pathway occurs in the mesophyll cells and bundle sheath cells which are specialized cells found in the leaves.

Steps Of The C4 Pathway

1. At the mesophyll cells, CO2 is fixed through the activity of PEP carboxylase into OAA. Later, it may be reduced to all other 4-carbon acids, for example, malic acid

2. The 4-carbon acid is moved into the bundle sheath cells into CO2 is released from the malate.

3. Now this CO2 diffuses into the chloroplasts of bundle sheath cells, which then undergoes the Calvin cycle and finally ends up as glucose.

Examples Of C4 Plants

Some of the common examples of C4 plants are:

1. Maize, or corn

2. Sorghum

3. Sugarcane

The 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.

Overview

CAM pathway helps plants so they can effectively capture and utilise carbon dioxide and conserve water efficiently. Hence making them very good at surviving in dry environments. Carbon fixation takes place in CAM plants in a manner quite different from either C3 or C4 plants.

Steps of the CAM Pathway

Night-time Carbon Fixation: Stomata open at night to let in CO2. The latter is converted into malic acid and then stored.

Daytime Utilization: During the day, the stomata close to save water and stored malic acid is converted back to CO2, which enters into the cycle of Calvin for producing sugar.

Examples Of CAM Plants

Some of the common examples of C4 plants are:

1. Cacti

2. Orchids

3. Euphorbias

Differences Between C3, C4, and CAM Pathways

The summary of the differences between the C3, C4 and Cam pathway:

C3, C4, CAM Pathways NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Carbon fixation

• Difference between C3, C4 & CAM Pathways

Practice Questions for NEET

Q1. Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:

1. C3 plants

2. C4 plants

3. C2 plants

4. C3 and C4 plants

Correct answer: 2) C4 Plants

Explanation:

Phosphoenolpyruvate (PEP), a three-carbon chemical found in mesophyll cells, is the main CO2 acceptor.
Bicarbonate ions are the form in which the CO2 is received.
Bicarbonate is created when carbonic anhydrase hydrates CO2.
The PEP carboxylase or PEPcase enzyme is in charge of this fixing.
It is crucial to note that the RuBisCO enzyme is absent from mesophyll cells.
The mesophyll cells produce the C4 acid OAA.
Other 4-carbon compounds, such as aspartic acid or malic acid, are subsequently formed in the mesophyll cells and transferred to the bundle sheath cells.

Hence, the correct answer is option 2) C4 plants.

Q2. Which of the following enzymes catalyses primary CO2 fixation in the C4 pathway?

1. Alsolase

2. PEP carboxylase

3. RuBP carboxylase

4. Isomerase

Correct answer: 2) PEP carboxylase

Explanation:

During the C4 pathway, CO2 combines with 3 the carbon compound phosphoenol pyruvate to form 4 the carbon compound oxaloacetic acid. This reaction is catalysed by PEP carboxylase. The formation of oxaloacetic acid occurs in the mesophyll cells and helps minimize photorespiration. Oxaloacetic acid is then converted into malate or aspartate, which is transported to bundle sheath cells for further processing. This adaptation allows C4 plants, such as maize and sugarcane, to maintain high photosynthetic efficiency in arid environments.

Hence, the correct answer is option 2) PEP carboxylase

Q3. Identify the incorrect statement in relation to C4 photosynthesis

1. Kranz anatomy is an essential feature for c4 plants

2. C4 plants have higher water use efficiency than C3 plants

3. Photorespiration can be minimized when C4 the pathway is in operation

4. Conversion of oxaloacetate to malate occurs in the bundle sheath cells

Correct answer: 4) Conversion of oxaloacetate to malate occurs in the bundle sheath cells

Explanation:

The mesophyll cells, not the bundle sheath cells, in C4 plants are where oxaloacetate is converted to malate. After that, malate is sent to the bundle sheath cells, which release CO₂ in preparation for the Calvin cycle. C4 plants have Kranz anatomy, which aids in the fixation of CO2. Compared to C3 plants, C4 plants use water more efficiently. Photorespiration is reduced via the C4 pathway.

Hence, the correct answer is option 4)Conversion of oxaloacetate to malate occurs in the bundle sheath cells.

Also Read:

• MCQs on C4 Pathway

• Difference between C3 and C4 Pathway

• Cyclic Photophosphorylation