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 require for sources of energy.

Overview Of C3, C4, And Cam Pathways

The key differences between C3, C4, and CAM are how the plants capture carbon dioxide and the nature of photosynthesis products.

• C3 pathway: 3-phosphoglyceric acid is the first product to be yielded in carbon fixation.

• C4 pathway: The first product to be produced is an oxaloacetic acid 4-carbon molecule, and again it enters into the Calvin cycle.

• CAM pathway: Carbon dioxide is fixed in the night and stored as malic acid, which is used in the daytime.

What Is The C3 Pathway?

Another name for the C3 pathway is the Calvin Cycle; it is the most common form of carbon fixation. It takes place in most plants that are mainly found in temperate climates. 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. Carboxylation: CO2 is fixed by the enzyme RuBisCO as the result of the reaction between CO2 and the RuBP (ribulose bisphosphate), which produces PGA.
2. Reduction: 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. Regeneration: Some of the G3P molecules are used in regenerating the RuBP so that it can pick up more CO2. The cycle has to turn six times to produce one molecule of glucose, and through these six turns a total of 6 CO2, 18 ATP, and 12 NADPH are consumed.

Examples Of C3 Plants

Some examples of plants in the C3 category are:

1. Beans

2. Spinach

3. Sunflower

4. Rice

5. Cotton

Detailed Explanation Of The C4 Pathway

It 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.

C4 Pathway Characteristics

Primary Product: A 4-carbon molecule; oxaloacetic acid.

Location: 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

The common examples of these plants are :

1. Maize, or corn

2. Sorghum

3. Sugarcane

What Is Cam Pathway?

The CAM pathway is the special adaptation in plants that belong to dry environments.

Key Features Of The Cam Pathway

Fixation at Noon: In CAM plants carbon fixation takes place during the night when the cool climate and humidity prevail.

Storage of Malic Acid: Carbon is fixed by storing it in malic acid and then stored in the vacuoles of the plant overnight.

Steps Involved In 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

Common examples of plants that involve CAM are:

1. Cacti

2. Orchids

3. Euphorbias

Summary Of The Differences Between The C3, C4 And Cam Pathways

Initial Products:

1. C3: 3-phosphoglycerate PGA)

2. C4: Oxaloacetic acid (OAA)

3. CAM: Malic acid (night) and PGA (day)

Timing Of Carbon Fixation:

1. C3: All processes occur during the day.

2. C4: Carbon fixation occurs in the day but the initial step takes place in mesophyll cells.

3. CAM: Carbon fixation at night, Calvin Cycle during the day

Environmental Adaptation:

1. C3: Cooler, wetter climates.

2. C4: Specialised in warm sunny environments

3. CAM: Well suited for arid conditions to cut down water loss.

Photorespiration

1. C3: Higher rates of photorespiration

2. C4: Lower rates of photorespiration

3. CAM: Takes place but reduced due to night-time CO2 fixation