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Introduction
Green plants utilise the energy from sunlight to synthesize their food and emit oxygen – this is called photosynthesis. It has two sub-steps:
- Light-dependent Light Reaction: The chloroplasts in green plants capture sunlight through a series of reactions, resulting in the synthesis of ATP and NADPH. These two high-energy chemical compounds are required to produce glucose from CO2 in the next phase of dark reactions
- Light independent Dark Reaction: These reactions do not need sunlight and use the energy molecules generated during the light reactions. Light-independent dark reactions are the ones that synthesize food for plants
C3 Plants
The dark reaction cycle of photosynthesis is known as the Calvin-Benson-Bassham cycle after the scientists discovered it. It is also known as the C3 or the Reductive pentose phosphate cycle. The plants that exhibit the Calvin cycle are known as C3 plants. 95% of green plants, trees, shrubs and herbs are C3 plants.
Crops like wheat, barley, rice and soya beans are C3 plants. These plants do not have photosynthetic adaptations to reduce photorespiration. Photorespiration is a metabolism process in C3 plants that helps in energy dissipation where the stomata are closed due to water stress during the daytime.
The C3 plants get this name as the first stable product formed is a three-carbon atom compound or C3 compound during the dark reaction.
C4 Plants
C4 plants have adaptive photorespiration adaptations to reduce photorespiration that separate initial CO2 fixation and the C3 or the Calvin cycle in space. The Calvin cycle and the light-dependent reactions are physically separated and performed in different cell types in these plants.
C4 plants contain a spongy tissue in the middle of the leaf where the light-dependent reactions occur in the mesophyll cells where the Calvin cycle occurs in special bundle-sheath cells around the leaf veins.
The C4 plants get their name from the first stable product formed in the C4 cycle, a 4-carbon or C4 compound.
Comparing C3 and C4 Plants
Let’s look at the table below for comparing C3 and C4 plants based on their distinct features and the way they tackle photosynthesis:
| Feature | C3 Plants | C4 Plants |
| Photosynthesis Dark Reaction | Use C3 Cycle | Use C4 cycle |
| First Stable Intermediate Product | Phosphoglyceric acid or 3- carbon acid | Oxaloacetate or 4 carbon acid, later reduced to malate |
| Location of Occurrence of Photosynthesis | Mesophyll cells on the leaves’ surface of plants | Both in mesophyll and bundle sheath cells of leaves. |
| Photosynthesis Steps | All dark reaction steps take place in mesophyll cells | Initial steps in mesophyll cells and remaining steps in bundle sheath cells |
| Plant Diversity | 95% of green plants | Remaining 5% of green plants |
| Type of Environment required for plants | Cool, wet temperate climates | Dry and tropical climates |
| Kranz Anatomy | Absent | Present |
| Bundle Sheath Cells | No chloroplasts | Contain chloroplasts |
| Chloroplast Type | Granite only | Granite as well as Agranal chloroplasts |
| Photorespiration | Unsuppressed | Suppressed |
| CO2 Acceptor | Only one | Two- primary & secondary |
| CO2 Fixation | Slow | Fast |
| CO2 Fixation Times | Only once | Twice |
| Photosynthesis Occurrence | Stomata need to be open | Happens with closed stomata as well |
| Origin | Evolved around 2.5 billion years ago | More recently, it evolved around 12 million years ago. |
Comparison between C3 and C4 Pathways
The picture above gives a comparison between C3 and C4 pathways. Let us look into these differences in detail in this section.
Different plants use different pathways for carbon fixation. For example, during photosynthesis, CO2 and water combine to give carbohydrates or sugar molecules. This reaction is known as carbon fixation.
There are two pathways based on the first product formed during carbon fixation.
- C3 Pathway or the Calvin Cycle
- C4 Pathway or the Hatch and Slack Pathway
The table below gives a comparison between C3 and C4 pathways:
| Feature | C3 Pathway | C4 Pathway |
| Number of Steps | Three steps:
| Two steps:
|
| Presence in Plants | All plants | Tropical plants |
| First Stable Compound | C3 compound | C4 compound |
| Carboxylase enzyme | RuBisCO | PEP carboxylase and RuBisCO |
| Carbon Fixation | Only once | Double fixation happens |
These pathways constitute salient points for comparing C3 and C4 plants.
CAM Plants
Other than C3 and C4 plants, other plants are adapted to dry and dry environments. These plants belong to the family of plants, the Crassulaceae, after the name of the scientist who first discovered the pathway.
These include cacti, orchids, and pineapples that use the CAM pathway or the Crassulacean acid metabolism to minimize photorespiration.
In CAM plants, the stomata are open at night. Therefore, the CO2 enters the stomata present in the leaves at night, which is then converted into 4C organic acid (malate). In the Calvin or the C3 cycle during the day, these organic acids are decomposed to release CO2 when the stomata are closed.
These CAM plants are adapted to hot and dry conditions with closed stomata during the hotter parts of the day.
Comparison between C3, C4, and CAM Plants
Now let’s look at an overall comparison between C3, C4, and CAM plants in the table below to understand how each differs from another.
| Feature | C3 Plants | C4 Plants | CAM Plants |
| CO2 Fixation & Calvin Cycle Separation | No separation only happens in mesophyll cells | Happens in mesophyll cells and bundle sheath cells | Time-based,CO2 fixation at night and Cavin cycle during the day |
| Photorespiration Rate | High | Low | Negligible |
| Number of Pathway Steps | One | Two | Two |
| Stomata Open Time | Day | Day | Night |
| Adaptation Environment | Cool, wet temperate climate | Hot, tropical climate | Very hot and dry desert climate |
Conclusion
While C3 and C4 plants differ on various points, they also have similarities. For example, while comparing C3 and C4 plants, both plants need sunlight to fix energy and use it to make carbohydrates. The process of photosynthesis in both types of plants occurs in the chloroplasts. It consists of the same light reactions. The major difference for both plant types lies in how the dark reaction is conducted.
