Photosynthesis is the essential process through which autotrophs, including plants, produce food and energy. The process consists of different stages, one of which is the Calvin cycle. Different plants use specific carbon fixation pathways to convert CO2 into an organic compound. C3 plants use the C3 pathway, while C4 plants make use of the C4 pathway in the dark reaction. Both processes and involved plants differ in their characteristics.
Keep reading to learn the differences between C3 and C4 plants.
Comparison Table
| Factors | C3 Plants | C4 Plants |
| Definition | Follow C3 pathway | Follow C4 pathway |
| Ratio | 95% | 5% |
| Steps | One step | Two-step |
| CO2 Fixation | Once | Twice |
| Kranz Anatomy | Absent | Present |
| Protein Content | High | Low |
| Primary Molecule | 3-phosphoglyceric acid | Oxaloacetate |
| Rate of Photorespiration | High | Low |
| Center of Cycle Reaction | Mesophyll cells | Mesophyll and bundle-sheath cells |
| Climatic Region | Cool-weather | Tropical temperature |
| Examples | Rice, barley, peanuts, spinach | Maize, sugarcane, pearl millet, sorghum |
What are C3 Plants?
Carbon fixation occurs in most plants and algae, by the enzyme rubisco, which utilizes the first carbon and converts it into 3 atoms of the carbon-containing compound. These plants are called C3 plants because the first organic compound generated is the three-carbon compound, i.e., 3PGA. The C3 path is the most fundamental type of photosynthesis. 95% of plants use the C3 pathway, and are also known as temperate plants. This is a series of chemical reactions in which plants take CO2 into a cycle and convert this carbon dioxide within the chloroplast. Just like chloroplast, mitochondria also plays a crucial role in the production of ATP in plants.
Steps of C-3 Pathway
- Plants take carbon dioxide from their stomata, and an enzyme called Rubisco, uses the carbon to bind it with sugars.
- The initial product of pathway C3 is 3-phosphoglycerate (3PGA), a three-carbon compound.
- Further, in the second and third reactions, glucose is formed from 3-PGA with the help of ATP and NADPH.
- Then the three-carbon compound turns into nucleotides, amino acids, and complex sugars (starches).
- Plants utilize carbohydrates produced during the cycle for energy purposes.
Advantages
- The C3 cycle operates effectively as long as sufficient CO2 is added.
- The process helps store energy for an extended period.
- These plants have high protein content.
- They may be annual, i.e., yearly and perennial, i.e., have more than two years of life cycles.
Disadvantages
- If the carbon dioxide supply is not enough, plants work less effectively. Thus, it is not suitable for dry areas.
- The leaves of C3 plants lack Kranz anatomy.
- When stomata are closed during dry weather to prevent water loss, Rubisco adds O2.
Examples of C3 Plants
Plants producing energy through the C3 process are soy oats, wheat, barley, and rice.
What are C4 Plants?
Plants that follow the C4 carbon fixation pathway in dark reactions are called C4 plants. They are also defined as “plants that fix carbon dioxide into a compound which contains the 4-carbon atom.” The process is also known as the Hatch and Slack pathway, named after two Scientists, Marshall Davidson Hatch and Charles Roger Slack, who discovered the cycle in the 1960s. It is also referred to as the C4 pathway, for the molecules produced are 4 carbon atoms.
Steps of C4 Pathway
Two types of cells participate in the cycle: sheath cells and mesophyll cells. Leaves containing bundle sheaths are known to have Kranz anatomy, and these cells are responsible for completing the C4 cycle.
- The first stable compound formed is oxaloacetate which is a 4-carbon compound.
- Carbon dioxide is collected from the atmosphere into mesophyll cells, and a 3-carbon compound (PEP) reacts to produce oxaloacetate. An enzyme called PEP carboxylase catalyzes the reaction through specific catalysts.
- Further, oxaloacetate containing 4-carbon forms malic acid or aspartic acid.
- Malic acid/malate is transferred into other cells called bundle sheaths. CO2 is removed from malate’s breakdown. The reaction is referred to as de-carboxylation, and 3-carbon acid is produced.
- The carbon dioxide is then fixed by an enzyme called Rubisco and transformed into sugars via the Calvin cycle, exactly as in the C3 Calvin pathway.
Advantages
- The plants grow in dry tropical communities and ecosystems with very high temperatures.
- The loss of photorespiration rate is extremely low.
Disadvantages
- The C4 pathway occurs less in plants that grow in cooler regions
Examples
Maize and sorghum are examples of C4 plants.
Differences Between C3 and C4 Plants
Definition
C3 Plants
Plants that follow the C3 pathway in dark reactions are called C3 plants.
C4 Plants
On the other hand, C4 plants follow the C4 pathway in dark reactions of the Calvin cycle.
Ratio
C3 Plants
In general, 95% of green plants are C3 plants.
C4 Plants
While approximately 5% of the green plants are C4 plants.
Steps
C3 Plants
A C3 pathway consists of only 1 step.
C4 Plants
On the other hand, two steps are involved in the C4 pathway.
CO2 Fixation
C3 Plants
Carbon dioxide fixation occurs only once in C3 plants.
C4 Plants
At the same time, carbon dioxide fixation occurs twice in C4 plants.
Leaf Anatomy
C3 Plants
Leaves of C3 plants possess a typical morphology and lack Kranz anatomy.
C4 Plants
However, C4 plant leaves have a special type of structure called Kranz anatomy.
Protein Content
C3 Plants
C3 plants contain high protein content.
C4 Plants
Whereas C4 plants contain less protein content than C3 plants.
Primary Molecule
C3 Plants
C3 plants produce 3-phosphoglyceric acid (PGA) is a 3-carbon compound.
C4 Plants
Contrarily, oxaloacetate is the first stable 4-carbon-containing compound which later is converted into malate.
Photorespiration Rate
C3 Plants
The rate of photorespiration is high in C3 plants.
C4 Plants
Alternatively, the photorespiration Rate in C4 plants is extremely low.
Cycle Reaction Center
C3 Plants
All the steps of the cycle occur in mesophyll cells of C3 plants
C4 Plants
Yet, initial steps take place in mesophyll cells, and later further steps are carried out in specialized cells called bundle-sheath cells.
Regions
C3 Plants
C3 plants need cool weather to carry out their normal functions.
C4 Plants
Nevertheless, C4 plants grow in dry or tropical ecosystems.
Examples
C3 Plants
Rice, wheat, oats, barley, cotton, peanuts, and spinach are examples of C3 plants.
C4 Plants
Examples of C4 plants include maize, sugarcane, pearl millet, and sorghum.
The Bottom Line
C3 and C4 plants are widely found in our biomes and ecosystems and are essential to different types of biodiversity. They have a specific morphology and structure contributing to particular functions. The main difference between C3 and C4 plants is that C3 plants follow the C3 pathway in dark reactions of the Calvin cycle. On the other hand, C4 plants follow the C4 pathway. They vary in their anatomy, protein content, locations, and function. However, they are critical to food chains and food webs as they act as producers.
FAQs
Which best describes the difference between C3 and C4 plants?
The main difference between C3 and C4 plants is the difference in their pathway. It arises due to the difference in their structure. C3 plants perform the process in mesophyll cells as the bundle sheath cells do not have chloroplasts. On the other hand, C4 plants contain chloroplasts in sheath cells.
Why are C3 plants more efficient than C4?
C4 plants are more efficient than C3 plants as they increase the CO2 concentration around the fixating enzyme Rubisco.
Is C4 photosynthesis faster than C3?
Yes, photosynthesis in C4 plants is faster than in C3 plants. The rate of photosynthesis efficiency is 50% higher in C4 plants.
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