The key difference between C3 and C4 plants is that the C3 plants form a three-carbon compound as the first stable product of the dark reaction while the C4 plants form a four-carbon compound as the first stable product of the dark reaction.
Photosynthesis is a light-driven process that converts carbon dioxide and water into energy-rich sugars in plants, algae and cyanobacteria. During the light reaction of photosynthesis, photolysis of water molecules occurs. As a result of photolysis of water, oxygen liberates as a byproduct. After the light reaction, the dark reaction starts and it synthesizes carbohydrates by fixing carbon dioxide. However, oxygen generated from the light reaction can bind with the main enzyme of the dark reaction which is RuBP oxygenase-carboxylase (Rubisco) and carry out photorespiration. Photorespiration is a process that wastes energy and decreases carbohydrate synthesis. Therefore, in order to prevent photorespiration, there are three different ways that dark reaction occurs in plants to prevent the meeting of oxygen with Rubisco. Hence, depending on the way that dark reaction takes place, there are 3 types of plants; namely, C3 plants, C4 plants, and CAM plants.
What are C3 Plants?
About 95% of the plants on earth are C3 plants. As the name indicates, they carry out C3 photosynthetic mechanism that is Calvin cycle. C3 photosynthesis is thought to have arisen nearly 3.5 billion years ago. These plants are mostly woody and round leaf plants. In these plants, carbon fixation takes place in the mesophyll cells that are just beneath the epidermis.
Carbon dioxide enters from the atmosphere to the mesophyll cells through the stomata. Then the dark reaction starts. The first reaction is the fixing of carbon dioxide with Ribulose bisphosphate into phosphoglycerate which is a three-carbon compound. In fact, it is the first stable product of the C3 plants. Ribulose bisphosphate carboxylase (Rubisco) is the enzyme that catalyzes this carboxylation reaction in plants. Likewise, the Calvin cycle occurs cyclically while producing carbohydrates.
Compared to C4 plants, C3 plants are inefficient regarding their photosynthetic mechanism. It is because of the occurrence of photorespiration in C3 plants. Photorespiration occurs due to the oxygenase activity of Rubisco enzyme. Oxygenation of Rubisco works in the opposite direction to carboxylation, effectively undoes photosynthesis by wasting large amounts of carbon originally fixed by the Calvin cycle at great expense, and results in loss of carbon dioxide from the cells that are fixing carbon dioxide. Likewise, interaction with oxygen and carbon dioxide occurs at the same site on Rubisco. These competing reactions normally run at a ratio of 3:1 (carbon: oxygen). Thus, it is clear that photorespiration is a light stimulated process that consumes oxygen and evolves carbon dioxide.
What are C4 Plants?
C4 plants are present in dry and high-temperature areas. Approximately 1% of plant species have C4 biochemistry. Some examples of C4 plants are corn and sugarcane. As the name indicates, these plants carry out the C4 photosynthetic mechanism. C4 photosynthesis is thought to have arisen nearly 12 million years ago; long after the evolution of C3 mechanism. C4 plants may be better adapted now, as the current carbon dioxide levels are much lower than 100 million years ago.
C4 plants are much more efficient at capturing carbon dioxide. Furthermore, C4 photosynthesis is found in both monocot and dicot species. In contrast to C3 plants, the first stable product formed during photosynthesis is oxaloacetic acid, which is a four-carbon compound. Most importantly, the leaves of these plants show a special type of anatomy called “Kranz Anatomy”. There is a circle of bundle sheath cells with chloroplasts around vascular bundles by which C4 plants can be identified.
In this pathway, carbon dioxide fixation occurs twice. In the mesophyll cell cytoplasm, CO2 first fixes with phosphoenolpyruvate (PEP), which acts as a primary acceptor. The reaction is catalyzed by PEP carboxylase enzyme. Then PEP converts into malate and then into pyruvate liberating CO2. And, this CO2 again fixes for the second time with Ribulose bisphosphate, to form 2 phosphoglycerate to carry out Calvin cycle.
What are the Similarities Between C3 and C4 Plants?
- Both C3 and C4 plants fix carbon dioxide and produce carbohydrates.
- They carry out a dark reaction.
- Also, both types of plants carry out the same light reaction.
- Furthermore, they have chloroplasts to carry out photosynthesis.
- Their photosynthetic equation is similar.
- Moreover, RuBP involves in the dark reaction of both types of plants.
- Both plants produce phosphoglycerate.
What is the Difference Between C3 and C4 Plants?
C3 plants produce phosphoglyceric acid as the first stable product of the dark reaction. It is a three-carbon compound. On the other hand, C4 plants produce oxalo-acetic acid as the first stable product of the dark reaction. It is a four-carbon compound. Therefore, this is the key difference between C3 and C4 plants.
Furthermore, photosynthetic efficiency of C3 plants is less than the photosynthetic efficiency of C4 plants. It is due to the photorespiration seen in C3 plants which is negligible in C4 plants. Thus, it is another difference between C3 and C4 plants. When considering the structural differences, C3 plants do not have two types of chloroplasts and Kranz anatomy in leaves. On the other hand, C4 plants have two types of chloroplasts, and they show Kranz anatomy in leaves. Hence, it is also a difference between C3 and C4 plants.
Moreover, a further difference between C3 and C4 plants is that the C3 plants fix carbon dioxide only once while C4 plants fix carbon dioxide twice. Due to this fact, C assimilation is less in C3 plants while C assimilation is high in C4 plants. Not only that, C4 plants can carry out photosynthesis when the stomata are closed and under very high light concentrations and low CO2 concentrations. However, C3 plants are unable to carry out photosynthesis when the stomata are closed and under very high light concentrations and low CO2 concentrations. Therefore, this is also a significant difference between C3 and C4 plants. Furthermore, C3 plants and C4 plants differ from the first carbon dioxide acceptor. RuBP is the CO2 acceptor in C3 plants while PEP is the first CO2 acceptor in C4 plants.
Summary – C3 vs C4 Plants
C3 and C4 are two types of plants. C3 plants are very common while C4 plants are very rare. The key difference between C3 and C4 plants depends on the first carbon product that they produce during the dark reaction. C3 plants carry out the Calvin cycle and produce three-carbon compound as the first stable product while the C4 plants carry out C4 mechanism and produce four carbon compound as the first stable product. Furthermore, C3 plants show less photosynthetic efficiency while C4 plants show high photosynthetic efficiency. Moreover, C3 plants do not have Kranz anatomy in leaves, and also they don’t have two types of chloroplasts. On the other hand, C4 plants have Kranz anatomy in their leaves, and also they have two types of chloroplasts. Thus, this is the summary of C3 and C4 plants.
1. Szczepanik, et al. “On the Mechanism of C 4 Photosynthesis Intermediate Exchange between Kranz Mesophyll and Bundle Sheath Cells in Grasses.” OUP Academic, Oxford University Press, 28 Mar. 2008. Available here
2. Study.com, Study.com. Available here
1.”Simplified photorespiration diagram”By Rachel Purdon – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”HatchSlackpathway2″By Adenosine (talk) – HatchSlackpathway.svg, (CC BY-SA 2.5) via Commons Wikimedia