Calvin Cycles vs Hatch and Slack Cycle
Difference between C3 and C4 Cycles: Photosynthesis is one of the vital events in the earth in which the green plants fix the energy from the sunlight and synthesis nutrients with carbon dioxide and water. Almost all living things on earth, either directly or indirectly, depend on photosynthesis for energy. The process of photosynthesis in plants is completed in two major pathways, a light dependent ‘Light Reaction’ and a light independent ‘Dark Reaction’.
In the light reaction, the chlorophyll molecules in the plants absorb energy from sunlight and synthesize energy rich chemical molecules such as ATP and reduced coenzymes (NADPHH+). In the dark reaction, this energy rich molecules are used up for the synthesis of carbohydrates from carbon dioxide. The first describe dark reaction pathway, better known as Calvin cycle (Melvin Calvin who discovered this pathway), is called C3 cycle.
For a considerable period of time, the Calvin cycle (C3 cycle) was thought to be the only dark reaction pathway in plants. Later, a new pathway of dark reaction called Hatch and Slack pathway or C4 cycle was described in some plants. Both these cycles (C3 and C4 cycles) show many similarities and differences. The present post describes the similarities and differences between C3 cycle and C4 cycle of the dark reaction of photosynthesis.
Similarities between C3 cycle and C4 cycle
Ø Both C3 and C4 cycles are pathways of dark reaction of photosynthesis.
Ø Both are light independent reactions.
Ø Both C3 and C4 cycle requires energy from ATP or reduced coenzymes.
Ø Both C3 and C4 plants accept carbon dioxide to perform dark reaction.
Ø End products of C3 and C4 cycle are similar.
Ø Both C3 and C4 cycle requires RuBP and RUBISCO to complete the pathway.
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Difference between C3 and C4 Cycles
Sl. No. C3 Cycle (Calvin Cycle) 1 C3 cycle is commonly known as Calvin Cycle (Melvin Calvin described it first). C4 cycle is commonly known as Hatch and Slack pathway (in honor of Marshall Davidson Hatch and C. R. Slack who elucidated this pathway). 2 Examples of C3 plants: Wheat, Rye, Oats, Rice, Cotton, Sunflower, Chlorella. Examples of C4 plants: Maize, Sugarcane, Sorghum, Amaranthus. 3 Leaves of C3 plants do not have Kranz anatomy. Leaves of C4 plants possess Kranz anatomy. 4 C3 plants are cool season plants, commonly seen in cool and wet areas (temperate areas). C4 plants are warm season plants, commonly seen in dry areas (tropical areas). 5 The C3 cycle is present in all plants The C4 cycle is present only in C4 plants 6 First stable product in C3 cycle is a 3 carbon (3C, hence the name) compound – Phosphoglyceric Acid (PGA). First stable product in C4 cycle is a 4 carbon (4C, hence the name) compound – Oxaloacetic Acid (OAA). 7 The CO2 acceptor in the C3 cycle is RuBP (Ribulose-1,5-bisphosphate), RuBp is a 5 carbon compound. The first CO2 acceptor in the C4 cycle is PEP (Phosphoenolpyruvate). PEP is a 3 carbon compound. 8 The first enzyme in C3 cycle is RUBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). The first enzyme in C4 cycle is PEP carboxylase. 9 The first enzyme RUBISCO has high affinity towards oxygen Frist enzyme PEP carboxylase does not have any affinity for oxygen. 10 Increased oxygen concentration has an inhibitory effect on C3 cycle. Concentration of oxygen does not have any inhibitory role in C4 cycle. 11 C3 cycle requires 18 ATP molecules to synthesize one molecule of glucose. C4 cycle requires 30 ATP molecules to synthesize one molecule of glucose. 12 The complete steps of C3 cycle are executed in the mesophyll cells only. The mesophyll cells will only do the initial steps of C4 cycle, the rest are completed in the bundle sheath cells. 13 In C3 cycle, the carbon dioxide fixation takes place only at one place. In C4 cycle, the carbon dioxide fixation takes places twice (first in mesophyll cells, second in bundle sheath cells). 14 Only a single type of chloroplasts is involved in C3 cycle. All chloroplasts are granal. Two types of chloroplasts are involved in C4 cycle. Granal in mesophyll cells and agranal in bundle sheath cells. 15 RuBP is the only CO2 acceptor in C3 cycle. There are two CO2 acceptors in C4 cycle. Phosphoenolpyruate (PE) is the primary CO2 acceptor in C4 cycle. 16 There is no secondary CO2 acceptor in C3 cycle. In C4 plants, there is a secondary CO2 acceptor. RuBP is the secondary CO2 acceptor in C4 plants. 17 In C3 plants, the bundle sheath cells do not contain chloroplasts. In C4 plants, the bundle sheath cells contain chloroplasts. 18 In C3 plants, the light and dark reactions of photosynthesis occur in a single location. In C4 plants, the light and dark reactions of the photosynthesis are physically separated and completed in two different locations. 19 C3 plants can perform photosynthesis only when the stomata are open. C4 plants can do photosynthesis even in the closed condition of stomata. 20 The optimum temperature for photosynthesis in C3 plants is very low. The optimum temperature for photosynthesis in C4 plants is high. 21 The rate of photorespiration is very high in C3 plants. The photorespiration is altogether absent in C4 plants (if present very little). 22 C3 cycle is less efficient in Photosynthetic energy fixation due to the presence of photorespiration. C4 cycle is more efficient than C3 cycle in photosynthesis due to the absence of photorespiration. 23 The carbon dioxide compensation point is high in C3 cycle (about 50 ppm). The carbon dioxide compensation point is low in C4 cycle (2 to 5 or even 0 ppm). 24 In high light intensity, the rate of CO2 evolution is high in C3 plants. In the high light intensity, the rate of CO2 evolution is very low in C4 plants. 25 The water loss per g of biomass produced with C3 cycle is high (450 to 950). The water loss per g of biomass produced with C4 cycle is low (250 – 350).
oxaloacetic acid is not a first stable compound formed in the c4 cycle. It is unstable and quickly reduced to form malic acid which is more stable.