Difference Between C3 and C4 Cycle

Main Difference – C3 vs C4 Cycle

C3 and C4 cycle are two types of cyclic reactions that occur as the dark reaction of the photosynthesis. Photosynthesis is the production of simple organic molecules, glucose from inorganic molecules, carbon dioxide and water, using sunlight as the energy source. During photosynthesis, the light reaction is followed by the dark reaction. C3 cycle is also called Calvin cycle, whereas the C4 cycle is called Hatch-Slack cycle. The main difference between C3 and C4 cycle is the first stable compound produced by these reactions; the first stable compound produced in the C3 cycle is a three carbon compound called 3-phosphoglyceric acid (PGA) whereas the first stable compound produced in the C4 cycle is a four carbon compound called oxaloacetic acid (OAA). 

This article explores,

1. What is C3 Cycle
      – Characteristics, Process, Function
2. What is C4 Cycle
      – Characteristics, Process, Function
3. What is the difference between C3 and C4 Cycle

What is C3 Cycle

C3 cycle is one of the two reaction pathways that can occur in the dark reaction of photosynthesis. It occurs in all plants. Three steps are observed in C3 cycle. During the first step, carbon dioxide is fixed into ribulose 1,5-bisphosphate, forming an unstable six carbon compound, which is then hydrolyzed into three carbon compound, 3-phosphoglycerate. Carbon dioxide fixation is catalyzed by the enzyme, rubisco, which is found in the stromal surface of the thylakoid membrane in the chloroplast. The fixation of carbon dioxide is the rate-limiting step of the C3 cycle. Due to the catalytic imperfection of the enzyme, rubisco, the enzyme reacts with molecular oxygen by a process called photorespiration. Two molecules of 3-phosphoglycerate are formed by the first step of C3 cycle per a single carbon dioxide fixation. During the second step, one molecule of 3-phosphoglycerate is reduced, forming three types of hexose phosphates: fructose 6-phosphate, glucose 6-phosphate and glucose 1-phosphate. Remaining 3-phosphoglycerate is recycled, forming ribulose 1, 5-bisphosphate. The C3 cycle is shown in figure 1.

Figure 1: C3 Cycle

What is C4 Cycle

C4 cycle is the other reaction pathway occurring in the dark reaction of photosynthesis. The plants, growing in hot and dry environments like sugar cane, corn, and crabgrass, utilize C4 pathway during photosynthesis. The gas exchanging stomata pores are kept close most of the day in these plants in order to reduce the excessive loss of moisture in dry and hot conditions. Thereby, the carbon dioxide concentration inside plant leaves is also reduced by the progression of C3 cycle. When carbon dioxide concentration is low, photorespiration is enhanced, reducing the efficiency of photosynthesis. In order to increase the efficiency of photosynthesis during dry and hot conditions, those C4 plants conduct the C4 cycle.

Two types of cells are involved in the C4 Cycle: mesophyll cells and bundle sheath cells. Vascular tissue of the leaf is surrounded by bundle sheath cells. The structure of the leaf of C4 plants is described by Kranz anatomy. Phosphoenol pyruvate reacts with carbon dioxide in the mesophyll cells, forming oxaloacetate, which is a four carbon compound. The reaction is catalyzed by phosphoenol pyruvate carboxylase enzyme, which is insensitive to oxygen. Oxaloacetate is then reduced to malate, which is transferred into bundle sheath cells. In the bundle sheath cells, malate is decarboxylated by removing the carbon dioxide, entering into the C3 cycle. The C4 cycle is shown in figure 2.

Figure 2: C4 Cycle

Difference Between C3 and C4 Cycle

First Stable Compound

C3 Cycle: The first stable compound produced in the C3 cycle is a three-carbon compound called 3-phosphoglyceric acid.

C4 Cycle: The first stable compound produced in the C4 cycle is a four carbon compound called oxaloacetic acid.

First Observation

C3 Cycle: C3 cycle was first observed by Melvin Calvin.

C4 Cycle: C4 cycle was first observed by Hatch and Slack.

Alternative Names

C3 Cycle: C3 cycle is called Calvin cycle.

C4 Cycle: C4 cycle is called Hatch-Slack cycle.

Presence

C3 Cycle: C3 cycle is found in all plants.

C4 Cycle: C4 cycle is found only in C4 plants like sorghum and maize.

Primary Carbon Dioxide Acceptor

C3 Cycle: The primary carbon dioxide acceptor is a five carbon compound, Ribulose bi phosphate (RUBP).

C4 Cycle: The primary carbon dioxide acceptor is a three carbon compound, phosphoenol pyruvic acid (PEP).

Carboxylase Enzyme

C3 Cycle: The carboxylase enzyme is Rubisco in C3 plants.

C4 Cycle: The carboxylase enzymes are PEP carboxylase and Rubisco.

Carbon Fixation

C3 Cycle: A single carbon fixation occurs in C3 cycle.

C4 Cycle: Double carbon fixations occur in C4 cycle.

Efficiency in Carbon Fixation

C3 Cycle: Carbon fixation is less efficient and slow in C3 cycle.

C4 Cycle: Carbon fixation is more efficient and fast in C4 cycle.

Requirements of the Carbon Fixation

C3 Cycle: The fixation of a single carbon molecule requires 3 ATP and 2 NADH.

C4 Cycle: The fixation of a single carbon fixation requires 5 ATP and 3 NADH.

Types of Chloroplasts

C3 Cycle: Granular chloroplasts are involved in the C3 cycle.

C4 Cycle: Granular and agranular chloroplasts are involved in the C4 cycle.

Kranz Anatomy in Leaves

C3 Cycle: Kranz anatomy is absent in leaves of the C3 plants.

C4 Cycle: Kranz anatomy is present in leaves of the C4 plants.

Cells

C3 Cycle: C3 cycle is carried out by mesophyll cells.

C4 Cycle: C4 cycle is carried out by both mesophyll cells and bundle sheath cells.

Optimum Temperature

C3 Cycle: The optimum temperature of the C3 cycle is 20-25 degrees of Celsius.

C4 Cycle: The optimum temperature of the C4 cycle is 30-45 degrees of Celsius.

At Very Low Carbon Dioxide Concentrations

C3 Cycle: C3 cycle is unable to proceed at very low carbon dioxide concentrations.

C4 Cycle: C4 cycle can proceed at very low carbon dioxide concentrations.

Effect of Oxygen

C3 Cycle: C3 cycle is inhibited by oxygen.

C4 Cycle: No inhibition of c4 cycle is observed with C4 cycle.

Sunlight

C3 Cycle: C3 cycle can be saturated with sunlight.

C4 Cycle: C4 cycle does not saturate with sunlight.

Photorespiration

C3 Cycle: Considerable amount of photorespiration is observed in C3 cycle.

C4 Cycle: Negligible amount of photorespiration is observed in C4 cycle.

Conclusion

C3 and C4 cycle are the two types of dark reactions that occur during photosynthesis. C3 cycle occurs in all plants at 20-25 degrees Celsius, whereas the C4 cycle occurs only in C4 plants at 30-45 degrees Celsius. During C3 cycle, a single carbon fixation event is observed whereas, during C4 cycle, two carbon fixation events are observed. Photorespiration occurs during C3 cycle but negligible amounts of photorespiration occur during C4 cycle. The efficiency of C3 cycle is low when compared to the efficiency of C4 cycle. The main difference between C3 and C4 cycle is the number of carbons in the first stable compound produced in each cycle.

Reference:
1. Berg, Jeremy M. “The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 16 Apr. 2017.
2. Lodish, Harvey. “CO2 Metabolism during Photosynthesis.” Molecular Cell Biology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 16

Image Courtesy:
1. “Calvin-cycle4” By Mike Jones – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “HatchSlackpathway2” By HatchSlackpathway.svg: *HatchSlackpathway.png: Adenosine derivatives work: Jamousederivative work: Adenosine (talk) – HatchSlackpathway.svg (CC BY-SA 2.5) via Commons Wikimedia