Main Difference – C3 vs C4 plants
C3 and C4 plants are two types of plants using C3 and C4 cycles during the dark reaction of photosynthesis respectively. Around 95% of plants on earth are C3 plants. Sugar cane, sorghum, maize, and grasses are C4 plants. Leaves of the C4 plants exhibit Kranz anatomy. C4 plants are capable of photosynthesizing even in low concentrations of carbon dioxide as well as in hot and dry conditions. Therefore, the efficiency of photosynthesis in C4 plants is higher than its efficiency in C3 plants. The main difference between C3 and C4 plants is that single fixation of carbon dioxide is observed in C3 plants and double fixation of carbon dioxide is observed in C4 plants.
This article explores,
1. What are C3 Plants
– Definition, Characteristics, Features, Examples
2. What are C4 Plants
– Definition, Characteristics, Features, Examples
3. What is the difference between C3 and C4 Plants
What are C3 Plants
C3 plants use Calvin cycle as their mechanism for dark reaction in photosynthesis. The first stable compound produced in the Calvin cycle is 3-phosphoglycerate. Since 3-phosphoglycerate is a three carbon compound, Calvin cycle is called the C3 cycle. C3 plants directly fix carbon dioxide by the enzyme, ribulose bisphosphate carboxylase (rubisco). This fixation occurs in the chloroplasts of mesophyll cells. C3 cycle occurs in three steps. During the first step, carbon dioxide is fixed into the five carbon sugar, ribulose 1,5-bisphosphate, which is alternatively hydrolyzed into 3-phosphoglycerate. Some of the 3-phosphoglycerate are reduced into hexose phosphates like glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate during the second step. The remaining 3-phosphoglycerate are recycled, forming ribulose 1,5-phosphate.
The optimum temperature range of C3 plants is 65-75 degrees Fahrenheit. When the soil temperature reaches 40-45 degrees Fahrenheit, C3 plants start growing. Therefore, C3 plants are called cool-season plants. The efficiency of photosynthesis become low with the increasing temperature. During the spring and fall, C3 plants become productive due to the high soil moisture, shorter photoperiod, and cool temperature. During summer, C3 plants are less productive due to the high temperature and less soil moisture. C3 plants can be either annual plants like wheat, oats, and rye or perennial plants like fescues and orchard. A cross section of the leaf of Arabidopsis thaliana, which is a C3 plant is shown in figure 1. Bundle sheath cells are shown in pink color.
Figure 1: Arabidopsis thaliana leaf
What are C4 Plants
C4 plants use Hatch-Stack cycle as their reaction mechanism in the dark reaction of photosynthesis. The first stable compound produced in the Hatch-Stack cycle is oxaloacetate. Since oxaloacetate is a four-carbon compound, Hatch-Stack cycle is called the C4 cycle. C4 plants fix carbon dioxide twice, in mesophyll cells and then in bundle sheath cells, by the enzymes, phosphoenol pyruvate carboxylase and ribulose bisphosphate carboxylase (rubisco) respectively. Phosphoenol pyruvate in the mesophyll cells is condensed with carbon dioxide, forming the oxaloacetate. This oxaloacetate becomes malate in order to transfer into bundle sheath cells. Inside the bundle sheath cells, malate is decarboxylated, making carbon dioxide available for Calvin cycle in these cells. Then carbon dioxide is fixed for the second time inside the bundle sheath cells.
The optimum temperature of C4 plants is 90-95 degrees Fahrenheit. C4 plants start growing at 60-65 degrees Fahrenheit. Therefore, C4 plants are called tropical or warm season plants. C4 plants are more efficient in gathering carbon dioxide and water from the soil. The gas exchanging stomata pores are kept close during most hours of the day in order to reduce the excessive loss of moisture in dry and hot conditions. Annual C4 plants are corn, pearlmillet, and sudangrass. Perennial C4 plants are bermudagrass, Indian grass and switchgrass. Leaves of the C4 plants exhibit Kranz anatomy. Photosynthesizing bundle sheath cells cover the vascular tissues of the leaf. These bundle sheath cells are surrounded by mesophyll cells. A cross section of a maize leaf, exhibiting Kranz anatomy is shown in figure 2.
Figure 2: Maize leaf
Difference Between C3 and C4 Plants
Alternative Names
C3 Plants: C3 plants are called cool season plants.
C4 Plants: C4 plants are called warm season plants.
Kranz Anatomy
C3 Plants: Leaves of the C3 plants lack Kranz anatomy.
C4 Plants: Leaves of the C4 plants possess Kranz anatomy.
Cells
C3 Plants: In C3 plants, the dark reaction is carried out by mesophyll cells. Bundle sheath cells lack chloroplasts.
C4 Plants: In C4 plants, the dark reaction is carried out by both mesophyll cells and bundle sheath cells.
Chloroplasts
C3 Plants: Chloroplasts of C3 plants are monomorphic. C3 plants only contain granular chloroplasts.
C4 Plants: Chloroplasts of C4 plants are dimorphic. C4 plants contain both granular and agranular chloroplasts.
Peripheral Reticulum
C3 Plants: Chloroplasts of C3 plants lack a peripheral reticulum.
C4 Plants: Chloroplasts of C4 plants contain a peripheral reticulum.
Photosystem II
C3 Plants: Chloroplasts of the C3 plants consists of PS II.
C4 Plants: Chloroplasts of the C4 plants do not consist of PS II.
Stomata
C3 Plants: Photosynthesis is inhibited when stomata are closed.
C4 Plants: Photosynthesis occurs even when stomata are closed.
Carbon Dioxide Fixation
C3 Plants: A single carbon dioxide fixation occurs in C3 plants.
C4 Plants: Double carbon dioxide fixations occurs in C4 plants.
Efficiency in the Carbon Dioxide Fixation
C3 Plants: Carbon dioxide fixation is less efficient and slow in C3 plants.
C4 Plants: Carbon dioxide fixation is more efficient and fast in C4 plants.
Efficiency of Photosynthesis
C3 Plants: Photosynthesis is less efficient in C3 plants.
C4 Plants: Photosynthesis is efficient in C4 plants.
Photorespiration
C3 Plants: Photorespiration occurs in C3 plants when the carbon dioxide concentration is low.
C4 Plants: No photorespiration is observed at low carbon dioxide concentrations.
Optimum Temperature
C3 Plants: The optimum temperature range of C3 plants is 65-75 degrees Fahrenheit.
C4 Plants: The optimum temperature range of C4 plants is 90-95 degrees Fahrenheit.
Carboxylase Enzyme
C3 Plants: The carboxylase enzyme is rubisco in C3 plants.
C4 Plants: The carboxylase enzyme is PEP carboxylase and rubisco in C4 plants.
First Stable Compound in the Dark Reaction
C3 Plants: The first stable compound produced in the C3 cycle is a three-carbon compound called 3-phosphoglyceric acid.
C4 Plants: The first stable compound produced in the C4 cycle is a four carbon compound called oxaloacetic acid.
Protein Content of the Plant
C3 Plants: C3 plants contain a high protein content.
C4 Plants: C4 plants contain low protein content compared to C3 plants.
Conclusion
C3 and C4 plants use distinct metabolic reactions during the dark reaction of photosynthesis. C3 plants use Calvin cycle whereas the C4 plants use Hatch-Slack cycle. In C3 plants, the dark reaction occurs in mesophyll cells by fixation of carbon dioxide directly into ribulose 1,5-bisphosphate. In C4 plants, carbon dioxide is fixed into phosphoenol pyruvate, forming malate in order to transfer into bundle sheath cells where Calvin cycle occurs. Therefore, carbon dioxide is fixed twice in C4 plants. In order to adjust into the C4 mechanism, the leaves of C4 plants exhibit Kranz anatomy. The efficiency of photosynthesis is high in C4 plants when compared to C3 plants. C4 plants are capable of carrying out photosynthesis even after the stomata are closed. Therefore, the main difference between C3 and C4 plants is their metabolic reactions, operating during the dark reaction of photosynthesis.
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 Apr. 2017.
Image Courtesy:
1. “Cross section of Arabidopsis thaliana, a C3 plant” By Ninghui Shi – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Cross section of maize, a C4 plant” By Ninghui Shi – Own work, (CC BY-SA 3.0) via Commons Wikimedia
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