What is the Difference Between C3 C4 and CAM Photosynthesis

The main difference between C3 C4 and CAM photosynthesis is that C3 photosynthesis produces a three-carbon compound via the Calvin cycle, and C4 photosynthesis produces an intermediate four-carbon compound, which split into a three-carbon compound for the Calvin cycle, whereas CAM photosynthesis gathers sunlight during the day and fix carbon dioxide at night. Furthermore, the majority of plants undergo C3 photosynthesis, while C4 photosynthesis occurs in around 3% of vascular plants, including crabgrass, sugarcane, corn, etc. Meanwhile, CAM photosynthesis occurs in plants adapted to dry environments, including cacti and pineapples. 

C3, C4, and CAM photosynthesis are three types of photosynthesis pathways with different modes of Calvin cycles. They have different mechanisms to combat photorespiration. C3 plants do not have special features to combat photorespiration, while C4 plants minimize photorespiration by performing carbon dioxide fixation and Calvin cycle in separate cells. CAM plants, on the other hand, minimize photorespiration by performing carbon dioxide fixation and Calvin cycle at separate times. 

Key Areas Covered 

1. What is Photosynthesis
     – Definition, Light Reaction, Dark Reaction
2. What is C3 Photosynthesis
     – Definition, Dark Reaction, Importance
3. What is C4 Photosynthesis
     – Definition, Dark Reaction, Importance
4. What is CAM Photosynthesis
     – Definition, Dark Reaction, Importance
5. What are the Similarities Between C3 C4 and CAM Photosynthesis
     – Outline of Common Features
6. What is the Difference Between C3 C4 and CAM Photosynthesis
     – Comparison of Key Differences

Key Terms 

CAM Photosynthesis, Calvin Cycle, C3 Photosynthesis, C4 Photosynthesis, Photorespiration

What is Photosynthesis 

Photosynthesis is the cellular process in green plants responsible for fixing light energy from the sunlight in order to synthesize simple carbohydrates with the use of atmospheric carbon dioxide and water. It is a process that occurs in chloroplasts.

Figure 1: Photosynthesis

Furthermore, photosynthesis proceeds through two steps: light reaction and dark reaction. Typically, in the light reaction, chlorophylls absorb energy from the sunlight and synthesize two types of energy-rich molecules: ATP and the coenzyme, NADPH2. In contrast, in the dark reaction, these two energy-rich molecules are used for the synthesis of carbohydrates by fixing carbon dioxide. Moreover, there are three types of dark reactions occurring in plants depending on the environmental conditions. They are C3, C4, and CAM photosynthesis. 

What is C3 Photosynthesis 

C3 photosynthesis is the main type of photosynthesis occurring in every photosynthetic plant. Generally, it undergoes the standard mechanism of the Calvin cycle, following the light reaction. Therefore, the first step of the Calvin cycle is the carbon dioxide fixation in the C3 photosynthesis. Here, carbon dioxide is fixed into ribulose 1,5-bisphosphate, forming an unstable six-carbon compound, which is then hydrolyzed into the three-carbon compound, 3-phosphoglycerate. Here, the first stable product of C3 photosynthesis is a three-carbon compound, hence the name.

Figure 2: C3 Photosynthesis – Regular Calvin Cycle

The enzyme RuBisCO in the stromal surface of the thylakoid membrane in the chloroplast catalyzes the above reaction. Due to the catalytic imperfection of RuBisCO, it reacts highly with molecular oxygen in a process called photorespiration. Furthermore, carbon dioxide fixation results in two molecules of 3-phosphoglycerate. During the second step, one molecule of 3-phosphoglycerate undergoes a reduction in order to form three types of hexose phosphates: fructose 6-phosphate, glucose 6-phosphate, and glucose 1-phosphate. Also, the remaining 3-phosphoglycerate is recycled, forming ribulose 1, 5-bisphosphate.   

What is C4 Photosynthesis 

C4 photosynthesis is another form of photosynthesis mainly occurring in tropic plants. Typically, the gas exchanging stomata pores remain closed most of the day time in these plants in order to reduce the excessive loss of moisture in dry and hot conditions. Therefore, the carbon dioxide concentration inside plant leaves is not sufficient for the progression of the C3 cycle, which in turn enhances photorespiration, reducing the efficiency of photosynthesis. Thus, in order to increase efficiency during dry and hot conditions, those plants conduct C4 photosynthesis.

Figure 3: C4 Photosynthesis – Dark Reaction

Furthermore, Kranz anatomy describes the structure of the leaf of C4 plants. Basically, two types of cells are present in the C4 plant leaf. They are mesophyll cells and bundle sheath cells. Bundle sheath cells surround the vascular tissue. In the mesophyll cells, phosphoenolpyruvate reacts with carbon dioxide, forming oxaloacetate, which is a four-carbon compound. Here, phosphoenolpyruvate carboxylase is the enzyme which catalyzes carbon dioxide fixation. However, it is insensitive to oxygen; hence, photorespiration is minimized with respect to the carbon dioxide fixation by rubisco. After that, oxaloacetate reduces into malate, which is then transferred into bundle sheath cells. In the bundle sheath cells, malate undergoes decarboxylation by removing the carbon dioxide, entering into the C3 cycle. 

What is CAM Photosynthesis 

CAM photosynthesis is the third form of photosynthesis occurring in plants under semi-arid conditions. Usually, water is one of the two factors required for photosynthesis while the second being carbon dioxide. However, the water loss of these plants is high. Therefore, they highly store water inside the plant and become thick. Also, they have a wax coating in order to reduce evaporation. In general, succulents such as cacti, sedum, jade, orchids, and agave are the semi-arid plants. 

Figure 4: CAM Photosynthesis -Dark Reaction

Furthermore, these plants keep their stomata closed during the day time. Instead, they open at night and take carbon dioxide at night. Then, this carbon dioxide is fixed into phosphoenolpyruvate, forming oxaloacetate by C4 photosynthesis. Next, this oxaloacetate is converted into malate and is stored until daylight returns. After that, malate moves into mesophyll cells in order to undergo the regular Calvin cycle. 

Similarities Between C3 C4 and CAM Photosynthesis  

• C3,  C4, and CAM photosynthesis are three types of photosynthetic processes which occur in plants. 
• Generally,  photosynthesis is the cellular process of the plant responsible for fixing energy from the sunlight in order to synthesis small organic compounds such as glucose from carbon dioxide and water.  
• Both the light reaction and the Calvin cycle occur similarly in each type of photosynthesis. However, they differ by the method of carbon fixation. 

Difference Between C3 C4 and CAM Photosynthesis 

Definition 

C3 photosynthesis refers to a major type of photosynthesis which produces a three-carbon compound via the Calvin cycle while C4 photosynthesis refers to a type of photosynthesis which produces an intermediate four-carbon compound, which split into a three-carbon compound for the Calvin cycle. In contrast, CAM photosynthesis refers to another type of photosynthesis, which gathers sunlight during the day and fix carbon dioxide at night.  

Occurrence 

The majority of plants undergo C3 photosynthesis; C4 photosynthesis occurs in around 3% of vascular plants including crabgrass, sugarcane, corn, etc. while CAM photosynthesis occurs in plants adapted to dry environments including cacti and pineapples.  

Cells Involved in 

C3 photosynthesis only occurs in mesophyll cells, and C4 photosynthesis occurs in mesophyll cells and bundle sheath cells while CAM photosynthesis occurs in mesophylls. 

First Stable Product 

3-Phosphoglycerate (3-PGA) is the first stable compound produced in the C3 photosynthesis, oxaloacetate (OAA) is the first stable compound produced in C4 photosynthesis while CAM plants produce 3-phosphoglycerate (3-PGA) at daytime and oxaloacetate (OAA) at night. 

Requirements for the Dark Reaction

Moreover, C3 plants require 12 NADPH, and 18 ATPs for the dark reaction, C4 plants require 12 NADPH and 30 ATPs for the dark reaction while CAM plants require 12 NADPH and 39 ATPs for the dark reaction. 

Optimum Temperature 

The optimum temperature for the C3 photosynthesis is 15-25 °C, and the optimum temperature for the C4 photosynthesis is 30-40 °C while the optimum temperature for the CAM photosynthesis is > 40 °C. 

Stomata Opening During the Day time 

Stomata remain opened during the day time in both C3 and C4 photosynthesis while stomata remain closed during the night. 

Carboxylation Enzyme 

RuBP carboxylase is the carboxylation enzyme in the C3 photosynthesis, PEP carboxylase is the enzyme in mesophyll, and RuBP carboxylase is the enzyme in bundle sheath cells in C4 photosynthesis while PEP carboxylase is the enzyme active in the dark and RuBP carboxylase is the enzyme active in the day time in CAM photosynthesis. 

Initial Carbon Dioxide Acceptor 

Ribulose-1,5-bisphosphate (RuBP) is the initial carbon dioxide acceptor in the C3 photosynthesis, phosphoenolpyruvate (PEP) is the initial carbon dioxide acceptor in both C4 photosynthesis and CAM photosynthesis. 

Leaf Anatomy 

Leaf anatomy is typical in C3 photosynthesis; Kranz anatomy is present in the C4 photosynthesis while the leaf anatomy is xeromorphic in CAM photosynthesis. 

Photorespiration 

Moreover, C3 plants do not have special features to combat photorespiration, and C4 plants minimize photorespiration by performing carbon dioxide fixation and Calvin cycle in separate cells while CAM plants minimize photorespiration by performing carbon dioxide fixation and Calvin cycle in separate times. 

Conclusion 

C3 photosynthesis is the main form of photosynthesis that produces a three-carbon compound in the Calvin cycle. Generally, it occurs in all photosynthetic plants. However, photorespiration occurs in C3 plants at higher rates. In contrast, C4 photosynthesis is a type of photosynthesis which occurs in tropic plants. Also, it produces an intermediate four-carbon compound, which splits into a three-carbon compound for the Calvin cycle. It minimizes photorespiration as it performs carbon fixation and Calvin cycle in separate cells. On the other hand, CAM photosynthesis is another type of photosynthesis, which occurs in semi-arid conditions. Here, carbon dioxide fixation occurs at night. In addition, it minimizes photorespiration by performing carbon dioxide fixation and Calvin cycle in separate times. Therefore, the main difference between C3, C4, and CAM photosynthesis is the difference in carbon fixation, occurrence, and methods to minimize photorespiration. 

References:

1. Cornell, Brent. “C3, C4 And CAM Plants.” BioNinja, Available Here.

Image Courtesy:

1. “Simple photosynthesis overview” By Daniel Mayer (mav) – Own work (CC BY-SA 4.0) via Commons Wikimedia   
2. “Calvin-cycle4” By Mike Jones – Own work (CC BY-SA 3.0) via Commons Wikimedia   
3. “HatchSlackpathway2” By HatchSlackpathway.svg (CC BY-SA 2.5) via Commons Wikimedia   
4. “CAM” By The original uploader was Crenim at English Wikipedia. (CC BY-SA 3.0) via Commons Wikimedia