Cam Pathway – Process & Explanation

Key Concepts

• Photosynthesis – process, stages

• Light and dark reactions – process, stages, products

• C4 cycle – process, stages

• CAM cycle – significance, process, stages

• Comparison of C3, C4 and CAM plants

Physiological Metabolism – CAM Pathway 

Introduction:

Photosynthesis (photo–light; synthesis – to join) is the process through which green plants make use of energy from sunlight to make their own food. In the presence of sunlight, chloroplasts of green plants synthesize glucose using water and carbon dioxide. During photosynthesis, water gets oxidized, and carbon dioxide gets reduced to form carbohydrates. Photosynthesis forms the basis of all types of food chains and food webs. Photosynthesis produces starch and oxygen required for the survival of an organism. Therefore, photosynthesis supports all forms of life on Earth. 

Photosynthesis in Plants: 

Photosynthesis is a vital process that occurs in green plants. In the presence of sunlight, plants combine carbon dioxide and water to produce carbohydrates. The extremely significant by-product of photosynthesis is oxygen. In the presence of light energy, water is split to release oxygen. This process is known as the photolysis of water. During the process of photosynthesis, water molecule gets split up to release oxygen, and carbon dioxide forms carbohydrates. Oxygen produced during photosynthesis comes from a water molecule and not from carbon dioxide. In plants, photosynthesis takes place in the green part. Mostly green leaves carry out the photosynthesis process. Sometimes other green parts of plants, such as green stems and floral buds, are also involved in photosynthesis. Chloroplasts are specialized organelles in which the photosynthesis process actually takes place.  

Stages of Photosynthesis: 

Photosynthesis involves two important processes, 

1. Light-dependent reaction (Light reaction) 

2. Light-independent reaction (Dark reaction / Calvin cycle) 

Though the entire process of photosynthesis takes place in chloroplast, light and dark reactions occur at different sites. Light reaction takes place in grana, and the dark reaction takes place in stroma regions of the chloroplast.  

Light-Dependent Reaction: 

Light-dependent reaction is also known as the primary photochemical reaction or Hill’s reaction, or Arnon’s cycle. Light reaction takes place at a faster rate than dark reaction. Light energy is converted into chemical energy during a light-dependent reaction. Light dependent reaction occurs in the thylakoid (thylakoids are stacked up to form grana) region of the chloroplast. Thylakoids contain chlorophyll molecules. Chlorophyll pigments are green in color. These pigments absorb different wavelengths of light and convert them into chemical energy through photosynthesis. 

Light-dependent reaction involves four important stages. 

1. Absorption of light energy 

2. Splitting of water molecules 

3. Release of oxygen 

4. Formation of energy-carrying molecules – ATP and NADPH 

Products of Light Reaction: 

During light reaction, solar energy gets converted into chemical energy and is stored in the form of energy carrying molecules – ATP and NADPH. During light reaction, water is split up to release oxygen. High energy carrier molecules, ATP, and NADPH are required for the dark reaction, the next stage of photosynthesis.  

Dark Reaction/Calvin Cycle: 

The second step of photosynthesis is the light-independent reaction. It is also known as the dark reaction or Calvin cycle, or C3 cycle. Calvin cycle refers to the conversion of carbon dioxide to carbohydrate or the fixation of atmospheric carbon dioxide by plants through photosynthesis. Plants that undergo this cycle of reactions are known as C3 plants (Example: rice, wheat, barley, peanuts, spinach, etc.). 

Calvin cycle takes place in the stroma of chloroplasts in the absence of light. The overall purpose of the Calvin cycle is to convert atmospheric carbon dioxide into carbohydrate (sugars). Calvin cycle makes use of ATP and NADPH produced by the light reaction to convert carbon dioxide into carbohydrate.  

Phases of the Calvin Cycle: 

Calvin cycle involves three important phases. 

1. Carbon fixation 

2. Reduction 

3. Regeneration of ribulose1, 5-bisphosphate (RuBP) 

During the first phase of the Calvin cycle, carbon dioxide reacts with ribulose1, 5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose biphosphate carboxylase/oxygenase or RUBISCO(RuBp). Two molecules of three-carbon molecule known as 3 – phosphoglycerate (3– PGA) are produced during this phase. In the reduction phase of the Calvin cycle, 3-PGA molecules (from the carbon fixation phase) are converted into glyceraldehyde-3-phosphate (G3P). ATP and NADPH are utilized for this reaction. As NADPH gets reduced (donates electrons), this reaction is termed as reduction phase. During the third phase, ribulose1, 5-bisphosphate (RuBP) gets regenerated, and the cycle restarts. 

Products of Calvin Cycle: 

• After each turn of the Calvin cycle, one molecule of carbon gets fixed. 
• After three turns of the Calvin cycle, one molecule of G3P is exported into the cytoplasm. 
• G3P molecules that leave the Calvin cycle are used for the production of glucose/fructose/ sucrose/starch.  
• Two G3P molecules combine together to form one glucose molecule. Therefore, six turns of the Calvin cycle are required to produce one molecule of glucose. 

C4 Cycle (Hatch and Slack Pathway): 

In 1966, Marshall Davidson Hatch and Charles Roger Slack discovered the C4 cycle. C4 cycle is observed in plants present in the dry tropical region. In order to grow in a hot and dry environment, plants have adapted themselves by following the C4 pathway. The first stable product of the C4 cycle is oxaloacetate. It is a four-carbon compound. Hence the pathway is named C4 cycle.  

C4 cycle is a kind of adaptation by plants to survive in a hot and dry environment. C4 plants have more benefits than C3 plants. C4 plants do not exhibit the process of photorespiration. (Photorespiration refers to the oxidation of RuBp in the presence of oxygen). Photorespiration begins when RUBISCO takes up oxygen instead of carbon dioxide. In order to reduce the loss of water, C3 plants close their stomata (pores present in leaves). Photorespiration takes place during this time. C3 plants lose an enormous amount of energy through photorespiration. In order to avoid energy loss through photorespiration, certain plants adapt themselves to C4 and CAM pathways. The crop yield also increases through this adaptation. 

Process of C4 Cycle: 

In C4 plants, light and dark reactions occur in different regions. Light reaction takes place in the mesophyll cells, while dark reaction/C3 cycle takes place in the bundle-sheath cells. Calvin cycle/C3 cycle is commonly observed in all photosynthetic plants. Whereas the C4 cycle is observed only in C4 plants. C4 pathway is also observed to occur before the C3 pathway.  

In C4 plants, the C4 pathway takes place in the mesophyll cells, while the C3 pathway takes place in the bundle-sheath cells. Whereas in C3 plants Calvin cycle occurs in the mesophyll cells. C4 cycle involves four important steps. 

1. Carboxylation 

2. Break down 

3. Splitting  

4. Phosphorylation 

CAM (Crassulacean Acid Metabolism) Pathway: 

Photorespiration is an inefficient metabolic pathway through which a lot of energy is lost. Photorespiration begins when RUBISCO takes up oxygen instead of carbon dioxide. In order to reduce the loss of water, C3 plants close their stomata (pores present in leaves). Photorespiration takes place during this time. C3 plants lose an enormous amount of energy through photorespiration. In order to avoid energy loss through photorespiration, certain plants adapt themselves to C4 and CAM pathways. In C3 plants, approximately 25 % of photosynthesis is lost through photorespiration. 

Crassulacean Acid Metabolism (CAM) pathway is a special type of carbon fixation pathway. It is observed in plants that grow in dry, semi-arid, or xerophytic conditions. In order to survive in dry conditions, plants adapt to the CAM pathway. Plants avoid photorespiration by using the CAM pathway. The leaves of these plants are found to be succulent or fleshy. Scientists first observed this pathway in the Crassulaceae family of plants (Example: Bryophyllum, Sedum, Kalanchoe). The pathway has been named after this discovery. 

CAM pathway is found in Crassulaceae, Cactaceae, Agavaceae and Orchidaceae family of plants. Plants like cacti, orchids, and pineapple are found to exhibit CAM pathway. The stomata of these plants are observed to be scotoactive. Generally, plants lose their water content during the daytime through transpiration. In order to prevent water loss during the daytime, stomata remain closed and are open at night (scotoactive).  

Significance of CAM Pathway: 

• In order to reduce water loss in CAM plants, stomata remain closed during the day and open at night. Using this adaptation, CAM plants survive in extremely dry conditions. 
• CAM plants carry out carbon dioxide fixation in the dark. Hence, they are able to survive in light for a longer time without the uptake of carbon dioxide. 
• When stomata remain closed, succulent plants obtain carbon dioxide from malic acid. 
• CAM plants uptake carbon dioxide during the night. This limits photosynthesis. In addition, stored carbohydrates and organic acids also limit the photosynthesis process. Hence, the growth of CAM plants is generally slow. 
• CAM plants are drought resistant. They possess xerophytic adaptations (For example: thick fleshy leaves). 

Process of CAM Pathway: 

CAM plants adapt themselves to survive under extreme drought/dry conditions. CAM pathway involves two important steps. 

1. Acidification 

2. Deacidification 

Acidification: 

Acidification occurs during the night. In darkness, the glycolysis process converts stored carbohydrates into phosphoenol pyruvate (PEP). As the stomata of CAM plants remain open during the night, atmospheric carbon dioxide diffuses into the leaves. CAM plants fix the carbon dioxide using PEP. PEP gets carboxylated, and oxalo acetic acid (OAA) is produced. This reaction is catalyzed by PEP carboxylase. The OAA is then converted into malic acid in the presence of malic dehydrogenase enzyme. NADPH produced during glycolysis is used for this reaction. Malic acid gets accumulated in the vacuole and causes an increase in the acidity of the cells. 

Deacidification: 

The deacidification process occurs during the day. In order to prevent water loss, stomata remain closed during the day. At this time, malic acid undergoes decarboxylation to produce pyruvate and carbon dioxide. This reaction is catalyzed by malic enzyme. As malic acid is decarboxylated to pyruvate, there is a decrease in the acidity of the cell. Therefore, this process is termed deacidification. Carbon dioxide produced during the deacidification of malic acid enters the C3 cycle and aids the production of carbohydrates. Pyruvate is either used for the regeneration of PEP or converted into carbon dioxide through Kreb’s cycle.  

CAM pathway is the most useful pathway in succulent plants. In order to reduce water loss, the stomata of these plants remain closed during the day. Therefore, atmospheric carbon dioxide does not enter the leaves during the daytime. However, these plants carry out photosynthesis during the day with the help of carbon dioxide produced by the decarboxylation of malic acid. Atmospheric carbon dioxide is fixed in the form of malic acid during the night. During the day, when atmospheric carbon dioxide is not available, stored malic acid is broken down to release carbon dioxide, which aids photosynthesis. Thus, CAM pathway conserves energy and aids the survival of succulent plants. 

Difference between C3, C4, and CAM Plants: 

C3 Plants: 

In C3 plants, carbon fixation and Calvin cycle take place in the mesophyll cells located on the surface of leaves. 

C4 Plants: 

In C4 plants, Carbon fixation and Calvin cycle take place in different regions. Carbon fixation takes place in mesophyll cells, and the Calvin cycle takes place in bundle-sheath cells. 

CAM Plants: 

In CAM plants, carbon fixation and Calvin cycle take place in the mesophyll cells. However, these steps are separated by time. Carbon fixation takes place during the night, and the Calvin cycle takes place during the day. 

Summary

• Photorespiration is an inefficient metabolic pathway through which a lot of energy is lost

• In order to avoid energy loss through photorespiration, certain plants adapt themselves to C4 and CAM pathways.

• Crassulacean Acid Metabolism (CAM) pathway is a special type of carbon fixation pathway. It is observed in plants that grow in dry, semi-arid, or xerophytic conditions.

• In order to survive in dry conditions, plants adapt to the CAM pathway. Plants avoid photorespiration by using the CAM pathway.

• CAM pathway involves two important steps – acidification and deacidification

• In order to reduce water loss, the stomata of CAM plants remain closed during the day and open at night.

• Atmospheric carbon dioxide is fixed in the form of malic acid during the night. This process is known as acidification.

• During the daytime, stored malic acid is broken down to release carbon dioxide. This process is known as deacidification.

• In C3 plants, carbon fixation and Calvin cycle take place in the mesophyll cells located on the surface of leaves.

• In C4 plants, Carbon fixation and Calvin cycle take place in different regions. Carbon fixation takes place in mesophyll cells, and the Calvin cycle takes place in bundle-sheath cells.

• In CAM plants, carbon fixation and Calvin cycle take place in the mesophyll cells. However, these steps are separated by time. Carbon fixation takes place during the night, and the Calvin cycle takes place during the day.