Pathways of Carbon Fixation

Photorespiration

•  Occurrence: Photorespiration primarily occurs in C3 plants, which include a wide range of plant species like wheat, rice, and soybeans. It takes place in the chloroplasts of plant cells.

• RuBisCO Reaction: The key enzyme involved in photorespiration is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). While its primary function is to fix carbon dioxide during photosynthesis, it can also react with oxygen under certain conditions.

• Oxygen Uptake: When oxygen concentrations are relatively high and carbon dioxide concentrations are low, or when the stomata (tiny openings on the leaf surface) are partially closed, oxygen can compete with carbon dioxide for the active site of RuBisCO. This leads to the binding of oxygen to RuBisCO instead of carbon dioxide.

• Production of Phosphoglycolate: When oxygen is bound to RuBisCO, a two-carbon compound called phosphoglycolate is formed instead of the desired three-carbon compound during carbon fixation. This initiates the photorespiratory process.

• Peroxisome Involvement: Phosphoglycolate is then transported to peroxisomes, specialized organelles within the cell. In the peroxisomes, phosphoglycolate is converted into glycolate, releasing carbon dioxide and consuming energy in the process.

• Glycolate Conversion: Glycolate is subsequently transported to the mitochondria, where it undergoes further conversions to form glycine. These reactions also result in the release of carbon dioxide.

• Serine/Glycine Conversion: Glycine, produced from glycolate, can be converted into serine, another amino acid. This conversion takes place in the chloroplasts and requires the consumption of ATP.

• The Cost of Photorespiration: Photorespiration consumes energy in the form of ATP and reduces the overall efficiency of photosynthesis. It also results in the loss of fixed carbon in the form of carbon dioxide and the production of toxic byproducts within the plant.

• Negative Effects: Photorespiration can have detrimental effects on plants. It reduces the net production of carbohydrates and limits plant growth and productivity. It also leads to the production of reactive oxygen species, which can cause oxidative damage to the plant's cells.

• Evolutionary Significance: Photorespiration is believed to be an evolutionary relic from an earlier time when the Earth's atmosphere had different compositions, such as higher carbon dioxide levels and lower oxygen levels. The rise of oxygen levels in the atmosphere favored the development of alternative carbon fixation mechanisms, such as C4 and CAM pathways, which minimize photorespiration.

Calvin cycle 

Carbon Fixation:

a. Carbon dioxide (CO2) enters the stroma of the chloroplast, where the Calvin cycle takes place.

b. The enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between CO2 and a five-carbon compound called ribulose bisphosphate (RuBP).

c. This reaction results in the formation of two molecules of 3-phosphoglycerate (3-PGA), each containing three carbon atoms.

Reduction:

a. ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) molecules produced in the light-dependent reactions are used in the reduction step.

b. Each molecule of 3-PGA receives a phosphate group from ATP, becoming 1,3-bisphosphoglycerate (1,3-BPG).

c. NADPH donates high-energy electrons to 1,3-BPG, converting it into glyceraldehyde-3-phosphate (G3P).

d. For every three molecules of CO2 fixed, six molecules of G3P are produced. However, only one molecule of G3P exits the cycle to be used for glucose synthesis, while the remaining molecules continue to regenerate RuBP.

Regeneration:

a. The regeneration of RuBP is crucial to keep the Calvin cycle going.

b. Five out of every six molecules of G3P produced in the reduction step are used to regenerate RuBP.

c. The remaining G3P molecules are rearranged and converted into the initial molecule, RuBP, through a series of enzymatic reactions.

d. This regeneration process requires additional ATP molecules generated during the light-dependent reactions.

Exit of G3P:

a. One molecule of G3P out of every six molecules is available to the plant for further metabolic processes.

b. G3P can be used to synthesize glucose, which can be stored as starch or used for energy production.

c. G3P can also be used to produce other organic compounds essential for plant growth, such as fatty acids, amino acids, and nucleotides.

ATP Requirements:

a. The Calvin cycle requires ATP as an energy source for the various enzymatic reactions involved in carbon fixation, reduction, and regeneration.

b. ATP is generated in the light-dependent reactions through photophosphorylation, where light energy is used to convert ADP (adenosine diphosphate) to ATP.

NADPH Requirements:

a. NADPH, another product of the light-dependent reactions, provides high-energy electrons necessary for the reduction step of the Calvin cycle.

b. NADPH carries the electrons and hydrogen ions (H+) required to convert 3-PGA to G3P.

Continuous Cycle:

a. The Calvin cycle is a cyclic process, meaning it continues as long as there is an adequate supply of ATP, NADPH, and CO2.

b. The regenerated RuBP molecules can accept more CO2, initiating the cycle again.

C4 cycle

Initial Carbon Fixation:

a. C4 plants, such as maize, sugarcane, and certain grasses, have specialized leaf anatomy with two distinct types of cells: mesophyll cells and bundle sheath cells.

b. In the mesophyll cells, which are located on the outer layer of the leaf, carbon dioxide is initially fixed into a four-carbon compound called oxaloacetate (OAA) or malate.

c. The enzyme responsible for this initial fixation is phosphoenolpyruvate carboxylase (PEP carboxylase), which has a higher affinity for CO2 and a lower affinity for oxygen compared to RuBisCO.

Formation of C4 Acids:

a. The four-carbon compounds (OAA or malate) formed in the mesophyll cells are transported to the inner layer of the leaf, where the bundle sheath cells are located.

b. In the bundle sheath cells, the four-carbon compounds release CO2, thanks to the activity of decarboxylases.

c. The released CO2 then enters the Calvin cycle in the bundle sheath cells, where it can be fixed by RuBisCO to produce carbohydrates.

Regeneration of PEP:

a. After releasing CO2, the remaining three-carbon compounds in the bundle sheath cells are converted back into the original acceptor molecule, phosphoenolpyruvate (PEP).

b. This conversion requires ATP and occurs in a series of reactions collectively known as the C4 acid regeneration or C4 shuttle.

The concentration of CO2:

a. By separating the initial carbon fixation and the Calvin cycle in different cell types (mesophyll and bundle sheath cells), C4 plants can maintain a higher concentration of CO2 in the vicinity of RuBisCO.

b. This spatial separation reduces the competition between CO2 and oxygen, thereby minimizing photorespiration and improving carbon fixation efficiency, especially under high-temperature and high-light conditions.

Energy Requirements:

a. The C4 cycle requires additional energy in the form of ATP to drive the C4 acid regeneration reactions in the bundle sheath cells.

b. This ATP is mainly supplied by the light-dependent reactions of photosynthesis occurring in the mesophyll cells.

Efficiency and Adaptation:

a. The C4 cycle is considered an adaptation to environments with high light intensity, high temperatures, and limited water availability.

b. The spatial separation of carbon fixation and the Calvin cycle reduces water loss through closed stomata during hot and dry conditions.

c. The reduced photorespiration and increased efficiency of carbon fixation in C4 plants make them more adapted to arid and tropical climates.

 CAM (Crassulacean Acid Metabolism) cycle

Stomatal Opening and CO2 Fixation at Night:

a. CAM plants open their stomata during the night to take in atmospheric carbon dioxide (CO2).

b. This nocturnal opening of stomata reduces water loss due to lower temperatures, higher humidity, and lower evaporative demand.

c. The CO2 is fixed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) in the mesophyll cells of the leaf.

d. PEP carboxylase has a high affinity for CO2, allowing efficient carbon fixation even at low CO2 concentrations.

Formation of Organic Acids:

a. The fixed CO2 is converted into organic acids, primarily malate or sometimes oxaloacetate, within the mesophyll cells.

b. The conversion of CO2 into organic acids occurs in the presence of PEP carboxylase.

c. These organic acids are then stored in vacuoles within the mesophyll cells until the following day.

Stomatal Closure and Organic Acid Decarboxylation during the Day:

a. During the day, when environmental conditions are hot and dry, CAM plants close their stomata to minimize water loss through transpiration.

b. The closed stomata reduce the entry of CO2 into the leaf.

c. The stored organic acids are decarboxylated (release CO2) within the mesophyll cells.

d. This decarboxylation is catalyzed by the enzyme malic enzyme, resulting in the release of CO2 for use in the Calvin cycle.

Carbon Fixation via the Calvin Cycle:

a. The released CO2 from the decarboxylation of organic acids enters the Calvin cycle within the chloroplasts of the mesophyll cells.

b. The CO2 is fixed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), initiating the Calvin cycle.

c. The fixed CO2 is converted into organic molecules, primarily sugars, which serve as a source of energy and building blocks for the plant.

Energy Requirements:

a. The CAM cycle requires ATP generated during the light-dependent reactions of photosynthesis to drive the various enzymatic reactions involved in carbon fixation, organic acid decarboxylation, and other metabolic processes.

Adaptation to Arid Environments:

a. The CAM cycle is an adaptation to arid or semi-arid environments where water availability is limited.

b. Opening stomata at night allows CAM plants to take up CO2 while minimizing water loss during the hotter and drier daytime conditions.

c. Stomatal closure during the day reduces transpiration, conserving water.

d. The storage of organic acids during the night and their subsequent decarboxylation during the day provide a source of CO2 for the Calvin cycle, allowing photosynthesis to continue even when stomata are closed.

Examples of CAM Plants:

a. CAM photosynthesis is found in various plants, including succulents like cacti, certain species of the Crassulaceae family (e.g., Kalanchoe, Crassula), orchids, bromeliads, and agaves.

b. These plants have evolved the CAM pathway as an adaptation to arid or water-limited environments.