Photosynthesis - 2

C4 plants possess dimorphic chloroplasts - bundle sheath chloroplasts are larger and lack grana, while mesophyll chloroplasts are smaller with well-developed grana. This Kranz anatomy is an adaptation for efficient carbon fixation. Bulliform cells and parallel venation are general monocot features, while night-opening stomata characterize CAM plants.

RUBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is Earths most abundant protein. It catalyzes carbon fixation in photosynthetic organisms plants, algae, cyanobacteria comprising up to 50% of leaf soluble protein.

Dark reaction is also called Blackman’s reaction.

One hexose (6C) requires 6 Calvin cycles. Each turn fixes 1 CO2 (3C), so 6 turns are needed to build one 6-carbon glucose molecule from 6 CO2 molecules.

Fixing one CO2 in the Calvin cycle consumes 3 ATP and 2 NADPH. This energy powers carbon reduction and RuBP regeneration.

Malic acid is a 4-carbon compound.

Aldolase and transketolase are key enzymes regenerating RuBP. Aldolase forms carbon-carbon bonds; transketolase rearranges sugar phosphates to restore RuBP for continued CO2 fixation.

C4 plants like sorghum and millet use PEP carboxylase to concentrate CO2 at RUBISCO, minimizing photorespiration. This makes them more efficient than C3 plants like wheat and paddy.

PEP carboxylase has no oxygenase activity, so it does not catalyze photorespiration. Unlike RUBISCO, it exclusively fixes CO2 even in high O2 conditions, making C4 photosynthesis more efficient.

C4 plants consume 30 ATP per hexose but generate 18 through the Calvin cycles, resulting in 12 net ATP required. C3 plants need only 18 net ATP.

Six 5C RUBP are regenerated.

The Calvin cycle enzymes are located in the stroma of chloroplasts. This is where carbon fixation and carbohydrate synthesis occur during the dark reactions of photosynthesis. Grana contain chlorophyll for light reactions, while stomata are pores for gas exchange and mitochondria handle cellular respiration.

Cells without chloroplasts cannot perform photosynthesis or produce oxygen through photosynthetic electron transport. They may still consume oxygen through respiration, require water for metabolic processes, and can utilize carbohydrates obtained from other sources. Only photosynthetic cells containing chloroplasts can evolve oxygen.

RuBisCO functions as an oxygenase under high temperature and high oxygen concentration conditions, leading to photorespiration. Under normal conditions, it acts primarily as a carboxylase fixing CO2. Low temperature or low oxygen favor carboxylation, while the combination of high temperature and high O2 shifts its activity toward oxygenation.

Peroxisomes are essential organelles in photorespiration, where they process glycolate produced by RuBisCO oxygenase activity. The photorespiratory pathway involves three organelles: chloroplast (where glycolate is produced), peroxisome (where glycolate is converted to glycine), and mitochondria (where glycine is decarboxylated). Ribosomes, spherosomes, and centrosomes are not involved in this process.

In Crassulacean Acid Metabolism (CAM) plants, CO2 is fixed at night into oxaloacetate, which is then converted to malic acid and stored in vacuoles. Malic acid is the first stable product of this carbon fixation pathway. PGA is the first stable product in C3 plants, while citric and succinic acids are involved in the Krebs cycle, not CAM photosynthesis.

CAM plants open stomata at night to fix CO2 into malic acid, which simultaneously reduces water loss through transpiration, prevents photoinhibition by avoiding excess light absorption during the day, and allows efficient recycling of internally released CO2 during daylight hours. This nocturnal stomatal opening strategy is a key adaptation to arid conditions.

 'Kranz anatomy’ is a special structure in the leaves of plants that have a C₄ pathway of carbon dioxide fixation. The leaves contain a ring of mesophyll cells, containing a few small chloroplasts concerned with the initial fixing of carbon dioxide, surrounding a sheath of parenchyma cells.

C4 plants use phosphoenolpyruvate (PEP) as the primary CO2 acceptor in mesophyll cells, whereas C3 plants use ribulose-1,5-bisphosphate (RuBP) directly in the Calvin cycle. The end products of both pathways are ultimately carbohydrates, and both use similar photosynthetic pigments. The key difference is the initial CO2 fixation mechanism and the spatial separation of carbon fixation in C4 plants.

It takes place in photosynthetic cells

Two carbon fixations separated by time in CAM plants.

DCMU (Diuron) inhibits electron transport from Photosystem II to plastoquinone, which blocks both non-cyclic photophosphorylation (linear electron flow) and oxygen evolution from water splitting. This herbicide specifically targets the photosynthetic electron transport chain. It does not directly inhibit dark reactions or promote photorespiration.

Produced as glucose but transports as sucrose

In bacterial photosynthesis evolution of oxygen does not takes place.