C3, C4 and CAM Photosynthetic Pathways - Class XI

In the C3 cycle (Calvin cycle), the regeneration of 5 molecules of RuBP requires ATP. Specifically, 1 ATP is consumed per RuBP regenerated during the phosphorylation of Ru5P to RuBP, so 5 molecules of RuBP require 5 ATP molecules.

Sodium (Na) is an essential micronutrient specifically required by C4 plants for processes like the regeneration of phosphoenolpyruvate (PEP) through the pyruvate-phosphate dikinase pathway, whereas C3 plants do not require it.

PEPcase is the primary carboxylase in C4 plants. Its activity is sensitive to low temperatures, which can limit the rate of photosynthesis in these plants in cold conditions.

In Kranz anatomy, bundle sheath cells contain large chloroplasts with well-developed grana (or sometimes reduced), while mesophyll cells have smaller chloroplasts with well-developed grana. The statement that mesophyll cells have large chloroplasts and more developed grana than bundle sheath cells is generally false in the context of C4 specialization.

In the Hatch-Slack (C4) cycle, the primary CO2 acceptor is phosphoenolpyruvate (PEP). This reaction is catalyzed by the enzyme PEP carboxylase to form oxaloacetate.

C4 plants are classified into three subgroups based on the specific enzyme used for the decarboxylation of the C4 acid in the bundle sheath cells: NADP-ME, NAD-ME, and PEP-CK types.

C3 plants possess a higher CO2 compensation point, a higher rate of photorespiration, and initially use RuBisCO for carbon fixation via the Calvin cycle. Bundle sheath cells are characteristic of C4 plants, not C3 plants, which lack Kranz anatomy.

C4 plants are more efficient than C3 plants primarily because they virtually eliminate photorespiration. They concentrate CO2 around RuBisCO in bundle sheath cells, preventing the oxygenase activity that wastes fixed carbon.

The family Crassulaceae is famous for containing species that exhibit CAM photosynthesis, but C3 and C4 species are also found within this diverse family.

Both C4 and CAM plants fix CO2 into organic acids like malate via PEP carboxylase. However, while C4 plants separate these steps spatially between mesophyll and bundle sheath cells, CAM plants separate them temporally between day and night.

In carbon fixation 5 molecules of C)@ are combines or fixed with 6 molecules; Ribulose Bisphosphate(RuBP) to make 12 molecules of PGA.

Kranz anatomy is observed in $C _4$ grasses like Maize, Sorghum and Sugarcane. This is to reduce the inefficiencies of RuBisCO that can catalyse the reaction with $O _2$ rather than $CO _2$ resulting in photorespiration than photosynthesis. To overcome this, $CO _2$ is first as a four carbon compound and then made available to RuBisCO, This is a spatial and temporal separation of reactions into bundle sheath cells and a ring of mesophyll cells surrounding the bundle sheath. Sunflower has only $C _3$ cycle of photosynthesis and does not exhibit Kranz anatomy.

According to photosynthetic quantum yield requirements, the reduction of one molecule of CO2 and the evolution of one molecule of O2 typically requires 8 light quanta (photons) distributed across both photosystems in standard conditions.

C4 plants exhibit Kranz anatomy where bundle sheath cells contain large, agranal chloroplasts (lacking grana) to facilitate the Calvin cycle without oxygen interference.

In C$ _3$ plants, three molecules of ribulose monophosphate (RuMP), 5C compound, are phosphorylated by ATP to form three molecules of ribulose bisphosphate (RuBP). RuBP is the first CO$ _2$ acceptor. It absorbs CO$ _2$ in presence of enzyme RuBP-carboxylase to form transitory or additive 6C compound, which is unstable. Three molecules of additive 6C compound immediately split up by hydrolysis into six molecules of 3C compound called 3-phosphoglyceric acid (3-PGA). C$ _3$ plants can do photosynthesis only at high concentrations of CO$ _2$. In  C$ _4$ plants, the CO$ _2$ is not directly absorbed by RuBP in presence of enzyme RuBP carboxylase as this enzyme helps to absorb CO$ _2$ having concentration of 50 ppm or more. However, C$ _4$ plants, due to high temperature and high light intensity, keep their stomata partially closed due to which the CO$ _2$ concentration goes down to 2 ppm. Hence the C$ _4$ plants can conduct photosynthesis at low concentrations of CO$ _2$.

Plants use carbon dioxide during photosynthesis, the process whereby the plant converts the energy from the sun into a chemical carbohydrate molecule.

Plants such as sugarcane and sorghum fix atmospheric carbon dioxide into phosphoenol pyruvic acid to produce oxaloacetic acid( a C$ _4$ acid) in the presence of phosphoenol pyruvate carbooxylase(PEP carboxylase). Such plants are called C$ _4$ plants.

In C$ _4$ plants, the primary product of OAA is mediated by PEP carboxylase, so it is relatively less limited by atmospheric CO$ _2$ levels.

So, the correct answer is 'The primary fixation of CO$ _2$ is mediated via PEP carboxylase'.

Photosynthesis is a process that converts $CO _2$ into organic compounds and starch (carbohydrate) using energy from sunlight. $O _2$ is also released during this process.

C4 plants are highly efficient at capturing CO2 and reach their photosynthetic saturation point at lower concentrations compared to C3 plants, typically around 360 microliters per liter.