Answer: cytoplasm
in C4 plants where is carbon dioxide fixed?

C4 carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants when supplied with CO 2 incorporate the C label into four-carbon molecules first. C4 fixation is an addition to the ancestral and more common C3 carbon fixation. The main carb…

C4 carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants when supplied with CO 2 incorporate the C label into four-carbon molecules first. C4 fixation is an addition to the ancestral and more common C3 carbon fixation. The main carboxylating enzyme in C3 photosynthesis is called RuBisCO and catalyses two distinct reactions with CO 2 (carboxylation) and with oxygen (oxygenation) which gives rise to the wasteful process of photorespiration. C4 photosynthesis reduces photorespiration by concentrating CO 2 around RuBisCO. To ensure that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen C4 leaves generally differentiate two partially isolated compartments called mesophyll cells and bundle-sheath cells. CO 2 is initially fixed in the mesophyll cells by the enzyme PEP carboxylase which reacts the three carbon phosphoenolpyruvate (PEP) with CO 2 to form the four carbon oxaloacetic acid (OAA). OAA can be chemically reduced to malate or transaminated to aspartate. These diffuse to the bundle sheath cells where they are decarboxylated creating a CO 2-rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate (PYR) together with about half of the phosphoglycerate (PGA) produced by Rubisco diffuse back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP). This exchange of metabolites is essential for C4 photosynthesis to work. On the one hand these additional steps require more energy in the form of ATP used to regenerate PEP. On the other concentrating CO 2 all...

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