(A) There is competition between the Calvin-Benson cycle (photosynthesis) and photorespiration in C3 plants because the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) is involved in both processes. When carbon dioxide is plentiful, the Calvin-Benson cycle converts carbon dioxide and water into carbohydrate (sugar) and oxygen: the energy needed to drive this process comes from the sun via ATP and NADPH molecules. However, when levels of carbon dioxide are low, photorespiration consumes energy and carbon dioxide without producing any carbohydrates. Acronyms are spelt out at the end of the caption: the number of carbon atoms in a molecule of the metabolite is given on the right, and the number of molecules involved in the reaction is on the left. (B) In C3 plants the Calvin-Benson cycle and photorespiration both occur in mesophyll cells, which are next to the pores (shown in yellow) that allow the plant to exchange gases with the atmosphere (and also allow water vapour to escape from the plant). Different organelles within the cells are illustrated in different colours: chloroplasts in green; peroxisome in blue; mitochondria in pink. (C) In C4 plants mesophyll cells absorb atmospheric gases and harness energy from the sun using chlorophyll, while the fixation of carbon dioxide by RuBisCO generally takes place in bundle sheath cells, which have no direct interactions with the atmosphere. Two variations of the C4 cycle are shown: reactions unique to the enzyme NADP-ME are labelled 1, and those unique to enzyme NAD-ME are labelled 2. Acronyms: ALA, ASP, GLY and SER are all amino acids; PEPC (phosphoenolpyruvate carboxylase) is an enzyme; the other metabolites are BPGA (1,3-bisphosphoglycerate), G3P (glyceraldehyde 3-phosphate), GLYC (glycerate), GLYCLT (glycolate), GLX (glyoxylate), MAL (malate), OAA (oxaloacetate), PEP (phosphoenolpyruvate), PG (phosphoglycolate), PGA (3-phosphoglycerate), PYR (pyruvate), R5P (ribulose 5-phosphate) and RuBP (ribulose-1,5-bisphosphate).