• In the light-independent stage of photosynthesis, carbon dioxide combines with RuBP to form a six-carbon compound, which immediately splits to form two three-carbon molecules
• Plants that do this are known as C3 plants
• In some plants, such as maize and sorghum (tropical grasses), the first compound that is produced in the light-independent stage is a four-carbon molecule
• Plants that do this are known as C4 plants

Adaptations of C4 plants for high rates of carbon fixation at high temperatures

• The enzyme rubisco catalyses the reaction of carbon dioxide with RuBP
• However, under certain conditions, it can also catalyse the reaction of oxygen with RuBP
• This process (known as photorespiration) results in less photosynthesis taking place, as less RuBP is available to combine with carbon dioxide
• This mainly occurs at high temperatures and high light intensities (conditions often found at low altitudes in tropical parts of the world
• Tropical grasses, such as maize and sorghum, have evolved an adaptation to minimise photorespiration – they keep RuBP and rubisco separated from high oxygen concentrations:
  • The cells that contain RuBP and rubisco are arranged around the vascular bundles and are known as bundle sheath cells. They have no direct contact with air inside the leaf
  • Carbon dioxide is absorbed by a different group of cells, known as mesophyll cells, that are in contact with air
  • Inside the mesophyll cells, carbon dioxide is combined with a three-carbon compound known as phosphoenolpyruvate (PEP) to form a four-carbon compound known as oxaloacetate, which is then converted to another four-carbon compound known as malate
  • Malate then enters the bundle sheath cells, where carbon dioxide is removed from the malate molecules and delivered to RuBP by rubisco in the normal way
  • The light-independent reaction then continues in the same way as in a C3 plant

• C4 plants have also evolved an adaptation to growing in hot climates – their enzymes generally have higher optimum temperatures than those of C3 plants