Question

What is Kranz anatomy? Write a note on photosynthesis of those plants in which this anatomy is found.

Answer 1

$C_4$ plants have Kranz anatomy which is the special structure of leaves where the tissue equivalent to the spongy mesophyll cells is clustered in a ring around the leaf veins, outside the bundle-sheath cells. The bundle-sheath cells contain large chloroplasts whereas the spongy mesophyll cells have a few chloroplasts.

$C_4$ carbon fixation or the Hatch-Slack pathway is a photosynthetic process in $C_4$ plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. $C_4$ refers to the four-carbon molecule that is the first product of this type of carbon fixation.

$C_4$ overcomes photorespiration where the enzyme RuBisCO wastefully fixes oxygen rather than carbon dioxide. This is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen. Carbon dioxide is shuttled from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells, carbon dioxide is released due to decarboxylation. $C_4$ plants use PEP carboxylase to capture more carbon dioxide in the mesophyll cells. PEP carboxylase (three carbons) binds to carbon dioxide to make oxaloacetic acid (OAA). OAA then makes malate (four carbons). Malate enters bundle sheath cells and releases the carbon dioxide. This requires more energy in the form of ATP.

Using this extra energy, $C_4$ plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or carbon dioxide.