Question

Explain Kranz anatomy.

Answer 1

Hint: Photorespiration is also called photosynthetic carbon oxidation cycle. It protects C-3 plants from photo oxidative damage. In C-4 plants, it is avoided.

Complete answer:
Kranz anatomy: (Kranz, meaning wreath or necklace) $C_4$ plants show Kranz anatomy. The mesophyll is rot differentiated into palisade and spongy tissue. It is homogenous. Each vascular bundle is surrounded by a ring or wreath of radially arranged large bundle sheath cells. These cells contain agranal chloroplasts, i.e. chloroplasts are without grana. These chloroplasts are bigger in size, less in number and are with only stroma. The chloroplasts in mesophyll cells contain grana chloroplasts. They are smaller in size, more in number, with abundant grana and very less stroma. Thus, chloroplasts show dimorphism in C-4 plants. The enzyme, PEP carboxylase is present in mesophyll chloroplast and RUDP or RUBP carboxylase (RUBISCO) in bundle sheath chloroplast.

The reactions occurring in $C_4$ pathway are completed in two parts and at two different sites.
> Part-1 (reaction in Mesophyll cells) consists of carboxylation and reduction.
> Part-2 (reaction in Bundle sheath cells) consists of decarboxylation and second $CO_2$ fixation.

Additional information: In certain tropical plants, CO, is not directly absorbed by RUBISCO. Due to low concentration of CO, they follow another pathway. In this alternative pathway, the first stable compound is a 4-C compound, Oxaloacetic acid and hence it is called C4 pathway (or dicarboxylic acid cycle.) The plants like maize, sugarcane, jowar, Amaranthus, etc. have this C4 pathway, hence all such plants are called C4 plants.

H.P.Kortschak reported this alternative method of CO fixation for the first time (in 1965) in sugarcane. In 1970 M.D. Hatch and C.R. Slack outlined the entire series of reactions; hence it is called Hatch and Slack pathway or HSK pathway.

Note: C-4 pathway has evolved in arid plants to maintain efficiency of photosynthesis under adverse conditions. Photorespiration is avoided in C-4 plants.