Question

Diagram and discuss C4 photosynthetic modifications. Include selective advantages they confer. Also, how and when they confer an advantage. Please, thank you.

Answer 1

C4 photosynthetic modifications are shown by C4 plants as they have a special type of leaf anatomy .They can tolerate high temperature .They show a response to high intensity of heat and they lack a wasteful process called photorespiration .Thus C4 plants show greater productivity and higher yield as compared to C3 plant

The C4 pathway requires the presence of two types of cells i,e. mesophyll cells and bundle sheath cells .The particularlarge cells around the vascular bundle of C4 plants are called bundle sheath cells .These cells may form several layers around the vascular bundle,they are characterised by having large number of chloroplast, and grana are absent ,thich walls impervious to gaseous exchange and no intercellular spaces .This special anatomy of leaves of the C4 plants is called the 'kranz anatomy '.'Kranz ' means wreath and is the reflection of the arrangement of cells .

So,the C4 photosynthetic modifications  plants follows the Hatch-Slack pathway .It is a cyclic process .The primary CO2 acceptor is a three carbon molecule phosphophenol pyruvate (PEP) and it is present in mesophyll cells .The enzyme that catalyses this CO2 fixation is PEP carboxylase or PEPcase .The mesophyll cells of C4 plants lack the enzyme RuBisCo .The 4-Carbon oxaloacetic acid (OAA) is formed in the mesophyll cells .It is then converted to other four-carbon compunds like malic acid or aspartic acid in the mesophyll cells itself ,these are then transported to the bundle sheath cells .In this bundle sheath cells ,these C4 acids are broken doen to release CO2 and a three carbon molecule .The CO2 release in the bundle sheath cells enters the C3 or the calvin cycle  .

The bundle sheath cells are rich in an enzyme RuBisCo but lacks PEPcase .The three carbon molecule is transported back to the mesophyll cells where it is converted to PEP again with the help of cold senitive enzyme ,called PEP synthetase, thus completing the cycle .

Thus the basic pathway that results in the formation of the sugars ,the Calvin pathway is common to C3 and C4 plants .There is no net gain or loss of NADPH in C4 cycle .

The digrammatic representation of C4 or Hatch and Slack pathway is

The advantages of C4 plant modifications are :-

a) They can tolerate saline conditions due to abundant occurence of organic acids (malic acid and oxaloacetic acid) in them which lowers the water potential than that of the soil.

b) Concentric arrangements of cells in leaf produces smaller area in relation to volume for better water utilisation .

c) They can perform photosynthesis even when their stomata are closed due to the presence of strong CO2  fixing enzyme i,e. PEPcase .

d) They lack the wasteful process photorespiration in which there is no production of ATP and NADPH .

e)The rate and effeciency of photosynthesis is higher and greater in C4  plant .

C4 plants adapt for living in hot and arid climate .They maintain low oxygen levels in their leaves by keeping the stomata close to prevent water loss .

Example of C4 plants are:- Sugercane,Maize,Sorghum ,Amaranthus etc.