Question

Describe C4 photosynthesis. Why is it important?

Answer 1

C​​​​​​4 carbon fixation or Hatch- Slack pathway is a photosynthetic process some plants. C4 photosynthesis is one three known process of Carbon fixation. C​​​​​​4 refers to the four carbon molecule that is the first product of this type of carbon fixation. The name Hatch- Slack pathway is in honour of Marshall Davidson Hatch and Charles Roger Slack, who elucidated it in Australia.

Importance of C4 photosynthesis : C4 fixation is elaboration of the more common C3 carbon fixation. C4 overcomes the tendency of ' RUBisCO' to wastefully fix oxygen rather than Carbon dioxide in the process of photorespiration ( photo respiraton is a wasteful pathway that occurs when the RUBisCO acts on oxygen rather than Carbon dioxide). This is achieved by ensuring that RUBisCO works in environment where there is a lot of carbon dioxide and very little oxygen. Many plants which live in hot and dry conditions use C4 photosynthesis to enhance the efficiency of RUBisCO and so that they don't have to keep their stomato open so much thus reduce the risk of dehydration. These plants are called ' C4plants'. Examples are Corn, Maize, Sugar cane. The C4 plants posses special antamoy called Kranz anatomy. The vascular bundles are surrounded by two rings of cells the inner ring called bundle sheath cells, contain starch rich choloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. The primary function of Kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby avoiding photorespiration.

C4 photosynthesis Steps:

1) Formation of Oxaloacetic acid: The primary acceptor of Carbon dioxide cycle is Phosphoenol pyurvic acid ( PEP). In mesophyll cells, the atmospheric CO2 first combines with water to form bicarbonate ion in presence of enzyme carbonic anhydrase. The CO​​​2  acceptor, Phosphoenol Pyurvate acid ( PEP), combine with CO2 and forms a 4 carbon acid called Oxaloacetic acid in presence of enzyme PEP carboxylase. The enzyme remain present in large amount of mesophyll cells.

2. Formation of malic acid and aspartic acid:

Oxaloacetic acid is quite unstable and is converted either to malic acid or aspartic acid. The Oxaloacetic acidis reduced to malic acid by using light generated NADHPH + H+ ​​​​​​. This reaction is catalysed by enzyme malic dehydrogenase. The Oxaloacetic acid can be also converted into aspartic acid in presence of enzyme aspartic transaminase.

The malic acid and aspartic acid are then transported to the choloroplasts of the bundle sheath.

3) Formation of Pyurvic acid: In the bundle sheath choloroplasts the malic acid undergoes oxidative decarboxylation to yield pyruvic acid and CO2 in presence of malic enzyme.

The CO2 and NADP+ H+ produced by Oxidative decarboxylation of malic acid enters to Calvin cycle. The Carbon dioxide combines with ribulose diphosphate ( RuDP) to yield 2 molecules of phosphoglyceric acid ( PGA) .

In a few C4 plants the aspartic acid undergoes transmission to form Oxaloacetic acid which is then decarboxylated to Pyurvic acid. The reaction is catalysed by aspartate transminase.

4) Formation of Phosphoenol Pyurvic acid ,( PEP) : The Pyurvic acid is produced by Oxidative decarboxylation is transported back to the mesophyll cells where it is phosphorylated to Phosphoenol Pyurvic acid in presence of enzyme Pyurvate phosphate dikinase. This enzyme is unusual because it splits one molecules of ATP into AMP and Ppi.