Question

Explain how photorespiration is avoided in C-4 plants.

Answer 1

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C4 plants like sugarcane have evolved to avoid the wasteful process of photorespiration unlike C3 plants. C4 plants fix carbon dioxide in mesophyll cells and then this fixed carbon in form of 4 carbon compounds namely malate or aspartate is shuttled to bundle sheath cells. 

C4 plants like sorghum, maize, sugarcane possess a special type of leaf anatomy in which two rings of cells are found around each of the vascular bundles. The inner ring, or the bundle sheath cells, contain chloroplasts which differ in form from those in the mesophyll cells in the outer ring. The chloroplast in the plant in therefore described as dimorphic. This anatomy is called Kranz anatomy. C4 plants  having kranz anatomy can tolerate high temperature and high light intensity, show greater productivity of biomass, and perform very less photorespiration.

Steps of C4 cycle 
Step1: PEP (Phosphoenolpyruvate) in mesophyll cells,  acts as the primary carbon dioxide acceptor  and form first stable  product, four carbon compound Oxaloacetic acid with the help of  enzyme, PEPcase.
Step2: A 4-carbon (C4) compound (either malic acid or aspartic acid) is formed in the mesophyll cells from OAA (oxalo acetic acid).
Step 3: C4 compound is transferred to bundle sheath cells.
Step 4: Decarboxylation of C4 compound in bundle sheath cells. The C4 compound which was transported is decarboxylated (CO2 is removed) to form a three carbon compound (C3 acid). CO2 is released inside the bundle sheath cell and is used for normal calvin cycle.
Step 5: Transfer of C3 acid to the mesophyll cells.
Step 6 : Regeneration of PEP from C3 acid in mesophyll cells.
 

Answer 2

The C4 plants have evolved by having special anatomical feature called bundle sheath cell in which CO₂ concentration is kept high so that RuBisCo  binds to CO₂ instead of O₂, thus functioning only as Carboxylase and not Oxygenase.

Explanation:

RuBisCo is a bi-functional enzyme. It catalyzes both the carboxylation and oxygenation of RuBP (Ribulose BisPhosphate). At lower CO₂ concentration, RuBisCo performs oxygenase activity on RuBP which produces one molecule of 3-Phosphoglycerate and one molecule of 2-Phosphoglycolate. 2-Phosphoglycolate cannot be utilized within the Calvin Cycle.

To prevent this loss, C-4 plants have evolved C-4 cycle ( the initial CO₂ fixation product is four carbon acid instead of a three carbon acid in C3 cycle).

The C4 plants have two distinct types of photosynthetic cells: bundle sheath cells and mesophyll cells. The enzyme RuBisCo is present only in Bundle Sheath cells where, using C4 cycle, a high concentration of CO₂ is maintained.

In bundle sheath cells and mesophyll cells, the C4 cycle completes in four stages:

1. Fixation of CO₂ by carboxylation of Phosphoenolpyruvate (PEP) in mesophyll cells to form a C4 acid (malate and/or aspertate).
2. transport of C4 acids to the bundle sheath cells
3. Decarboxylation of the C4 acid within the bundle sheath cell and generation of CO₂, which enters in C3 cycle.
4. Transport of C3 acid (pyruvate) that is formed by the decarboxylation step back to the mesophyll cell.