Carboxylase-oxygenase activity of Rubisco:

Rubisco is characterised by the fact that its active site can bind to both CO2 and O2. RuBisCO has a much greater affinity for CO2 when the CO2: O2 is nearly equal. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme. The oxygenase activity occurs at:

a. High O2 and low CO2 conditions

b. High temperatures.

c. High light intensity

d. Low Co2/O2 ratio

Photorespiration

In C3 plants some O2 does bind to RuBisCO, thus decreasing the photosynthetic efficiency. Photorespiration involves the coordination between three organelles: chloroplast peroxisome and mitochondria.

Series of events during photorespiration:

Chloroplast: RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon). Phosphoglycolate is further converted into glycolate through a series of reactions.

Peroxisome: Glycolate is trasnsported to the Peroxisomes where it gets converted into glyoxylate and then into Glycine and Hydrogen peroxide (H2O2) is generated as a byproduct.

Mitochondria: Glycine is further transported to Mitochondria where it under a series of reactions to produce another amino acid serine. During this conversion ammonia and carbon dioxide are released and energy is generated through respiration. ATP and NADPH are used in consumed thereby reducing the overall efficiency of energy production. Serine is further converted back into Glyceric acid that can be used in calvin cycle for carbon fixation. Briefly, serine enters peroxisome where it gets converted int hydropyruvic acid and then to glyceric acid. Glyceric acid is transported to Chloroplasts where it is converted to 3phosphoglyceric acid and can be used in calvin cycle for carbon fixation.

Disadvantages of photorespiration:

Photorespiration is a wasteful process and decrease the photosynthetic efficiency. There is neither synthesis of sugars, nor of ATP and NADPH. Rather, it results in the release of CO2 with the utilisation of ATP.

The C4 Pathway (Hatch and Slack pathway)

Plants that are adapted to dry tropical regions have the C4 pathway. it is an adaptation that allow these plants to overcome the in efficiences of the Calvin cycle under high temperatures and intense light conditions.

Some unique features of C4 plants: Kranz anatomy C4 plants exhibit a unique leaf anatomy (Kranz anatomy) where the mesophyll cells and bundle sheath cells are arranged in concentric manner around the vascular bundle.The outer layer of the cells consists of mesophyll cells and the inner layer consists of the wreath of bundle sheath cells. another feature of C4 plants is that the leaves show dimorphic chloroplast. Agranal and large chloroplast are present in bundle sheath cells whereas granal and small choloroplast are present in mesophyll cells.Rubisco enzyme is present in Bundle sheath cells and (PEP carboxylase) PEPcase is present in mesophyll cells.

Carbon dioxide fixation: It is known as c4 cycle as the first stable compound is a four carbon compound. The initial acceptor of CO2 is a three carbon compound, Phosphoenol Pyruvate (PEP), that combine with CO2 in the presence of the enzyme PEP carboxylase or PEPcase to form a four carbon compound Oxaloacetic acid (OAA).

Transport to bundle sheath cells: OAA is further converted into another four carbon compound, malic acid in the presence of the enzyme Malic dehydrogense or aspartic acid, in the presence of the enzyme transaminase, in the mesophyll cells itself. Both malic acid and aspartic acid are then transported to the bundle sheath cells through plasmodesmata.

Decarboxylation: In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule, pyruvate. malic acid is converted into pyruvate in the presence of malic enzyme. Transport back to mesophyll cells: Pyruvate is transported back to the mesophyll cells

Regeneration: pyruvate is converted to PEP again, thus, completing the cycle. The CO2 released in the bundle sheath cells enters the C3 or the Calvin pathway. The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase. Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4 plants.