Dark Reaction Lab Report

2-Dark Reaction

Three ecological varients that have different carbon dioxide incorporation mechanism have evolved during evolution that are C3, C4 and CAM plants. There is no net carbon dioxide assimilation until carbon dioxide during respiration is fully compensated by photosynthetic carbon dioxide fixation. The supply of carbon dioxide to the chloroplast occurs through diffusion process.

C3 Plants:

In these plants, first carbon dioxide fixation product is phosphoglycerate a three carbon compound. During light reaction, light is captured by (antenna complexes) PS II (P680) after excitation its electrons are ultimately transferred to PSI (P700) through a series of electron carriers. These electrons are then given to reaction centre to produce

RUBP + CO2 3-PGA

Reduction Phase:

It involves two reactions. Previously encountered in glycolysis. In first reaction, phosphorylation of 3-PGA by ATP to form 1,3-bisphosphoglycerate occurs .In second reaction, 1,3- bisphosphoglycerate is reduced to glyceraldehydes-3-phosphate by NADPH. In these two reactions all NADPH and two-third of the ATP are utilized to derive Calvin cycle Both ATP and NADPH activate the chloroplast

Overall 117 genera of 13 families contain C4 mechanism for CO2 fixation, out of these 11 families are dicotyledons are remaining are monocotyledons. Out of 117 genera 11 genera have both C3 and C4 species.

• Anatomical and Physiological Adaptations in C4Plants:

• C4 plants have distinct leaf anatomy. There are few chloroplast in mesophyll cells of C4 plants.Two types of cells i.e bundle sheath cells and mesophyll cells take part in photosynthesis associated with each other by a complicated process so malate is exported from mesophyll to bundle sheath cells and in turn pyruvate is imported. In mesophyll chloroplast PEP-carboxylase is present while in bundle sheath cells ribulose-1,5-bisphosphate carboxylase occurs. PEP-carboxylase has more affinity to combine with CO2 RUBP- carboxylase so C4 plants can utilize CO2 from weak concentration with help of PEP-carboxylase. Thus C4 plants are more efficient than C3 plants for increased photosynthesis under low CO2