Plants have evolved by developing mechanisms that promote entry of CO, into the Calvin Cycle (CC) to limit photorespiration when environmental conditions tend to favor the latter. This is because photorespiration 1) is energetically inefficient (consuming more ATP than the CC alone) and 2) it does not produce any sugars that can be "fed" into cellular respiration to produce lots more ATP. A) Under what environmental conditions does photorespiration "outcompete" that of the CC? Specifically, what mechanism is responsible for committing the cell to photorespiration and what type of plant does not have any "safeguards" to ensure that the CC is performed rather than photorespiration? B) Describe the mechanisms employed by two types of plants to minimize photorespiration and ensure that the CC is carried out even when environmental conditions are not in their favor. Be sure to identify 1) the type of plant, 2) its natural environment and how it affects the plant's ability to perform the CC, and 3) how it has adapted itself to minimize photorespiration.

A Dark reaction also called the Calvin cycle involves the reduction and fixation of carbon in plants. The series of biochemical reaction reduces CO2 into three carbon compounds (glyceraldehyde 3 phosphate). Calvin cycle takes place in stroma of chloroplast in eukaryotic organicism. In the prokaryotic cell it takes place in the cytosol. 3 molecules of CO2 are required for the synthesis of 3C compound. The key enzyme in this process is, RuBP oxygenase-carboxylase (rubisco). It attaches to the CO2 to follow the reduction process. Rubisco has a dual function. It reacts with both O2 and CO2. This side reaction initiates a pathway called photorespiration. a) under low CO2 concentration the rubisco enzyme readily attaches to the O2 molecules inducing oxygenation of RuBp hereby producing one molecule of 3 phosphoglycerate and one molecule of 2-phosphoglycolate. The by products formed cannot be used by the Calvin cycle hence the glycolate pathway is initiated. The pathway involves 3 subcellular compartments, chloroplast, peroxisomes and mitochondria functioning together to convert 2-phosphoglycolate into glycine followed by decarboxylation process to form serine, CO2 and NH3. Serine is then converted into 3-phosphoglycerate that enters the Calvin cycle. The release of CO2 and combustion of energy decreases the photosynthetic output. The C3 plants(terrestrial plants) have no special features to combat photorespiration. b) the two types of plants that are adapted to combat photorespiration are C4 plants and succulent plants The C4 plants include both dicots and monocots evolved to minimize the photorespiraton by maintaining high CO2 concentration by the help of two distinct photosynthetic cells, bundle sheath sand mesophyll cells. The increased CO2 concentration will keep the rubisco enzyme hooked hereby increasing the photosynthetic output. This wreath like two layered arrangement of chloroplast bearing cells surrounding vascular bundles is called kranz anatomy. Succulent plants such as cactus that are xerophytic dominantly found in arid or semi arid regions have evolved to prevent photorespiration by crassulacean acid metabolism(CAM). These plants open their stomata at night and close during day to minimize the loss of water and CO2 but it also prevents the entry of CO2. During night CO2 is converted into malic acid and stored in vacuoles. In day time when the light reaction provides ATP and NADPH CO2 is released from malate and fixed through Calvin cycle.