Daniel L. answered • 10/18/19
UC Berkeley Molecular Toxicology graduate, 4th year medical student
Great question.
TLDR;
A) Pyruvate + ATP -> PEP + AMP (catalyzed by PPDK), then PEP + CO2 -> oxaloacetate (catalyzed by PEP carboxylase).
B) Oxaloacetate is converted to malate in the mesophylls (primary layer of photosynthesis) and is transported to the bundle sheath cells in the vascular layer
C) Refer to last paragraph below.
D) Due to the competitive advantage that C4 offers versus C3 in dry and hot climates, the majority of plants utilizing this process are grasses (of the family Poaceae), shrubs and perennial herbs (of the family Amaranthaceae). I tend to think of C4 plants belonging into savannah and prairie biomes.
E) There are 3 instances which cause photorespiration to predominate over the Calvin cycle: too much light (which raises the O2 concentration in chloroplasts), too much heat (which lowers the solubility of CO2 in water (thus lowering its concentration), and too little water (leaves are cooled by evaporation of water, thus leading to the 2nd cause). In these instances, C4 significantly reduces the incidence of photorespiration and greatly increases the efficiency of carbon fixation.
Remember that the structure of plant leaves is composed of: epidermis (the outer layer, with a waxy cuticle), the mesophylls (where the bulk of photosynthesis occurs), and the vascular bundles. Classic photosynthesis has two simultaneous processes: the light reactions, which generate ATP, NADPH and O2 by oxidation of water, and the dark reactions which occur by fixation of CO2 to ribulose 1,5-biphosphate (RuBP), catalyzed by RuBisco. The problem with RuBisco is that it can do other, unsavory things (the scientific term is called enzyme promiscuity) one of which is the fixation of O2 instead of CO2 to RuBP in a process called photorespiration. Photorespiration is bad because it decreases the efficiency of carbon fixation in plants (meaning less glucose being produced). At ambient air and light, Rubisco will fixate about 3-4 CO2 for every one O2. However problems occur when the temperature gets too hot, when there is not enough water, or when the plant is exposed to too much sun. In these settings, O2 concentrations build-up relative to CO2 concentrations in the mesophylls and photorespiration predominates.
Plants that use the C4 cycle stop photorespiration from happening by concentrating CO2 where the Calvin Cycle occurs. In C3 plants, chloroplasts exist only in the mesophylls, but in C4 plants chloroplasts exist in both the mesophylls and the vascular bundles (around bundle sheath cells). In the mesophylls, C4 plants express an enzyme called PEP carboxylase. PEP carboxylase is different from RuBisco in that it almost exclusively fixes CO2. PEP carboxylase, as it name suggests, carboxylates (fixates CO2) onto PEP (Phosphoenolpyruvate), forming oxaloacetate. PEP is formed from pyruvate (catalyzed by pyruvate orthophosphate dikinase, PPDK), I mention this because pyruvate will show up later in the cycle. After forming oxaloacetate, it gets quickly converted to malate, and then transported to the bundle sheath cells (which also contain chloroplasts). In the bundle sheath cells, malate gets converted back to pyruvate through a decarboxylation reaction (catalyzed by malate dehydrogenase) and releases CO2, which then can be used by RuBisco in the Calvin Cycle. Pyruvate is then returned to the mesophylls, regenerating the cycle. Thus the C4 pathway is a cycle that transports CO2 from the upper layer of mesophylls to the lower layer of the vascular bundles. The downside to this pathway is that CO2 will need to be fixed twice to an organic carbon molecule (the first time onto PEP, and the second time onto RuBP in the Calvin cycle), costing 30ATP to fixate one molecule of CO2 versus 18ATP in plants which exclusively do C3.
Hope this helps,
Daniel
