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ARE YOU TRULY READY FOR THE second EXAM?

Ribulose bisphosphate (RuBP) can combing with either carbon dioxide or oxygen gas.  (a) What determines which of these two gases combines with RuBP?  (b) Is there any advantage in having RuBP combine with oxygen rather than with carbon dioxide?

(a)     An enzyme, RuBP carboxylase, catalyzes both the joining of carbon dioxide with RuBP and the joining of oxygen gas with RuBP. The two molecules are alternative substrates that compete with each other for the same active site on the enzyme. The more abundant of the two molecules gets to occupy the active site and subsequently joins with RuBP. When the concentration of carbon dioxide in the stroma of the chloroplast is high compared with the concentration of oxygen, then carbon dioxide joins with RuBP and the Calvin-Benson cycle is initiated. When the concentration of carbon dioxide is low relative to oxygen gas, then oxygen joins with RuBP and a pathway known as phororespiration is initiated. A high concentration of oxygen relative to carbon dioxide occurs whenever the stomata of a leaf remain closed while photosynthesis continues. WHY IS THIS SO???????
(b)     In photorespiration, oxygen gas combines with RuBP to form an unstable molecule that breaks down into glyceraldehyde phosphate and glycolic acid, a two-carbon molecule that is further catabolized into carbon dioxide. Although this is a form of respiration, no ATP is produced and much of the energy within RuBP is lost as heat. Photorespiration has no known BENEFICIAL function and is detrimental to a cell: RuBP is wasted and glucose production by the Calvin-Benson cycle is attenuated.

(a)     How does the C4 pathway differ from the C3 pathway. of carbon fixation? (b) What are the advantages and disadvantages of each pathway?

(a)     The C4 (or Hatch-Slack) pathway differs from the C3 pathway in that carbon dioxide combines first with phosphoenolpyruvic acid (PEP) to form a four-carbon molecule, oxaloacetic acid, whereas in the C3 pathway carbon dioxide combines first with ribulose bisphosphate to form two three-carbon molecules of  phosphoglyceric acid In a C4 plant, this initial fixation of carbon takes place in a a mesophylI cell, and the Benson-Calvin cycle occurs  in a different kind of cell, called a bundle sheath cell.

The enzyme that catalyzes attachment of carbon dioxide to PEP in the C4 pathway-PEP carboxylase-has a strong affinity for carbon dioxide and no affinity for oxygen gas. Thus, carbon fixation takes place, and the Calvin-Benson cycle proceeds rapidly even when carbon dioxide concentration is low because the bundle sheath cells are flooded with carbon dioxide from the mesophyll cells.
(b) A plant with the C4 pathway has a tremendous advantage over a plant with a C3 pathway.  When the weather is hot, arid dry, the leaves close their stomata to eliminate water loss. In a C3 plant this closure results in low carbon dioxide concentrations and, ultimately, both loss of RuBP and inhibition of the Calvin-Benson cycle by photorespiration. Is it clear why these two things happen?

There are no such disadvantages in a C4 plant during hot, dry weather-even with its stomata  are closed carbon dioxide is fixed because of the efficiency of PEP carboxylase in trapping and concentrating carbon dioxide. Consequently, the Calvin-Benson cycle continues to produce glucose and there is no loss of chemical energy through photorespiration.


The C3 pathway has an advantage over the C4 pathway, however, under cooler and wetter condition since then the stomata remain open and there is always sufficient carbon dioxide to combine directly with RuBP and drive the Calvin-Benson cycle. About half as much energy is needed to pump carbon dioxide directly into the Benson-Calvin cycle as is needed to pump it in the roundabout way used in the C4 pathway. Since less energy is expended, each day a C3 plant can use more ATP to synthesize glucose than a C4 plant can under these conditions.

Other than by a detailed biochemical study, how could you distinguish between a C3 and a C4 plant?

There are two distinct kinds of photosythetic cells in a C4 plant: (1) bundle sheath cells which provide the Calvin-Benson cycle and (2) MesophylI cells which provide carbon dioxide fixation through PEP carboxylase.  Plants with the C3 pathway, by contrast. have just a single type of photosynthetic cell: the mesophyll cell. Their bundle sheath cells do not contain chloroplasts.

     An alternative very efficient pathway for carbon fixation occurs in the cells of certain green plants, especially those found in hot, arid climates. The C4 pathway utilizes a number of four-carbon acids as intermediates. The function of this pathway is to pick up C02 at low concentrations in the outer layers of the leaf (the mesophyll) and transport it as a four-carbon intermediate (such as malic acid) to an inner layer of the leaf (the bundle-sheath layer). In the inner layer, the four-carbon intermediate breaks down, releasing the C02 it had picked up in the outer layer. This process delivers the C02 to the cells where it can be used in the Benson-Calvin cycle.
    Mesophyll has a high concentration of the enzymes for the C4 pathway and a low concentration of enzymes for the C3 pathway. Bundle-sheath cells are just the opposite.

The existence of these two systems makes it possible for those desert plants that have the enzymes for the C4 system to photosynthesize at a high rate even with the stomata partially closed, as would happen at rnidday when water loss through evaporation would be at its highest. You need to be able to explain why this is true.  

Give some thought to the light-dependent reactions of photosynthesis and what their basic purpose is and how the plant achieves these goals.
Think about the ways in which a mitochondrium is similar to a chloroplast both FUNCTIONALLY AND STRUCTURALLY and there important differences.
I am not feeling well and am leaving work.  This will have to do for the second exam.