Additional Major Concept Questions
2. A Z protein, associated with photosystem II and facing the thylakoid lumen, splits water into oxygen, hydrogen ions, and electrons. Two of these electrons are used to replace the missing electrons in chlorophyll P680. Oxygen leaves the chloroplast as a byproduct, and the protons remain in the thylakoid space, adding to the H+ gradient that powers chemiosmosis.
3. The electrons that leave photosystem II pass through the Q cycle, which transports protons from the stroma into the thylakoid lumen, thus creating an H+ gradient for chemiosmosis. Four protons are translocated into the transport chain. The electrons then move through plastocyanin, Pc, and other components of the electron transport chain, eventually replacing the two electrons that were lost by photosystem I when it was struck by photons.
4. The electrons from photosystem I pass through another elecron transport chain containing an iron-containing protein called ferredoxin (Fd). They then move to the enxyme NADP reductase that uses the two electrons and H+ ions from the stroma to reduce NADP+ to NADPH.
5. Protons that accumulate in the thylakoid lumen contribute to an electrochemical gradient that drives the phosphorylation of ADP to ATP. As protons move through the ATPase compolex from the thulakoid lumen into the stroma, ATP is fromed. Since light is reaquired for the establishment of the proton gradient, this process is called photophosphorylation. The ratio of protons translocated to ATP formed by chiosmosis has recently been found to be four H+ per ATP.
CO2 adds to an already existing 5 carbon molecule, ribulose 1,5-bisphosphate, RuBP, to form an unstable 6 carbon intermediate. This intermediate instantly splits into two 3-carbon molecules called 3-phosphoglycerate (PGA). Since the first compound to be produced contains three carbon atoms, the Calvin cycle is also known as C3 photosynthesis. These reactions are caralysed by the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco). Rubisco is a very large enzyme that works very slowly. Typical plant enzymes process about 1000 substrate molecules per second. Rubisco averages about three. Thus, many copies of the enzyme are needed to complete the reactions effficiently. It has been estimated that rubisco makes up about half of all the protein in a typical leaf causeing it to be the most abundant protein on Earth. This reaction is exergonic, owing to the high level of chemical potential energy in RuBP in relation to PGA.
Phase 2: Reduction Reactions
Each of the six molecules of PDA is phosphorylated by an ATP to form six molecules of 1,3-bisphosphoglycerate (1,3-BPG). Next, a pair of electrons from each of six molecules of 1,3-BPG to six molecules of glyceraldehyde 3-phosphate (G3P or PGAL), a sugar. These reactions are essentially the reverse of glycolosis. At this stage, one molecule of G3P exits the cycle as a final product.
Phase 3: RuBP Regeneration
In a series of enzyme-catalyzed reactions, the remaining five molecules of G3P are rearranged to regenerate three molecules of RuBP to complete the cycle. Three molecules of ATP are used in the process. With RuBP regenerated, the cycle may fix more CO2. As the cycle continues, the G3P molecules that leave are used to synthesize larger sugars, such as glucose and other carbohydrates. Three CO2 molecules must be fixed before one 3-carbon G3P molecule can be removed from the cycle in order to maintain the pool of intermediate molecules needed to sustain the cycle. This means that six turns of the cycle fix enough CO2 to produce the equivalent of one 6-carbon glucose molecule.
The overall equation for the Calvin cycle (per G3P produced):
3 RuBP + 3CO2 + 9ATP + 6NADPH + 5H2O ----> 9ADP + 8Pi + 6NADP+ + G3P + 3 RuBP
Thus, for the net synthesis of one G3P molecule, the Calvin cycle uses nine molecules of ATP and six molecules of NADPH. The light reactions are a source of ATP and NADPH. For every four electrons (from H2O) transferred in the light reactions, twelve protons are added to the H+ reservoir in the thylakoid lumen, resulting in th eproduction of three molecules of ATP. Thus, three ATP and two NADPH are produced in the light reactions, and three ATP and two NADPH are used in the Calvin cycle for every G3P produced.
Photosynthesis seems to be self-sufficient for its ATP and NADPH needs. However, the H+/ATP ratio is an approximation, and any mismatch in the production and use of ATP may be compensated for by the production of additional ATP by cyclic electrion flow. The relatibe rates of cyclic and noncyclic electron flow are regulated by the NADPH to NADP+ ratio in the stroma. When the ratio is high, (e.g., during high light intensities) cyclic electron flow is favoured because the availability of NADP+ will be low, slowing down noncyclic flow.
In the C4 pathway, the enzyme PEP carboxylase fixes CO2 by catalyzing the attachment of CO2 to three-carbon PEP, forming four-carbon OAA. This rection occurs in the cytoplasm, not the chloroplast. OAA is converted to malate (OAA and malate are also found in the KRebs cycle). Malate, a four carbon acid, diffuses from the mesophyll cells into bundle-sheath cells through cell-cell connections called plasmodesmata. Here, a carbon dioxide portion is removed from the malate molecule (decarboxylation). This reaction converts 4-carbon malate into 3-carbon pyruvate, which diffuses back into the mesophyll cell where it is converted into PEP (the compound we started with). This reaction is endergonic and uses one ATP. The CO2 that was removed from malate in the bundle-sheath cell enters the C3 Calvin cycle in a second fixation reaction, thistime catalyzed by rubisco in the bundle sheath cell.
This method of carbon fixation reduces the amount of photorespiration that take splace by continually pumping CO2 molecules from the mesophyll cells to the bundle-sheath cells (via malate), where rubisco brings them into the Calvin cycle. The concentration of CO2 in bundle-sheath cells is kept high so that CO2 outcompetes O2 in binding to rubisco. Photorespiration is minimized and sugar production is maximized. However, it costs the plant two ATP molecules to transport a CO2 molecule into a bundle-sheath cell. Since six CO2 molecules are processed by the Calvin cycle to produce the equivalent of one glucose molecule, the cell must expend the energy of 12 additional ATP are used to produce one glucose molecule, whereas, in C4 plants, 30 ATP molecules are used - almost twice as many. Nevertheless, the process is advantageous in hot tropical climates where photorespiration would otherwise convert more than half of the glucose produced back to CO2.
