The pigment molecules that capture energy from the sun are arranged in the thylakoid membranes of the chloroplasts in structurally separate units called photosystems. Hundreds of systems are present in each thylakoid. Each photosystem has a light collecting array of 200–300 molecules, the antenna complex, not unlike a satellite dish, which collects and focuses the photons into the reaction center where energy processing begins. The photons move from antenna molecule to antenna molecule without an energy loss, a resonance energy transfer.
There are two kinds of photosystems, Photosystem I (PsI) and Photosystem II (PsII). In the reaction center of PsI the light-absorbing pigment is a specialized chlorophyll a molecule that absorbs red light of 700 nanometer wavelength most efficiently, hence the designation P700. PsII reaction center chlorophyll a molecules absorb maximally at 680 nanometers and therefore are P680 molecules. Why twosystems? Apparently because one alone can't capture enough energy to power the carbon-fixation reactions and to supply the rest of the energy requirements of plant metabolism. By utilizing chlorophylls and accessory pigments with different absorption spectra and providing different mechanisms to change the light energy to chemical energy, plants meet their energy needs.
PsI and PsII are chemically linked. PsI was discovered and named before PsII was identified. The process, however, starts with PsII, not PsI, and works as follows.
Photosystem II actions. The energy absorbed by a P680 molecule of chlorophyll in the reaction center of PsII converts the molecule to its excited state and raises the energy level of an electron, which moves to an outer orbit and is lost from the molecule. An acceptor molecule in the adjacent electron transport system—a chain of alternately oxidized and reduced compounds—accepts it, and immediately moves it down the transport chain, losing some energy in each transfer. The P680 molecule, having lost an electron, is unstable; it returns to its stable ground state by drawing an electron from a water molecule using an as yet not clearly understood mechanism ofphotolysis. The P680 is then ready to accept another photon, and the process is repeated over and over.
The energy lost in the electron chain transfers is used to produce ATP (adenosine triphosphate), the molecule universally used by organisms as a quick energy source. To do so, proton pumps in the thylakoid membrane are activated by the high-energy electrons. They pump protons (H+) into the intermembrane space creating a (H+) gradient across the thylakoid membrane. As protons diffuse back down the gradient into the stroma (the space in the chloroplast), they pass through pores lined withATPsynthase, the enzyme that catalyzes the formation of ATP from ADP (adenosine diphosphate). In the passage, a phosphate group (Pi) is added to ADP forming a terminal high energy bond and ATP. The general term for this process isphotophosphorylation ( photo meaning light energy; phosphorylation indicating a phosphate group is added to an organic molecule).
The energy that reaches the end of the electron transport chain is transferred to the P700 chlorophyll in the reaction center of PsI where it replaces the electrons lost by P700 to the PsI electron transport chain.
Photosystem I actions. PsI is structurally similar to PsII and does part of the same things: It captures light energy (at a slightly different wavelength, 700 nm); transfers electrons down an electron chain; and captures the lost energy and uses it for short-term energy release and to power the carbon-fixing reactions in the next step of photosynthesis.
The enzymes used and products made differ in the two systems. In PsI the primary acceptor molecule of the electron chain is a special kind of chlorophyll a (A0). The energy transfers are extremely rapid, on the order of picoseconds (10−12) to femtoseconds (10−15), and almost 100 percent of the energy is captured in the transfer from P700 to A0. At least 3 more transfers occur in rapid succession before the vital transfer is made to the coenzyme, NADP+ (nicotinamide adenine dinucleotide phosphate). With this transfer, NADP+ acquires a negative charge and attracts a proton (H+) thus forming NADPH (reduced nicotinamide adenine dinucleotide phosphate). NADPH is important in cellular metabolism because it has great reducing ability: it readily releases its hydrogen atom, which drives various chemical reactions. At this point in photosynthesis, NADPH supplies the H+ that eventually reduces CO2 to a carbohydrate.
Role of water. The photo-oxidation ( photolysis) of water is a key reaction and the primary source of the earth's oxygen. The mechanism is not completely understood, but this much is clear: Electrons lost from the reaction center of PsII are replaced by electrons removed from water in a stepwise process. The P680 molecule removes electrons one at a time from an oxygen-evolving complex. When four have been removed from two water molecules, oxygen (O2) is released together with 4 protons (H+) and 4 electrons ( e−). The protons are added to the reservoir in the space between the thylakoid membranes and function in the proton pumps of ATP synthesis. Some of the oxygen is used in plant cell respiration; the rest is released to the atmosphere.
Cyclic and noncyclic photophosphorylation. The passage of electrons from water (PsII PsI NADP+) is a noncyclic electron flow—electrons that reach NADP+ are not returned to water. This passage is a phosphorylation process because ATP is generated along the way by the addition of a phosphate group (Pi) to ADP. Since solar energy runs the process, this is also a noncyclic photophosphorylation.
Cyclic photophosphorylation also occurs in PsI in a process that does not make NADPH, split water, or produce oxygen. Its sole end product is ATP (making it a photophosphorylation) and the electrons cycle, without involving PsII or NADP+. Thus, this is a cyclic photo-phosphorylation and also a cyclic electron flow. Since NADPH is not produced there is no reducing power for carbon fixation. Cyclic photophosphorylation seems designed for compilation of energy only. It apparently was the type of photosynthesis used by primitive organisms and is the method used today by some bacteria. Eukaryotes apparently retain the cyclic process as an evolutionary artifact.
Photorespiration, a type of respiration, occurs in many plants in bright light. It differs from cellular respiration, which occurs in the mitochondria, because it does not release energy, although it does use O2 and release CO2. Photorespiration occurs at the same time that photosynthesis is operating and uses some of the newly made carbohydrate for energy, thereby reducing the yield of photosynthesis by as much as 50 percent in some plants and conditions. The enzyme rubisco is responsible for this seeming anomaly of efficiency.
Rubisco doesn't recognize CO2 specifically enough. When O2 is abundant in the chloroplast (as it is when photosynthesis is operating) rubisco accepts the O2 rather than CO2 and catalyzes a series of different reactions, resulting in carbon being released and energy being expended for no net energy gain. The structure of the leaves adds to the problem: The stomata, which regulate both CO2 and O2 gas exchanges, regularly close during periods of high light and temperature. While water is thereby conserved, gas exchange is impeded; entrance of CO2 into the leaf is stopped as is the outward flow of O2, slowing photosynthesis, but adding more O2 in leaf tissues. Photorespiration apparently is an evolutionary vestige left over from the time when the atmosphere contained little oxygen; molecular changes in the structure of rubisco have not occurred.
Some plants, notably grasses of tropical origin, have developed a means of subverting photorespiration by using a method called the Hatch-Slack or C4 Pathway , named for its discoverers and its first product, oxaloacetic acid, a 4-carbon organic acid. A structural modification of the leaves accompanies the biochemical differences in this system: The leaf veins are surrounded by a cylinder of enlarged bundle sheath cellsaround which there usually is another layer of modified mesophyll cells. In cross-section view the two appear as rings of modified tissue, which gives the term Kranz anatomy(from the German word for wreath) to the leaf condition. The bundle sheath cells, in addition, contain functional chloroplasts unlike the sheath cells of C3 plants. The first part of the C4 pathway takes place in the mesophyll cells, the second part—which is an ordinary Calvin cycle—takes place in the bundle sheath cells.
The CO2 diffusing into the leaf reacts in the cytoplasm of the mesophyll cells withphosphoenolpyruvate (PEP) and its enzyme PEP carboxylase. The oxaloacetateformed is then reduced by NADPH to malate some of which is shunted into the bundle sheath cells. (Malate also can be converted to aspartate, an amino acid.) Malate is decarboxylated in the bundle sheath cells to pyruvate and CO2. This CO2 now enters the Calvin cycle where, as usual, it becomes fixed first as PGAL and from there either to sucrose or to starch. The pyruvate goes back to the mesophyll cells, reacts with ATP, and synthesizes more PEP.
C4 photosynthesis has both benefits and drawbacks.
C4 plants aren't adversely affected by the normal atmospheric (and leaf tissue) ratios of high O2:low CO2 concentrations that favor photorespiration because they produce CO2. Rubisco doesn't switch to O2, and there is no wasteful photorespiration in C4 plants.
C4 plants are better adapted than C3 plants to grow in hot, dry sites since they preserve cellular water loss with closed stomata in the heat of the day, but still have sufficient stores of CO2 within the plant to conduct worthwhile photosynthesis.
Photosynthesis in C4 plants, however, requires two to three times more ATP energy to fix a molecule of CO2 than it does in C3 plants because of the extra malate step.
The presence of C4 species in at least 19 different flowering plant families is evidence that the process is advantageous. The method apparently has arisen independently many times as a solution to a vexing environmental problem. Corn and sugar cane are commonly cultivated C4 plants.
Another adaptation of plants for life in arid environments is the CAM photosynthetic pathway. CAM plants use both C3 and C4 pathways, but unlike the C4 plants—which use two different types of cells—the CAM plants use only mesophyll cells and separate the time at which each pathway is run in the same cells.
CAM plants close their stomata during the day, which prevents water from leaving, but also prevents CO2 from entering. They open the stomata at night for gas exchange when humidity is higher and temperatures lower. As CO2 diffuses in, it combines with PEP in the cytoplasm of the mesophyll cells, forming oxaloacetic acid. The oxaloacetic acid is then reduced to malate, which accumulates in the cell vacuoles. This is the C4pathway, run in the dark of night with its product, malate, migrating only to the vacuole of the cell in whose cytoplasm it was made.
With the dawning day and rising temperatures, the stomata close once more, and photosynthesis commences. The malate is now transported back into the cytoplasm and is decarboxylated to pyruvate and CO2. The CO2 enters the chloroplast and is picked up in the Calvin cycle. PGAL and then sucrose or starch are produced.
The CAM plants are successful inhabitants of warm, arid sites and include species of 23 or more flowering plant families as well as a few ferns. Many are fleshy succulents like the stonecrops ( Crassulaceae) for which the C4 type was named, as well as many cacti, agaves, and lilies.