A form of energy coupling in which light energy is absorbed by special pigments (colored chemicals) and then this energy is used either to generate H+ ion gradients across membranes, and/or to raise ATP and NADH to their high energy forms. for example: Light energy + NAD --> NADH In addition to plants, many procaryotes have photosynthesis. This includes all Blue-Green Algae, ("Cyanobacteria") but also several (other) kinds of bacteria One interesting group of bacteria use rhodopsin as their photosynthetic pigment. (Rhodopsin is completely different from chlorophyll) Many procaryotes use chlorophyll, as do chloroplasts. "rhodopsin" is a protein bound to a certain chemical related to vitamin A. Rhodopsin is very similar in chemical structure to a protein which humans and other vertebrates use in our eye cells to detect light. The name of this human protein is also rhodopsin! Bacterial rhodopsin is purple, and the human version is often called "visual purple", even when it isn't quite that color! These bacteria often make whole lakes purple. (good examples are the salt-making lagoons south and east of the San Francisco airport!) Absorption of light energy by bacterial rhodopsin causes hydrogen ions to be pumped outward through the bacteria's plasma membranes. Then ATP synthase uses the energy of these hydrogen ions diffusing back inward... etc. to make ATP from ADP and phosphate! Experimental biologists sometimes transfer bacterial rhodopsin to other kinds of membranes, in order to generate H+ gradients, and be able to control these gradients by light illumination! In the eye, light absorption by rhodopsin also creates gradients of ion concentrations across plasma membranes, but for the purpose of controlling nerve signals, instead of to get energy. (Oddly enough, light inhibits signals; so the more signals initiated, the darker the cells "think" it is. Then the signals to the brain are based on relative brightness of light in adjacent parts of the retina!! We will cover this later.) Another side issue: How chemical structures are related to whether a given chemical will absorb light. (alternating double and single bonds, continuing a long way, especially when there are ionized or ionizable groups at the ends) Being a ring or not has nothing to do with this, despite what the textbook implies on page 140. If, in addition, this structure is rigid (can't bend) then the chemical will probably be fluorescent. (rigidity DOES often result from having lots of rings) Think about an electric field flipping all those double bonds over to the single bond side, with this flipping-over being how the light gets absorbed! (and also how it gets re-emitted) In plants, the chlorophyll is located in the chloroplasts! (although there are often special kind of chloroplasts that don't make chlorophyll! These are used for other purposes. Have I mentioned that all chloroplasts are produced by division of pre-existing chloroplasts?? Likewise mitochondria! So what Virchow said about cells is also true of these organelles! Chlorophyll is green because it absorbs both blue and red wave-lengths of light (but does not absorb much green light). There are (slightly) different kinds of chlorophyll, with slightly different absorption spectra (=graphs of relative amounts of light absorbed at different wave-lengths of light). "chlorophyll a" "chlorophyll b" "etc." Incidentally, chlorophyll is very fluorescent, because it is good at holding and transferring the absorbed energy. (And this fluorescence can be a royal pain for experimenters!) Chlorophyll can transfer the absorbed light energy, from molecule to molecule of chlorophyll, and sometimes from certain other pigments to chlorophyll, also. (as in red & brown algae) The light energy absorbed by chlorophyll is used to force several different chemical reactions to occur. > "Splitting" of H2O water molecules to release O2 (oxygen gas) > Driving ADP + P -> ATP and NAD ->NADH (conversion of energy-carrying molecules to high energy forms) > Pumping Hydrogen ions into the thylakoids; (innermost membrane sacks) so these ions can drive ATP synthase as they leak back out. In separate reactions, that can occur in the dark, energy from ATP and NADPH are used to capture ("fix") CO2 into sugars. Note that people often think about photosynthesis as making oxygen out of carbon dioxide, but really the oxygen released comes from water; and the CO2 goes into making sugar. To make things more complicated, Plants turn out to have two "photosystems" I & II ("Photosystem one" and "Photosystem two"); These each have their own slightly different sets of enzymes and chlorophylls; They absorb light at rather different wave-length maxima; (causing photosynthesis to be much more efficient when the light includes a combination of BOTH these wave-lengths) The energy from one gets added to that of the other (the Z pattern, etc.) First system two acts, and then system one! (Because they are numbered by which got discovered first!) These two systems evolved separately, in different ancestral groups, and there are bacteria that just have one or the other. System II: 680 nanometers light absorption maximum. Pumps hydrogen ions (like in mitochondria) Oxidizes water (splits it) to release Oxygen gas + H+ Molecules on inner face of grana = thylakoid membranes (pheophytin = chlorophyll lacking its Mg++ ion) System I: 700 nanometers light absorption maximum. Makes NADH from NAD Molecules on outer face of grana = thylakoid membranes What about the part of photosynthesis in which carbon dioxide is fixed and sugars are synthesized? These are called the "dark reactions" because they make no direct use of light, and can occur in the dark (& can also occur in light) The energy comes from ATP and NADH (which got made in the "light reactions", but which could have come from anywhere! If a plant had other sources of energy, then it could fix carbon using that energy, just as well) Ribose + CO2 -> 6 Carbon sugar -->two 3 Carbon chemicals (Notice how often ribose shows up in key biochemical reactions!) Really, what reacts with the carbon dioxide is phos-Ribose-phos etc. (Notice how often ribose phosphates show up in key biochemical reactions!) The enzyme that catalyses this reaction is "Rubisco" (ribulosebiphosphatetranscarbolase .... just learn rubisco!) It used to be called "rudi-P-kase", which I liked better There is more rubisco on earth than any other protein, because it is so important, and because its reaction is slow ( ~ 3 or 4 reactions per second per enzyme molecule) the 3-carbon sugar-like chemicals that are produced then go through a series of enzymatic reactions that generate another ribose (5-C sugar) so as to repeat the process called "the Calvin Cycle" named after the great Swiss Theologian, once dictator of Geneva. Really, it was discovered by a an American biologist in the late 1940s, using radioactively labeled carbon in carbon dioxide, and using paper chromatography to find out which chemicals became radioactive in plants taking up carbon-14. There are some nice illustrations in the textbook showing how this was done; which you don't need to know. The Calvin cycle also produces one 6 carbon sugar for every 6 carbon dioxides "fixed" from the air. Some plants have enzymes that fix CO2 into 4 carbon chemicals (in addition to having the Calvin cycle; NOT instead of regular 6-carbon photosynthesis, although some books give that impression) These "4C plants" live in hot dry places, and these reactions allow them to capture CO2 even at very low concentrations, and without losing too much water. Questions that you should be able to answer: 1) Do any forms of photosynthesis not use chlorophyll? 2) Does human vision depend on any chemicals that are sometimes used in photosynthesis? 3) Are the cyanobacteria the only procaryotes that are photosynthetic? 4) Do gradients of ion concentrations play any role in photosynthesis? What roles? 5) Does chemiosmosis play any role in photosynthesis? 6) What aspects of chemical structure are related to light absorption (in organic chemicals)? *7) If a chemical reaction caused a certain double bond to be converted to a single bond, would that make the chemical absorb more light? Or would it shift the absorption peak? Or make the chemical absorb less light? Why would this depend on whether this double bond was immediately next to other double bonds, or one atom away from other double bonds? 8) In what sense is what Virchow said about cells also true of certain organelles? Why? 9) What is believed to be the evolutionary origin of chloroplasts? What are some of the reasons for believing this? **10) Some organisms that have long been believed to be one-celled algae have turned out to be protozoan animals that have symbiotic algae (eucaryotic algae, that is) living in their cytoplasm; So does this mean that their chloroplasts are not descended from eucaryotes, or what? In other species of algae, these symbiotic algae were themselves animals that had symbiotic algae, with the same evolutionary sequence having been repeated up to 4 times! 11) Is chlorophyll green because it absorbs green light, or what? **If a chemical absorbed only green light what color would it be? 12) What is meant by chlorophyll a, chlorophyll b, etc.? 13) Why is it not surprising that chlorophyll is fluorescent? In terms of its normal function? *In terms of its molecular structures? Questions 14 through 20) What is the immediate source of energy that drives each of the following events in photosynthesis: 14) Formation of oxygen gas? 15) Fixing of carbon from carbon dioxide? 16) Pumping hydrogen ions across the innermost membranes of chloroplasts? 17) Synthesis of ATP? 18) Reduction of NAD to form NADH? 19) Fusion of ribose-diphosphate with carbon dioxide? 20) The "dark reactions"? 21) What sort of chemical is "rubisco"? What is its function? Why would it be lethal for a plant to have a mutation that prevented its formation? *22) If there were any kinds of plants that could survive without it, then what would need to be different about those plants? *23) Why do plants contain so much of this chemical? (combination of two facts) 24) In general terms, what is meant by saying that plants have two photosystems? 25) Why can plants make much more sugar when illuminated with a combination of 680 nm light and 700 nm light, more efficiently than when only one or the other wave-length is used. 26) What is the Calvin cycle? How is rubisco related to this cycle? *27) Would you expect C-4 plants to contain rubisco, or not? **28) How did discovery of the Calvin cycle depend on the Manhattan project, indirectly? (you can guess) 29) Do any organisms have just one or other of the photosystems? 30) What is the source of the oxygen gas that plants release when they absorb carbon dioxide and make sugar using its carbon? **31) Experimentally, how was that proven? Until then, what did most people assume must be the source of that oxygen?

 

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