Topic 11: PHOTOSYNTHESIS (17-18) OBJECTIVES: 1. Be able to differentiate the functional roles of the light vs. the dark reactions in photosynthesis. 2. Know the difference between absorption spectrum and action spectrum. 3. Understand the process of photoexcitation as relates to the behavior of electrons. 4. Understand the organization of photosystems and reaction centers and how light excitation results in noncyclic electron flow. Understand the role of water in this process and the overall products of noncyclic electron flow which are essential to the dark reactions. 5. Be able to differentiate noncyclic from cyclic electron flow and understand the forces driving flow in particular directions. 6. Know the roles of ATP and NADPH in the standard C3 (Calvin cycle) photosynthesis. 7. What is the role of Rubisco in this pathway? 8. Understand the phenomenon of photorespiration and how alternate modes of carbon dioxide fixation reduce photorespiration and/or water loss. autotrophy- the organism generates its own food; forms of autotrophy (1) photosynthesis- process by which the trapping of light energy is used to produce carbon compounds (2) chemosynthesis- process by which the energy present in the bonds of inorganic compounds (such as hydrogen sulfide, H2S) is used to produce compounds heterotrophy- the organism obtains its food from its environment Photosynthetic organisms- land plants, algae, unicellular protists like Euglena, cyanobacteria and purple sulfur bacteria; to a large extent the fundamental biochemical mechanisms are the same. We will focus on photosynthesis in plants. fig. 10.2- photosynthesis involves gas exchange (inward movement of CO2 and outward movement of O2), light absorption and the biochemical events of energy conversion and the eventual fixation of CO2 into carbohydrate. Typical site of these events is the leaf- stomata- gas exchange valves located on the underside of leaves; formed by cells known as guard cells which increase or decrease size of aperture based on cell volume changes mesophyl- interior portion of the leaf which contains the cells which carry on photosynthesis. chloroplasts- actual site of photosynthesis; highly compartmented organelle consisting of outer & inner membrane, stroma and grana consisting of stacks of thylacoids. 1fig. 10.4- overview of photosynthesis (1) light reactions- light energy is used to produce ATP and reducing power in the form of the reduced coenzyme known as NADPH; H20 is split yielding oxygen. (2) dark reactions (aka Calvin Cycle)- the ATP and NADPH are used to “fix” CO2 into C-C bonds ultimately forming sugar; produced ADP and NADP+ are recycled back to the light reactions. Light reactions fig. 10.9 - dissolved in the thylacoid membranes are pigments known as chlorophyll a and chlorophyll b; each molecule has a chemical structure consisting of a porphyrin ring in which a Mg2+ atom is “coordinated” and a long hydrocarbon tail. Like all pigments these molecules absorb certain colors of light and reflect others. fig. 10.5- visible spectrum; energy content is inversely proportional to wavelength fiig. 10.8 a & b- -10.8 a: absorption spectra for photosynthetic pigments; absorbance is an index of the amount of light that a molecule absorbs. The absorption spectrum shows the relationship between light absorbance and light quality (wavelength); chlorophylls absorb strongly in both the blue and red regions but not in the green region (plants look green because of the reflected green light which was not absorbed; see fig. 10.6) -10.8 b: action spectra; action spectra show the relationship between photosynthetic activity and light wavelength. Highest photosynthetic activity is in the same regions as highest light absorbance. fig. 10.4- overview again How does the absorption of light produce the forces that result in the splitting of water and the formation of ATP and NADPH? Photoexcitation of chlorophyll- fig. 10.10; absorption of a photon of light causes an electron to move to a higher energy level. That is, it moves from the ground state to a higher energy (“excited”) state. When the excited electrons returns to ground state energy is dissipated in the form of heat and/or light (fluorescence). However, when chlorophyll is present in thyalkoids, the molecules are organized into photosystems (photosystem = light gathering “antenna” complex consisting of molecules of chlorophyll a and b and carotenoid pigment). Instead of dissipating the energy by heat/fluorescence, the energy can be passed from one chlorophyll to another (fig. 10.11) until it reaches a molecule of chlorophyll a located in what is known as a reaction center ( reaction center = site where light-driven photosynthesis takes 2place). This molecule of chlorophyll a transfers its excited electron to another molecule known the primary electron acceptor. This is a redox reaction. Fig. 10.12 There are two kinds of photosystems- I and II ; reaction centers differ in terms of primary electron acceptor and associated proteins I- chlorophyll a molecule is known as P700 because it maximally absorbs at 700 nm [red] (its spectral properties are changed by the nature of the proteins it is near) II- chlorophyll a molecule is known as P680 (maximal absorption at 680 nm). ATP and NADPH production by noncyclic electron flow (fig. 10.12). 1. P680 absorbs light energy and excites electron to higher orbital which is transferred to the primary electron acceptor; this renders the chlorophyll a strongly oxidizing agent. 2. water is oxidized by P680 ultimately yielding oxygen (reduces P680) 3. electrons in the primary electron acceptor are passed down an electron transport chain 4. this transfer results in pumping of protons from the stroma into the interior of the thylakoid (fig. 10.15); this creates a proton (pH gradient). 5. when protons flow down their gradient, they pass through an ATP synthase and ATP is produced. This is known as photophosphorylation; in this case noncyclic photphosphorylation. Fig. 10.13- Shows a good mechanical analogy for how non-cyclic works 1. the electron is then transferred to P700 which is highly oxidized due to its loss of electron after light excitation. 2. the electron from P700 passes through a ferredoxin (Fd) and then NADP+ reductase uses the electrons to reduce NADP+ to NADPH. 3. Products of noncyclic electron flow- ATP and NADPH ATP production by cyclic
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