Heterotrophs: Organisms that depend on an external source of organic compounds

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1 Heterotrophs: Organisms that depend on an external source of organic compounds Autotrophs: Organisms capable of surviving on CO2 as their principle carbon source. 2 types: chemoautotrophs and photoautotrophs Chemoautotrophs: Utilize the chemical energy stored in inorganic molecules to convert CO2 into organic compounds. All are prokaryotes Photoautotrophs: Utilize the radiant energy of the sun to convert CO2 into organic compounds. They include plants and eucarytoic algae, protists, and several other prokaryotes Photosynthesis: a process in which energy from sunlight is transformed into chemical energy that is stored in carbohydrates and other organic molecules. During photosynthesis, relatively low-energy electrons are removed from a donor compound and converted into high-energy electrons using the energy absorbed from light. Cyanobacteria if an organism is going to carry out oxygenic (oxygenreleasing) photosynthesis, it has to generate a very strong oxidizing agent as part of its photosynthetic metabolism in order to pull tightly held electrons from water. The switch from H2O Photosynthetic vesicles H2S (or other reduced substrates) to H2O as the electron source for photosynthesis required an overhaul of the photosynthetic machinery. Cynaobacterium transformed into a chloroplast. As the chloroplast evolved, most of the genes that were originally present in the symbiotic cyanobacterium were either lost or transferred to the plant cell nucleus Chloroplasts are located predominantly in the mesophyll cells of leaves. Chloroplasts of higher plants are generally lens-shaped approximately 2 to 4 um wide and 5 to 10 um long, and typically numbering 20 to 40 per cell. chloroplasts arise by fission from preexisting chloroplasts (or their nonpigmented precursors, which are called proplastids). Chloroplasts were identified as the site of photosynthesis in 1881 in an ingenious experiment by the German biologist T. Engelmann

2 The outer covering of a chloroplast consists of an envelope composed of two membranes separated by a narrow space Like the outer membrane of a mitochondrion, the outer membrane of a chloroplast envelope contains several different porins The inner membrane of the envelope is highly impermeable; substances moving through this membrane do so only with the aid of a variety of transporters. Much of the photosynthetic machinery of the chloroplast including light-absorbing pigments, a complex chain of electron carriers, and an ATP-synthesizing apparatus is part of an internal membrane system The internal membrane of the chloroplast, which contains the energy-transducing machinery, is organized into flattened membranous sacs, called thylakoids. Thylakoids are arranged in orderly stacks called grana The space inside a thylakoid sac is the lumen, and the space outside the thylakoid and within the chloroplast envelope is the stroma Like the matrix of a mitochondrion, the stroma of a chloroplast contains small, double-stranded, circular DNA molecules and prokaryotic-like ribosomes. Depending on the organism, chloroplast DNA contains between about 60 and 200 genes involved in either gene expression (e.g., trnas, rrnas, ribosomal proteins) or photosynthesis Photosynthetic Metabolism: C. B. van Niel van Niel proposed an alternate mechanism based on his work with sulfur bacteria. Van Niel recognized that photosynthesis was essentially a process of oxidation reduction The van Niel proposal placed photosynthesis in a different perspective; it became, in essence, the reverse of mitochondrial respiration. Whereas respiration in mitochondria reduces oxygen to water, photosynthesis in chloroplasts oxidizes water to oxygen. The former process releases energy, so the latter process must require energy The events of photosynthesis can be divided into two series of reactions. During the first stage, the light-dependent reactions, energy from sunlight is absorbed and stored as chemical energy in two key biological molecules: ATP and NADPH During the second stage, the light independent reactions (or dark reactions as they are often called), carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP and NADPH molecules produced in the light-dependent reactions

3 Absorption of Light: Light travels in packets (or quanta) of energy called photons, which can be thought of as particles of light. The absorption of light is the first step in any photochemical process. When a photon is absorbed by a molecule, an electron becomes sufficiently energetic to be pushed from an inner to an outer orbital. The molecule is said to have shifted from the ground state to an excited state. Pigments are compounds that appear colored because they only absorb light of particular wavelength(s) within the visible spectrum. Leaves are green because their chloroplasts contain large quantities of the pigment chlorophyll, which absorbs most strongly in the blue and red, leaving the intermediate green wavelengths to be reflected to our eyes. Structure of a chlorophyll: Each molecule consists of two parts: (1) a porphyrin ring that functions in light absorption (2) a hydrophobic phytol chain that keeps the chlorophyll embedded in the photosynthetic membrane. An absorption spectrum is a plot of the intensity of light absorbed relative to its wavelength. The range of wavelengths absorbed by photosynthetic pigments situated within the thylakoids is further increased because the pigments are noncovalently associated with a variety of different polypeptides. Chlorophylls are the primary light-absorbing photosynthetic pigments,but terrestrial plants also contain orange and red accessory pigments called carotenoids Carotenoids absorb light primarily in the blue and green region of the spectrum while reflecting those of the yellow, orange, and red regions. Caratenoids produce the characteristic colors of carrots and oranges, and the leaves of some plants during the fall. Carotenoids have multiple functions: they act as secondary light collectors during photosynthesis, and they draw excess energy away from excited chlorophyll molecules and dissipate it as heat. Action spectrum is a plot of the relative rate (or efficiency) of photosynthesis produced by light of various wavelengths. Unlike an absorption spectrum, which simply measures the wavelengths of light that are absorbed by particular pigments, an action spectrum identifies the wavelengths that are effective in bringing about a given physiologic response. The action spectrum for photosynthesis follows the absorption spectrum of chlorophylls and carotenoids fairly closely, reflecting the participation of these pigments in the photosynthetic process.

4 The light-absorbing reactions of photosynthesis occur in large pigment protein complexes called photosystems. Two types of photosystems are required to catalyze the two lightabsorbing reactions utilized in oxygenic photosynthesis. One photosystem, photosystem II (PSII), boosts electrons from an energy level below that of water to a midway point The other photosystem, photosystem I (PSI), raises electrons from a midway point to an energy level well above that of NADP The two photosystems act in series, that is, one after the other. The reaction center of photosystem II is a chlorophyll dimer referred to as P680, P standing for pigment and 680 standing for the wavelength of light that this particular pair of chlorophylls absorbs most strongly. The reaction center of photosystem I is also a chlorophyll dimer and is referred to as P700 for comparable reasons.

5 Photophosporylsation: The conversion of one mole of CO2 to one mole of carbohydrate requires the input of three moles of ATP and two moles of NADPH The machinery for ATP synthesis in a chloroplast is virtually identical to that of a mitochondrion or a plasma membrane of an aerobic bacterium. As in those cases, the ATP synthase consists of a head (called CF1 in chloroplasts), which contains the catalytic site of the enzyme, and a base (CF0), which spans the membrane and mediates proton movement. The formation of ATP during the process of oxygenic photosynthesis is called noncyclic photophosphorylation because electrons move in a linear (i.e., noncyclic) path from H2O to NADP+ Daniel Arnon of the University of California, Berkeley, discovered that isolated chloroplasts were not only capable of synthesizing ATP from ADP but could do so even in the absence of added CO or NADP+ These experiments indicated that chloroplasts had a means for ATP formation that did not require most of thephotosynthetic reactions that would have led to oxygen production, CO2 fixation, or NADP+ reduction. The process Arnon had discovered was later called cyclic photophosphorylation and is a process that is carried out by PSI independent of PSII. Cyclic electron transport begins with the absorption of a quantum of light by PSI and transfer of a highenergy electron to the primary acceptor.