3 Photosynthesis - Overview The energy used by most* living cells ultimately comes from the sun, and is captured by plants, algae, or bacteria via photosynthesis. Two Reactions – light dependent reactions capture energy from sunlight use energy to produce ATP and NADPH – Calvin cycle (or light independent reactions) formation of organic molecules
4 Photosynthesis - Overview Anabolic (small molecules combined) Anabolic (small molecules combined) Endergonic (stores energy) Endergonic (stores energy) Carbon dioxide (CO 2 ) requiring process that uses light energy (photons) and water (H 2 O) to produce organic macromolecules (glucose). Carbon dioxide (CO 2 ) requiring process that uses light energy (photons) and water (H 2 O) to produce organic macromolecules (glucose). 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 glucose SUN photons
6 Plants have thick layer of cells (mesophyll) rich in chloroplasts. Flattened thylakoids in the chloroplasts are stacked into columns, grana. Light dependent reactions take place on the thylakoid membranes and generate ATP and NADPH for Calvin cycle, The stroma consists of enzymes that carry out Calvin cycle (light independent reactions)
8 Chloroplasts Internal membranes of chloroplasts, thylakoids, are organized into grana. – Thylakoid membranes house pigments for capturing light and the machinery to produce ATP. clustered together to form a photosystem acts as an antenna, gathering light energy harvested by multiple pigment molecules
10 Light and Reducing Power Light-dependent reactions of photosynthesis use the energy of light to reduce NADP to NADPH and to manufacture ATP. – Reducing power generated by splitting water is used to convert CO 2 into organic matter during carbon fixation.
12 Energy in Photons Energy content of a photon is inversely proportional to the wavelength of light. – Highest intensity photons, at the short- wavelength end of the electromagnetic spectrum, are gamma rays. – Ultraviolet light possesses considerably more energy than visible light. potent force in disrupting DNA
14 Absorption Spectra Photon absorption depends on its wavelength, and the chemical nature of the molecule it hits. – Each molecule has a characteristic absorption spectrum. range and efficiency of photons the molecule is capable of absorbing
15 Pigments Pigments are molecules that absorb light in the visible range. – green plant photosynthesis carotenoids chlorophyll chlorophyll a - main pigment chlorophyll b - accessory pigment
16 Absorption Spectra Fall colors are produced by carotenoids and other accessory pigments. During the spring and summer, chlorophyll in leaves masks the presence of the carotenoids and the other pigments. In the fall when fall arrives, leaves stop making chlorophyll and it no longer present to reflect the green light and leaves the carotenoids to reflect the yellow and orange that the carotenoids and other pigments do not absorb.
17 Absorption Spectra Carotenoids consist of carbon rings linked to chains of alternating single and double bonds. They can absorb photons with a wide range of energies, however they are not as efficient as transferring energy as chlorophyll. They assist in photosynthesis by capturing energy from light composed of wavelengths that are not efficiently absorbed by chlorophyll.
18 Absorption Spectra A typical carotenoid, β-carotene, has 2 carbon rings connected by a chain of 18 carbon atoms with alternating single and double bonds. Splitting a molecule of β-carotene into equal halves produces 2 molecules of vitamin A. Oxidation of vitamin A produces 2 molecules of retinal, the pigment used in vertebrate vision. This explain why carrots enhance vision.
19 Chlorophyll Chlorophylls absorb photons by means of an excitation process. – Photons excite electrons in the pigment’s ring structure, and are channeled away through alternating carbon-bond system. Wavelengths absorbed depend on the available energy levels to which excited electrons can be boosted.
22 Light-Dependent Reaction Stages Primary photoevent – A photon of light is captured – An electron within the pigment is excited Charge separation – Excitation energy is transferred to reaction center (specialized chlorophyll pigment) – Reaction center transfer energy to electron receptor Electron transport – Excited electron is shuttled along a series of electron carrier molecules. – Protons are transported across the membrane Chemiosmosis – Protons flow back through ATP synthase where chemiosmotic synthesis of ATP takes place (just like in aerobic respiration)
23 Photosystems Photosynthesis output increases linearly at low light intensities but lessens at higher intensities. – saturation point Photosystem - network of pigments that channels excitation energy gathered by any of the molecules to the reaction center – reaction center allows photon excitation to move away from chlorophylls and is the key conversion of light to chemical energy
24 Light to Chemical Energy Figure 10.11 – Converting light to chemical energy. The reaction center chlorophyll donates a light-energized electron to the primary electron acceptor, reducing it. The oxidized chlorophyll then fills the “hole” by oxidizing a donor molecule.
25 Photosystem Function Bacteria use a single photosystem. – electron is joined with a proton to make hydrogen – electron is recycled to chlorophyll
26 Photosystem Function Plants use two photosystems – photosystem I and II generate power to reduce NADP + to NADPH with enough left over to make ATP two stage process: photosystem II – I. noncyclic photophosphorylation ejected electrons end up in NADPH
32 Calvin Cycle Also referred to as C 3 photosynthesis – The key step in the Calvin cycle – the event that makes the reduction of CO 2 possible – is the attachment of CO 2 to a organic molecule RuBP – C 3 plants - ribulose 1,5-bisphosphate (RuBP) is carboxylated to form a three-carbon compound via rubisco activity – CO 2 binds to RuBP in a key process call carbon fixation forming 2 3-carbon molecules of 3- phosphoglycerate (PGA) – The enzyme that carries out this reaction, ribose biphosphate carboxylase/oxygenase (rubisco), is a very large, four-subunit enzyme present in the stroma.
37 Photorespiration In photorespiration, O 2 is incorporated into RuBP, which undergoes additional reactions that release CO 2. – decreased yields of photosynthesis – Rubisco has a second enzyme activity that interferes with the Calvin cycle, oxidizing RuBP. – This essentially undoes the Calvin cycle.
38 Photorespiration – Temperature effects the rate of carboxylation. – At 25˚ C the rate of carboxylation is 4 times greater then the rate of oxidation. – As temperature increass the rate of carboxylation decreases. – In hot arid conditions stoma close the leaf to conserve water. – The closing of the stoma cuts off the supply of CO 2 and does not allow O 2 produced to exit. – These conditions favor photorespiration
39 Photorespiration – Plants that use only the C 3 Photosynthesis (the Calvin Cycle) are called C 3 plants. – Other plants use C 4 photosynthesis in which phosphoenolpyruvate (PEP) is carboxylated to form a 4 carbon compound using the enzyme PEP carboxylase. – This enzyme has no oxidation activity and thus no photorespiration.
40 C 4 Pathway Plants adapted to warmer environments deal with the loss of CO 2 in two ways: – C 4 conducted in mesophyll cells, Calvin cycle in bundle sheath cells creates high local levels of CO 2 to favor carboxylation reaction of rubisco isolates CO 2 production spatially
42 Crassulacean Acid Metabolism (CAM) CAM is another strategy to decrease photorespiration in hot regions. – Used by succulents (cacti and pineapple) CAM plants open stomata during the night, and close them during the day to cut-down the loss of water vapor. – this opening of stomata is reverse of most plants – in the day high levels of CO 2 drive the Calvin cycle and minimize photorespiration – like C 4 plants, CAM plants utilize both C 4 and C 3 pathways. they differ from C 4 plants because they use the C 4 pathway at night and the C 3 pathway during the day in the same cells. – in C 4 plants the two pathways occur in different cells isolates CO 2 production temporalily
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