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10 Chapter Photosynthesis I. Photosynthesis when sunlight is used to manufacture carbohydrate a. Autotrophs organisms that make all of their own food from ions and simple molecules. i.e. plants b. Heterotrophs non-photosynthetic organisms such as humans and fungi and the bacterium E. coli who obtain the sugars and other macromolecules they need from other organisms c. CO2 + 2H20 + light energy = (CH2O)n + H20 + O2, energy from light is transformed to chemical energy in the C-H bonds of carbohydrates. d. Van Neil's purple sulfur bacteria experiment proves that CO2 and H20 do not combine directly and that O2 is not released from photosynthesis. i. O2 comes from H20 e. Calvin cycle the light-independent component of photosynthesis] i. Reduce carbon dioxide and result in the production of sugar ii. Takes place in the stroma of chloroplasts f. Electrons released during the formation of O2 are transferred to NADP+ and form NADPH and ATP. g. The reactions during the Calvin Cycle use the electrons from NADPH and potential energy from ATP to reduce CO2 to carbohydrate. II. Chloroplast a. Thylakoids vesicle-like structures which often contain interconnected stacks called grana, found on the interior of the chloroplast b. Lumen the space inside of thylakoid c. Stroma fluid filled space between the thyladkoids and the inner membrane. d. Thylakoid membranes are abundant in pigments i. Pigments are molecules that absorb only certain wavelengths of light other wavelengths are transmitted or reflected. ii. Chlorophyll is the most abundant pigment found in the thylakoid membrane, it absorbs blue and red light and reflects green light. e. Proplastids the colorless organelles from which chloroplasts are derived i. Plastids are a family of double membrane-bound organelles found in plants. 1. 3 major types of plastids a. Chloroplasts i. Responsible for photosynthesis b. Leucoplasts i. Often function are energy storehouses c. Chromoplasts i. Are brightly colored because they synthesize and hoard large amounts of orange, yellow or red pigments in their vacuoles. f. Electromagnetic energy in the form of sunlight is converted into chemical energy in the C-H bonds of sugar g. h. i. j. k. l. m. n. o. p. i. Wavelength the distance between two successive wave crests, determines the type of electromagnetic radiation. Shorter wave lengths contain more energy ii. Electromagnetic spectrum the range of wavelengths 1. visible light electromagnetic radiation that humans can see 400 to 710 nm iii. photons light exists in discrete packets When a photon strikes an object the photon may be absorbed, transmitted, or reflected. A pigment absorbs particular wavelengths of light. Reflected wavelengths are what we see Paper chromatography a technique used to separate pigments extracted from a leaf. i. Used to find that blue and red photons are the most effective at driving photosynthesis, chlorophyll a and b Accessory pigments, carotenoids absorb light and pass the energy on to chlorophyll i. Carotenes ii. Xanthophylls Carotenoids serve a protective function, plants without they lose their chlorophyll, turn white and die, and they do this by stabilizing the free radicals created when high energy photons knock electrons out of atoms. Flavonoids absorb ultraviolet radiation Chlorophyll molecules have two fundamental parts i. A long tail made up of isoprene subunits, the tail keeps the molecule embedded in the thylakoid membrane ii. A head that consists of a large ring structure with a magnesium atom in the middle, the head is where light is absorbed. Photons energy is transferred to an electron in the chlorophylls head If the difference between the possible energy states is the same as the energy in the photon, then the photon can be absorbed and an electron is excited to that energy state. Fluorescence if an excited electron falls back to its ground state, some of the absorbed energy is released as heat, while the rest is released as electromagnetic radiation. Photosystem In a thylakoid membrane, 200-300 chlorophyll molecules and accessory pigments such as carotenoids are grouped together in an array of proteins. Each photosystem has two major elements i. Antenna complex photo strikes pigment molecule which excites an electron, this energy is passed along to a nearby chlorophyll molecule, where another electron is excited in response. ii. Reaction center at the reaction center excited electrons are transferred to a molecule that acts as an electron acceptor. When the molecule becomes reduced, the electromagnetic energy is transformed to chemical energy. In the absence of light this process is endergonic, but when light excites electrons in chlorophyll to a high-energy state, the reactions become exogonic. III. IV. V. q. Enhancement effect when both wavelengths are present, the photosynthetic rate was much more than the sum of the rates produced by each wavelength independently. r. The enhancement effect occurs because photosynthesis is much more efficient when both photosystem are working s. together. Pheophytin structurally similar to chlorophyll but lacks a magnesium atom in its head region. When an electron in the reaction center chlorophyll is excited energetically, the electron binds to pheophytin and the reaction center chlorophyll is oxidized. i. Electrons that reach pheophytin are passed to an electron transport chain (similar to one in mitochondria) in the thylakoid membrane. ii. Plastoquinone (PQ) is lipid soluble and not anchored to a protein; therefore it is free to move from one side of the thylakoid membrane to the other. It receives electrons from pheophytin and carries them to the other side of the membrane and delivers them to more electronegative molecules in the chain. iii. PQ carries protons to the lumen side of the thylakoid membrane, creating a large proton gradient that will drive H+ out of the lumen into the stroma, which drives the production of ATP. iv. The flow of protons through the enzyme ATP synthase cause conformational changes that drive the phosphorylation of ADP. t. Photophosphorylation the light energy captured by chlorophyll in photosystem II is transformed to chemical energy stored in ATP. u. The electrons that enter photosystem II come from water. i. 2H20 + 4H+ + 4e- + O2 v. When electrons reach the end of photosystem IIs electron transport chain, they are passed to a small diffusible protein called plastocyanin which diffuses through the lumen and donates the electron to photosystem I. w. When the reaction center in photosystem I absorbs a photon, excited electrons are passed through a series of iron and sulfur containing proteins inside the photosystem, and then to a molecule called ferredoxin. The electrons then move to the enzyme ferredoxin/NADP+ oxidoreductase which transfer two electrons and a proton to form NADPH, which is an electron carrier that can donate electrons to other compounds and thus reduce them. The Z Scheme: Photosystems I and II Work Together Cyclic Photophosphorylation a. Photosystem I transfers electrons back to the electron transport chain, to produce extra ATP. This is used to reduce CO2 and produce sugars. Calvin Cycle a. Fixation phase CO2 reacts with ribulose bisphosphate (RuBP). This phase fixes carbon dioxide by attaching it to a more complex molecule. It also leads to the production of two molecules of 3-phosphoglycerate. Carbon fixation is the addition of carbon dioxide to an organic compound, putting CO2 into a biologically useful form. VI. VII. VIII. IX. i. Rubisco is the CO2 fixing enzyme, utilized 8 active sites where CO2 is fixed it is found in all photosynthetic organisms that use the Calvin Cycle to fix carbon, the most abundant enzyme on earth. 1. It is maladaptive a trait that reduces the fitness of individuals, it is slow and O2 competes with CO2 for the enzymes active sites. 2. photorespiration consumes oxygen and produces CO2 b. Reduction phase 3-phosphoglycerate is phosphorylated by ATP and then reduced by electrons from NADPH. The product is the phosphorylated sugar glyceraldehydes-3-phosphate (G3P). Some of the resulting G3P is drawn off to manufacture glucose and fructose, which are linked to form the disaccharide sucrose. c. Regeneration phase the rest of G3P keeps the cycle going by serving as the substrate for the third phase in the cycle: reactions that result in the regeneration of RUBP. How CO2 enters leaf a. Stoma - paired cells called guard cells border a pore i. Open stomata allows CO2 from the atmosphere to diffuse into the air filled spaces inside the leaf eventually moving into the chloroplasts of the cells. In some species CO2 fixation produces four-carbon sugars. (C4 photosynthesis) a. Three carbon compounds are to CO2 by PEP carboxylase. i. PEP carboxylase is common in mesophyll cells near the surface of leaves, while Rubisco is found in bundle-sheath cells that surround the vascular tissue in the interior of the leaf. 1. Vascular tissue conducts water and nutrients in plants. ii. PEP carboxylase fixes CO2 in mesophyll cells iii. The four-carbon organic acids that result travel to bundle sheath cells iv. The four-carbon organic acids release a CO2 molecule that Rubisco uses as a substrate to form 3-phosphoglycerate which initiates the Calvin cycle. v. Pathway is an adaptation that keeps CO2 concentrations in leaves high by acting as a CO2 pump, limiting the effects of photorespiration. CAM plants crassulacean acid metabolism a. CAM is a CO2 pump that acts an a preparatory step to the Calvin cycle b. During the night CAM plants open their stomata to take in CO2 G3P involves a series of other phosphorylated three carbon sugars, including the synthesis of glucose, and ends with the production of sucrose, or glucose molecules polymerize to form starch. Starch production occurs inside the chloroplast; sucrose synthesis takes place in the cytosol. a. When photosynthesis is slow sucrose is made, sucrose is water soluble and thus can be transported to other parts of the plant b. When photosynthesis is rapid glucose is used to synthesize starch ... View Full Document

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