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Lecture 6

Lecture 6.odt

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Western University
Biology 1002B
Tom Haffie

PRELECTURE 1. Be able to identify a catabolic versus and anabolic pathway. • Catabolic breakdown of more complex organic molecules into simpler substances. It releases energy (either catabolic destruction of glucose, or ATP --> ADP + P) • Anabolic simpler substances are combined to form more complex molecules. . It needs energy (either to create a molecule, like photosynthesis, or ADP + P --> ATP) 2. How is the structure of ATP linked to the fact that its hydrolysis is strongly exergonic? • Making ATP is endergonic. This is because after ATP hydrolysis to form ADP + P, we now are at a lower energy state and for ATP to be formed again it has to be fueled by catabolic pathways, eg respiration. this energy input allows ATP to be formed and thus we see that phosphorylation of ADP requires energy input (endergonic) to form ATP • The key to how ATP stores energy lies in its triphosphate group. These groups are highly negatively CHARGED and they repel on each other relatively strongly. Due to this instability, the molecule is often referred to as a coiled spring. The unstable bonds holding the phosphates together in the ATP molecule have a low activation energy and are, thus, easily broken by hydrolysis. (remember, hydrolysis is the addition of H2O) When these groups break, they transfer a considerable amount of energy. Another way of saying this is that the HYDROLYSIS OF ATP HAS A NEGATIVE DeltaG, and the energy released can be used to perform work. To understand it, just remember this. The synthesis of ATP from ADP + Pi is endergonic and is powered by exergonic cellular reactions. The hydrolysis of ATP to ADP + Pi is exergonic and the energy releases is used to power endergonic cellular functions such as muscle contraction. LECTURE OUTCOMES 1. Definition: b. P680: a protein complex in the thylakoid membrane that uses energy absorbed from sunlight to synthesize NADPH. It also absorbs light optimally at the wavelength of 680 nm. c. P700: a protein complex in the thylakoid membrane that uses energy absorbed from sunlight to synthesize ATP. It also absorbs light optimally at the wavelength of 700 nm d. Chemiosmosis: ability of cells to use the protein-motive force to do work. e. phases of Calvin cycle: Light independent reaction f. Photosystem: is to trap photon and oxidize chlorophyll by donating its electron to primary electron acceptor. g. cyclic electron flow: no PSII involvement, just PSI. No reduction of NADP+ to NADPH. 2. Structure of chloroplast The chloroplast is the organelle where photosynthesis occurs in photosynthetic eukaryotes. The organelle is surrounded by a double membrane. Inside the inner membrane is a complex mix of enzymes and water. This is called stroma and is important as the site of the dark reactions, more properly called the Calvin cycle. Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum and each of the flattened sacs which make up the granum is called a thylakoid. Each thylakoid has a series of photosystems and associated proteins. The photosystems contain chlorophyll and other pigments and all these associated structures in the thylakoid membrane are the site for the light reactions in which light energy is converted to chemical energy needed for the Calvin cycle in the dark reaction. As the light reactions proceed, the inside of the thlyakoid develops a high concentration of hydrogen ions, and this is important for the production of ATP by the chloroplast. 3. Source of electrons and products of electron transport • source of electrons of electron transport: NADPH • products of electron transport: make NADPH and ATP 4. Reaction catalyzed by rubisco • 1 CO2 + 1 RuBP = 2 3-phosphoglycerate (PGA) 5. Difference between carboxylation reaction and oxygenation (photorespiratory) reaction. Carboxylation • CO2 + RuBP = > 2 PGA Oxygenation • O2 + RuBP = > PGA + Phospholycolate (2 C). Phospholycolate- lost as CO2 by respiration Rubisco is the most abundant enzyme, very slow and inefficient at fixing CO2. Rubisco does not have a specific active site for CO2. Thus, O2 also can compete with CO2 for this active site. But now the active site is more specific for CO2 due to natural selection. 6. Implication of photorespiration of growth • carbon gain = growth 10. Mechanism by which Chlamydomonas concentrates carbon dioxide • HCO3- enters plasma membrane causing the plasma to have high concentration of HCO3-. The enzyme carbonic anhydrase helps to convert to CO2 in cytosol. Since CO2 is not charged, it rapidly difffuse
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