#34 Biochemistry Lecture (Photosynthesis) from Kevin Ahern's BB 350





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Published on Jun 5, 2013

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1. Control of LDLs in the bloodstream is the responsibility of liver LDL receptors. In the liver, the amount of LDLs that make it back (after being released as VLDLs) are an indication of how many VLDLs and LDLs the cells used. More LDLs back at the liver indicates less need by cells.
2. People who do not have functional LDL receptors on the liver have a genetic disease called familial hypercholesterolemia.

Highlights Photosynthesis
1. Photosynthesis is a process in plants and some bacteria that use energy from light to synthesize glucose using carbon dioxide and water as starting reagents. It accomplishes this in a process that is divided into light reactions and the dark reactions.
2. Photosynthesis occurs in organelles called chloroplasts. The thylakoid disks of the chloroplast are the sites where the light reactions of photosynthesis and the stroma is the location of the dark reactions.
3. Molecules involved in the capture of light energy are chlorophylls, which contain a porphyrin ring with a magnesium ion at the center (instead of the iron molecule found in hemoglobin).
4. In the light reactions of photosynthesis, 1) electrons are removed from water (producing oxygen); 2) ATP is produced by the process of photophosphorylation as electrons pass through the membrane of the thylakoids; and 3) NADPH is produced from NADP+ in the final reduction reaction.
5. Two photosystems act together to produce the reactions of photosynthesis. In photosystem II, electrons are excited by light. As these electrons are passed to the electron transport chain of the membrane, the ring extracts electrons from water.
6. Movement of electrons through the electron transport chain in the thylakoid membrane causes protons to be pumped INTO the thylakoid. This creates a proton gradient. Protons in the thylakoid move outside through a complex (same general structure as the mushroom-like complex with the same function in mitochondrial membranes). As protons move through, ATP is generated from ADP.
7. Electrons released from photosystem II eventually reach photosystem I where they are excited by sunlight of a different wavelength. These newly excited electrons are passed to ferredoxin and ultimately to NADP+, creating NADPH, the final product of the light reactions.
8. Quinones are molecules that help carry electrons in the thylakoid membrane.
9. The dark reactions of photosynthesis are where the glucose is synthesized. Thus, we can think of the light reactions as the ones where the energy necessary for making glucose is stored up and the dark reactions use energy from the light reactions to store that energy in glucose.
10. The dark reactions of photosynthesis are known as the Calvin cycle in honor of their discoverer, Melvin Calvin.
11. In the dark reactions, carbon dioxide is removed from the atmosphere. Carbon dioxide is first covalently attached to ribulose1,5bisphosphate (Ru1,5BP) to form a six carbon intermediate that breaks down to from two molecules of 3-phosphoglycerate (3-PG) for each molecule of carbon dioxide combined with Ru1,5BP. This reaction is catalyzed by the enzyme called RuBisco.
12. 3-PG is an intermediate in glycolysis and gluconeogenesis. Phosphorylation and reduction of it leads to glyceraldehyde-3-phosphate (G3P).
13. The pathway described to this point is that taken by plants known as C3 plants, by virtue of the fact that the first molecule made after fixation of carbon dioxide has three carbons. Another group of plants, known as C4 plants, fixes carbon dioxide to PEP and forms a four carbon molecule, oxaloacetate. Ultimately, this four carbon molecule is decarboxylated in another part of the plant, releasing carbon dioxide once again. This newly released carbon dioxide enters the Calvin cycle (same reactions here as C3 plants). This unique delivery system allows C4 plants to efficiently deliver CO2 to where it is used and allows them to avoid water loss.
14. Rubisco is a relatively inefficient enzyme. When it encounters molecular oxygen instead of CO2, it catalyzes a reaction that ultimately breaks ribulose 1,5 bisphosphate into 3-PG and glyoxylate.

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