Highlights Lecture #34 Spring 2017

Highlights Photosynthesis

1. Molecules involved in the capture of light energy are known as the chlorophylls. These molecules contain a porphyrin ring (like hemoglobin) with a magnesium ion at the center (instead of the iron molecule found in hemoglobin).

2. 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. NADP+ is therefore the terminal electron carrier in photosynthesis, whereas water is the electron source for the process.

3. There are two photosystems (I and II) that act together to produce the reactions of photosynthesis. Photosystem II is involved in the first set of reactions. Here electrons from the first porphyrin ring complex are excited by light. As these electrons are passed to the electron transport chain of the membrane, the ring extracts electrons from water, creating oxygen.

4. 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 (higher proton concentration in the thylakoid than the stroma). Protons in the thylakoid move outside through a proton translocating ATP synthase (PTAS) complex (same general structure as the mushroom-like complex with the same function in mitochondrial membranes). As protons move through the PTAS, ATP is generated from ADP (photophosphorylation).

5. 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.

6. Quinones are molecules that help carry electrons in the thylakoid membrane (and mitochondrial membrane too).

7. 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 (charging a battery) and the dark reactions as the ones that use energy from the light reactions to store that energy in another form – glucose.

8. The dark reactions of photosynthesis are also known as the Calvin cycle in honor of their discoverer, Melvin Calvin.

9. In the dark reactions, carbon dioxide is removed from the atmosphere in a process called fixation. Carbon dioxide is first covalently attached to ribulose1,5bisphosphate (RuBP) to form a six carbon intermediate that immediately 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 with the acronym RuBisco.

10. 3-PG is an intermediate in glycolysis and gluconeogenesis. Phosphorylation and reduction of it leads to glyceraldehyde-3-phosphate (G3P).

11. You do NOT need to know the individual reactions of the Calvin cycle, except for the first one I described where carbon dioxide is attached to RuBP.

12. 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 (surprise!) 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. It may also allow them to be more efficient in that the rubisco reaction is occurring in the plant at a location where the oxygen concentration is lower than it is where the rubisco reaction occurs in C3 plants.

13. 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. Thus, the glyoxylate cycle in plants makes a lot of sense.

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