Unit 3: Photosynthesis & Respiration

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Photosynthetic Pigments

Photosynthetic pigments are chemical compounds that absorb light energy and convert it into chemical energy. In plants, these are primarily found in the chloroplasts.

• Chlorophyll a (Chl a): The primary photosynthetic pigment present in all oxygen-evolving photosynthetic organisms. It acts as the reaction center.
• Chlorophyll b (Chl b): An accessory pigment that broadens the spectrum of light that can be used for photosynthesis.
• Xanthophylls: Yellowish accessory pigments that also provide photoprotection to the plant by dissipating excess light energy.
• Carotenes: Orange-colored accessory pigments that assist in light harvesting and protect chlorophyll from photo-oxidation.

Photosystem I and II

Photosystems are functional and structural units involved in photosynthesis that absorb light and transfer electrons.

• Photosystem I (PS I): Contains a reaction center chlorophyll a molecule known as P700, which has an absorption peak at 700 nm. It is primarily involved in the production of NADPH.
• Photosystem II (PS II): Contains a reaction center chlorophyll a molecule known as P680, with an absorption peak at 680 nm. It is responsible for the photolysis of water and the evolution of oxygen.
• Antenna Molecules: These are accessory pigments that capture light energy and funnel it toward the reaction center.
• Reaction Center: The specific site within a photosystem where the energy of a photon is converted into the movement of an electron.

Electron Transport and ATP Synthesis

The movement of electrons through the photosystems creates the energy needed for ATP synthesis.

• Electron Transport Chain (ETC): Electrons move from PS II to PS I through a series of carriers (Plastoquinone, Cytochrome b6f, Plastocyanin).
• Mechanism of ATP Synthesis: According to the chemiosmotic hypothesis, the movement of electrons leads to the accumulation of protons in the thylakoid lumen. The flow of these protons back into the stroma through the ATP synthase enzyme drives the synthesis of ATP from ADP and inorganic phosphate.

C3, C4, and CAM Pathways

These are different strategies used by plants to fix carbon dioxide from the atmosphere.

Photorespiration

Photorespiration is a process where the enzyme Rubisco binds with oxygen instead of carbon dioxide, leading to a loss of fixed carbon and energy.

• It occurs primarily in C3 plants under high temperature and high oxygen conditions.
• It is considered a "wasteful" process because it produces no ATP or sugar.
• C4 plants have evolved to minimize photorespiration by concentrating CO2 in bundle sheath cells.

Glycolysis, Anaerobic Respiration, and TCA Cycle

Respiration is the process by which plants break down food to release energy.

Glycolysis

Occurs in the cytosol and is common to both aerobic and anaerobic respiration. One molecule of glucose is broken down into two molecules of pyruvic acid, with a net gain of 2 ATP.

Anaerobic Respiration

Occurs in the absence of oxygen. Pyruvate is converted into either ethanol and CO2 (alcoholic fermentation) or lactic acid (lactic acid fermentation).

TCA Cycle (Krebs Cycle)

Occurs in the mitochondrial matrix. Pyruvate enters the cycle after being converted to Acetyl-CoA. It produces CO2, ATP, NADH, and FADH2.

Advanced Respiratory Pathways

• Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane where NADH and FADH2 are oxidized to produce ATP via the electron transport system and ATP synthase.
• Glyoxylate Cycle: A variation of the TCA cycle that allows plants (especially in fatty seeds) to convert fats into carbohydrates.
• Oxidative Pentose Phosphate Pathway (OPPP): An alternative pathway to glycolysis for glucose oxidation that generates NADPH and five-carbon sugars for biosynthetic reactions.

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