Metabolism and Energy Transformations

Overview of Metabolism

Metabolism refers to all chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways, where the product of one reaction serves as the substrate for the next.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

Forms of Energy

• Kinetic energy: Energy of motion.

• Thermal energy: Kinetic energy associated with random movement of atoms or molecules (heat).

• Potential energy: Stored energy due to position or structure.

• Chemical energy: Potential energy available for release in a chemical reaction.

Thermodynamics in Biology

• First Law: Energy cannot be created or destroyed, only transformed (conservation of energy).

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

• Entropy (S): A measure of disorder or randomness.

Free Energy and Reactions

• Free energy (G): The portion of a system’s energy that can perform work.

• Change in free energy () determines whether a process is spontaneous.

• Exergonic reactions: Release energy (), spontaneous.

• Endergonic reactions: Absorb energy (), non-spontaneous.

ATP and Energy Coupling

• ATP (adenosine triphosphate): Main energy currency of the cell.

• Energy coupling: Using exergonic processes to drive endergonic ones, often via ATP hydrolysis.

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, forming a phosphorylated intermediate.

Enzymes and Catalysis

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Active site: Region on the enzyme where the substrate binds.

• Induced fit: Enzyme changes shape to fit the substrate more closely.

• Enzyme activity is affected by temperature, pH, and inhibitors.

• Feedback inhibition: End product of a pathway inhibits an earlier step, regulating the pathway.

Cellular Respiration

Overview and Equation

Cellular respiration is the process by which cells extract energy from glucose to produce ATP. The overall equation is:

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions transfer energy via electron carriers such as NAD+ and FAD.

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytosol; glucose is split into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH.

• Pyruvate Oxidation: Pyruvate is converted to Acetyl CoA, producing NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation: Includes the electron transport chain (ETC) and chemiosmosis, producing most of the ATP.

Electron Transport Chain and Chemiosmosis

The ETC is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, ultimately reducing O2 to H2O. The energy released pumps protons into the intermembrane space, creating a proton gradient used by ATP synthase to generate ATP (chemiosmosis).

Electron Carriers: NAD+ and NADH

• NAD+: Oxidized form, accepts electrons during glycolysis and the citric acid cycle.

• NADH: Reduced form, carries electrons to the ETC.

Fermentation

• Occurs when oxygen is not present.

• Glycolysis is followed by fermentation pathways (e.g., lactic acid or alcohol fermentation) to regenerate NAD+.

• Produces much less ATP than aerobic respiration (2 ATP per glucose).

• Lactate produced in muscles can be transported to the liver for conversion back to glucose.

Photosynthesis

Light Reactions

Light reactions occur in the thylakoid membranes of chloroplasts. They capture solar energy to produce ATP and NADPH, releasing O2 as a byproduct.

• Photosystems: Complexes of chlorophyll and proteins that absorb light (Photosystem II and I).

• Photon: A particle of light energy.

• Accessory pigments expand the range of light absorption.

The Calvin Cycle (Dark Reactions)

• Occurs in the stroma of the chloroplast.

• Uses ATP and NADPH from the light reactions to fix CO2 into organic molecules (e.g., glucose).

• Called "dark reactions" because they do not require light directly.

Photosystems and Electron Flow

• Photosystem II absorbs light, exciting electrons that are passed down an electron transport chain to Photosystem I.

• Photosystem I further energizes electrons, which are used to reduce NADP+ to NADPH.

• The reaction-center complex contains a primary electron acceptor that initiates electron flow.

Wavelengths and Pigments

• Chlorophyll a and b are the main pigments; accessory pigments include carotenoids.

• Photosynthesis is driven primarily by blue and red wavelengths of light.