Energy and Metabolism

Exergonic vs. Endergonic Reactions

• Exergonic reactions release energy; products have less free energy than reactants. These reactions are spontaneous (ΔG < 0).

• Endergonic reactions require an input of energy; products have more free energy than reactants. These reactions are non-spontaneous (ΔG > 0).

• Catabolic pathways break down molecules and are generally exergonic (e.g., cellular respiration).

• Anabolic pathways build complex molecules and are generally endergonic (e.g., photosynthesis).

• Entropy (S) is a measure of disorder; the second law of thermodynamics states that entropy of the universe tends to increase.

• Gibbs Free Energy (ΔG) determines whether a reaction is spontaneous:

• Examples: Hydrolysis of ATP is exergonic; synthesis of glucose from CO2 is endergonic.

First and Second Laws of Thermodynamics

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

• Second Law: Every energy transfer increases the entropy of the universe; some energy is lost as heat.

Types of Energy

• Potential energy: Stored energy due to position or structure (e.g., chemical bonds).

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Chemical energy: Potential energy available for release in a chemical reaction (e.g., glucose).

• Examples: Water behind a dam (potential), rolling ball (kinetic), ATP (chemical).

Enzymes and Their Function

How Enzymes Work

• Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• They do not change the ΔG of a reaction or get consumed in the process.

• Enzymes bind substrates at their active site to form an enzyme-substrate complex.

• Enzyme activity can be affected by temperature, pH, and substrate concentration.

Enzyme Kinetics and Regulation

• Increasing enzyme concentration increases reaction rate (if substrate is not limiting).

• Increasing substrate concentration increases rate up to a maximum (saturation point).

• Competitive inhibitors bind to the active site, blocking substrate binding.

• Non-competitive inhibitors bind elsewhere, changing enzyme shape and reducing activity.

• Feedback inhibition occurs when the end product of a pathway inhibits an earlier step, regulating pathway activity.

Enzyme-Catalyzed Reaction Diagram

• With enzyme: Lower activation energy, faster reaction.

• Without enzyme: Higher activation energy, slower reaction.

ATP: The Energy Currency

• ATP (adenosine triphosphate) stores and transfers energy in cells.

• Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate ().

• The energy comes from the repulsion of negatively charged phosphate groups.

• ATP powers cellular work by coupling exergonic and endergonic reactions.

Redox Reactions

Oxidation and Reduction

• Oxidation: Loss of electrons (or hydrogen); molecule becomes more positive.

• Reduction: Gain of electrons (or hydrogen); molecule becomes more negative.

• In cellular respiration and photosynthesis, follow the movement of electrons (often with hydrogen atoms).

• Example: In respiration, glucose is oxidized, oxygen is reduced.

Cellular Respiration

Stages of Cellular Respiration

• Glycolysis: Occurs in cytoplasm; glucose → 2 pyruvate, 2 ATP, 2 NADH.

• Pyruvate oxidation: Mitochondrial matrix; pyruvate → acetyl-CoA, CO2, NADH.

• Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix; acetyl-CoA → CO2, NADH, FADH2, ATP.

• Electron Transport Chain (ETC): Inner mitochondrial membrane; NADH/FADH2 donate electrons, O2 is final electron acceptor, H2O formed, H+ pumped to intermembrane space.

• Chemiosmosis: H+ flows back through ATP synthase, generating ATP.

Electron Carriers

• NAD+ and FAD are reduced to NADH and FADH2 as they accept electrons.

Overall Equation for Aerobic Respiration

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses O2 as final electron acceptor; produces up to ~30-32 ATP per glucose.

• Anaerobic respiration: Uses other molecules (not O2) as final electron acceptor; less ATP produced.

• Fermentation: Occurs when O2 is absent; glycolysis followed by conversion of pyruvate to lactic acid or ethanol; regenerates NAD+.

• Types of fermentation: Lactic acid (muscle cells, some bacteria), alcoholic (yeast).

Role of Oxygen and CO2

• Oxygen is the final electron acceptor in the ETC; forms water.

• CO2 is produced during pyruvate oxidation and the citric acid cycle as a waste product.

Electron Transport Chain and Chemiosmosis

• ETC is located in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 pass through protein complexes, pumping H+ into the intermembrane space.

• Chemiosmosis: H+ flows back through ATP synthase, driving ATP production.

• Similar process occurs in photosynthesis (chloroplast thylakoid membrane).

Photosynthesis

Overall Reaction and Redox

• CO2 is reduced to glucose; H2O is oxidized to O2.

Primary Producers

• Organisms (e.g., plants, algae, cyanobacteria) that produce organic molecules from CO2 using light energy.

Photosynthetic Pigments

• Chlorophyll a is the main pigment; absorbs blue and red light, reflects green.

• Accessory pigments (chlorophyll b, carotenoids) broaden absorption spectrum.

• Absorbed light powers electron flow; reflected light is not used in photosynthesis.

Stages of Photosynthesis

• Light reactions: Occur in thylakoid membranes; convert light energy to ATP and NADPH, produce O2 from H2O.

• Calvin cycle (dark reactions): Occur in stroma; use ATP and NADPH to fix CO2 into glucose.

• H+ gradient is established across the thylakoid membrane (high inside thylakoid space).

Photosystems

• Photosystem II (PSII): Absorbs light, splits H2O to release electrons, O2, and H+; electrons passed to ETC.

• Photosystem I (PSI): Receives electrons from ETC, absorbs light, electrons reduce NADP+ to NADPH.

• Source of electrons: H2O (PSII); destination: NADPH (PSI).

RUBISCO and Carbon Fixation

• RUBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that fixes CO2 in the Calvin cycle.

• It catalyzes the addition of CO2 to ribulose bisphosphate (RuBP).

Photorespiration, C4, and CAM Pathways

• Photorespiration: Occurs when RUBISCO fixes O2 instead of CO2, reducing photosynthetic efficiency.

• C4 plants: Use spatial separation to minimize photorespiration (e.g., maize).

• CAM plants: Use temporal separation (open stomata at night) to conserve water (e.g., cacti).

Comparison Table: Aerobic Respiration vs. Anaerobic Respiration vs. Fermentation

Summary

• Energy flows through biological systems via redox reactions, ATP, and electron carriers.

• Enzymes regulate metabolic pathways, and their activity is tightly controlled.

• Respiration and photosynthesis are linked by the flow of electrons and the production/consumption of ATP and NAD(P)H.

• Adaptations such as C4 and CAM pathways help plants survive in challenging environments.