Energy, Metabolism, and Enzymes: Study Guide for General Biology

Chapter 6 – Energy, Metabolism, and Enzymes

Introduction

This chapter explores the fundamental principles of energy transformation in biological systems, the nature of metabolic pathways, and the role of enzymes in facilitating cellular reactions. Understanding these concepts is essential for grasping how cells obtain, store, and use energy.

Energy in Biological Systems

• Energy is the capacity to cause change, do work, or move matter against opposing forces. It exists in various forms, such as kinetic and potential energy.

• Kinetic energy is the energy of motion (e.g., diffusion of ions through a channel).

• Potential energy is stored energy due to position or structure (e.g., water held behind a dam, chemical bonds in molecules).

• Example: The energy stored in glucose is potential energy; when glucose is metabolized, this energy is released as kinetic energy and heat.

Thermodynamics and Free Energy

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

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

• Free energy (G): The portion of a system's energy that can perform work at constant temperature and pressure. Symbol: G.

• Change in free energy (ΔG): Determines whether a reaction is spontaneous.

  • For exergonic reactions (energy-releasing), ΔG is negative.

  • For endergonic reactions (energy-requiring), ΔG is positive.

• Equation: $\Delta G = \Delta H - T\Delta S$

  • Where ΔH = change in enthalpy (total energy), T = temperature (Kelvin), ΔS = change in entropy.

Metabolic Pathways

• Anabolic pathways: Build complex molecules from simpler ones (require energy input).

• Catabolic pathways: Break down complex molecules into simpler ones (release energy).

• Example: Cellular respiration is a catabolic pathway; photosynthesis is an anabolic pathway.

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

ATP: The Energy Currency of the Cell

• Adenosine triphosphate (ATP): Main energy carrier in cells.

• Hydrolysis of ATP to ADP and inorganic phosphate (Pi) releases energy: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$ $\Delta G = -7.3\ \text{kcal/mol}$

• Energy coupling: ATP hydrolysis is used to drive endergonic reactions.

Enzymes and Catalysis

• Enzyme: A biological catalyst, usually a protein, that speeds up reactions by lowering the activation energy (Ea).

• Catalyst: A substance that increases the rate of a reaction without being consumed.

• Activation energy (Ea): The initial energy input required to start a reaction.

• Enzymes do not change ΔG; they only lower Ea.

• Induced fit: The enzyme changes shape slightly to fit the substrate more closely, as shown in the enzyme-substrate complex diagram.

• Substrate: The reactant molecule upon which an enzyme acts.

• Cofactors and coenzymes: Non-protein helpers required for enzyme activity. Cofactors are inorganic (e.g., Mg2+), coenzymes are organic (e.g., NAD+).

• Enzyme specificity: Determined by the shape and chemical environment of the active site, which is influenced by protein structure.

Enzyme Regulation

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

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

• Factors affecting enzyme activity: pH, temperature, substrate concentration.

• Enzymes function best at optimal conditions (e.g., pepsin in the stomach at pH 2).

Metabolism and Cellular Respiration

• Fermentation vs. aerobic respiration: Fermentation occurs without oxygen, produces less ATP; aerobic respiration uses oxygen, produces more ATP.

• Redox reactions: Involve transfer of electrons. Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain).

• Electron carriers: NAD+ (oxidized form) and NADH (reduced form) shuttle electrons during cellular respiration.

• Final electron acceptor: In aerobic respiration, oxygen is the final electron acceptor in the electron transport chain (ETC).

• Oxidative phosphorylation: ATP is produced using energy from the ETC and a proton gradient across the mitochondrial membrane.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP, occurs in glycolysis and the citric acid cycle.

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytosol; breaks glucose (6C) into two pyruvate (3C) molecules. Net gain: 2 ATP, 2 NADH.

• Pyruvate oxidation: Converts pyruvate to Acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes glucose oxidation, produces ATP, NADH, FADH2, and CO2.

• Oxidative phosphorylation: Includes ETC and chemiosmosis; produces most ATP.

Summary Table: Comparison of Fermentation and Aerobic Respiration

Process	Oxygen Required?	ATP Yield (per glucose)	Final Electron Acceptor	End Products
Fermentation	No	2	Organic molecule (e.g., pyruvate or acetaldehyde)	Lactic acid or ethanol + CO2
Aerobic Respiration	Yes	~30-32	Oxygen (O2)	CO2 and H2O

Key Equations

• Cellular respiration overall equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

• ATP hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

Additional info:

• Enzyme activity can be regulated by allosteric sites, covalent modification, or changes in gene expression.

• Cells avoid equilibrium by constantly exchanging materials and energy with their environment.

• ATP synthase uses the proton gradient generated by the ETC to synthesize ATP from ADP and Pi (chemiosmosis).