Section 5: Metabolism

Introduction to Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism, enabling it to transform matter and energy to sustain life. These reactions are organized into metabolic pathways, which are sequences of enzymatically catalyzed steps.

• Fuel: Organic molecules (such as glucose) serve as fuel for metabolic processes.

• Energy: Metabolism provides the energy required for cellular activities.

• Work: Cells use energy to perform work, including chemical, transport, and mechanical tasks.

• Biochemistry: The study of the chemical processes underlying metabolism.

Key Concepts in Metabolism

• Free-energy change (ΔG): Determines whether a reaction occurs spontaneously.

• ATP: Powers cellular work by coupling exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

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

• Catabolic pathways: Yield energy by oxidizing organic fuels (e.g., cellular respiration and fermentation).

• Fermentation and anaerobic respiration: Enable cells to produce ATP without oxygen.

Spontaneous Reactions and Free Energy

Free Energy (ΔG) and Spontaneity

The free energy change (ΔG) of a reaction indicates whether it can occur spontaneously. Only reactions with a negative ΔG are spontaneous, meaning they can proceed without an input of energy.

• ΔG is negative if:

  • ΔH (enthalpy) is negative: The system releases heat (gives up enthalpy).

  • ΔS (entropy) is positive: The system increases in disorder (gives up order).

  • Both ΔH negative and ΔS positive.

• Every spontaneous process decreases the system's free energy.

• Spontaneous processes can be harnessed to do cellular work.

Equation:

Definitions:

• ΔG: Change in free energy

• ΔH: Change in enthalpy (total energy)

• T: Temperature in Kelvin

• ΔS: Change in entropy (disorder)

Energy and Metabolism

Exergonic and Endergonic Reactions

Metabolic reactions can be classified based on their energy changes:

Exergonic Reactions

• Proceed with a net release of free energy.

• ΔG is negative; reaction is spontaneous.

• The magnitude of ΔG indicates the maximum amount of work the reaction can perform.

• Example: Cellular respiration:

  • kJ/mol

Endergonic Reactions

• Absorb free energy from their surroundings.

• ΔG is positive; reaction is not spontaneous.

• The magnitude of ΔG is the amount of energy required to drive the reaction.

• Reactions are exergonic in one direction and endergonic in the other.

• Example: Photosynthesis (requires energy input from sunlight):

  • kJ/mol

ATP and Energy Coupling

ATP Powers Cellular Work

ATP (adenosine triphosphate) is the primary energy currency of the cell. It powers cellular work by coupling exergonic reactions (which release energy) to endergonic reactions (which require energy).

• Three types of cellular work:

  • Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules).

  • Transport work: Pumping substances across membranes against their concentration gradients.

  • Mechanical work: Movement (e.g., muscle contraction, movement of vesicles).

• Energy coupling: The use of an exergonic process to drive an endergonic one, often mediated by ATP.

Structure and Hydrolysis of ATP

• ATP consists of a ribose sugar, adenine (a nitrogenous base), and three phosphate groups.

• The bonds between phosphate groups are high-energy bonds due to repulsion between negatively charged phosphates.

• Hydrolysis of ATP (breaking the terminal phosphate bond) releases energy:

  • kJ/mol (standard conditions)

• The released energy is used to drive endergonic reactions in the cell.

The ATP Cycle

• ATP is regenerated by phosphorylation of ADP, using energy from exergonic reactions (e.g., cellular respiration).

• This cycle couples the cell's endergonic and exergonic reactions, maintaining a supply of ATP for cellular work.

Enzymes and Metabolic Reactions

Enzymes as Biological Catalysts

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed in the process. They lower the activation energy (Ea) required for reactions to proceed.

• Enzyme names often end in -ase or -zyme (e.g., sucrase).

• Enzymes are highly specific for their substrates (the reactants they act on).

• They do not change the ΔG of a reaction, only the rate at which equilibrium is reached.

Activation Energy and the Transition State

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

• Enzymes lower Ea, allowing reactions to occur at biological temperatures.

• The transition state is an unstable intermediate state during the reaction.

Enzyme Specificity and Mechanism

• Active site: The region of the enzyme where the substrate binds.

• Induced fit: The enzyme changes shape slightly to fit the substrate more closely, enhancing its catalytic activity.

• The enzyme-substrate complex undergoes a catalytic cycle, converting substrates to products.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for activity. Higher temperatures increase reaction rate up to a point, after which the enzyme may denature.

• pH: Each enzyme has an optimal pH. For example, pepsin (a stomach enzyme) works best at pH 2, while most enzymes function optimally at pH 6-8.

Cellular Respiration and Fermentation

Catabolic Pathways and ATP Production

Catabolic pathways break down complex molecules, releasing energy that is used to synthesize ATP. The main catabolic pathways are cellular respiration and fermentation.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic respiration: Uses oxygen as the final electron acceptor; most efficient pathway.

• Anaerobic respiration: Uses substances other than oxygen as the final electron acceptor.

Redox Reactions in Metabolism

• Redox (oxidation-reduction) reactions: Involve the transfer of electrons from one molecule to another.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Reducing agent: Donates electrons and becomes oxidized.

• Oxidizing agent: Accepts electrons and becomes reduced.

• Example:

Electron Carriers and the Electron Transport Chain

• Electrons from glucose are transferred to NAD+, forming NADH.

• NADH carries electrons to the electron transport chain, a series of proteins in the mitochondrial membrane.

• As electrons move down the chain, energy is released and used to produce ATP.

Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytosol; breaks glucose into two pyruvate molecules, producing ATP and NADH.

2. Pyruvate oxidation and the citric acid cycle: Pyruvate is oxidized to acetyl CoA, which enters the citric acid cycle; more ATP, NADH, and FADH2 are produced.

3. Oxidative phosphorylation: Electron transport chain and chemiosmosis generate most of the cell's ATP.

ATP Synthesis Mechanisms

• Oxidative phosphorylation: Accounts for about 90% of ATP production; uses the electron transport chain and chemiosmosis.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from an intermediate substrate; occurs in glycolysis and the citric acid cycle.

Fermentation and Anaerobic Respiration

• Anaerobic respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).

• Fermentation: Extension of glycolysis; regenerates NAD+ by transferring electrons from NADH to pyruvate or its derivatives.

• Types of fermentation:

  • Alcohol fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.

  • Lactic acid fermentation: Pyruvate is reduced to lactate; NAD+ is regenerated. Occurs in muscle cells during strenuous activity.

Comparison of Fermentation and Cellular Respiration

Types of Organisms Based on Oxygen Use

• Obligate anaerobes: Only use fermentation or anaerobic respiration; oxygen is toxic.

• Facultative anaerobes: Can use either fermentation or aerobic respiration (e.g., yeast, E. coli, muscle cells).

• Obligate aerobes: Require oxygen for respiration (e.g., brain cells).

Example:

During strenuous exercise, muscle cells switch to lactic acid fermentation when oxygen is scarce, producing lactate and regenerating NAD+ to allow glycolysis to continue.