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Chapter 8: An Introduction to Metabolism

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Thermodynamics and Biological Processes

• Laws of Thermodynamics:

  • First Law: Energy can be transferred and transformed but cannot be created or destroyed.

  • Second Law: Energy transformations increase the entropy (disorder) of the universe.

Metabolism Overview

• Definition: The totality of an organism's chemical reactions. Metabolism arises from the orderly interactions between molecules.

• Metabolic Pathways: Specific molecules are altered through a series of steps, each catalyzed by specific enzymes.

Types of Metabolic Pathways

Catabolic Pathways

• Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration breaks down glucose).

Anabolic Pathways

• Consume energy to build complex molecules from simpler ones (e.g., protein synthesis from amino acids).

Energy Flow in Organisms

• Catabolic pathways are “downhill” reactions, providing energy for “uphill” anabolic reactions.

• Bioenergetics: The study of energy flow in living organisms.

Forms of Energy

Kinetic Energy

• Associated with motion; e.g., water gushing through a dam.

Thermal Energy

• Kinetic energy related to the random movement of atoms; transferred as heat.

Potential Energy

• Energy due to location or structure; e.g., water behind a dam or glucose molecules.

Chemical Energy

• Potential energy available for release in chemical reactions. Complex molecules like glucose are rich in chemical energy.

Energy Transformation

• Energy can be converted from one form to another, such as when chemical energy from food is converted to kinetic energy of muscle movement.

Key Laws of Energy Transformation

Thermodynamics

• Isolated Systems: Cannot exchange energy or matter with surroundings.

• Open Systems: Can exchange energy and matter; organisms are open systems and absorb energy from their environments.

Thermodynamic Principles

First Law of Thermodynamics

• The energy of the universe is constant; the principle of conservation of energy.

Second Law of Thermodynamics

• Every energy transfer increases entropy; living organisms increase disorder in surroundings through metabolism.

Biological Order and Disorder

• Cells create ordered structures from less organized materials.

• This order is balanced by catabolic processes that release heat and small molecules.

• Evolution of complex organisms does not violate the second law; localized decreases in entropy can occur as long as overall entropy increases.

Free-Energy Changes in Reactions

• Gibbs Free Energy (G): Portion of a system's energy that can work under uniform temperature and pressure.

  • G = H - TS (where T is temperature in Kelvin).

• Spontaneity: Reactions with negative G are spontaneous; reactions with zero or positive G are nonspontaneous.

Equilibrium in Metabolism

• Reactions in closed systems eventually reach equilibrium and can perform no work.

• Metabolic reactions in living cells are reversible and do not reach equilibrium, allowing for continual work.

ATP and Cellular Work

• ATP (adenosine triphosphate): Powers cellular work by coupling exergonic reactions to endergonic reactions.

  • Types of Cellular Work:

    • Chemical Work: Driving endergonic reactions.

    • Transport Work: Pumping substances across membranes.

    • Mechanical Work: Movement and contractions in cells.

• Cells use ATP hydrolysis to drive endergonic reactions, often through phosphorylation.

Enzymes in Metabolism

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

• Substrate Specificity: Enzymes bind to substrates, forming enzyme-substrate complexes to convert substrates to products.

Regulation of Enzymatic Activity

• Allosteric Regulation: Regulatory molecules bind to enzymes at sites other than the active site, modifying enzyme activity.

• Feedback Inhibition: End products of metabolic pathways inhibit their own production, preventing waste.

Localization of Enzymes

• Enzymes may be localized within compartments in eukaryotic cells to ensure effective metabolic pathways.