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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.