Chapter 8: An Introduction to Metabolism – Study Notes

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes, and are essential for maintaining life by transforming matter and energy.

Metabolic Pathways

• Metabolic pathway: A series of chemical reactions that convert a starting molecule into a product through enzyme-catalyzed steps.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis from amino acids).

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy in Biological Systems

Energy is the capacity to cause change and exists in various forms relevant to biological processes.

Kinetic and Potential Energy

• Kinetic energy: Energy associated with motion (e.g., moving water turning turbines).

• Thermal energy: Kinetic energy due to random movement of atoms or molecules; transferred as heat.

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

• Chemical energy: A form of potential energy stored in chemical bonds, available for release in chemical reactions (e.g., glucose breakdown).

The Laws of Thermodynamics in Biology

Thermodynamics describes how energy is transformed and transferred in biological systems.

The First Law of Thermodynamics

• Energy can be transferred and transformed, but not created or destroyed (principle of conservation of energy).

• Living organisms are open systems, exchanging energy and matter with their surroundings.

The Second Law of Thermodynamics

• Every energy transfer or transformation increases the entropy (disorder) of the universe.

• Some energy is lost as heat during transformations, becoming unavailable to do work.

• Spontaneous processes increase entropy and do not require energy input; nonspontaneous processes decrease entropy and require energy input.

Biological Order and Disorder

• Cells create order from less organized materials, but overall, the entropy of the universe increases.

• Energy flows into ecosystems as light and exits as heat, maintaining order locally while increasing disorder globally.

Free Energy and Spontaneity of Reactions

Free energy (Gibbs free energy, G) is the portion of a system’s energy available to do work at constant temperature and pressure. The change in free energy (ΔG) determines whether a reaction is spontaneous.

Gibbs Free Energy Equation

• ΔG = ΔH – TΔS

• ΔG: Change in free energy

• ΔH: Change in enthalpy (total energy)

• T: Temperature in Kelvin

• ΔS: Change in entropy

Spontaneous reactions: ΔG < 0 (negative), energetically favorable. Nonspontaneous reactions: ΔG ≥ 0 (zero or positive), require energy input.

Free Energy, Stability, and Equilibrium

• Systems move toward lower free energy (greater stability).

• Equilibrium is the state of maximum stability; no net change occurs.

• Cells are not at equilibrium; they maintain a constant flow of materials to do work.

Exergonic and Endergonic Reactions

• Exergonic reactions: Release free energy (ΔG < 0), spontaneous.

• Endergonic reactions: Absorb free energy (ΔG > 0), nonspontaneous.

Equilibrium and Metabolism

Metabolic reactions in cells are kept out of equilibrium, allowing continuous work and life processes.

• Closed systems reach equilibrium and can do no work.

• Open systems (like cells) maintain a flow of materials, preventing equilibrium.

• Catabolic pathways release free energy in a series of steps, with products of one reaction serving as reactants for the next.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) powers cellular work by coupling exergonic and endergonic reactions.

Structure and Hydrolysis of ATP

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

• Hydrolysis of ATP (removal of a phosphate group) releases energy used to drive cellular work.

• The high energy of ATP is due to the repulsion between negatively charged phosphate groups.

How ATP Performs Work

• ATP hydrolysis powers chemical, transport, and mechanical work in cells.

• Phosphorylation (transfer of a phosphate group) makes molecules more reactive.

• ATP hydrolysis can cause conformational changes in proteins, enabling movement or transport.

The ATP Cycle

• ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions.

• This cycle couples energy-releasing and energy-consuming processes in the cell.

Regulation of Metabolism

Cells regulate metabolic pathways to maintain efficiency and respond to changing conditions.

Allosteric Regulation of Enzymes

• Allosteric regulation involves regulatory molecules binding to sites other than the active site, affecting enzyme activity.

• Enzymes may be stabilized in active or inactive forms by activators or inhibitors.

• Cooperativity is a form of allosteric regulation where substrate binding increases enzyme activity at other sites.

Feedback Inhibition

• End products of metabolic pathways can inhibit earlier steps, preventing overproduction and conserving resources.

Localization of Enzymes Within the Cell

• Enzyme compartmentalization within organelles or complexes increases metabolic efficiency and regulation.

• Some enzymes are fixed in specific locations, while others form multienzyme complexes.

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