Chapter 8: An Introduction to Metabolism – Study Notes

Metabolism and Thermodynamics in Biology

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling the transformation of matter and energy. It is an emergent property of life, arising from the orderly interactions between molecules.

• Metabolic Pathways: A series of chemical reactions where a specific molecule is altered stepwise to produce a final product. Each step is catalyzed by a specific enzyme.

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration).

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones (e.g., protein synthesis, glycogen synthesis).

• Enzymes: Biological catalysts that speed up specific reactions without being consumed.

The Laws of Thermodynamics

Biological processes are governed by the laws of thermodynamics, which describe energy transformations and the tendency toward disorder.

• First Law (Conservation of Energy): Energy can be transferred and transformed, but not created or destroyed.

• Second Law (Entropy): Every energy transfer increases the entropy (disorder) of the universe. Some energy is lost as heat and becomes unavailable to do work.

Forms of Energy in Biological Systems

Kinetic, Potential, and Chemical Energy

Energy exists in various forms and is essential for cellular work. Living cells transform energy from one form to another.

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

• Thermal Energy: Kinetic energy from random movement of atoms/molecules; transfer is called heat.

• Light Energy: Used in photosynthesis.

• Potential Energy: Energy due to location or structure (e.g., water behind a dam, arrangement of electrons in bonds).

• Chemical Energy: Potential energy available for release in a chemical reaction (e.g., glucose breakdown).

Free Energy and Spontaneity of Reactions

Free Energy Change (ΔG)

The change in free energy (ΔG) determines whether a reaction occurs spontaneously. Free energy is the portion of a system’s energy that can do work under constant temperature and pressure.

• Equation: $\Delta G = \Delta H - T \Delta S$ Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (total energy) $\Delta S$ = change in entropy $T$ = temperature in Kelvin

• Spontaneous Processes: Occur without energy input; $\Delta G$ is negative.

• Nonspontaneous Processes: Require energy input; $\Delta G$ is zero or positive.

• Stability: Systems with higher free energy are less stable and tend to become more stable (lower free energy).

Exergonic and Endergonic Reactions

Chemical reactions are classified based on their free-energy changes:

• Exergonic Reactions: Net release of free energy; $\Delta G$ is negative; occur spontaneously.

• Endergonic Reactions: Absorb free energy; $\Delta G$ is positive; nonspontaneous.

ATP and Energy Coupling

ATP Structure and Function

ATP (adenosine triphosphate) is the cell’s primary energy currency, mediating energy coupling between exergonic and endergonic reactions.

• Structure: Composed of ribose (sugar), adenine (nitrogenous base), and three phosphate groups.

• Hydrolysis: Energy is released when the terminal phosphate bond is broken by hydrolysis.

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.

ATP in Cellular Work

Cells use ATP to perform three main types of work:

• Chemical Work: Driving endergonic reactions.

• Transport Work: Pumping substances across membranes against their concentration gradient.

• Mechanical Work: Moving structures within the cell, such as motor proteins and vesicles.

The ATP Cycle

ATP is regenerated by phosphorylation of ADP, using energy from catabolic (exergonic) reactions. This cycle couples energy-yielding and energy-consuming processes.

• Equation: $\text{ADP} + P_i \rightarrow \text{ATP}$ (phosphorylation)

• Energy Source: Exergonic breakdown reactions (catabolism).

Enzymes and Activation Energy

Activation Energy Barrier

Every chemical reaction requires an initial input of energy to break bonds, known as activation energy (EA).

• Activation Energy (EA): The energy required to initiate a reaction.

• Exergonic Reactions: Products have less free energy than reactants, but require EA to proceed.

Enzymes as Biological Catalysts

Enzymes are proteins that lower the activation energy barrier, speeding up reactions without being consumed. They do not alter the free energy change (ΔG) of the reaction.

• Substrate: The reactant an enzyme acts upon.

• Enzyme-Substrate Complex: Formed when the enzyme binds to its substrate.

• Active Site: The region on the enzyme where the substrate binds.

• Induced Fit: The enzyme changes shape slightly to fit the substrate, enhancing catalysis.

Factors Affecting Enzyme Activity

Environmental Effects

Enzyme activity is influenced by temperature, pH, and the presence of cofactors or inhibitors.

• Optimal Temperature: Each enzyme has a temperature at which it functions best.

• Optimal pH: Each enzyme has a pH at which it is most active (e.g., pepsin in stomach pH 2, trypsin in intestine pH 8).

• Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme activity. Organic cofactors are called coenzymes (e.g., vitamins).

• Inhibitors: Chemicals that reduce enzyme activity. Competitive inhibitors bind to the active site, while noncompetitive inhibitors bind elsewhere, altering enzyme shape.

Summary Table: Exergonic vs. Endergonic Reactions

Type of Reaction	ΔG	Spontaneity	Energy Flow	Example
Exergonic	Negative	Spontaneous	Energy released	Cellular respiration
Endergonic	Positive	Nonspontaneous	Energy absorbed	Photosynthesis

Key Terms and Definitions

• Metabolism: The sum of all chemical reactions in an organism.

• Catabolic Pathway: Pathway that breaks down molecules and releases energy.

• Anabolic Pathway: Pathway that builds molecules and consumes energy.

• Enzyme: Protein catalyst that speeds up reactions by lowering activation energy.

• ATP: Adenosine triphosphate, the main energy carrier in cells.

• Activation Energy (EA): The energy required to start a reaction.

• Free Energy (G): Energy available to do work.

• ΔG: Change in free energy during a reaction.

• Entropy (S): Measure of disorder or randomness.

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Absorbs energy; nonspontaneous.

• Cofactor: Nonprotein helper for enzyme activity.

• Coenzyme: Organic cofactor (often a vitamin).

• Competitive Inhibitor: Binds to active site, blocking substrate.

• Noncompetitive Inhibitor: Binds elsewhere, changing enzyme shape.