Metabolism: Transforming Matter and Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions within a cell, including those that generate or consume energy. It is an emergent property of life, arising from the orderly interactions between molecules.

• 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).

• ATP: Serves as the energy currency, linking catabolic and anabolic processes.

Metabolic Pathways

Metabolic pathways consist of a series of enzyme-catalyzed reactions, transforming a starting molecule into a product through intermediate steps.

• Each step is catalyzed by a specific enzyme.

• Pathways can be linear or branched, depending on the cellular needs.

Forms of Energy and Thermodynamics

Types of Energy

Energy is the capacity to cause change and is essential for cellular work.

• Kinetic energy: Energy of motion.

• Potential energy: Energy due to position or structure.

• Chemical energy: Potential energy stored in chemical bonds.

The Laws of Thermodynamics

Biological processes obey the laws of thermodynamics, which govern energy transformations.

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

• Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Free Energy and Spontaneity

Gibbs Free Energy (ΔG)

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

• ΔG = ΔH - TΔS Where ΔH is enthalpy, T is temperature (Kelvin), and ΔS is entropy.

• Spontaneous reactions have negative ΔG.

• Equilibrium is the state of maximum stability; systems move toward equilibrium spontaneously.

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

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

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

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) is a nucleotide composed of ribose, adenine, and three phosphate groups. It is a key molecule for energy transfer in cells.

• Hydrolysis of ATP releases energy by breaking the terminal phosphate bond.

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

ATP Coupling and Cellular Work

ATP powers cellular work by coupling exergonic reactions (energy-releasing) to endergonic reactions (energy-consuming).

• Chemical work: Drives endergonic reactions.

• Transport work: Pumps substances across membranes.

• Mechanical work: Moves cellular structures.

• Phosphorylation transfers a phosphate group to another molecule, increasing its energy.

Enzymes: Biological Catalysts

Role and Function of Enzymes

Enzymes are proteins that act as catalysts, speeding up metabolic reactions by lowering activation energy barriers.

• Enzymes are highly specific for their substrates.

• They do not change ΔG; they only accelerate reactions.

Enzyme Structure and Specificity

The active site of an enzyme binds to the substrate, forming an enzyme-substrate complex. The induced-fit model describes how the enzyme changes shape to enhance catalysis.

• Substrate concentration affects reaction rate; saturation occurs when all active sites are occupied.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by temperature, pH, cofactors, and inhibitors.

• Each enzyme has optimal temperature and pH.

• Cofactors (inorganic or organic) assist enzyme function.

• Inhibitors can be competitive (bind active site) or noncompetitive (bind elsewhere).

Regulation of Enzyme Activity

Allosteric Regulation

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

• Can stabilize active or inactive forms of the enzyme.

• Cooperativity occurs when substrate binding increases enzyme activity (e.g., oxygen binding to hemoglobin).

Feedback Inhibition

Feedback inhibition occurs when the end product of a metabolic pathway inhibits an earlier step, preventing overproduction and conserving resources.

Summary Table: Exergonic vs. Endergonic Reactions

Key Equations

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