Metabolism: Transforming Matter and Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions within a cell, including those that consume or generate energy. It is an emergent property of life, arising from the orderly interactions between molecules. Metabolic pathways consist of a series of steps, each catalyzed by a specific enzyme, transforming a starting molecule into a product.

• 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).

• Metabolism is regulated to maintain cellular function and efficiency.

Metabolic Pathways

Metabolic pathways are stepwise series of reactions, each catalyzed by a specific enzyme. This organization allows for regulation and efficiency in cellular processes.

• Each step is catalyzed by a unique enzyme.

• Pathways can be linear, branched, or cyclical.

Energy and Thermodynamics in Biology

Forms of Energy

Energy is the capacity to cause change and is essential for cellular work. It exists in various forms:

• Kinetic energy: Energy of motion.

• Potential energy: Energy due to position or structure.

• Chemical energy: Potential energy stored in chemical bonds.

Thermodynamics and Biological Systems

Organisms are open systems, exchanging energy and matter with their surroundings. The laws of thermodynamics govern energy transformations in biological processes:

• 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 of Reactions

Gibbs Free Energy ()

Free energy is the portion of a system's energy available to do work. The change in free energy () determines whether a reaction is spontaneous:

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin

• Spontaneous reactions have

Free Energy, Stability, and Equilibrium

Systems move toward equilibrium, a state of maximum stability. Spontaneous changes decrease free energy and increase stability.

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

• Exergonic reactions: Release free energy (), occur spontaneously.

• Endergonic reactions: Absorb free energy (), are nonspontaneous.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) consists of ribose, adenine, and three phosphate groups. It is a nucleotide and serves as the primary energy carrier in cells.

ATP Hydrolysis and Energy Release

ATP releases energy when its terminal phosphate bond is broken by hydrolysis. The energy is used to drive cellular work.

ATP Coupling and Cellular Work

ATP powers cellular work by coupling exergonic reactions (energy-releasing) to endergonic reactions (energy-consuming). Phosphorylation transfers a phosphate group to another molecule, enabling endergonic reactions.

The ATP Cycle

ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions. This cycle couples energy-yielding and energy-consuming processes.

Enzymes: Biological Catalysts

Role and Function of Enzymes

Enzymes are proteins that act as catalysts, speeding up metabolic reactions by lowering activation energy barriers. They are not consumed in the reaction.

Activation Energy and Reaction Progress

Activation energy () is the initial energy required to start a reaction. Enzymes lower $E_A$, allowing reactions to proceed at moderate temperatures.

Enzyme Specificity and Induced Fit

Enzymes are highly specific, binding to their substrates at the active site. The induced-fit model describes how the enzyme changes shape to enhance catalysis.

Factors Affecting Enzyme Activity

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

• Optimal temperature and pH: Each enzyme has conditions where it functions best.

• Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme activity.

• Enzyme inhibitors: Chemicals that reduce enzyme activity, either competitively or noncompetitively.

Regulation of Enzyme Activity

Allosteric Regulation

Allosteric regulation involves binding of regulatory molecules at sites other than the active site, affecting enzyme activity. This can either activate or inhibit the enzyme.

Feedback Inhibition

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

Example: The binding of oxygen to hemoglobin illustrates cooperativity, a form of allosteric regulation where binding at one site affects binding at others.