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.

• Anabolic pathways build complex molecules from simpler ones, consuming energy.

• ATP (adenosine triphosphate) is the central molecule linking catabolic and anabolic reactions.

Metabolic Pathways

Metabolic pathways are stepwise series of reactions, each catalyzed by a specific enzyme. These pathways transform a starting molecule into a product through multiple intermediate steps.

• Each step is catalyzed by a unique enzyme.

• Pathways can be linear, branched, or cyclical.

Forms of Energy and Thermodynamics

Types of Energy

Energy is the capacity to cause change and can be used to do work. It exists in several forms:

• 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 are governed by the laws of thermodynamics:

• 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 that can perform work. The change in free energy (ΔG) determines whether a reaction is spontaneous.

• ΔG = ΔH - TΔS where ΔH is enthalpy, T is temperature in Kelvin, and ΔS is entropy.

• Spontaneous reactions have negative ΔG.

• Nonspontaneous reactions have positive 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, ΔG < 0, spontaneous.

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

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP consists of ribose, adenine, and three phosphate groups. It belongs to the class of nucleotides and is used to make RNA.

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

• The triphosphate tail is highly energetic due to repulsion between negative charges.

ATP Coupling and Cellular Work

ATP hydrolysis powers cellular work by coupling exergonic and endergonic reactions. Phosphorylation transfers a phosphate group to another molecule, driving endergonic reactions.

• Chemical work: Synthesis of macromolecules.

• Transport work: Pumping substances across membranes.

• Mechanical work: Movement of cilia, muscle contraction.

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 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 this barrier, allowing reactions to proceed at moderate temperatures.

Enzyme Specificity and Induced Fit

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

Factors Affecting Enzyme Activity

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

• Optimal temperature: Human enzymes ~37°C; thermophilic bacteria ~75°C.

• Optimal pH: Pepsin (stomach) ~2; Trypsin (intestine) ~8.

• Cofactors: Nonprotein helpers (metal ions or coenzymes).

• Inhibitors: 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. This can stabilize either the active or inactive form of the enzyme.

Feedback Inhibition

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

Example: Oxygen binding to hemoglobin illustrates cooperativity, a form of allosteric regulation.

Additional info: These notes expand on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.