Metabolism and Metabolic Pathways

Introduction to Metabolism

Metabolism is the sum of all chemical reactions that occur within a living organism. It provides the energy and molecular building blocks needed for growth, repair, and maintenance.

• Metabolism: The totality of an organism’s chemical reactions.

• Metabolic pathways are series of chemical reactions that either build complex molecules (anabolism) or break them down (catabolism).

Catabolic vs. Anabolic Pathways

• 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, photosynthesis).

• Example: The breakdown of glucose during cellular respiration is catabolic; the synthesis of proteins from amino acids is anabolic.

Energy in Biological Systems

Types of Energy

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Thermal energy: Kinetic energy associated with the random movement of atoms or molecules; transferred as heat.

• Potential energy: Stored energy due to position or structure (e.g., chemical bonds).

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

Energy Transformations and Thermodynamics

• Energy can be transformed from one form to another but cannot be created or destroyed (First Law of Thermodynamics).

• Every energy transfer increases the entropy (disorder) of the universe (Second Law of Thermodynamics).

Free Energy and Chemical Reactions

• Free energy (G): The portion of a system’s energy that can perform work at constant temperature and pressure.

• The change in free energy () determines whether a reaction is spontaneous.

• Equation: where = change in enthalpy (total energy), = temperature in Kelvin, = change in entropy.

• If is negative, the reaction is spontaneous (exergonic); if positive, the reaction is non-spontaneous (endergonic).

• Systems are more stable when is low; high indicates less stability.

Equilibrium in Chemical Reactions

• A chemical reaction at equilibrium has no net change in the concentration of reactants and products.

• Cells are not at equilibrium; they are open systems with constant flow of materials.

Exergonic and Endergonic Reactions

Definitions and Differences

• Exergonic reaction: Releases free energy; is negative; spontaneous.

• Endergonic reaction: Requires input of energy; is positive; non-spontaneous.

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Coupling of Reactions

• Cells couple exergonic reactions (like ATP hydrolysis) to drive endergonic processes (such as biosynthesis).

ATP: The Cell’s Energy Currency

Structure and Function of ATP

• ATP (adenosine triphosphate): Composed of adenine, ribose, and three phosphate groups.

• ATP stores energy in the bonds between its phosphate groups.

• Hydrolysis of ATP releases energy:

• This reaction is exergonic and powers cellular work.

ATP Hydrolysis and Phosphorylation

• Energy released from ATP hydrolysis is used to drive endergonic reactions by phosphorylation (transferring a phosphate group to another molecule).

• Phosphorylation changes the shape and activity of target molecules, enabling cellular work.

Enzymes and Catalysis

What Are Enzymes?

• Enzymes: Biological catalysts, usually proteins, that speed up chemical reactions without being consumed.

• They lower the activation energy (Ea) required for a reaction to proceed.

• Catalyst: A substance that increases the rate of a chemical reaction without being permanently altered.

How Enzymes Work

• Enzymes have a specific active site where substrates bind.

• The enzyme-substrate complex stabilizes the transition state, lowering activation energy.

• Enzymes are highly specific for their substrates due to the shape and chemical environment of the active site.

• Enzymes are not used up in reactions and can catalyze multiple cycles.

Enzyme Specificity and Regulation

• Each enzyme catalyzes a specific reaction or set of related reactions.

• Specificity is determined by the compatibility between the enzyme’s active site and the substrate’s shape and chemical properties.

• Enzyme activity can be regulated by factors such as temperature, pH, cofactors, and inhibitors.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; too high or too low reduces activity.

• pH: Each enzyme has an optimal pH range; deviations can denature the enzyme.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required for enzyme activity.

• Inhibitors: Molecules that decrease enzyme activity by blocking the active site (competitive) or altering enzyme shape (noncompetitive).

Cellular Work Powered by ATP

Types of Cellular Work

• Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules).

• Transport work: Pumping substances across membranes against concentration gradients.

• Mechanical work: Movement (e.g., muscle contraction, movement of chromosomes during cell division).

Summary Table: Exergonic vs. Endergonic Reactions

Key Concepts and Applications

• Metabolic pathways are tightly regulated to maintain cellular balance.

• ATP is the universal energy currency, linking exergonic and endergonic reactions.

• Enzymes are essential for life, enabling reactions to occur rapidly and under mild conditions.

Additional info: Some explanations and examples were expanded for clarity and completeness, based on standard biology textbook content.