Metabolism: An Overview

Definition and Role

Metabolism refers to all chemical reactions that occur within living organisms to maintain life. These reactions are organized into metabolic pathways, which are sequences of chemical reactions, each catalyzed by a specific enzyme.

• Metabolic Pathway: A series of chemical reactions where the product of one reaction becomes the substrate for the next.

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

• Example: Cellular respiration is a metabolic pathway that breaks down glucose to produce energy.

Types of Metabolic Pathways

Anabolic Pathways

Anabolic pathways build complex molecules from simpler ones, requiring energy input. These are essential for growth and repair.

• Example: Synthesis of proteins from amino acids.

• Process: Dehydration reactions join monomers to form polymers, releasing water.

Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing energy that can be used by the cell.

• Example: Cellular respiration breaks down glucose into carbon dioxide and water, releasing energy.

Energy in Biological Systems

Definition of Energy

Energy is the capacity to do work or cause change. In biological systems, energy is required for processes such as movement, synthesis of molecules, and transport across membranes.

Types of Energy

• Kinetic Energy: Energy of motion. Example: Muscle contraction, movement of molecules.

• Potential Energy: Stored energy due to position or structure. Example: Chemical energy stored in bonds.

• Chemical Energy: A form of potential energy available for release in chemical reactions.

• Thermal Energy: Energy associated with random movement of atoms or molecules; often referred to as heat.

Thermodynamics in Biology

First Law of Thermodynamics

The First Law of Thermodynamics states that energy can be transferred and transformed, but it cannot be created or destroyed. This is also known as the principle of conservation of energy.

• Equation: (Change in energy equals heat added minus work done)

Second Law of Thermodynamics

The Second Law of Thermodynamics states that every energy transfer or transformation increases the entropy (disorder) of the universe.

• Entropy: A measure of disorder or randomness.

• Energy transformations are never 100% efficient; some energy is lost as heat.

Free Energy and Spontaneity

Gibbs Free Energy ()

Free energy is the portion of a system's energy that can perform work when temperature and pressure are uniform. The change in free energy () determines whether a reaction is spontaneous.

• Equation:

• If is negative, the reaction is spontaneous (exergonic).

• If is positive, the reaction is non-spontaneous (endergonic).

Exergonic vs. Endergonic Reactions

ATP: The Energy Currency of the Cell

Structure and Function

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP releases energy:

• kcal/mol (exergonic reaction)

• ATP is regenerated from ADP and inorganic phosphate in an endergonic reaction:

Energy Coupling

Cells use energy coupling to drive endergonic reactions by pairing them with exergonic reactions, often using ATP hydrolysis.

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

• Example: Synthesis of glutamine from glutamic acid and ammonia is coupled to ATP hydrolysis.

Enzymes and Activation Energy

Role of Enzymes

Enzymes are biological catalysts that lower the activation energy () required for reactions, increasing reaction rates without being consumed.

• Enzymes bind to specific substrates at their active site, forming an enzyme-substrate complex.

• Enzyme specificity is determined by the shape and chemical properties of the active site.

Mechanisms of Enzyme Action

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Directly participating in the reaction

Factors Affecting Enzyme Activity

• Temperature: Increased temperature speeds up reactions to a point, but excessive heat denatures enzymes.

• pH: Each enzyme has an optimal pH range.

Regulation of Metabolic Pathways

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Allosteric Regulation

Enzymes can be regulated by molecules binding to sites other than the active site (allosteric sites), stabilizing either the active or inactive form.

• Allosteric activators: Increase enzyme activity.

• Allosteric inhibitors: Decrease enzyme activity.

Feedback Inhibition

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction and conserving resources.

• Example: ATP allosterically inhibits an enzyme in its own synthesis pathway.

Summary Table: Key Concepts in Metabolism

Additional info: The study notes expand on brief points and diagrams from the original slides, providing definitions, examples, and context for key metabolic concepts relevant to a General Biology course.