Metabolism: Transforming Matter & Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism, enabling it to grow, reproduce, maintain structure, and respond to environmental changes. These reactions are organized into metabolic pathways, where each step is catalyzed by a specific enzyme.

• Metabolic Pathway: A series of chemical reactions beginning with a specific molecule and ending with a product. Each step is facilitated by a unique enzyme.

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler ones (e.g., cellular respiration).

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones (e.g., protein synthesis).

Example: Cellular respiration is a catabolic pathway that breaks down glucose to produce ATP.

Energy: A Review

Energy is the capacity to perform work, such as moving objects against a force or rearranging matter. In biological systems, energy transformations are essential for life processes.

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

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

Example: Food contains chemical energy, which is a form of potential energy that can be converted to kinetic energy for movement.

Laws of Energy Transformation (Thermodynamics)

First Law of Thermodynamics (Conservation of Energy)

The first law states that energy cannot be created or destroyed, only transformed from one form to another. In biological systems, chemical energy from food is converted into kinetic energy and heat.

• Equation:

Example: During cellular respiration, the chemical energy in glucose is converted to ATP, heat, and work.

Second Law of Thermodynamics

Every energy transfer increases the entropy (disorder) of the universe. Some energy is always lost as heat during transformations, making processes less efficient.

• Entropy: A measure of disorder or randomness.

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

Example: When muscles contract, not all chemical energy is converted to movement; some is lost as heat.

Free Energy and Cellular Work

Free Energy (Gibbs Free Energy)

Free energy () is the portion of a system's energy that can perform work. Changes in free energy determine whether a reaction is spontaneous.

• Exergonic Reaction: Releases energy (), spontaneous.

• Endergonic Reaction: Requires energy input (), nonspontaneous.

Equation: Where = change in enthalpy, = temperature (K), = change in entropy.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) stores and transfers energy for cellular work. It powers chemical, transport, and mechanical work by coupling exergonic and endergonic reactions.

• ATP Structure: Adenine, ribose, and three phosphate groups.

• ATP Hydrolysis: Releases energy by breaking a phosphate bond, forming ADP and inorganic phosphate ().

• Phosphorylation: Transfer of a phosphate group to another molecule, activating it for work.

Example: ATP powers muscle contraction and active transport across membranes.

Enzymes: Role & Regulation

What are Enzymes?

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions without being consumed. They are highly specific due to their unique shapes.

• Active Site: The region on the enzyme where the substrate binds.

• Substrate: The reactant an enzyme acts upon.

• Induced Fit: The enzyme changes shape to better fit the substrate upon binding.

Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

How Enzymes Work

Enzymes lower the activation energy (EA) required for reactions, allowing them to proceed faster. They do not change the overall free energy () of the reaction.

• Orient substrates correctly.

• Strain substrate bonds.

• Provide a favorable microenvironment.

• Covalently bond to the substrate.

Factors Affecting Enzyme Activity

• Temperature and pH: Each enzyme has optimal conditions for activity. Deviations can denature the enzyme.

• Cofactors: Non-protein helpers required for enzyme function. Organic cofactors are called coenzymes (e.g., vitamins).

• Inhibitors: Molecules that decrease enzyme activity.

Enzyme Inhibition

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive Inhibitors: Bind elsewhere, changing the enzyme's shape and reducing activity.

• Allosteric Regulation: Regulatory molecules bind to a site other than the active site, stabilizing the enzyme in active or inactive forms.

• Feedback Inhibition: The end product of a pathway inhibits an earlier step, preventing overproduction.

Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway when enough isoleucine is present.

Summary Table: Types of Metabolic Pathways

Key Terms

• Metabolism

• Catabolic pathway

• Anabolic pathway

• Enzyme

• ATP

• Activation energy

• Allosteric regulation

• Feedback inhibition