CHAPTER 6: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, allowing it to manage material and energy resources. These reactions are organized into metabolic pathways, which are essential for cellular function and survival.

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

• Metabolic pathways: Series of chemical reactions catalyzed by enzymes, transforming molecules stepwise.

• Example: The breakdown of glucose in cellular respiration is a metabolic pathway.

Types of Metabolic Pathways

Metabolic pathways are classified based on whether they release or consume energy.

• Catabolic pathways: Release energy by breaking down complex molecules into simpler compounds. Example: Digestive enzymes break down food to release energy.

• Anabolic pathways: Consume energy to build complex molecules from simpler ones. Example: Amino acids link to form muscle proteins.

Energy in Biological Systems

Forms of Energy

Energy is the capacity to do work. In biological systems, energy exists in various forms and can be transformed from one type to another.

• Kinetic energy (KE): Energy associated with motion.

• Heat (thermal energy): A form of kinetic energy due to the random movement of atoms or molecules.

• Potential energy (PE): Stored energy due to position or structure.

• Chemical energy: A type of potential energy available for release in a chemical reaction.

• Energy conversion: Energy can be converted from one form to another (e.g., chemical to mechanical to electrical).

Thermodynamics in Biology

Basic Principles

Thermodynamics is the study of energy transformations. Biological systems obey the laws of thermodynamics.

• Closed system: Isolated from its surroundings (e.g., liquid in a thermos).

• Open system: Energy and matter can be transferred between the system and its surroundings. Organisms are open systems.

First Law of Thermodynamics

The energy of the universe is constant. Energy can be transferred and transformed, but cannot be created or destroyed. This is also known as the principle of conservation of energy.

• Example: Chemical energy in food is converted to kinetic energy for movement.

Second Law of Thermodynamics

Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat.

• Entropy: A measure of disorder or randomness.

• Example: Heat released during muscle contraction increases entropy.

Free Energy and Chemical Reactions

Free Energy (G)

Free energy is the portion of a system’s energy that can perform work. The change in free energy () determines whether a reaction is spontaneous.

• Exergonic reaction: Releases energy; spontaneous.

• Endergonic reaction: Requires energy; nonspontaneous.

Energy Coupling in Cells

Cells perform work by coupling exergonic and endergonic reactions. The energy released from exergonic reactions is used to drive endergonic processes.

• Types of cellular work:

  • Mechanical work: Movement (e.g., muscle contraction).

  • Transport work: Pumping substances across membranes.

  • Chemical work: Building macromolecules.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the main energy source for cellular processes. It consists of adenine, ribose, and three phosphate groups.

• ATP hydrolysis: Breaking the bonds between phosphate groups by hydrolysis releases energy.

• The energy released comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.

How ATP Performs Work

The exergonic release of phosphate from ATP is used to power endergonic cellular work. After hydrolysis, ATP becomes ADP (adenosine diphosphate).

• Example: ATP phosphorylates key reactants, making them more reactive.

Enzymes and Catalysis

Role of Enzymes

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy required to start a reaction.

• Catalyst: Substance that increases the rate of a reaction without being consumed.

• Enzyme: Biological catalyst, usually a protein.

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

Enzyme-Substrate Interaction

The reactant that an enzyme acts on is called its substrate. The enzyme binds to its substrate at the active site, forming an enzyme-substrate complex.

• Active site: Region on the enzyme where the substrate binds.

• Induced fit: Enzyme changes shape to fit the substrate snugly, enhancing catalysis.

The Catalytic Cycle of an Enzyme

Enzymes facilitate the conversion of substrates to products through a series of steps:

1. Substrates enter the active site.

2. Substrates are held in the active site by weak interactions.

3. The active site lowers activation energy and speeds up the reaction.

4. Substrates are converted to products.

5. Products are released; enzyme is ready for another cycle.

Factors Influencing Enzyme Activity

Environmental Factors

Enzyme activity can be affected by temperature, pH, and the presence of chemicals.

• Optimal temperature: Each enzyme has a temperature at which it functions best.

• Optimal pH: Each enzyme has a pH range where activity is maximal.

Cofactors and Coenzymes

Some enzymes require nonprotein helpers for activity.

• Cofactors: Inorganic helpers (e.g., Zn, Fe, Cu).

• Coenzymes: Organic helpers (e.g., vitamins).

Enzyme Inhibitors

Enzyme activity can be regulated by inhibitors.

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

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a shape change that renders the active site nonfunctional.

Regulation of Enzyme Activity

Allosteric Regulation

Allosteric regulation involves the binding of regulatory molecules to a site other than the active site, affecting enzyme function.

• Activator: Stabilizes the active form of the enzyme.

• Inhibitor: Stabilizes the inactive form.

• Cooperativity: Binding of one substrate triggers a shape change in other active sites, increasing catalytic activity.

Feedback Inhibition

In feedback inhibition, the end product of a metabolic pathway shuts down the pathway by binding to the allosteric site of an enzyme, preventing waste and increasing efficiency.

• Example: Isoleucine inhibits the first enzyme in its synthesis pathway when present in sufficient quantity.

Summary Table: Types of Enzyme Inhibition

Additional info: These notes expand on the original slides and handwritten content, providing definitions, examples, and academic context for key concepts in metabolism and enzyme function.