Chapter 6: An Introduction to Metabolism

Energy of Life

All living organisms are powered by chemical reactions that transform energy and matter. Cells function as chemical factories, carrying out thousands of reactions every second to sustain life.

• Metabolism refers to all chemical reactions occurring within an organism.

• Cells extract energy from sugars through cellular respiration and use that energy to perform work.

• Some organisms, such as fireflies and dinoflagellates, convert chemical energy into light—a process called bioluminescence.

• Example: Fireflies use bioluminescence to produce light for communication and mating.

Metabolic Pathways

Metabolism is organized into metabolic pathways, which are sequences of chemical reactions where each step is catalyzed by a specific enzyme.

• A metabolic pathway begins with a specific molecule (reactant) and ends with a product.

• Each step in the pathway is facilitated by a unique enzyme.

• Pathways can be catabolic (breaking down molecules to release energy) or anabolic (building complex molecules from simpler ones, consuming energy).

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

Catabolic vs. Anabolic Pathways

Forms of Energy

Energy is the capacity to cause change or perform work. It exists in various forms, some of which can be converted from one to another.

• Kinetic Energy: Energy associated with motion (e.g., running water).

• Thermal Energy: Kinetic energy due to random movement of atoms or molecules; transferred as heat.

• Light Energy: Can be harnessed to perform work, such as driving photosynthesis.

• Potential Energy: Stored energy due to position or structure (e.g., water behind a dam).

• Chemical Energy: Potential energy available for release in chemical reactions (e.g., energy stored in glucose bonds).

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Biological systems are open systems, exchanging energy and matter with their surroundings.

• First Law of Thermodynamics: Energy can be transferred and transformed, but cannot be created or destroyed.

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

• Spontaneous processes increase entropy and do not require energy input.

• Organisms create ordered structures from less organized materials, but overall entropy of the universe increases.

• Example: Building a protein from amino acids decreases entropy locally, but increases entropy globally.

Entropy Comparison Table

Free Energy, Stability, and Equilibrium

Free energy () is the portion of a system's energy that can perform work when temperature and pressure are uniform. Changes in free energy determine the spontaneity of reactions.

• Stable systems have lower free energy.

• Reactions with negative change in free energy () are spontaneous.

• At equilibrium, no net work can be performed; living cells avoid equilibrium to remain alive.

Equation:

Exergonic and Endergonic Reactions

Metabolic reactions are classified based on their energy changes.

• Exergonic Reactions: Release energy; spontaneous; is negative.

• Endergonic Reactions: Require energy input; nonspontaneous; is positive.

• Example: ATP hydrolysis is exergonic; protein synthesis is endergonic.

ATP and Cellular Work

Adenosine triphosphate (ATP) is the cell's main energy currency, coupling exergonic and endergonic reactions.

• ATP hydrolysis releases energy by breaking a phosphate bond:

• Energy released is used to drive cellular work (chemical, transport, mechanical).

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

• ATP is regenerated by adding a phosphate group to ADP, using energy from catabolic reactions.

Enzymes and Metabolic Reactions

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy ().

• Enzymes are usually proteins; some RNA molecules (ribozymes) also act as enzymes.

• Enzymes are specific for their substrates due to their unique 3D structure.

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

• Enzyme-substrate complex forms when the enzyme binds its substrate.

• Enzymes do not affect ; they only speed up reactions that would occur eventually.

Activation Energy Equation:

= Energy required to start a reaction

Enzyme Specificity and Catalysis

• Enzyme specificity results from the fit between the active site and the substrate ("lock and key" or "induced fit" models).

• Substrates are held in the active site by weak interactions (e.g., hydrogen bonds).

• Enzymes can orient substrates, strain substrate bonds, provide a favorable microenvironment, or participate directly in the reaction.

• Enzyme activity can be increased by raising substrate concentration until saturation is reached.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions and regulatory molecules.

• Temperature and pH: Each enzyme has optimal conditions for activity.

• High temperatures can denature enzymes, reducing activity.

• Cofactors: Non-protein helpers (inorganic ions or organic coenzymes, such as vitamins) required for enzyme function.

• Inhibitors: Molecules that reduce enzyme activity; can be reversible or irreversible.

Types of Enzyme Inhibitors

Regulation of Metabolic Pathways

Cells regulate metabolism by controlling enzyme activity and gene expression.

• Allosteric Regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme function (can inhibit or activate).

• Enzymes may have multiple subunits that oscillate between active and inactive forms.

• Feedback Inhibition: End product of a pathway inhibits an enzyme earlier in the pathway, preventing overproduction.

• Example: Isoleucine synthesis is inhibited by its own accumulation.

Additional info: These notes expand on the provided slides and text, adding definitions, examples, and academic context for clarity and completeness.