Chapter 8: An Introduction to Metabolism

Big Idea

All of the chemical reactions that take place inside cells, including those that consume or generate energy, are collectively referred to as the cell’s metabolism. Metabolism is essential for maintaining life, enabling cells to grow, reproduce, and respond to their environment.

8.1: An Organism’s Metabolism Transforms Matter and Energy

Metabolic Pathways

Metabolism consists of metabolic pathways, which are series of chemical reactions where a specific molecule is altered in a stepwise manner to form a final product. Each step is catalyzed by a specific enzyme.

• 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).

Forms of Energy

• Energy is the capacity to cause change or do work.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure.

• Chemical energy: A form of potential energy stored in chemical bonds.

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Biological systems are considered open systems because they exchange energy and matter with their surroundings.

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

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

8.2: The Free-Energy Change of a Reaction Tells Us Whether or Not the Reaction Occurs Spontaneously

Gibbs Free Energy (ΔG)

Free energy (G) is the portion of a system’s energy that can perform work at constant temperature and pressure. The change in free energy (ΔG) determines whether a process is spontaneous.

• ΔG = ΔH – TΔS

• ΔH: Change in enthalpy (total energy)

• ΔS: Change in entropy (disorder)

• T: Temperature in Kelvin

• If ΔG < 0, the process is spontaneous (energetically favorable).

• If ΔG > 0, the process is nonspontaneous (requires energy input).

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release free energy (ΔG < 0), occur spontaneously.

• Endergonic reactions: Absorb free energy (ΔG > 0), nonspontaneous.

8.3: ATP Powers Cellular Work by Coupling Exergonic Reactions to Endergonic Reactions

Structure and Function of ATP

ATP (adenosine triphosphate) is the cell’s main energy currency. It consists of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups. ATP is a nucleotide.

ATP Hydrolysis and Energy Coupling

• Hydrolysis of ATP (breaking off a phosphate group) releases energy that can be used to drive endergonic reactions.

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

ATP in Cellular Work

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

• Transport work: Pumping substances across membranes.

• Mechanical work: Movement, such as muscle contraction or movement of cilia.

The ATP Cycle

ATP is regenerated by adding a phosphate group to ADP. This cycle couples energy-yielding (catabolic) and energy-consuming (anabolic) processes.

8.4: Enzymes Speed Up Metabolic Reactions by Lowering Energy Barriers

Role and Function of Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions without being consumed. They lower the activation energy required for reactions to proceed.

Activation Energy and Transition State

• Activation energy (EA): The initial energy needed to start a reaction.

• Enzymes lower EA, making reactions proceed faster at cellular temperatures.

Enzyme Specificity and Induced Fit

• Each enzyme acts on a specific substrate, forming an enzyme-substrate complex.

• The active site is the region where the substrate binds.

• Induced fit: The enzyme changes shape slightly to fit the substrate more snugly, enhancing catalysis.

Factors Affecting Enzyme Activity

• Substrate concentration: Higher concentrations increase reaction rate until the enzyme is saturated.

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

• pH: Each enzyme has an optimal pH.

• Cofactors: Nonprotein helpers (e.g., metal ions, vitamins).

• Inhibitors: Chemicals that reduce enzyme activity (competitive or noncompetitive).

8.5: Regulation of Enzyme Activity Helps Control Metabolism

Allosteric Regulation

Allosteric regulation occurs when a molecule binds to an enzyme at a site other than the active site, causing a change in enzyme activity. This can either inhibit or stimulate the enzyme.

Cooperativity

Cooperativity is a form of allosteric regulation where the binding of one substrate molecule increases the activity at other active sites (e.g., oxygen binding to hemoglobin).

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.

Summary: Metabolism is a complex network of chemical reactions regulated by enzymes, energy transformations, and feedback mechanisms. Understanding these processes is fundamental to biology and essential for comprehending how cells function and maintain life.