Chapter 8: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a living organism, enabling the transformation of matter and energy. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes, and are essential for maintaining life.

• Metabolism: The sum total of all chemical reactions in an organism.

• Emergent property: Metabolism arises from the orderly interactions between molecules.

• Metabolic pathway: A sequence of chemical reactions, each catalyzed by a specific enzyme, transforming a starting molecule into a product through several intermediate steps.

• Enzyme: A macromolecule (usually a protein) that acts as a catalyst to speed up a specific chemical reaction.

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

Types of Metabolic Pathways

Catabolic and Anabolic Pathways

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

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

• Anabolic pathways: Consume energy to build complex molecules from simpler ones (e.g., synthesis of proteins from amino acids).

• Catabolic pathways are often described as "downhill" reactions, while anabolic pathways are "uphill." Energy released from catabolic reactions powers anabolic reactions.

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy

Energy in Biological Systems

Energy is the capacity to cause change and exists in various forms relevant to biological processes.

• Kinetic energy: Energy associated with motion (e.g., water flowing through a dam turns turbines).

• Thermal energy: Kinetic energy due to the random movement of atoms or molecules; transfer of thermal energy is called heat.

• Potential energy: Energy possessed due to location or structure (e.g., water behind a dam, energy stored in chemical bonds).

• Chemical energy: A form of potential energy stored in chemical bonds, available for release in a chemical reaction (e.g., glucose).

• Energy can be converted from one form to another (e.g., chemical energy from food to kinetic energy for movement).

Laws of Thermodynamics in Biology

First Law of Thermodynamics

The first law, also known as the principle of conservation of energy, states that energy can be transferred and transformed, but cannot be created or destroyed.

• Living organisms demonstrate this law by transforming energy from one form to another (e.g., light to chemical energy in photosynthesis).

Second Law of Thermodynamics

The second law states that every energy transfer or transformation increases the entropy (disorder) of the universe.

• Some energy is always lost as heat, becoming unavailable to do work.

• Entropy: A measure of molecular disorder or randomness.

• Living organisms increase the disorder of their surroundings through metabolism (e.g., breakdown of food releases heat and small molecules like CO2).

• Spontaneous processes increase entropy and occur without energy input; nonspontaneous processes require energy input.

Order and Disorder in Biological Systems

• Organisms create ordered structures from less organized materials (e.g., amino acids assembled into proteins).

• The increase in order within living systems is balanced by the catabolic breakdown of organized matter, releasing heat and small molecules.

• Overall, the total entropy of the system and its surroundings increases, in accordance with the second law.

Free Energy and Spontaneity

Free-Energy Change ()

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

• The equation for free-energy change is:

• = change in free energy

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin (K)

• If is negative, the process is spontaneous; if $\Delta G$ is zero or positive, the process is nonspontaneous.

Free Energy, Stability, and Equilibrium

• Systems with higher free energy are less stable and tend to change to become more stable (lower free energy).

• At equilibrium, forward and reverse reactions occur at the same rate, and the system is at maximum stability.

• Processes can perform work only when moving toward equilibrium.

Exergonic and Endergonic Reactions

Classification of Chemical Reactions

• Exergonic reactions: Release free energy (), are spontaneous, and the products have less free energy than the reactants.

• Endergonic reactions: Absorb free energy (), are nonspontaneous, and the products have more free energy than the reactants.

• The magnitude of indicates the maximum amount of work an exergonic reaction can perform or the energy required to drive an endergonic reaction.

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Equilibrium and Metabolism

Metabolic Reactions and Equilibrium

• In closed systems, reactions eventually reach equilibrium and can do no work.

• Cells are open systems, allowing materials to flow in and out, preventing metabolic equilibrium and enabling continuous work.

• Catabolic pathways release free energy in a series of reactions, with products of one reaction serving as reactants for the next, maintaining a steady state.

ATP and Cellular Work

ATP: Structure and Function

• ATP (adenosine triphosphate): Composed of ribose (sugar), adenine (nitrogenous base), and three phosphate groups.

• ATP is used for energy coupling and as a building block for RNA.

• Energy is released from ATP when the terminal phosphate bond is broken by hydrolysis.

• The high potential energy of ATP is due to the repulsion between the negatively charged phosphate groups.

How ATP Powers Cellular Work

• ATP hydrolysis powers three main types of cellular work:

  • Chemical work: Driving endergonic reactions.

  • Transport work: Pumping substances across membranes against gradients.

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

• Energy coupling: The use of an exergonic process (ATP hydrolysis) to drive an endergonic one.

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

ATP Cycle

• ATP is regenerated by the addition of a phosphate group to ADP (adenosine diphosphate).

• The energy for this phosphorylation comes from catabolic (exergonic) reactions.

• This cycle couples energy-yielding and energy-consuming processes in the cell.

Regulation of Enzyme Activity

Enzyme Regulation and Metabolic Control

• Cells regulate metabolism by controlling enzyme activity and gene expression.

• Allosteric regulation: A regulatory molecule binds to an enzyme at one site, affecting its function at another site. This can inhibit or stimulate enzyme activity.

• Most allosterically regulated enzymes are made from multiple subunits, each with its own active site.

• Cooperativity: Substrate binding to one active site stabilizes the active form for all other sites, amplifying the enzyme's response.

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction and waste of resources.

Localization of Enzymes Within the Cell

• Enzyme compartmentalization brings order to metabolic pathways.

• Some enzymes form multienzyme complexes or are fixed in specific cellular locations (e.g., mitochondrial membranes).

• In eukaryotic cells, certain enzymes are localized within specific organelles (e.g., mitochondria for cellular respiration).