BackChapter 6: An Introduction to Metabolism – Study Notes
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Chapter 6: An Introduction to Metabolism
Overview of Metabolism
Metabolism encompasses all chemical reactions occurring within a living organism. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes, and are essential for maintaining life by transforming matter and energy.
Metabolic Pathway: Begins with a specific molecule and ends with a product, with each step catalyzed by a unique 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).
Bioenergetics: The study of how energy flows through living organisms.
Forms of Energy
Energy is the capacity to cause change and exists in various forms relevant to biological systems.
Kinetic Energy: Energy of motion (e.g., muscle movement).
Thermal Energy: Kinetic energy associated with random movement of atoms or molecules; heat is thermal energy in transfer.
Light Energy: Can be harnessed by organisms (e.g., photosynthesis).
Potential Energy: Energy due to location or structure (e.g., water behind a dam, chemical bonds).
Chemical Energy: A form of potential energy stored in molecules, available for release in chemical reactions.
Thermodynamics and Biological Systems
The Laws of Thermodynamics
Thermodynamics is the study of energy transformations. Biological systems obey two fundamental laws:
First Law (Conservation of Energy): Energy can be transferred and transformed, but not created or destroyed.
Second Law: Every energy transfer or transformation increases the entropy (disorder) of the universe.
First Law of Thermodynamics
Energy in food is converted to other forms, but the total amount remains constant.
Example: Light energy from the sun is converted to chemical energy in plants, then transferred through food chains.
Second Law of Thermodynamics
Some energy is always lost as heat during energy transformations, increasing entropy.
Spontaneous processes increase the entropy of the universe and do not require energy input.
Nonspontaneous processes decrease entropy and require energy input.
Biological Order and Disorder
Living organisms create ordered structures from less organized materials, but overall, the entropy of the universe increases. Organisms are islands of low entropy in a universe tending toward disorder.
Free Energy and Metabolism
Free-Energy Change (ΔG), Stability, and Equilibrium
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 reaction is spontaneous.
ΔG < 0: Spontaneous reaction (can perform work).
ΔG > 0: Nonspontaneous reaction (requires energy input).
At equilibrium, ΔG is at its lowest value, and no work can be done.
Equation:
Where: = change in enthalpy (total energy) = temperature in Kelvin = change in entropy
Exergonic and Endergonic Reactions
Metabolic reactions are classified based on their energy changes:
Exergonic Reaction: Proceeds with a net release of free energy (ΔG < 0); spontaneous.
Endergonic Reaction: Absorbs free energy from surroundings (ΔG > 0); nonspontaneous.
Equilibrium and Metabolism
Cells are open systems and never reach equilibrium, allowing continuous work. Catabolic pathways release free energy in a series of reactions, with products of one reaction serving as reactants for the next.
ATP and Cellular Work
ATP: Structure and Function
Adenosine triphosphate (ATP) is the cell’s main energy currency. It consists of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups. ATP hydrolysis releases energy used to power cellular work.
How ATP Performs Work
ATP powers cellular work by coupling exergonic reactions (ATP hydrolysis) to endergonic reactions. This is often achieved by transferring a phosphate group to another molecule (phosphorylation), making it more reactive.
ATP in Transport and Mechanical Work
ATP hydrolysis can cause changes in protein shape and binding ability, powering transport across membranes and movement of cellular structures.
The ATP Cycle
ATP is regenerated by the addition of a phosphate group to ADP, using energy from catabolic reactions. This cycle is essential for maintaining cellular energy balance.
Enzymes and Metabolic Reactions
Enzymes as Catalysts
Enzymes are biological catalysts, usually proteins, that speed up metabolic reactions by lowering activation energy barriers without being consumed in the process.
Activation Energy
Activation energy (EA) is the initial energy required to start a chemical reaction. Enzymes lower this barrier, allowing reactions to proceed at cellular temperatures.
How Enzymes Work
Enzymes bind substrates at their active sites, forming enzyme-substrate complexes. The active site lowers activation energy by orienting substrates, straining bonds, providing a favorable environment, or forming temporary covalent bonds.
Substrate Specificity and Induced Fit
Enzymes are highly specific, recognizing only their substrates. The active site changes shape to fit the substrate more closely, a phenomenon known as induced fit.
The Catalytic Cycle
The enzyme’s active site binds substrates, converts them to products, and releases the products, making the enzyme available for another cycle.
Factors Affecting Enzyme Activity
Enzyme activity is influenced by temperature, pH, and the presence of specific chemicals. Each enzyme has optimal conditions for activity.
Cofactors and Coenzymes
Cofactors are nonprotein helpers required for enzyme function. They can be inorganic (e.g., metal ions) or organic (coenzymes, often derived from vitamins).
Enzyme Inhibitors
Enzyme inhibitors regulate enzyme activity:
Competitive Inhibitors: Bind to the active site, blocking substrate binding.
Noncompetitive Inhibitors: Bind elsewhere, changing the enzyme’s shape and reducing activity.
Regulation of Enzyme Activity
Cells regulate metabolism by controlling enzyme activity through:
Allosteric Regulation: Regulatory molecules bind to sites other than the active site, stabilizing active or inactive forms of the enzyme.
Feedback Inhibition: The end product of a pathway inhibits an enzyme early in the pathway, preventing overproduction.
Organization of Enzymes Within the Cell
Enzymes are often organized within specific cellular compartments or structures, facilitating efficient metabolic pathways. For example, enzymes for cellular respiration are located in mitochondria.
Summary Table: Key Concepts in Metabolism
Concept | Description | Example |
|---|---|---|
Catabolic Pathway | Breaks down molecules, releases energy | Cellular respiration |
Anabolic Pathway | Builds molecules, consumes energy | Protein synthesis |
Exergonic Reaction | Releases free energy (ΔG < 0) | Glucose oxidation |
Endergonic Reaction | Requires energy input (ΔG > 0) | Photosynthesis |
ATP | Main energy currency of the cell | Muscle contraction |
Enzyme | Biological catalyst | Sucrase |
Allosteric Regulation | Regulation by binding at a site other than the active site | Phosphofructokinase in glycolysis |
Feedback Inhibition | End product inhibits pathway | Isoleucine synthesis |
