Energy, Metabolism, and Enzyme Function in Cells

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Energy and Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions within an organism, allowing it to transform matter and energy. These reactions are organized into metabolic pathways, where each step is catalyzed by a specific enzyme.

Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., hydrolysis).
Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., dehydration synthesis).

Diagram of a metabolic pathway with sequential enzyme-catalyzed steps

Forms of Energy

Energy exists in various forms and can be converted from one form to another:

Kinetic energy: Energy of motion.
Thermal energy: Kinetic energy associated with the random movement of atoms or molecules.
Potential energy: Stored energy due to position or structure.
Chemical energy: Potential energy available for release in a chemical reaction.

Thermodynamics in Biological Systems

The Laws of Thermodynamics

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 it cannot be created or destroyed (principle of conservation of energy).
Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat.

Bear illustrating first and second laws of thermodynamics: chemical energy in food, kinetic energy, heat, CO2, and H2O

Energy Conversions and Entropy

Useful energy is stored in highly organized matter. When used, the overall entropy of the universe increases. Living organisms increase the disorder of their surroundings through metabolism, but local decreases in entropy (e.g., building complex molecules) are offset by greater increases in the surroundings.

Car engine converting chemical energy in gasoline to kinetic energy and heat

Free Energy and Spontaneity

Gibbs 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 < 0: Spontaneous process (releases free energy, increases stability).
ΔG > 0: Nonspontaneous process (requires input of energy).
ΔG = G_{final} - G_{initial}

Examples of spontaneous processes: gravitational motion, diffusion, chemical reaction

ATP and Energy Coupling

Structure and Function of ATP

Adenosine triphosphate (ATP) is the main energy currency of the cell. It consists of adenine, ribose, and three phosphate groups. The hydrolysis of ATP releases energy that can be used to drive cellular work.

Structure of ATP molecule showing three phosphate groups, ribose, and adenine

The ATP Cycle

ATP is regenerated by the addition of a phosphate group to adenosine diphosphate (ADP). Energy from catabolic reactions is used to synthesize ATP, which is then used to power cellular work.

Diagram of the ATP cycle: energy from catabolism regenerates ATP, which is used for cellular work

Exergonic and Endergonic Reactions

Exergonic reactions: Release free energy (ΔG < 0), are spontaneous.
Endergonic reactions: Absorb free energy (ΔG > 0), are nonspontaneous.

Graphs comparing exergonic and endergonic reactions: energy released vs. energy required

Enzymes and Metabolic Pathways

Role of Enzymes

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy barrier. They do not affect the overall change in free energy (ΔG).

Graph showing activation energy barrier and effect of enzyme on reaction progress Graph comparing activation energy with and without enzyme

Enzyme Structure and Function

Substrate: The reactant an enzyme acts on.
Active site: The region on the enzyme where the substrate binds.
Induced fit: The enzyme changes shape to better fit the substrate, enhancing catalysis.

Diagram of enzyme-substrate complex and catalytic cycle

Factors Affecting Enzyme Activity

Temperature: Each enzyme has an optimal temperature for activity.
pH: Each enzyme has an optimal pH.
Chemicals: Certain chemicals can enhance or inhibit enzyme activity.
Cofactors: Nonprotein helpers (inorganic ions or organic coenzymes, e.g., vitamins).

Enzyme Inhibition and Regulation

Competitive inhibitors: Bind to the active site, blocking substrate binding.
Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.
Allosteric regulation: Enzymes have active and inactive forms stabilized by activators or inhibitors binding to regulatory sites.
Cooperativity: Substrate binding to one active site stabilizes favorable conformational changes at other subunits.

Diagram of competitive and noncompetitive inhibition Diagram of allosteric activation and inhibition in a multisubunit enzyme Diagram of cooperativity in enzyme regulation

Feedback Inhibition

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway. This prevents the cell from wasting resources by overproducing the product.

Diagram of feedback inhibition in a metabolic pathway

Summary Table: Key Concepts in Energy and 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, spontaneous	ATP hydrolysis
Endergonic Reaction	Requires energy input, nonspontaneous	Glucose synthesis
Enzyme	Speeds up reactions by lowering activation energy	Sucrase catalyzing sucrose hydrolysis
Feedback Inhibition	End product inhibits pathway	Isoleucine synthesis pathway