Metabolism, Energy, and Enzyme Function in Cells: Study Guide

Metabolism and Energy Transformations

Overview of Metabolism

Metabolism refers to the totality of an organism’s chemical reactions, organized into metabolic pathways. These pathways manage the material and energy resources of the cell.

• Anabolic pathways: Build complex molecules from simpler ones; require energy input.

• Catabolic pathways: Break down complex molecules into simpler ones; release energy.

• Examples: Photosynthesis (anabolic), Cellular respiration (catabolic).

Energy in Biological Systems

Energy is the capacity to cause change or do work. It exists in various forms, such as kinetic and potential energy.

• Kinetic energy: Energy of motion.

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

• Chemical energy: Potential energy available for release in a chemical reaction.

• Example: Water at the top of a dam has potential energy; glucose stores chemical energy.

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Two laws are especially important in biology:

• First Law: Energy cannot be created or destroyed, only transformed.

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

• Implication: Energy conversions are never 100% efficient; some energy is lost as heat.

Free Energy and Spontaneity of Reactions

Gibbs Free Energy ($\Delta G$)

The change in free energy ($\Delta G$) of a system determines whether a reaction occurs spontaneously.

• Formula: $\Delta G = \Delta H - T\Delta S$

• Spontaneous reactions: $\Delta G < 0$ (exergonic)

• Non-spontaneous reactions: $\Delta G > 0$ (endergonic)

• Symbol: $\Delta G$ represents the change in free energy.

Exergonic vs. Endergonic Reactions

• Exergonic: Release energy; spontaneous (e.g., cellular respiration).

• Endergonic: Require energy input; non-spontaneous (e.g., photosynthesis).

• Example: Hydrolysis of ATP is exergonic; synthesis of ATP is endergonic.

ATP and Energy Coupling

Structure and Function of ATP

Adenosine triphosphate (ATP) is the cell’s main energy currency. It consists of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP: Releases energy by breaking the terminal phosphate bond.

• Reaction: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

• Energy coupling: ATP hydrolysis drives endergonic reactions by transferring a phosphate group.

Energy Coupling in Cells

• Cells use ATP to couple exergonic and endergonic reactions.

• Example: Sodium-potassium pump uses ATP hydrolysis to transport ions against their concentration gradients.

Enzymes and Metabolic Pathways

Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy ($E_a$).

• Catalyst: Substance that increases reaction rate without being consumed.

• Activation energy ($E_a$): The energy required to start a reaction.

• Effect of enzymes: Lower $E_a$, but do not affect $\Delta G$.

Enzyme Structure and Function

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

• Substrate: The reactant acted upon by the enzyme.

• Induced fit: Enzyme changes shape to better fit the substrate.

• Product: The molecule(s) produced by the reaction.

Steps of Enzyme Action

1. Substrate enters active site.

2. Enzyme-substrate complex forms.

3. Induced fit occurs.

4. Substrate is converted to product(s).

5. Product(s) released.

6. Enzyme is ready for another cycle.

Factors Affecting Enzyme Activity

• Substrate concentration: Higher concentration increases rate up to a point.

• pH: Each enzyme has an optimal pH.

• Temperature: Rate increases with temperature up to an optimum; high temperatures can denature enzymes.

• Cofactors and coenzymes: Non-protein helpers; cofactors are inorganic (e.g., Mg2+), coenzymes are organic (e.g., NAD+).

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site, blocking substrate.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape.

• Allosteric regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme activity.

• Cooperativity: Substrate binding to one active site affects binding at other sites (e.g., hemoglobin).

Regulation of Enzyme Activity

Feedback Inhibition

Feedback inhibition is a common regulatory mechanism in metabolic pathways. The end product of a pathway inhibits an enzyme involved earlier in the pathway.

• Purpose: Prevents overproduction of the end product.

• Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway.

Summary Table: Energy Transformations in Key Biological Processes

Process	Starting materials, and their relative energy level	Ending materials, and their relative energy level	Will this process occur spontaneously (without input of energy)?
Water spilling over a dam	Water at the top has a higher energy	Water at the bottom has a lower energy	Yes
Photosynthesis	CO2 and H2O (low energy)	Glucose and O2 (high energy)	No (requires energy input from sunlight)
Cellular Respiration	Glucose and O2 (high energy)	CO2 and H2O (low energy)	Yes (releases energy)
Synthesis of ATP	ADP and Pi (lower energy)	ATP (higher energy)	No (requires energy input)
Hydrolysis of glucose	Glucose (high energy)	CO2 and H2O (low energy)	Yes (releases energy)
Hydrolysis of ATP	ATP (high energy)	ADP and Pi (lower energy)	Yes (releases energy)

Key Terms and Definitions

• Metabolism: All chemical reactions in an organism.

• Anabolic pathway: Builds molecules; requires energy.

• Catabolic pathway: Breaks down molecules; releases energy.

• Enzyme: Protein catalyst that speeds up reactions.

• Substrate: Reactant acted upon by an enzyme.

• Active site: Region on enzyme where substrate binds.

• Product: Molecule(s) produced by a reaction.

• Allosteric regulation: Regulation by binding at a site other than the active site.

• Feedback inhibition: End product inhibits an earlier step in a pathway.

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard biology curriculum.