Metabolism

Definition and Overview

Metabolism encompasses all the chemical reactions occurring within cells or organisms. These reactions include both the synthesis (anabolism) and breakdown (catabolism) of molecules, as well as energy transformations necessary for life.

• Anabolism: Building larger molecules from smaller ones (requires energy).

• Catabolism: Breaking down larger molecules into smaller ones (releases energy).

• Energy intermediates: ATP, NADH, FADH2 are used to transfer energy within metabolic pathways.

Energy in Biological Systems

Types of Energy

Energy is essential for cellular processes and can be classified into two main types:

• Potential Energy: Stored energy due to position or structure (e.g., energy in chemical bonds, concentration gradients).

• Kinetic Energy: Energy of motion (e.g., movement of molecules, muscle contraction).

Physical Laws Governing Energy

Biological systems obey the laws of physics and chemistry, especially:

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Systems tend toward disorder (entropy) unless energy is input. Life on Earth is sustained by energy from the sun.

Chemical Reactions and Free Energy

Exergonic and Endergonic Reactions

Chemical reactions in cells either release energy (exergonic) or require energy input (endergonic).

• Exergonic reactions: Release free energy; products have less free energy than reactants ().

• Endergonic reactions: Absorb free energy; products have more free energy than reactants ().

ATP: The Energy Currency of Cells

Structure and Function

ATP (adenosine triphosphate) is a nucleic acid molecule that stores and transfers energy in cells.

• Consists of a base (adenine), a 5-carbon sugar (ribose), and three phosphate groups.

• The bonds between phosphate groups are high-energy bonds; breaking them releases energy for cellular work.

ATP Cycle

• Energy from exergonic reactions is used to attach a phosphate to ADP, forming ATP.

• ATP hydrolysis releases energy to drive endergonic reactions.

Redox Reactions

Oxidation and Reduction

Many energy conversions involve oxidation-reduction (redox) reactions, where electrons (and their energy) are transferred between molecules.

• Oxidation: Loss of electrons (often involves removal of hydrogen).

• Reduction: Gain of electrons (often involves addition of hydrogen).

• Redox reactions occur simultaneously; one molecule is oxidized, another is reduced.

• Mnemonic: "LEO the lion says GER" (Loss of Electrons is Oxidation, Gain of Electrons is Reduction).

Enzymes

Role and Properties

Enzymes are biological catalysts, usually proteins (some are RNA called ribozymes), that speed up chemical reactions by lowering activation energy.

• Enzymes are highly specific; each reaction requires a unique enzyme.

• Enzymes are not consumed or altered in the reaction.

• Enzymes do not provide energy; they facilitate reactions.

Mechanism of Action

Enzymes bind substrates at their active site, positioning them for the reaction. Sometimes, the enzyme changes shape for a better fit (induced fit).

Classes of Enzymes

Enzymes are classified based on the type of reaction they catalyze.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimum temperature; activity decreases if too high or too low.

• pH: Each enzyme has an optimum pH; activity decreases outside this range.

Enzyme Concentration and Substrate Concentration

• Increasing enzyme concentration increases reaction rate (if substrate is abundant).

• Increasing substrate concentration increases reaction rate up to a point, after which enzymes become saturated.

Coenzymes and Cofactors

• Coenzymes: Organic molecules (often vitamins) required for enzyme function.

• Cofactors: Inorganic ions (often minerals) required for enzyme function.

Enzyme Inhibition

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

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Metabolic Pathways

Organization and Regulation

Metabolic pathways are sequences of enzyme-catalyzed reactions, where the product of one reaction becomes the substrate for the next.

• Pathways are regulated by controlling enzyme production (genetic control), physical environment (temperature, pH), and biochemical factors (coenzymes, cofactors, inhibitors).

• Most pathways are regulated by negative feedback; excess product inhibits pathway activity.

Complexity of Metabolism

• Cells contain thousands of metabolic reactions; scientists specialize in studying specific pathways.

Summary Table: Key Concepts in Energy, Enzymes, and Metabolism

Additional info: Academic context was added to clarify definitions, mechanisms, and regulatory principles. All images included are directly relevant to the adjacent explanations and reinforce key concepts.