Atoms, Molecules, and Bonds: Chemical Reactions and Properties of Water

Atoms, Molecules, and Bonds: Recap

Learning Objectives

• Distinguish the three main types of chemical reactions

• Discuss the functions of enzymes and factors that affect enzyme activity

• Describe the inorganic properties of water

• Describe the emergent properties of water

Chemical Reactions

Definition and Representation

Chemical reactions are fundamental processes in biological systems, occurring when chemical bonds are formed, rearranged, or broken. These reactions are represented by chemical equations, which symbolically show the transformation of substances.

• Reactants: Substances entering into the reaction together.

• Products: Resulting chemical end products.

• Balanced equations: Show the amounts of reactants and products, ensuring conservation of mass.

Types of Chemical Reactions

Synthesis (Combination) Reactions

Synthesis reactions involve the combination of atoms or molecules to form larger, more complex molecules. These are essential for building cellular structures and macromolecules.

• Anabolic processes: Synthesis reactions are used in anabolic (building) processes.

• Example: Formation of proteins from amino acids.

Decomposition Reactions

Decomposition reactions break down molecules into smaller molecules or their constituent atoms. This is the reverse of synthesis and is crucial for energy release and recycling cellular components.

• Catabolic processes: Decomposition reactions are involved in catabolic (bond-breaking) processes.

• Example: Breakdown of glycogen into glucose molecules.

Exchange (Displacement) Reactions

Exchange reactions involve both synthesis and decomposition, where bonds are both made and broken. These reactions are important for transferring functional groups and energy within cells.

• Bonds made and broken: Both synthesis and decomposition occur simultaneously.

• Example: Transfer of phosphate group from ATP to glucose.

Energy Flow in Chemical Reactions

Exergonic vs. Endergonic Reactions

Chemical reactions can be classified based on energy flow:

• Exergonic reactions: Net release of energy; products have less potential energy than reactants. Common in catabolic and oxidative reactions.

• Endergonic reactions: Net absorption of energy; products have more potential energy than reactants. Common in anabolic reactions.

Rate of Chemical Reactions

Factors Affecting Reaction Rate

The speed of chemical reactions is influenced by several factors:

• Temperature: Increased temperature usually increases reaction rate.

• Concentration of reactants: Higher concentration increases rate.

• Particle size: Smaller particles increase rate.

• Catalysts: Substances that increase reaction rate without being consumed. Enzymes are biological catalysts.

Inorganic Compounds: Water

Properties and Biological Importance

Water is the most abundant inorganic compound in living cells, accounting for 60%-80% of cell volume. Its unique properties are essential for life.

• High heat capacity: Absorbs and releases heat slowly, preventing sudden temperature changes.

• High heat of vaporization: Requires large amounts of heat to evaporate, providing a cooling mechanism.

• Polar solvent properties: Dissolves and dissociates ionic substances; forms hydration layers around charged molecules (e.g., proteins).

• Reactivity: Participates in hydrolysis and dehydration synthesis reactions.

• Cushioning: Protects organs from physical trauma (e.g., cerebrospinal fluid cushions nervous system organs).

Summary Table: Types of Chemical Reactions

Type	Description	Biological Example
Synthesis	Combining smaller molecules to form larger ones	Protein synthesis from amino acids
Decomposition	Breaking down larger molecules into smaller ones	Glycogen breakdown to glucose
Exchange	Bonds are both made and broken	ATP transfers phosphate to glucose

Summary Table: Properties of Water

Property	Biological Significance
High heat capacity	Maintains stable internal temperature
High heat of vaporization	Enables cooling via evaporation
Polar solvent	Dissolves ionic and polar substances
Reactivity	Participates in metabolic reactions
Cushioning	Protects organs from trauma

Key Equations

General Chemical Reaction Equation

Symbolic representation of a chemical reaction:

$\text{Reactants} \rightarrow \text{Products}$

Energy Change in Reactions

Exergonic and endergonic reactions:

$\Delta G < 0$ (Exergonic: energy released) $\Delta G > 0$ (Endergonic: energy absorbed)

Additional info:

• Enzymes lower the activation energy required for reactions, increasing reaction rates without being consumed.

• Water's polarity and hydrogen bonding are responsible for its unique properties, which are critical for maintaining homeostasis in living organisms.