Water: Properties and Biological Importance

Hydrogen Bonding and Water Structure

Water's unique properties arise from its molecular structure and the hydrogen bonds that form between molecules.

• Hydrogen bonds: Weak attractions between an electronegative atom (like oxygen) and a hydrogen atom.

• Hydrogen bonds are crucial for maintaining the three-dimensional shapes of large biological molecules (e.g., proteins, DNA).

• Hydrogen bonds can form between water molecules and other polar molecules, making water an efficient solvent.

• Hydrophilic (water-loving) substances: Ions and polar molecules that dissolve easily in water due to hydrogen bonding.

• Hydrophobic (water-fearing) substances: Uncharged, nonpolar compounds that do not dissolve in water and interact through hydrophobic interactions.

Water: Polarity and Attraction

• Water molecules are polar, leading to strong intermolecular attractions (hydrogen bonds).

• Cohesion: Tendency of water molecules to stick together, resulting in high surface tension and contributing to capillary action.

• Adhesion: Attraction between water molecules and other substances, aiding in capillary action.

• Water is less dense as a solid (ice) than as a liquid due to the organization of hydrogen bonds, allowing ice to float and insulate aquatic environments.

Water's Capacity for Absorbing Energy

• Water has a high specific heat: the amount of energy needed to raise the temperature of 1 gram of water by 1°C.

• Extensive hydrogen bonding means water resists temperature changes, aiding in homeostasis.

• Heat of vaporization: Water requires a lot of energy to evaporate, helping organisms cool through sweating or transpiration.

Chemical Reactions and Energy

Acid-Base Chemical Reactions

Chemical reactions in biology often involve acids and bases, which affect the pH and function of biological systems.

• Acids: Substances that give up protons (H+) during chemical reactions, increasing hydronium ion concentration.

• Bases: Substances that acquire protons or reduce hydronium ion concentration.

• The chemistry of life is sensitive to acidic and basic conditions.

Chemical Reactions and Equilibrium

• Chemical reactions occur when bonds are broken and formed, often written as equations:

• Chemical equilibrium: The point at which the forward and reverse reactions occur at the same rate.

• Equilibrium can be disturbed by changing reactant/product concentrations or temperature.

Energy in Chemical Reactions

• Energy: The capacity to do work or supply heat.

• Exists as potential energy (stored in chemical bonds) and kinetic energy (energy of motion).

• In molecules, potential energy depends on the position of shared electrons in covalent bonds.

• The first law of thermodynamics: Energy is conserved; it can be transferred or transformed but not created or destroyed.

• The second law of thermodynamics: Entropy (disorder) always increases in a closed system.

Spontaneity of Chemical Reactions

• Spontaneous reactions proceed without continuous external input.

• Driven by two factors:

  • Products have lower potential energy than reactants.

  • Products are more disordered (higher entropy).

Metabolic Pathways

• A metabolic pathway is a series of chemical reactions, each catalyzed by a specific enzyme.

• Two types:

  • Anabolic pathways: Build complex molecules (require energy).

  • Catabolic pathways: Break down complex molecules (release energy).

• Metabolic pathways are regulated to meet the needs of the cell.

Carbon and Organic Molecules

Carbon: The Backbone of Life

• Carbon atoms form the backbone of most biological molecules.

• Can form four covalent bonds, allowing for branching, rings, and chains of various lengths.

• Forms polar bonds (C–O) and nonpolar bonds (C–H).

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical behavior of those molecules.

• Common functional groups: hydroxyl, carbonyl, carboxyl, amino, phosphate, sulfhydryl.

• Functional groups confer specific properties, such as acidity, basicity, or reactivity.

Macromolecules: Structure and Function

Polymers and Monomers

• Large biological molecules (macromolecules) are polymers made by linking smaller units called monomers.

• Condensation (dehydration) reactions: Join monomers by removing water.

• Hydrolysis reactions: Break polymers into monomers by adding water.

• Enzymes control these reactions in cells.

Proteins

Proteins are the most diverse biological molecules, involved in nearly every function in the body.

• Composed of amino acids (20 types), each with a core structure and a unique side chain (R group).

• The sequence of amino acids (primary structure) determines the protein's shape and function.

• Secondary structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape formed by interactions between side chains.

• Quaternary structure: Association of multiple polypeptide chains.

• Denaturation: Loss of 3D structure (and function) due to heat, pH changes, or chemicals; some proteins can renature.

Protein Functions

• Enzymatic proteins: Catalyze chemical reactions (e.g., digestive enzymes).

• Defensive proteins: Protect against disease (e.g., antibodies).

• Storage proteins: Store amino acids (e.g., casein in milk).

• Transport proteins: Move substances (e.g., hemoglobin transports oxygen).

• Hormonal proteins: Coordinate activities (e.g., insulin regulates blood sugar).

• Receptor proteins: Respond to chemical stimuli (e.g., nerve cell receptors).

• Contractile and motor proteins: Movement (e.g., actin and myosin in muscles).

• Structural proteins: Support (e.g., collagen, keratin).

Nucleic Acids

Structure and Function

• Nucleic acids are polymers of nucleotides.

• Two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleotide monomers are joined by phosphodiester bonds.

• DNA is double-stranded (antiparallel), with complementary base pairing (A with T, C with G).

• RNA is usually single-stranded and can have various functions (e.g., mRNA, tRNA, rRNA).

Carbohydrates

Monosaccharides and Polysaccharides

• Carbohydrates are monomers or polymers of monosaccharides (simple sugars).

• General formula: multiples of CH2O.

• Monosaccharides (e.g., glucose, fructose) are a ready source of energy.

• Polysaccharides (e.g., starch, glycogen, cellulose) are formed by linking monosaccharides via glycosidic bonds.

• Functions: energy storage, structural support, precursors for other molecules.

Variation in Sugars

• Location of carbonyl group

• Number of carbons

• Spatial arrangement of atoms

• Linear and ring forms

Lipids

Structure and Function

• Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Fats store energy, phospholipids form cell membranes, and steroids serve as hormones.

• Phospholipid bilayers exhibit selective permeability, allowing some substances to cross while blocking others.

Types of Lipids

• Saturated fats: No double bonds between carbon atoms; solid at room temperature.

• Unsaturated fats: One or more double bonds; liquid at room temperature.

Summary Table: Macromolecules and Their Monomers