Chapter 2 (PART TWO)

Exchange Reactions and Catalysis

• Exchange reactions (such as acid–base reactions) occur when parts of molecules switch places by breaking chemical bonds and forming new ones.

• Reversible reactions are those in which the products can change back into the reactants; they are symbolized by double arrows: AB + CD \rightleftharpoons AD + CB.

• Catalysts influence the speed of chemical reactions without being used up in the process; catalysts in the body are called enzymes.

Acids and Bases

• Electrolytes are substances that release ions in water; these can carry electric charge in the body.

• When ionically bound substances are put into water, they dissociate, because the slightly positive ends of water molecules attract negative ions, and the slightly negative ends attract positive ions; the ions then interact with the water molecules.

• Electrolytes that release hydrogen ions in water are called acids.

• Electrolytes that release ions that combine with hydrogen ions in water are called bases.

• The concentrations of ions in the body are very important to physiology, since they affect chemical reactions that control many physiological functions.

• pH represents the concentration of hydrogen ions in solution. For the hydrogen ion balance we use: pH = -\log_{10} [H^+] ,\quad [H^+] = 10^{-pH}.

• The pH scale ranges from 0 to 14; a pH of 7 indicates a neutral solution with equal numbers of hydrogen and hydroxide ions.

• pH < 7 indicates acidity (more H⁺ than OH⁻); the lower the pH, the more acidic.

• pH > 7 indicates basic/alkaline conditions (more OH⁻ than H⁺); the higher the pH, the more basic.

• Between each whole number on the pH scale there is a tenfold difference in hydrogen ion concentration; e.g., a solution with pH 3 has ten times more H⁺ than a solution with pH 4.

• Buffers are chemicals that combine with excess acids or bases to help minimize pH changes in body fluids.

The Dissociation of NaCl in Water (Conceptual Image)

• When an ionically bonded substance is put into water, the charged ions are attracted to the slightly charged ends of the polar water molecules.

• This dissociates the substance, and the ions become surrounded by water molecules.

• The substance is now called an electrolyte, since it can carry an electric current.

The pH Scale (Continued)

• The higher the H⁺ concentration, the lower the pH and the higher the acidity; the lower the H⁺ concentration, the higher the pH and the lower the acidity (higher alkalinity).

2.6: Chemical Constituents of Cells

• Chemicals in nature are divided into two categories: organic and inorganic.

• Organic molecules contain both hydrogen and carbon; many dissolve in water but do not release ions; these are nonelectrolytes.

• Examples of organic substances in the body: carbohydrates, lipids, proteins, and nucleic acids.

• Inorganic substances: all other compounds that usually dissolve in water and release ions; examples include water, oxygen, carbon dioxide, and salts.

Inorganic Substances: Water

• Water is the most abundant compound in living things and makes up about two-thirds of the weight of a human adult.

• Water is an important solvent; most metabolic reactions occur in water. A solvent is a substance in which other substances dissolve.

• Water is important in transporting solutes in the body because it is the major component of blood and other body fluids.

• Water absorbs and transports heat through the body.

Other Inorganic Compounds

• Oxygen: needed to release energy from nutrients; this energy drives cellular metabolism; inhaled into the lungs.

• Carbon Dioxide: waste product from energy-releasing metabolic reactions; exhaled from the lungs.

• Salts: compounds consisting of oppositely charged ions; salts provide essential ions (e.g., Na⁺, Cl⁻, K⁺, Ca²⁺, Mg²⁺, PO₄³⁻, CO₃²⁻, HCO₃⁻, SO₄²⁻) that play important roles in nerve impulse conduction, muscle contraction, and transport across cell membranes.

Common Inorganic Substances (Table Summary)

• Water: \text{H}_2\text{O} — medium for biochemical reactions; major component of body fluids; regulates body temperature; helps transport chemicals.

• Oxygen: \text{O}_2 — used in energy release from glucose molecules.

• Carbon Dioxide: \text{CO}_2 — waste product from metabolism; reacts with water to form carbonic acid.

• Bicarbonate ions: \text{HCO}_3^{-} — helps maintain acid–base balance.

• Calcium ions: \text{Ca}^{2+} — necessary for bone tissue, muscle contraction, blood clotting.

• Carbonate ions: \text{CO}_3^{2-} — component of bone tissue.

• Chloride ions: \text{Cl}^- — major extracellular negative ion.

• Hydrogen ions: \text{H}^+ — determine pH of internal environment.

• Magnesium ions: \text{Mg}^{2+} — component of bone; required for certain metabolic processes.

• Phosphate ions: \text{PO}_4^{3-} — required for synthesis of ATP and nucleic acids; component of bone; helps maintain polarization of cell membranes.

• Potassium ions: \text{K}^+ — polarization of cell membranes.

• Sodium ions: \text{Na}^+ — polarization of cell membranes; helps maintain water balance.

• Sulfate ions: \text{SO}_4^{2-} — helps maintain polarization of cell membranes.

Organic Substances: Carbohydrates

• Provide energy for cellular activities and materials for synthesizing various cell structures.

• Contain carbon, hydrogen, and oxygen; typically with twice as many hydrogen as oxygen atoms.

• Monosaccharides (simple sugars): contain 5–6 carbon atoms; examples: glucose, fructose, galactose, ribose, deoxyribose.

• Disaccharides (double sugars): consist of two simple sugars; examples: lactose, sucrose, maltose.

• Polysaccharides (many simple sugars): glycogen, starch.

Glucose Structural Sketch (Conceptual)

• Glucose molecules can exist as straight-chain or ring structures in solution.

Organic Substances: Lipids

• Lipids are organic substances insoluble in water.

• Major types: triglycerides (fats), phospholipids, and steroids.

• Triglycerides store energy for cellular function; comprised of glycerol plus three fatty acids.

• Fatty acids can be saturated (all single carbon–carbon bonds) or unsaturated (one or more double bonds).

• Phospholipids: glycerol with two fatty acids and a phosphate group; phosphate "head" is hydrophilic, fatty acid "tail" is hydrophobic; essential in cellular membranes.

• Steroids: four fused carbon rings; cholesterol is a key example used to synthesize sex hormones and adrenal hormones.

Lipid Structures (Conceptual)

• Triglyceride: glycerol backbone + three fatty acids.

• Phospholipid: glycerol + two fatty acids + phosphate group; hydrophilic head, hydrophobic tail.

• Steroid: four-ring structure; cholesterol is a base scaffold.

Organic Substances: Proteins

• Proteins contain carbon, hydrogen, oxygen, and nitrogen; many also contain sulfur.

• Functions: structural materials, energy sources, hormones, receptors on cell membranes, antibodies, and enzymes.

• Built from amino acids, each with a carboxyl group, an amino group, and an R group (side chain).

• About 20 different amino acids occur in proteins; amino acids bind together in polypeptide chains ranging from fewer than 100 to more than 5,000 amino acids.

Structure of Amino Acids (Key Points)

• General structure: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen, and an R group attached to a central carbon.

• R group ("rest of molecule") varies among amino acids, giving each its properties.

• Examples:

  • Cysteine: R group contains sulfur.

  • Phenylalanine: R group is an aromatic ring.

Levels of Structure in Proteins

• Primary structure: sequence of amino acids.

• Secondary structure: pleated or twisted coil due to hydrogen bonds between amino acids.

• Tertiary structure: unique three-dimensional folding due to attractions between amino acids in different parts of the chain.

• Quaternary structure: present when a protein consists of more than one polypeptide chain (e.g., hemoglobin has four polypeptide chains).

Protein Conformation and Denaturation

• Confirmation: the unique 3D shape of a protein, determined by hydrogen and covalent bonds, dictates protein function; it can be long fibers or globular.

• Denaturation: irreversible disruption of a protein’s shape, leading to loss of function; caused by pH changes, excessive temperature changes, radiation, or chemicals.

• Example: hard-boiling an egg denatures the albumin.

Organic Substances: Nucleic Acids

• Nucleic acids form genes and participate in protein synthesis; they are large organic molecules.

• Composed of building blocks called nucleotides (each nucleotide contains a 5-carbon sugar, a phosphate group, and one of five nitrogenous bases).

• Nucleic acid molecules are chains of nucleotides.

Structure of a Nucleotide

• Each nucleotide consists of a 5-carbon sugar, a phosphate group, and one of five nitrogenous bases.

Types of Nucleic Acids

• RNA (ribonucleic acid):

  • Usually single-stranded.

  • Functions in protein synthesis.

  • Sugar: ribose.

• ATP (Adenosine Triphosphate): a modification of RNA that contains three phosphate groups; stores and provides energy for chemical reactions in the body.

• DNA (deoxyribonucleic acid):

  • Double-stranded, twisted into a spiral, held together by hydrogen bonds.

  • Stores the molecular/genetic code used to synthesize proteins.

  • Sugar: deoxyribose.

Nucleic Acids: Visual and Functional Reference

• Figure references show typical structures of RNA and DNA, ATP structure, and nucleotide components.

Organic Compounds in Cells (Table Summary)

• Carbohydrates

  • Elements: C,\ H,\ O

  • General forms: Monosaccharide, Disaccharide, Polysaccharide

  • Functions: Provide energy, contribute to cell structure

  • Examples: Glucose, Sucrose, Glycogen

• Lipids

  • Elements: C,\ H,\ O (often P in phospholipids)

  • General forms: Triglyceride, Phospholipid

  • Functions: Provide energy, contribute to cell structure

  • Examples: Fats, Cholesterol

• Proteins

  • Elements: C,\ H,\ O,\ N (often S)

  • General form: Polypeptide chain

  • Functions: Provide cell structure, enzymes, and energy

  • Examples: Albumins, Hemoglobin

• Nucleic Acids

  • Elements: C,\ H,\ O,\ N,\ P

  • General form: Polynucleotide chain

  • Functions: Store information for protein synthesis; control cell activities

  • Examples: RNA, DNA