Chemical Composition of the Body: Molecular Interactions and Organic Molecules

Chemical Composition of the Body

Atoms, Ions, and Chemical Bonds

Understanding the chemical principles underlying physiological processes is essential for studying human physiology. Atoms, ions, and chemical bonds form the foundation of molecular interactions in the body.

• Atom: The smallest unit of an element, consisting of a nucleus (protons and neutrons) and orbiting electrons.

• Atomic Mass: Equal to the number of protons plus neutrons.

• Atomic Number: Equal to the number of protons.

• Electron Orbitals: Energy levels (shells) surrounding the nucleus; electrons fill shells starting closest to the nucleus.

• Valence Electrons: Electrons in the outermost shell; participate in chemical bonding.

• Isotopes: Atoms of the same element with different numbers of neutrons; some are radioactive and used in medical testing.

Chemical Bonds

Chemical bonds are formed when electrons interact between atoms. The type of bond depends on how electrons are shared or transferred.

• Covalent Bonds: Valence electrons are shared between atoms. If shared equally, the bond is nonpolar (e.g., H2); if shared unequally, the bond is polar (e.g., water).

• Ionic Bonds: One atom donates electrons to another, forming ions. The donor becomes a cation (positive), and the receiver becomes an anion (negative). Ionic compounds dissociate in water, forming hydration spheres.

• Hydrogen Bonds: Weak bonds between polar molecules, based on attraction between electropositive hydrogen and electronegative oxygen or nitrogen. Responsible for surface tension, capillarity, and protein/DNA structure.

Acids, Bases, and pH

Acids and bases are defined by their ability to donate or accept protons (H+). The pH scale measures the concentration of hydrogen ions in a solution.

• Acid: Proton donor; pH < 7.

• Base: Proton acceptor; pH > 7.

• Neutral Solution: Equal concentrations of H+ and OH-; pH = 7.

• pH Scale: Ranges from 0 (strong acid) to 14 (strong base).

• Buffer: Stabilizes pH; consists of a weak acid and a weak base. In blood, the bicarbonate/carbonic acid system prevents acidosis and alkalosis.

Buffer Equation:

$\mathrm{HCO_3^- + H^+ \leftrightarrow H_2CO_3}$

Additional info: The lungs and kidneys also regulate blood pH by removing CO2 and adjusting bicarbonate levels.

Organic Molecules

Four Categories of Organic Molecules

Organic molecules are essential for cellular structure and function. They are classified into four main groups:

• Carbohydrates: Energy storage and structural support; composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio.

• Lipids: Energy storage, membrane structure, and signaling; hydrophobic due to nonpolar hydrocarbon chains.

• Proteins: Chains of amino acids; provide structure, catalyze reactions, and regulate cell functions.

• Nucleic Acids: Chains of nucleotide subunits; store and express genetic information (DNA, RNA).

Carbohydrates

Carbohydrates are classified by the number of sugar units:

• Monosaccharides: Simple sugars with one carbon ring (e.g., glucose, fructose, galactose).

• Disaccharides: Two monosaccharides joined by a covalent bond (e.g., sucrose, maltose, lactose).

• Polysaccharides: Chains of monosaccharides (e.g., starch, glycogen, cellulose, chitin).

Dehydration Synthesis: Forms covalent bonds by removing water.

Hydrolysis: Breaks covalent bonds by adding water.

Lipids

Lipids are hydrophobic molecules with diverse functions:

• Triglycerides: Composed of glycerol and three fatty acids; can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Phospholipids: Amphipathic molecules with a polar phosphate group and nonpolar fatty acid chains; major component of cell membranes.

• Steroids: Four-ring structure; includes cholesterol, steroid hormones, vitamin D, and bile salts.

• Prostaglandins: Regulatory molecules derived from fatty acids; involved in inflammation, blood clotting, and other functions.

Proteins

Proteins are polymers of amino acids, each with a unique sequence and structure:

• Amino Acids: Contain an amino group, carboxyl group, and variable R group; 20 types exist.

• Primary Structure: Sequence of amino acids in a polypeptide chain.

• Secondary Structure: Alpha helix or beta sheet formed by hydrogen bonds.

• Tertiary Structure: 3D folding due to interactions between R groups; determines function.

• Quaternary Structure: Multiple polypeptide chains bonded together (e.g., hemoglobin).

• Conjugated Proteins: Proteins combined with other molecules (e.g., glycoproteins, lipoproteins).

Nucleic Acids

Nucleic acids are polymers of nucleotides, which consist of a five-carbon sugar, phosphate group, and nitrogenous base.

• DNA: Double-stranded; contains deoxyribose sugar and four bases (adenine, thymine, cytosine, guanine). Follows the Law of Complementary Base Pairs.

• RNA: Single-stranded; contains ribose sugar and uracil instead of thymine. Types include mRNA, tRNA, and rRNA.

• Other Nucleotides: ATP and GTP (energy carriers), cAMP (regulation), NAD and FAD (co-enzymes).

Summary Table: Common Biological Ions

Positive Ion	Symbol	Negative Ion	Symbol
Hydrogen ion	H+	Chloride ion	Cl-
Sodium ion	Na+	Hydroxide ion	OH-
Calcium ion	Ca2+	Nitrate ion	NO3-
Aluminum ion	Al3+	Carbonate ion	CO32-
Ammonium ion	NH4+	Sulfate ion	SO42-
		Phosphate ion	PO43-

Summary Table: Common Biological Compounds

Compound	Formula	Compound	Formula
Sodium chloride	NaCl	Methane	CH4
Sodium hydroxide	NaOH	Glucose	C6H12O6
Carbon monoxide	CO	Water	H2O
Carbon dioxide	CO2	Hydrogen sulfide	H2S
Nitric oxide	NO	Sulfur dioxide	SO2
Nitrogen dioxide	NO2	Ammonia	NH3
Nitrous oxide	N2O	Calcium carbonate	CaCO3
Nitric acid	HNO3