Chemical Composition of the Body

Atoms are the smallest units of matter that can undergo chemical change.
The nucleus, at the center of an atom, contains protons (+ charge) and neutrons (no charge).
Atomic mass is the sum of protons and neutrons.
Atomic number is the number of protons in an atom.
In a neutral atom, the number of protons equals the number of electrons.
Isotopes vary in the number of neutrons but have the same atomic number and thus the same number of protons. They vary in atomic mass.
A chemical element includes all isotopic forms of a given atom.
Hydrogen has three isotopes:
- Most common: one proton, one electron
- Deuterium: one proton, one neutron, one electron
- Tritium: one proton, two neutrons, one electron
There are 106 chemical elements.
Four elements crucial to living organisms are carbon (C), nitrogen (N), oxygen (O), and hydrogen (H).
Electrons, outside the nucleus, are negatively charged and occupy orbitals surrounding the nucleus.
Valence electrons are in the outermost orbital and participate in chemical reactions if the orbital is incomplete. They form chemical bonds.
Orbitals, also called shells or energy levels, are where electrons are usually found.
- First shell: 2 electrons
- Second shell: 8 electrons
- Third shell: usually 8 electrons
Atoms share electrons in covalent bonds, which are the strongest bonds.
Electrons are equally distributed between the two identical atoms in nonpolar bonds. In polar bonds, electrons are not shared equally
Water molecules (H2O) exhibit polar covalent bonds.
- Oxygen pulls electrons toward itself, creating a partial negative charge.
- Hydrogen atoms have a slight positive charge.
Ionic bonds form when one or more valence electrons are transferred completely from one atom to another.
- The first atom, losing electrons, becomes a positively charged cation.
- The second atom, gaining electrons, becomes a negatively charged anion.
Cations and anions attract, forming ionic compounds. Ionic bonds are weaker than polar bonds. Ionic compounds dissociate easily when dissolved in H2O. For example NaCl forms Na+ and Cl- in solution.
Hydrophilic substances form hydration spheres with water (polar covalent).
Hydrophobic substances cannot form hydration spheres with water (nonpolar covalent).
Hydrogen bonds occur between a hydrogen atom with a slight positive charge and a more electronegative atom in another molecule. This is the case in water molecules.
Acids are molecules that release protons (H+). Bases are negatively charged ions that can combine with H+.
pH is the measure of H+ concentration in a solution. Normal blood pH is 7.35-7.45. A buffer system is a set of molecules that act against changes in H+ concentration.
Organic molecules contain carbon and hydrogen, and carbon has 4 electrons in its outer orbital, allowing it to bond with other atoms. Organic chemistry focuses on carbon-containing molecules. Millions of known organic compounds exist, and carbon bonds readily to many different elements.
Hydrocarbons are organic molecules containing only carbon and hydrogen. They can be linear, cyclic, or aromatic.
Functional groups are reactive groups attached to a hydrocarbon backbone in organic molecules.
- Examples: carbonyl, amino, hydroxyl, carboxyl, sulfhydryl, phosphate. The presence of functional groups dictates a molecule's class. For example: ketones, aldehydes, organic acids, and alcohols.
Stereoisomers are molecules with the same atoms in the same sequence, but with different spatial orientations, notably of functional groups.
Four main classes of organic molecules are lipids, carbohydrates, proteins, and nucleic acids.
A lipid is a diverse group of nonpolar hydrophobic organic molecules insoluble in water, often composed of hydrocarbons.
Different types of lipids include: hydrocarbons, fatty acids, triglycerides, phospholipids, steroids, and prostaglandins. In general, lipids provide insulation as well as energy.
A carbohydrate is an organic molecule containing carbon, hydrogen, and oxygen with a general formula of (C₆H₁₀O₅)₆. Carbohydrates supply energy and contribute to structural support in the body.
Monosaccharides are the simple sugars, such as glucose, fructose and galactose.
Disaccharides are formed by joining two monosaccharides through dehydration synthesis, which removes a water molecule to form a covalent bond. Disaccharides frequently exist as transport sugars in blood/bodily fluids. Examples are sucrose, maltose, and lactose.
Polysaccharides are comprised of many monosaccharides covalently joined. Types include starch, glycogen, and cellulose.
Proteins are large molecules composed of amino acids, providing structural support and functions. They consist of a set 20 amino acids, with varying functional groups. The sequential joining of these amino acids determines the protein's primary structure, secondary structure (alpha helices and beta-sheets), and tertiary structure (final 3D form). The association of multiple proteins defines quaternary structure. Conjugated proteins combine protein with other molecules such as carbohydrates, lipids, or hemes.
Nucleic acids, DNA and RNA, are polymers of nucleotides. DNA carries heredity info via nucleotide sequencing, while RNA plays a role in DNA-directed cellular activities. Both contain pentose sugars (deoxyribose in DNA and ribose in RNA). Four bases exist that form hydrogen bonds between nucleotide chains (A-T; C-G) The precise organization of these base pairs determines what instructions are conveyed.