Chapter 2: The Chemical Basis of Life – Study Notes

Chapter 2: The Chemical Basis of Life

Elements, Atoms, and Compounds

All living organisms are composed of matter, which consists of chemical elements. These elements combine in various ways to form compounds, which are substances made of two or more different elements in a fixed ratio.

• Element: A substance that cannot be broken down into other substances by chemical means. About 25 elements are essential for human life.

• Compound: A substance consisting of two or more elements combined in a fixed ratio (e.g., H2O, NaCl).

• Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96% of the weight of most living organisms.

Trace Elements

Trace elements are required by an organism in only minute quantities but are essential for health and biological function.

• Examples: Iron (Fe), Iodine (I), Fluoride (F).

• Fluoride is added to water and dental products to reduce tooth decay.

• Other trace elements are added to food to preserve it, enhance nutrition, or improve appearance.

Atoms: Structure and Properties

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of three subatomic particles: protons, neutrons, and electrons.

• Protons (+): Positively charged particles found in the nucleus.

• Neutrons (0): Neutral particles also found in the nucleus.

• Electrons (−): Negatively charged particles orbiting the nucleus in electron shells.

• Atomic number: Number of protons in an atom, unique to each element.

• Mass number: Sum of protons and neutrons in the nucleus.

• Atomic mass: Approximately equal to the mass number.

Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are radioactive, meaning they decay spontaneously, releasing particles and energy.

• Stable isotopes: Do not change over time (e.g., Carbon-12, Carbon-13).

• Radioactive isotopes: Unstable and can be used as tracers in biological research and medicine (e.g., PET scans).

• Applications: Radioactive tracers help monitor the fate of atoms in living organisms but can also pose health risks.

	Carbon-12	Carbon-13	Carbon-14
Protons	6	6	6
Neutrons	6	7	8
Electrons	6	6	6
Mass number	12	13	14

Chemical Bonds

Electron Shells and Chemical Properties

Electrons occupy specific energy levels called electron shells. The chemical properties of an atom are determined by the distribution of electrons, especially in the outermost shell (valence shell).

• Atoms with incomplete outer shells tend to interact with other atoms to achieve stability, forming chemical bonds.

Covalent Bonds

A covalent bond forms when two atoms share one or more pairs of electrons. Covalent bonds can be nonpolar (equal sharing) or polar (unequal sharing).

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2).

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Ionic Bonds

An ionic bond is formed when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other.

• Ion: An atom or molecule with an electrical charge due to the loss or gain of electrons.

• Example: Sodium chloride (NaCl) forms when a sodium atom donates an electron to a chlorine atom.

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and an electronegative atom (often oxygen or nitrogen) in another molecule. These bonds are crucial in the structure and properties of water and biological macromolecules.

• Hydrogen bonds give water its unique properties, such as cohesion, adhesion, and high specific heat.

Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds, converting reactants into products. Matter is neither created nor destroyed in these reactions; it is rearranged.

• Example: Formation of water from hydrogen and oxygen.

$2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$

Water’s Life-Supporting Properties

Cohesion, Adhesion, and Surface Tension

Water molecules stick together (cohesion) and to other substances (adhesion) due to hydrogen bonding. Surface tension is a measure of how difficult it is to break the surface of a liquid.

• Cohesion: Responsible for water transport in plants and the formation of droplets.

• Adhesion: Helps water cling to cell walls and other surfaces.

Temperature Moderation

Water moderates temperature due to its high specific heat, which is a result of hydrogen bonding. When water evaporates, it cools the surface (evaporative cooling).

• Thermal energy: Energy associated with the random movement of atoms and molecules.

• Heat: Transfer of thermal energy from a warmer to a cooler body.

• Temperature: Measures the intensity of heat.

Ice: Solid Water is Less Dense

Unlike most substances, water is less dense as a solid than as a liquid. This is due to the stable hydrogen bonds that form a crystalline structure in ice, spacing the molecules farther apart.

• This property allows ice to float, insulating aquatic life in cold climates.

Water as a Solvent

Water is known as the "solvent of life" because its polarity allows it to dissolve a wide variety of substances, forming aqueous solutions.

• Solution: A liquid mixture in which the minor component (solute) is uniformly distributed within the major component (solvent).

• Aqueous solution: A solution in which water is the solvent.

Acids, Bases, and pH

The chemistry of life is sensitive to acidic and basic conditions. The pH scale measures the concentration of hydrogen ions (H+) in a solution.

• Acid: A substance that increases the H+ concentration of a solution (pH < 7).

• Base: A substance that reduces the H+ concentration (pH > 7).

• Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions.

Additional info: The pH scale is logarithmic; each unit represents a tenfold difference in H+ concentration. For example, a solution at pH 4 has 10,000 times more H+ than a solution at pH 8.