The Chemical Context of Life

Atoms, Molecules, and Biological Organization

All living organisms are composed of atoms, which combine to form molecules. These molecules assemble into organelles, cells, and ultimately, complex organisms. Understanding the chemical basis of life is essential for studying biology.

• Atoms: The smallest units of matter that retain the properties of an element.

• Molecules: Two or more atoms held together by chemical bonds.

• Organelles: Specialized structures within cells, composed of various molecules.

• Cells: The basic unit of life, containing organelles and surrounded by a membrane.

Elements Essential to Life

Life depends on a limited number of chemical elements. Four elements—oxygen, carbon, hydrogen, and nitrogen—make up about 96% of living matter. Other elements are required in smaller amounts.

Atomic Structure and Properties

Subatomic Particles

Atoms are composed of three types of subatomic particles: protons, neutrons, and electrons.

Atomic Number, Mass Number, and Isotopes

The atomic number is the number of protons in an atom and defines the element. The mass number is the sum of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons.

• Atomic Number (Z): Number of protons

• Mass Number (A): Number of protons + neutrons

• Isotopes: Atoms with the same atomic number but different mass numbers

Electron Shells and Energy Levels

Electrons occupy specific energy levels or shells around the nucleus. The chemical reactivity of an atom is determined by the electrons in the outermost shell, known as valence electrons.

• The first shell holds up to 2 electrons.

• The second shell holds up to 8 electrons.

• The third shell holds up to 8 electrons.

• Atoms are most stable when their valence shell is full (the "octet rule").

Chemical Bonds and Interactions

Ionic Bonds

Ionic bonds form when one atom donates an electron to another, resulting in oppositely charged ions that attract each other. This typically occurs between atoms with large differences in electronegativity.

• Cation: Positively charged ion (loses electron)

• Anion: Negatively charged ion (gains electron)

• Example: Sodium (Na) donates an electron to chlorine (Cl) to form sodium chloride (NaCl).

Covalent Bonds

Covalent bonds form when two atoms share one or more pairs of electrons. These bonds can be single, double, or triple, depending on the number of shared electron pairs.

• Single bond: Sharing one pair of electrons

• Double bond: Sharing two pairs of electrons

• Triple bond: Sharing three pairs of electrons

• Polar covalent bond: Unequal sharing of electrons, resulting in partial charges (δ+ and δ-)

• Nonpolar covalent bond: Equal sharing of electrons, no charge difference

Electronegativity and Bond Polarity

Electronegativity is the ability of an atom to attract shared electrons. The difference in electronegativity between two atoms determines the type of bond formed:

• Nonpolar covalent bond: Electronegativity difference < 0.5

• Polar covalent bond: Electronegativity difference between 0.5 and 1.7

• Ionic bond: Electronegativity difference > 1.7

Example: In water (H2O), oxygen is more electronegative than hydrogen, resulting in a polar covalent bond.

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between atoms:

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Mnemonic: O.I.L. R.I.G. (Oxidation Is Loss, Reduction Is Gain)

Biological Molecules and Their Chemical Properties

Major Classes of Biomolecules

Cells are composed of four major classes of biomolecules: lipids, proteins, nucleic acids, and carbohydrates. Each class has distinct chemical properties and biological functions.

• Lipids: Hydrophobic molecules, major components of cell membranes

• Proteins: Polymers of amino acids, perform structural and enzymatic functions

• Nucleic acids: DNA and RNA, store and transmit genetic information

• Carbohydrates: Sugars and polysaccharides, provide energy and structural support

Cellular Locations of Biomolecules

Different biomolecules are found in specific cellular locations:

• Lipids: Predominantly in cell membranes and organelle membranes

• DNA (nucleic acids): Located in the nucleus (eukaryotes) or nucleoid region (prokaryotes)

• Proteins: Found throughout the cell, including the cytoplasm, membranes, and organelles

Summary Table: Types of Chemical Bonds

Key Concepts

• Atoms interact via bonds to form molecules, which are essential for the structure and function of biomolecules.

• Bonds and molecular polarity are determined by differences in electronegativity.

• The chemical properties of biomolecules impart functional capabilities to cells and organisms.