The Chemical Basis of Life: Atoms, Bonds, and Water

The Chemical Basis of Life

Holistic vs. Reductionist Approaches

Biology can be studied from a holistic perspective, which considers the complexity of entire systems, or a reductionist perspective, which breaks down systems into their component parts for detailed study. Both approaches are valuable for understanding biological phenomena.

Matter, Atoms, and Elements

Definitions and Key Concepts

• Matter: Anything that has mass and occupies space.

• Element: A substance that cannot be broken down into other substances by chemical means.

• Atom: The smallest unit of an element that retains its properties.

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

• Compound: A substance consisting of two or more different elements combined in a fixed ratio.

Elements in the Human Body

Living organisms are composed primarily of a few key elements. The most abundant elements in the human body are oxygen, carbon, hydrogen, and nitrogen.

Element	Symbol	Percentage of Body Weight
Oxygen	O	65.0
Carbon	C	18.5
Hydrogen	H	9.5
Nitrogen	N	3.3
Calcium	Ca	1.5
Phosphorus	P	1.0
Potassium	K	0.4
Sulfur	S	0.3
Sodium	Na	0.2
Chlorine	Cl	0.2
Magnesium	Mg	0.1

Parts of an Atom

Atomic Structure

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles found in the nucleus.

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

Atomic Properties

• Atomic Number: Number of protons in an atom, determines the element.

• Atomic Mass (Mass Number): Sum of protons and neutrons in the nucleus.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Radioisotope: An isotope with an unstable nucleus that emits radiation.

• Ion: An atom or molecule with a net electric charge due to loss or gain of electrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Electron Shells: Energy levels where electrons are found; the outermost is the valence shell.

Electrons and Chemical Bonds

Valence Shells and Stability

The outermost shell of an atom, called the valence shell, determines its chemical reactivity. Atoms tend to share, gain, or lose electrons to achieve a full valence shell, leading to the formation of chemical bonds.

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. Covalent bonds are strong and common in biological molecules.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a covalent bond. Differences in electronegativity between atoms can lead to polar covalent bonds, where electrons are shared unequally, creating partial charges within the molecule.

Ionic Bonds

Ionic bonds form when one atom transfers electrons to another, resulting in the formation of oppositely charged ions that attract each other. For example, sodium (Na) donates an electron to chlorine (Cl), forming sodium chloride (NaCl).

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to a highly electronegative atom (such as oxygen or nitrogen) and another electronegative atom. These bonds are crucial for the structure and properties of water and biological macromolecules.

Van der Waals Interactions

Van der Waals interactions are weak, transient attractions between molecules or parts of molecules that result from transient local partial charges. These interactions are important in the three-dimensional structure of proteins and other macromolecules.

Chemical Reactions and Bonds

Chemical Reactions

• A chemical reaction involves the making and breaking of chemical bonds, converting reactants into products.

• Some reactions are reversible, meaning they can proceed in both directions.

• Chemical equilibrium is reached when the forward and reverse reactions occur at the same rate.

Example: The reaction of hydrogen and oxygen to form water: $2\ \mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\ \mathrm{H}_2\mathrm{O}$

Water and Hydrogen Bonding

Cohesion and Adhesion

Water molecules are held together by hydrogen bonds, resulting in cohesion (attraction between water molecules) and adhesion (attraction between water and other substances). These properties are essential for processes like water transport in plants.

Temperature Moderation

Water has a high specific heat capacity due to hydrogen bonding, allowing it to absorb and release large amounts of heat with minimal temperature change. This property helps stabilize temperatures in organisms and environments.

Density of Water

Water is less dense as a solid (ice) than as a liquid because hydrogen bonds form a crystalline structure in ice, spacing molecules farther apart. This allows ice to float on water, insulating aquatic life in cold climates.

The Universal Solvent

• Solvent: The dissolving agent in a solution (e.g., water).

• Solute: The substance dissolved in the solvent.

• Solution: A homogeneous mixture of solute and solvent.

Water and pH

pH and Ionization

Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH-). The pH scale measures the concentration of H+ ions in a solution:

• pH < 7: Acidic (higher H+ concentration)

• pH = 7: Neutral (equal H+ and OH-)

• pH > 7: Basic (lower H+ concentration)

Acids and Bases

• Acids: Substances that increase H+ concentration in solution (e.g., HCl).

• Bases: Substances that decrease H+ concentration, either by absorbing H+ (e.g., NH3) or releasing OH- (e.g., NaOH).

The pH Scale

The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral. Each unit represents a tenfold difference in H+ concentration.

pH Buffers

Buffers are substances that minimize changes in pH by absorbing or releasing H+ or OH- ions. Biological systems rely on buffers to maintain stable internal pH for proper function.

Molecular Structure and Function

Structure Determines Function

The structure of molecules, determined by the types and arrangements of bonds, is critical to their function in living organisms. Understanding atomic interactions helps explain the behavior of biological molecules.

Example: Endorphin and Morphine

Both endorphin (a natural pain-relieving molecule) and morphine (a drug) have similar shapes, allowing them to bind to the same receptors in the brain and produce similar effects.

Key Terms

• Matter

• Element

• Atom

• Molecule

• Compound

• Protons

• Atomic number

• Atomic mass

• Electrons

• Ions

• Neutrons

• Isotope

• Chemical bonds

• Ionic bonds

• Covalent bonds

• Hydrogen bonds

• Chemical reaction

• Reversible reaction

• Chemical equilibrium

• Solution

• Solvent

• Solute

• pH

• Acids

• Bases

• Buffers