The Chemical Basis of Life: Foundations for General Biology

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The Chemical Basis of Life

Why Do Biologists Need to Study Chemistry?

Understanding chemistry is essential for biology because all biological processes are rooted in chemical interactions. The structure and function of living organisms depend on the arrangement and behavior of atoms and molecules.

Molecules are the fundamental units at the chemical level of biological organization.
The properties of organisms arise from the arrangement of atoms into molecules and the arrangement of molecules into higher-order structures.
There is a direct relationship between molecular structure and biological function. For example, the arrangement of atoms in a hemoglobin molecule enables it to carry oxygen in red blood cells.

Elements, Atoms, and Isotopes

Elements and Atoms

All matter is composed of elements, which are substances that cannot be broken down into simpler substances by chemical reactions. Each element consists of only one type of atom.

Atom: The smallest unit of matter that retains the properties of an element.
Element: A pure substance made of only one kind of atom.
There are about 92 naturally occurring elements, but only a small subset are essential for life (e.g., carbon, hydrogen, oxygen, nitrogen).

Essential Elements of Life

Living organisms require certain elements in large amounts (major elements) and others in trace amounts.

Element	Symbol	Approximate % of Human Body
Oxygen	O	65%
Carbon	C	18%
Hydrogen	H	10%
Nitrogen	N	3%
Other elements (Ca, P, K, S, Na, Cl, Mg, trace elements)	-	4%

Additional info: Trace elements such as iron (Fe) and iodine (I) are required in minute quantities but are vital for functions like oxygen transport and hormone production.

Atomic Structure

An atom consists of a nucleus containing protons and neutrons, surrounded by electrons in electron shells.

Proton: Positively charged particle in the nucleus.
Neutron: Electrically neutral particle in the nucleus.
Electron: Negatively charged particle orbiting the nucleus.
The number of protons defines the atomic number and the element.
Atoms are electrically neutral when they have equal numbers of protons and electrons.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Some isotopes are stable, while others are radioactive (unstable and decay over time, emitting radiation).
Radioactive isotopes can be used as tracers in medical diagnosis and research, but excessive exposure can be harmful.
Example: Carbon-12 and Carbon-13 are stable; Carbon-14 is radioactive and used in radiocarbon dating.

Chemical Bonds

Types of Chemical Bonds

Atoms combine to form compounds through chemical bonds, which result from interactions between electrons in their outermost shells (valence electrons).

Covalent Bonds: Atoms share pairs of electrons. Can be single, double, or triple bonds.
Nonpolar Covalent Bonds: Electrons are shared equally between atoms of similar electronegativity.
Polar Covalent Bonds: Electrons are shared unequally, resulting in partial charges (e.g., in water molecules).
Ionic Bonds: Electrons are transferred from one atom to another, creating ions that are attracted to each other by opposite charges.
Hydrogen Bonds: Weak attractions between a slightly positive hydrogen atom in one molecule and a slightly negative atom (often oxygen or nitrogen) in another molecule.

Electronegativity

Electronegativity is an atom's ability to attract electrons in a chemical bond. The greater the electronegativity, the more strongly an atom pulls shared electrons toward itself.

Oxygen and nitrogen are highly electronegative; hydrogen and carbon are less so.
Differences in electronegativity determine whether a bond is nonpolar covalent, polar covalent, or ionic.

Summary Table: Types of Chemical Bonds

Bond Type	Electron Behavior	Relative Strength	Example
Nonpolar Covalent	Shared equally	Strong	O2, H2
Polar Covalent	Shared unequally	Strong	H2O
Ionic	Transferred	Moderate	NaCl
Hydrogen	Attraction between partial charges	Weak	Between water molecules

The Chemistry of Water

Structure and Polarity of Water

Water (H2O) is a polar molecule, with a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This polarity leads to hydrogen bonding between water molecules.

Hydrogen bonds are responsible for many of water's unique properties.

Emergent Properties of Water

Water's structure gives rise to several properties critical for life:

Cohesion: Water molecules stick together due to hydrogen bonding, contributing to surface tension and the transport of water in plants.
Moderation of Temperature: Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change. This helps stabilize temperatures in organisms and environments.
Expansion Upon Freezing: Ice is less dense than liquid water because hydrogen bonds form a crystalline structure, allowing ice to float and insulate aquatic life.
Versatility as a Solvent: Water dissolves many substances due to its polarity, making it an excellent solvent for biological reactions.

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances that have an affinity for water (e.g., salts, sugars, polar molecules).
Hydrophobic: Substances that do not interact well with water (e.g., oils, nonpolar molecules).

Acids, Bases, and Buffers

Acids and Bases

Acids and bases affect the concentration of hydrogen ions ([H+]) in a solution.

Acid: A substance that donates H+ ions, increasing [H+].
Base: A substance that accepts H+ ions or donates OH- ions, decreasing [H+].

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 (most acidic) to 14 (most basic), with 7 being neutral.

pH is defined as:
Each unit change in pH represents a tenfold change in [H+].
Biological fluids typically have a pH close to 7.

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions as needed. They are crucial for maintaining stable pH in biological systems.

Example: The carbonic acid-bicarbonate buffer system in blood.
Buffer reaction:
This equilibrium helps resist changes in pH when acids or bases are added.

Summary Table: Acids, Bases, and Buffers

Substance	Effect on [H+]	pH Range	Example
Acid	Increases	0-6.9	Gastric juice
Base	Decreases	7.1-14	Household bleach
Buffer	Stabilizes	Near 7	Blood (bicarbonate system)

Key Takeaways

All living things are made of matter, which is composed of atoms held together by chemical bonds.
Chemical bonds (covalent, ionic, hydrogen) determine the structure and function of molecules.
Water's unique properties support life on Earth.
Biological systems are sensitive to acids and bases, and buffers help maintain pH stability.