The Chemical Basis of Life: Study Notes for General Biology

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The Chemical Basis of Life

Why Biologists Need to Study Chemistry

Understanding chemistry is essential for biologists because all biological processes are rooted in chemical interactions. The molecular level is the foundation of biological organization, and the properties of living organisms arise from the arrangement and interactions of atoms and molecules.

Molecules are the smallest units of biological function.
The arrangement of atoms in a molecule determines its function (e.g., the structure of hemoglobin enables oxygen transport in blood).
Biological function is closely tied to molecular structure.
All living things are composed of matter, which consists of chemical elements.

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 an element, composed of subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge).
Atomic number: Number of protons in the nucleus; defines the element.
Mass number: Sum of protons and neutrons in the nucleus.
Atoms are electrically neutral when the number of protons equals the number of electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

Stable isotopes: Nucleus does not change over time.
Radioactive isotopes: Nucleus decays, emitting particles and energy.
Radioactive isotopes are used in medicine (e.g., cancer treatment) and research (e.g., tracing biological pathways).
Example: Carbon-12 and Carbon-13 are stable; Carbon-14 is radioactive.

Essential Elements of Life

Although many elements exist, only a subset are essential for life. These are classified by their abundance and biological importance.

Element Group	Examples	Role
Major Elements (96.3%)	C, H, O, N	Form the bulk of living matter
Minor Elements (3.7%)	Ca, P, K, S, Na, Cl, Mg	Required in smaller amounts
Trace Elements (<0.01%)	Fe, I, Zn, Cu, etc.	Vital for specific functions (e.g., Fe in hemoglobin)

Additional info: Trace elements, though present in minute quantities, are crucial for health (e.g., iodine for thyroid hormone production).

Chemical Bonds

Types of Chemical Bonds

Atoms combine to form compounds through chemical bonds, which are interactions that hold atoms together.

Covalent bonds: Atoms share electrons; can be single, double, or triple bonds.
Ionic bonds: Electrons are transferred from one atom to another, creating ions that attract each other.
Hydrogen bonds: Weak attractions between a slightly positive hydrogen atom and a slightly negative atom (often oxygen or nitrogen) in another molecule.

Covalent Bonds

Nonpolar covalent bonds: Electrons are shared equally (e.g., O2 molecule).
Polar covalent bonds: Electrons are shared unequally, resulting in partial charges (e.g., H2O molecule).
Electronegativity: The tendency of an atom to attract electrons. Higher electronegativity means stronger attraction.

Example: In water, oxygen is more electronegative than hydrogen, leading to a polar covalent bond.

Ionic Bonds

Formed when one atom donates an electron to another, resulting in oppositely charged ions (e.g., Na+ and Cl- in NaCl).
Ionic compounds are held together by electrostatic attraction.

Hydrogen Bonds

Occur between molecules, not within them.
Weaker than covalent or ionic bonds, but crucial for the structure and properties of water and biological molecules (e.g., DNA, proteins).

Chemistry of Water

Structure and Properties of Water

Water is essential for life due to its unique chemical properties, which arise from its molecular structure and hydrogen bonding.

Water molecule (H2O): Polar, with partial negative charge on oxygen and partial positive charges on hydrogens.
Hydrogen bonds: Form between water molecules, leading to emergent properties.

Emergent Properties of Water

Cohesion: Water molecules stick together due to hydrogen bonding, resulting in surface tension.
Adhesion: Water molecules stick to other substances, aiding in transport in plants.
Moderation of temperature: Water has a high specific heat, absorbing and releasing heat slowly.
Expansion upon freezing: Ice is less dense than liquid water, allowing it to float and insulate aquatic life.
Versatility as a solvent: Water dissolves many substances due to its polarity, making it a universal solvent.

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances that have an affinity for water (e.g., salts, sugars).
Hydrophobic: Substances that repel water (e.g., oils, fats).

Acids, Bases, and Buffers

Acids and Bases

Acids and bases affect the concentration of hydrogen ions (H+) in a solution, influencing pH.

Acid: Donates H+ ions, increasing [H+].
Base: Accepts H+ ions or releases OH-, decreasing [H+].

pH Scale: Measures acidity from 0 (most acidic) to 14 (most basic). Neutral pH is 7.

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

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions.

Crucial for maintaining stable pH in living organisms.
Example: The carbonic acid-bicarbonate buffer system in blood:

When the solution becomes too acidic, the reaction shifts left; when too basic, it shifts right.

Summary Table: Types of Chemical Bonds

Bond Type	Definition	Strength	Example
Covalent (Nonpolar)	Equal sharing of electrons	Strong	O2 molecule
Covalent (Polar)	Unequal sharing of electrons	Strong	H2O molecule
Ionic	Transfer of electrons, attraction between ions	Strong (in dry conditions)	NaCl
Hydrogen	Attraction between partial charges on molecules	Weak	Between water molecules

Key Equations

Dissociation of water:

Carbonic acid-bicarbonate buffer:

Conclusion

All living things are made of matter, which consists of atoms held together by chemical bonds. The unique properties of water and the regulation of acids and bases are essential for life on Earth.