Chemical Context of Life

Chemistry – Study of Matter

All living organisms are composed of matter, and understanding its properties is fundamental to biology. The laws of chemistry apply to both living and nonliving systems, and the interactions of matter's building blocks determine the structure and function of biological molecules.

• Matter: Anything that occupies space and has mass.

• Chemistry: The study of matter and its interactions.

• Understanding matter helps explain biological processes and the properties of larger structures.

Elements and Compounds

Definitions and Properties

Matter is composed of elements and compounds. Elements are pure substances, while compounds are combinations of elements in fixed ratios, often with properties distinct from their constituent elements.

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

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

• Compounds have characteristics different from those of their elements. Example: Sodium (Na, a reactive metal) and chlorine (Cl, a toxic gas) combine to form sodium chloride (NaCl, table salt), which is safe to eat.

Elements of Life

Essential and Trace Elements

Of the 92 naturally occurring elements, only a subset is essential for life. Four elements make up the majority of living matter, while others are required in smaller amounts.

• Essential Elements: About 20-25% of elements are required for life.

• Major Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) constitute about 96% of living matter.

• Other Important Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), and Magnesium (Mg) make up most of the remaining 4%.

• Trace Elements: Required in minute quantities (e.g., iron, iodine, copper).

Atoms – Building Blocks of Matter

Atomic Structure

Each element consists of unique atoms, which are the smallest units retaining the properties of an element. Atoms are composed of subatomic particles.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles also in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus in a 'cloud'.

• Atomic Number: Number of protons in the nucleus (defines the element).

• Mass Number: Sum of protons and neutrons.

• Atomic Mass: Total mass of an atom, measured in Daltons (Da).

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Isotopes and Radioactivity

Atoms of the same element may differ in neutron number, forming isotopes. Some isotopes are unstable and radioactive.

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

• Radioactive Isotope: Decays spontaneously, emitting particles and energy.

• Application: Radioactive tracers are used in medicine to track metabolic processes.

Chemical Properties – Electrons

Energy Levels and Electron Shells

The chemical behavior of an atom is determined by the distribution of electrons in electron shells. Electrons possess potential energy based on their position relative to the nucleus.

• Electron Shell: Energy level where electrons are found.

• Orbital: Three-dimensional space where an electron is found 90% of the time.

• Each shell has a specific number of orbitals and electrons.

Valence Electrons and Chemical Reactivity

Valence electrons are those in the outermost shell and determine an atom's chemical properties. Atoms with incomplete valence shells are reactive and tend to form chemical bonds.

• Atoms with full valence shells are chemically inert.

• Other atoms interact to achieve full valence shells, often through chemical bonding.

Chemical Bonds

Ionic Bonds

Ionic bonds form when atoms transfer electrons, resulting in charged ions that attract each other.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• Ionic Bond: Attraction between cation and anion.

• Example: Sodium (Na) transfers an electron to chlorine (Cl), forming Na+ and Cl-, which combine to make NaCl (table salt).

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. These bonds are fundamental to the structure of biological molecules.

• Single Bond: Sharing one pair of electrons.

• Double Bond: Sharing two pairs of electrons.

• Triple Bond: Sharing three pairs of electrons.

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

• Molecular Formula: Shows the number of each type of atom (e.g., H2O).

• Structural Formula: Shows which atoms are bonded (e.g., H-O-H).

• Lewis Dot Structure: Shows valence electrons and bonding (e.g., :O:H).

Electronegativity and Bond Polarity

Electronegativity is an atom's ability to attract electrons in a covalent bond. Differences in electronegativity lead to polar and nonpolar covalent bonds.

• Nonpolar Covalent Bond: Electrons are shared equally (similar electronegativity).

• Polar Covalent Bond: Electrons are shared unequally (different electronegativity), resulting in partial charges.

• High Electronegativity: F, O, N, Cl

• Medium Electronegativity: C, H

• Low Electronegativity: Metals (important for ionic bonds)

Intermolecular Forces

Types of Interactions

Large biological molecules are stabilized by weak intermolecular forces, which allow for temporary interactions essential for biological function.

• Dipole-Dipole: Attraction between polar molecules.

• Hydrogen Bonds: Special dipole-dipole interaction involving hydrogen bonded to highly electronegative atoms (O or N).

• Van der Waals Forces: Weak attractions due to transient electron movements; collectively can be significant (e.g., gecko's toe hairs).

Molecular Shape and Function

Shape Determines Function

The shape of a molecule, determined by the positions of its atoms and orbitals, is crucial for its biological function. Covalent bonds can lead to specific molecular shapes through orbital hybridization.

• Molecular shape affects how biological molecules interact (e.g., enzyme-substrate binding, hormone-receptor interaction).

• Shape and charge together determine molecular recognition and function.

• Example: Endorphins and morphine have similar shapes, allowing both to bind to endorphin receptors.

Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the rearrangement of atoms by making and breaking chemical bonds. Reactants are transformed into products.

• Reactants: Starting molecules in a chemical reaction.

• Products: Resulting molecules after the reaction.

• Example: Photosynthesis:

• Photosynthesis converts carbon dioxide and water into glucose and oxygen using sunlight.

• Chemical reactions are reversible; products can become reactants in the reverse reaction.

• Chemical Equilibrium: Reached when forward and reverse reactions occur at the same rate; concentrations of reactants and products remain constant.

Summary of Key Terms

• Matter

• Element

• Compound

• Atom

• Isotope

• Energy

• Chemical Equilibrium

Additional info: These notes expand on the original lecture slides and text, providing definitions, examples, and context for key concepts in the chemical basis of life, suitable for General Biology students.