The Chemistry of Life

Introduction

The study of biology requires an understanding of the chemical principles that underlie all living systems. This section explores the elements found in living organisms, the structure of atoms, and the types of chemical bonds that form the basis of biological molecules.

Elements in Living Systems

Essential Elements

• Element: A substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.

• Essential elements: Elements required for an organism’s survival. Humans require about 25 essential elements, while plants may require fewer.

• Major elements in the human body: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) make up approximately 96% of body mass.

• Trace elements: Elements required in minute quantities (e.g., iron, iodine, zinc).

Example: Oxygen is essential for cellular respiration, while iron is necessary for oxygen transport in blood.

Matter and Its Composition

Definition and Properties

• Matter: Anything that takes up space and has mass. All living and non-living things are composed of matter.

• Biotic factors: Living components of an ecosystem (e.g., plants, animals, fungi, bacteria).

• Abiotic factors: Non-living components (e.g., rocks, water, air).

Atoms: The Building Blocks of Matter

Structure of Atoms

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

• Subatomic particles:

  • Proton: Positively charged particle, mass ≈ 1 Dalton, located in the nucleus.

  • Neutron: Neutral particle, mass ≈ 1 Dalton, located in the nucleus.

  • Electron: Negatively charged particle, mass ≈ 1/2000 Dalton, found in orbitals around the nucleus.

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

• Mass number (A): Sum of protons and neutrons in the nucleus.

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

Example: Hydrogen has three isotopes: protium (no neutrons), deuterium (one neutron), and tritium (two neutrons).

Properties of Compounds

Atomic Structure and Bonding

• Compound: A substance consisting of two or more elements combined in a fixed ratio (e.g., H2O, NaCl).

• Emergent properties: Compounds have characteristics different from those of their constituent elements.

• Properties of a compound: Determined by the types of atoms present and how they are bonded together.

Example: Formic acid (CH2O2) contains carbon, hydrogen, and oxygen. Its properties depend on the arrangement and bonding of these atoms.

Chemical Bonds

Types of Chemical Bonds

• Covalent bond: Sharing of a pair of valence electrons between atoms. Can be single, double, or triple bonds.

• Ionic bond: Attraction between oppositely charged ions (cation and anion) formed by the transfer of electrons.

• Hydrogen bond: Weak attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

• Van der Waals interactions: Weak attractions due to transient local partial charges.

• Hydrophobic interactions: Nonpolar substances aggregate to avoid contact with water.

Electronegativity and Bond Polarity

• Electronegativity: The tendency of an atom to attract electrons in a covalent bond.

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2, O2).

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Example: In water, oxygen is more electronegative than hydrogen, creating a polar molecule with partial positive and negative charges.

Molecular Shape and Function

Importance of Shape

• Molecular shape: Determined by the positions of atoms’ orbitals; crucial for the function of biological molecules.

• Shape determines function: Molecules with similar shapes can have similar biological effects (e.g., morphine and endorphins).

Example: The shape of a neurotransmitter allows it to bind to specific receptors in the brain.

Chemical Reactions

Formation and Breaking of Bonds

• Chemical reaction: The making and breaking of chemical bonds, leading to changes in the composition of matter.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of the reaction.

• Reversible reactions: Reactions that can proceed in both directions.

Example:

Factors Affecting Reaction Rates

• Temperature: Heating reactants increases reaction rates by increasing molecular collisions.

• Catalysts: Substances that increase reaction rates without being consumed. In biological systems, most catalysts are proteins called enzymes.

• Concentration: Higher concentrations of reactants increase the likelihood of collisions and reactions.

Chemical Equilibrium

• Equilibrium: The point at which the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.

Example:

Summary Table: Types of Chemical Bonds

Additional info: Some details, such as the specific percentages of elements in the human body and the full list of trace elements, were inferred based on standard biology knowledge.