Atoms, Molecules, and Water: Foundations of Biological Chemistry

Nomenclature and Key Terms

This section introduces essential terminology for understanding the chemical basis of life. Mastery of these terms is foundational for further study in biology and biochemistry.

• Chemical bonds: Interactions that hold atoms together in molecules (hydrogen, covalent, ionic).

• Elements: Pure substances consisting of only one type of atom.

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

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

• Compound: Substance formed from two or more different elements in a fixed ratio.

• Hydration shell: The layer of water molecules that surrounds ions or polar molecules in solution.

• Hydrophobic: Substances that do not interact favorably with water.

• Hydrophilic: Substances that interact well with water.

• Solvent – Solute – Solution: The solvent dissolves the solute to form a solution.

• pH: A measure of hydrogen ion concentration; indicates acidity or basicity.

• Buffer: A substance that minimizes changes in pH.

• Hydroxide ion (OH-): A negatively charged ion made of oxygen and hydrogen.

• Proton (H+): A hydrogen ion; a single proton with a positive charge.

Learning Outcomes

By the end of this section, students should be able to:

• Recognize and compare the structural organization of cells: atoms, molecules, compounds.

• Describe different chemical bonds: ionic, covalent, hydrogen.

• Explain the molecular structure of water and its solvent properties.

• Draw the dissociation of water into ions.

• Explain acids and bases.

• Compare acidity and basicity of solutions using the pH scale.

• Predict buffer reaction equilibrium based on pH or solution components.

Levels of Analysis in Biology and Size Range

Biological Scale and Units

Biology examines structures from the molecular to the organismal level. Understanding size scales is crucial for appreciating biological complexity.

• 1 m = 1000 mm

• 1 mm = 1000 μm

• 1 μm = 1000 nm

• 0.1 nm = 1 Å (angstrom)

Cells represent the lowest structural level capable of performing all activities of life.

All Living Things Are Composed of Matter

Definition of Matter

All living organisms are made of matter, which is anything that takes up space and has mass.

• Matter: Composed of elements and compounds.

Cell Composition

Elements in Biological Systems

All matter, including biological systems, is composed of elements.

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

• 92 natural elements exist; 25 are essential for life.

• 96% of living matter is made of four main elements: carbon (C), oxygen (O), nitrogen (N), and hydrogen (H).

• The remaining 4% consists of elements such as sulfur (S), calcium (Ca), potassium (K), and phosphorus (P).

• Trace elements (e.g., iron (Fe), zinc (Zn), copper (Cu)) are required in minute amounts.

Atoms, Molecules, and Compounds

Definitions and Relationships

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

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

• Molecule: Two or more atoms held together by covalent bonds; can be the same or different elements.

Example: Water (H2O) is a molecule and a compound; oxygen gas (O2) is a molecule but not a compound.

Structural Organization of Cells

Hierarchy of Biological Structure

• Atoms (mainly C, H, O) form the foundation.

• Molecules and Compounds (lipids, proteins, etc.) are formed by combinations of atoms.

• Cells are formed by combinations of compounds and molecules.

Properties of Elements: Formation of Molecules and Compounds

Subatomic Particles

Atoms are composed of subatomic particles, which determine their chemical behavior.

• Neutrons: No electrical charge.

• Protons: Positive charge.

• Electrons: Negative charge.

Atomic Structure and Chemical Behavior

• The chemical behavior of an atom is determined by the distribution of electrons in its electron shells.

• The periodic table arranges elements by increasing atomic number, reflecting the sequential addition of protons and electrons.

• Atoms of different elements differ in their number of subatomic particles.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Electron Shells and Chemical Bonds

• Electrons are arranged in shells around the nucleus.

• The outermost shell (valence shell) determines an atom's chemical reactivity.

• Atoms form chemical bonds to achieve stable electron configurations.

Key Equation:

Additional info: Later sections (not shown) likely cover types of chemical bonds, water properties, acids, bases, and buffers in more detail.