Atoms, Molecules, and Water: Foundations of Cell Composition and Chemical Bonds

Atoms, Molecules, and Water

Nomenclature and Key Terms

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

• Chemical bonds: Interactions that hold atoms together in molecules and compounds. Types include hydrogen, covalent, and ionic bonds.

• Elements: Pure substances that cannot be broken down by chemical means.

• Atoms: The smallest unit of an element retaining its chemical 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: Layer of water molecules surrounding a solute in solution.

• Hydrophobic: Repelled by water; nonpolar substances.

• Hydrophilic: Attracted to water; polar substances.

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

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

• Buffer: Substance that minimizes changes in pH.

• Hydroxide ion (OH-): Contributes to basicity in solutions.

• Proton (H+): Hydrogen ion; central to acid-base chemistry.

Learning Outcomes

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

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

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

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

• Draw the dissociation of a water molecule into ions.

• Explain what acids and bases are.

• Compare and explain the acidity and basicity of different solutions based on the pH scale.

• Evaluate and solve buffer problems: predict buffer reaction equilibrium based on changing pH or solution components.

Levels of Analysis in Biology and Size Range

Biological Scale and Measurement Units

Biology examines structures from the molecular to the organismal level. Understanding size scales is crucial for appreciating cellular and molecular processes.

• 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.

Composition of Living Things

Matter and Its Role in Biology

All living things are composed of matter, which is anything that takes up space and has mass. Biological systems are organized from atoms to molecules to cells.

• Matter: Anything with mass and volume.

• Elements: Fundamental substances; cannot be broken down further.

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

• 96% of living matter is made of four main elements: Carbon (C), Oxygen (O), Nitrogen (N), Hydrogen (H).

• The remaining 4% consists of elements like Sulfur (S), Calcium (Ca), Phosphorus (P), and trace elements (e.g., Iron (Fe), Zinc (Zn), Copper (Cu)).

Atoms, Molecules, and Compounds

Atoms are the building blocks of matter. Molecules and compounds are formed by the combination of atoms through chemical bonds.

• Atom: Smallest unit of matter retaining element properties.

• Compound: Two or more different elements combined in a fixed ratio (e.g., H2O, NaCl).

• Molecule: Two or more atoms held together by covalent bonds; can be the same or different elements (e.g., O2, H2O).

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

Structural Organization of Cells

Cells are formed from combinations of molecules and compounds, which in turn are formed from atoms.

• Atoms (C, H, O, N) are 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 and ability to form bonds.

• Neutrons: No electrical charge; found in the nucleus.

• Protons: Positive charge; found in the nucleus.

• Electrons: Negative charge; orbit the nucleus in electron shells.

Example: Hydrogen atom has 1 proton and 1 electron; oxygen atom has 8 protons, 8 neutrons, and 8 electrons.

Electron Shells 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 the number of subatomic particles.

• Electron configuration influences bonding and reactivity.

Example: Atoms with full outer electron shells are chemically inert (e.g., noble gases).

Table: Comparison of Subatomic Particles

Particle	Charge	Location
Proton	+1	Nucleus
Neutron	0	Nucleus
Electron	-1	Electron shell/orbit

Additional info:

• Understanding the structure and properties of atoms is essential for grasping how chemical bonds form and how biological molecules interact.

• Water's unique properties as a solvent are central to cellular processes and life itself.