Biology: Cells to AP Biology · Module 1 of 15
The Chemistry of Life
The atoms, bonds, water properties and macromolecules every living cell is built from.
Atoms, Elements & Bonding in Living Things
- Atoms are made of protons, neutrons and electrons; the number of protons (atomic number) determines the element.
- Ionic bonds form when electrons transfer between atoms; covalent bonds form when atoms share electrons — most bonds in living things are covalent.
- Hydrogen bonds are weak attractions involving a hydrogen atom already covalently bonded to one electronegative atom (like O or N) and another electronegative atom nearby — they can form between separate molecules (e.g. water to water) or within a single large molecule (e.g. the backbone hydrogen bonds that hold a protein's alpha helix together), and are crucial for water's properties and DNA's double-helix shape.
Why are covalent bonds, not ionic bonds, the main bonds holding together molecules like glucose and DNA?
Solution. Covalent bonds involve sharing electrons between the nonmetal atoms (carbon, hydrogen, oxygen, nitrogen) common in biological molecules, forming stable connections; ionic bonds mainly occur between metals and nonmetals forming charged ions, which is far less common in organic molecules.
Water & Its Properties
- Water is polar (unequal charge distribution), letting it form hydrogen bonds with other water molecules and dissolve many substances — the "universal solvent."
- Cohesion (water sticking to itself) creates surface tension; adhesion (water sticking to other surfaces) helps water move up plant stems.
- Water has a high specific heat, meaning it resists temperature change — this helps organisms and climates stay stable.
Why can some insects walk on the surface of a pond?
Solution. Hydrogen bonds between water molecules create cohesion, forming surface tension strong enough to support the insect's weight without breaking.
Macromolecules: Carbs, Lipids, Proteins, Nucleic Acids
- Carbohydrates are built from monosaccharide monomers; they provide quick energy and structural support (e.g. cellulose).
- Lipids (fats, oils, waxes) are mostly nonpolar and store long-term energy; phospholipids form cell membranes.
- Proteins are built from amino acid monomers folded into specific 3D shapes; they do most of the cell's work (enzymes, transport, structure).
- Nucleic acids (DNA, RNA) are built from nucleotide monomers and store and transmit genetic information.
Which macromolecule would you expect an enzyme to be made of, and why?
Solution. Enzymes are proteins — their specific folded 3D shape creates an active site that fits a particular substrate, letting them speed up a specific reaction.
Enzymes & Biological Reactions
- Enzymes are biological catalysts (usually proteins) that lower the activation energy a reaction needs, speeding it up without being used up.
- An enzyme's active site binds a specific substrate (the "induced fit" model).
- Enzyme activity is affected by temperature and pH; extreme conditions can denature (unfold) an enzyme, destroying its function.
Why might a fever over 104°F (40°C) be dangerous to the human body?
Solution. High temperatures can denature enzymes — unfolding their 3D shape so the active site no longer fits its substrate, disrupting the chemical reactions the body depends on.
Common mistake: An enzyme is not "used up" or permanently changed by a reaction — it's reused to catalyze more reactions of the same type.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What determines which element an atom is?
2. Which type of bond involves atoms sharing electrons?