Unit 1 establishes the chemical foundation for everything else in AP Biology. The behavior of living systems traces back to the structure of atoms, the polar nature of water, and the way carbon links together to form large molecules. In this unit you will see a recurring theme: structure determines function. The arrangement of atoms in a molecule, the bonds between them, and the overall shape that results all dictate what a molecule can do inside a cell.
You will focus on the properties of water, the elements common to living things, and the four major classes of biological macromolecules — carbohydrates, lipids, proteins, and nucleic acids — including how they are built and broken down.
Although about 25 elements occur in living organisms, just six make up roughly 96% of living matter: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S), often abbreviated CHNOPS. These elements are central because they form stable covalent bonds and combine into the molecules of life.
Carbon is especially versatile: with four valence electrons it can form four covalent bonds, building long chains, branches, and rings. This bonding flexibility is why carbon serves as the backbone of organic molecules. Nitrogen and phosphorus deserve special attention — nitrogen is found in amino acids and nucleotides, while phosphorus is a key component of phospholipids, ATP, and the sugar-phosphate backbone of nucleic acids.
A water molecule (H2O) is polar: oxygen is more electronegative than hydrogen, so it pulls the shared electrons closer, giving the oxygen end a partial negative charge and the hydrogen ends a partial positive charge. This uneven distribution of charge across the molecule makes water a polar covalent molecule, even though the molecule as a whole is electrically neutral.
The bent shape of the molecule and the polarity together are responsible for nearly all of water's life-supporting properties. Because so much of biology happens in aqueous solution, understanding water's polarity is essential.
Polarity allows water molecules to form hydrogen bonds with one another — weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of a neighbor. Although each hydrogen bond is weak, collectively they produce water's emergent properties.
Cohesion is the attraction between water molecules, which produces high surface tension and helps water move upward in plants (transpiration). Adhesion is attraction between water and other surfaces. Water also has a high specific heat and high heat of vaporization, allowing it to resist temperature change and to cool surfaces through evaporation. Finally, ice is less dense than liquid water because hydrogen bonds lock molecules into an open crystal lattice, so ice floats.
Water's polarity makes it an excellent solvent for charged and polar substances. Polar and ionic substances that dissolve in water are hydrophilic ("water-loving"), while nonpolar substances that do not dissolve are hydrophobic ("water-fearing"). When an ionic compound such as table salt dissolves, water molecules surround each ion, forming hydration shells that pull the ions apart.
This selective solubility matters biologically: it explains why nutrients and ions travel dissolved in blood and cytoplasm, and why hydrophobic regions, such as the interior of a membrane, exclude water and water-soluble molecules.
Carbohydrates are made of carbon, hydrogen, and oxygen, typically in a ratio of about 1:2:1. Their monomers are monosaccharides (simple sugars) such as glucose, fructose, and galactose. Two monosaccharides join to form a disaccharide (for example, sucrose), and many join to form a polysaccharide.
Polysaccharides serve two main roles: energy storage and structure. Starch (in plants) and glycogen (in animals) store energy, while cellulose (in plant cell walls) and chitin (in fungal walls and arthropod exoskeletons) provide structural support. The same glucose monomer can build either storage or structural polymers depending on how the units are linked, illustrating that subtle structural differences create large functional differences.
Lipids are a diverse group united by being largely hydrophobic (nonpolar). Unlike the other three classes, lipids are not true polymers. Triglycerides (fats and oils) consist of a glycerol molecule bonded to three fatty acids. Saturated fatty acids have no carbon-carbon double bonds, pack tightly, and are usually solid at room temperature; unsaturated fatty acids have one or more double bonds that create kinks, keeping them liquid.
Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails. This amphipathic structure causes them to self-assemble into bilayers in water, forming the basis of cell membranes. Steroids, such as cholesterol, share a four-ring carbon skeleton and serve roles in membranes and as hormones.
Proteins are polymers of amino acids, of which there are 20 standard types. Each amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable R group (side chain) that determines its chemical properties. Amino acids are joined by peptide bonds to form polypeptides.
Protein shape arises across four levels. Primary structure is the linear sequence of amino acids. Secondary structure consists of local folding into alpha helices and beta pleated sheets, stabilized by hydrogen bonds along the backbone. Tertiary structure is the overall three-dimensional shape formed by interactions among R groups (including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges). Quaternary structure occurs when two or more polypeptide subunits combine, as in hemoglobin. Because shape determines function, conditions that disrupt these interactions — heat or extreme pH — can cause denaturation and loss of function.
Nucleic acids — DNA and RNA — store and transmit genetic information. Their monomers are nucleotides, each built from a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. DNA bases are adenine, thymine, guanine, and cytosine; RNA uses uracil in place of thymine.
Nucleotides link through phosphodiester bonds to form a sugar-phosphate backbone with directionality: one end is the 5' (five-prime) end and the other is the 3' (three-prime) end. The carbons of the sugar are numbered, and new nucleotides are added only to the 3' end, so strands are always synthesized in the 5' → 3' direction. In double-stranded DNA, the two strands are antiparallel, running in opposite directions and held together by hydrogen bonds between complementary base pairs.
All four macromolecule classes (and lipids in part) are assembled and disassembled by the same paired reactions. Dehydration synthesis (a condensation reaction) joins two monomers by removing a molecule of water, forming a covalent bond between them. Hydrolysis does the reverse: it adds a water molecule to break a bond, separating monomers.
These reactions are central to metabolism. Cells use dehydration synthesis to build storage molecules and structural components, and they use hydrolysis — for example, in digestion — to break large molecules into usable subunits. Recognizing whether water is consumed or released tells you which direction a reaction is going.
Practise exam-style questions on this topic.