What you'll learn
All life is built on chemistry — from the water that fills cells to the large molecules that store energy and information. This is the focus of the Chemistry of Life unit (Unit 1 of AP Biology). For AP Biology you need to understand the properties of water, the structure and function of the four biological macromolecules, and how their structure relates to their function. This guide covers water and its properties, carbon and functional groups, the four macromolecules, and the structure–function relationship. By the end you should be able to explain why water is essential to life and describe the biological macromolecules.
Key terms and definitions
Polarity — An uneven distribution of charge in a molecule, as in water.
Hydrogen bond — A weak attraction between a slightly positive and a slightly negative region of molecules.
Macromolecule — A large biological molecule (carbohydrate, lipid, protein, or nucleic acid).
Monomer — A small unit that joins with others to form a polymer.
Polymer — A large molecule made of many monomers.
Dehydration synthesis — Joining monomers by removing a water molecule.
Hydrolysis — Breaking a polymer by adding a water molecule.
Enzyme — A protein that catalyses (speeds up) a biological reaction.
Core concepts
Water and its properties
Water is essential to life, and its special properties come from its polarity — the oxygen atom is slightly negative and the hydrogen atoms slightly positive, so water molecules attract each other through hydrogen bonds. This gives water several life-supporting properties:
- Cohesion and adhesion — water molecules stick to each other (cohesion) and to other surfaces (adhesion), allowing, for example, water to move up plants.
- High specific heat — water resists temperature change, helping organisms and environments stay stable.
- High heat of vaporisation — evaporation removes a lot of heat, allowing cooling (such as sweating).
- Ice is less dense than liquid water — so ice floats, insulating the water below.
- A good solvent — its polarity lets it dissolve many substances, so it is the medium for reactions in cells.
Carbon and functional groups
Life is carbon-based. Carbon can form four covalent bonds, allowing it to build large, complex and varied molecules, including chains and rings. Attached to these carbon skeletons are functional groups — specific groups of atoms (such as hydroxyl, carboxyl, amino and phosphate) that give molecules their particular chemical properties. The arrangement of carbon and functional groups determines a molecule's structure and behavior.
Building and breaking macromolecules
The large biological molecules are polymers built from smaller monomers:
- Dehydration synthesis (condensation) joins monomers together by removing a water molecule.
- Hydrolysis breaks polymers apart by adding a water molecule.
These two opposite reactions are how cells build and break down the macromolecules as needed.
The four macromolecules
There are four classes of biological macromolecule:
- Carbohydrates — made of monosaccharides (such as glucose). They provide energy (glucose) and structure (cellulose in plants).
- Lipids — including fats, phospholipids and steroids. They store energy, form cell membranes (phospholipids), and act as hormones. Lipids are not true polymers but are grouped here.
- Proteins — made of amino acid monomers joined by peptide bonds. They have an enormous range of functions: enzymes, structure, transport, defence and signalling. A protein's function depends on its folded shape.
- Nucleic acids — DNA and RNA, made of nucleotide monomers. They store and carry genetic information.
Structure determines function
A central theme of AP Biology is that structure determines function. This is clear in the macromolecules: the sequence of amino acids determines how a protein folds, and its shape determines its function — for example, an enzyme's active site must have the right shape to fit its substrate. Similarly, the structure of phospholipids (with water-attracting heads and water-repelling tails) allows them to form cell membranes. Understanding this relationship is key to the whole course.
Enzymes
Enzymes are proteins that act as biological catalysts, speeding up reactions by lowering the activation energy needed. Each enzyme has an active site with a shape specific to its substrate. Enzyme activity is affected by temperature and pH: each enzyme has an optimum, and extremes can denature it (change its shape so the substrate no longer fits). Enzymes are essential because they allow the reactions of life to happen quickly at body temperature.
Acids, bases and pH in living systems
The chemistry of life also depends on pH, a measure of how acidic or basic a solution is, on a scale from 0 (very acidic) to 14 (very basic), with 7 being neutral. Acids release hydrogen ions (H⁺) and lower the pH, while bases accept hydrogen ions and raise it. Living systems are very sensitive to pH, because enzymes and other proteins only work within a narrow range — a change in pH can denature them. To keep pH stable, organisms use buffers, which resist changes in pH by absorbing or releasing hydrogen ions. For example, buffers in the blood keep it close to a pH of about 7.4. Understanding pH and buffering explains why maintaining a stable internal chemical environment (homeostasis) is essential for life.
Monomers and polymers across the macromolecules
It is worth seeing the monomer–polymer pattern that runs through the macromolecules, because it ties the unit together. Carbohydrates are polymers (polysaccharides such as starch, glycogen and cellulose) built from monosaccharide monomers like glucose. Proteins are polymers (polypeptides) built from amino acid monomers, of which there are twenty, joined by peptide bonds. Nucleic acids are polymers built from nucleotide monomers, each made of a sugar, a phosphate and a base. Lipids are the exception — they are large molecules but not true polymers of repeating monomers. Recognising that three of the four groups follow the same build-from-monomers pattern, joined by dehydration synthesis and broken by hydrolysis, makes the whole unit easier to remember and connects structure to how these molecules are made.
Worked examples
Example 1: Why ice floats matters
Explain why the fact that ice is less dense than water is important for life. Because ice is less dense than liquid water, it floats and forms a layer on top of ponds and lakes. This insulates the water below, allowing organisms to survive the winter in the liquid water beneath the ice.
Example 2: Dehydration synthesis
Describe how two monomers are joined into a polymer. In dehydration synthesis (condensation), the two monomers are joined by removing a molecule of water, forming a covalent bond between them. Repeating this builds a polymer. Hydrolysis reverses this by adding water to break the bond.
Example 3: Structure and function of a protein
Explain why the shape of a protein is important for its function. A protein's function depends on its three-dimensional shape, which is determined by its amino acid sequence. For example, an enzyme's active site must have a shape complementary to its substrate; if the shape is wrong (or the protein is denatured), it cannot perform its function.
Example 4: Identifying a macromolecule
A molecule is made of amino acid monomers and folds into a specific shape to act as a catalyst. What type of macromolecule is it? A protein — specifically an enzyme. Proteins are made of amino acids, and their folded shape gives them their function, including acting as catalysts.
Common mistakes and how to avoid them
A common error is not linking water's properties to its polarity and hydrogen bonding. Its cohesion, high specific heat and solvent properties all come from these, so mention them.
Students often confuse dehydration synthesis and hydrolysis. Dehydration synthesis removes water to join monomers; hydrolysis adds water to break them. Remember "hydro-lysis" = split with water.
Another mistake is mixing up the macromolecules and their monomers. Learn them: carbohydrates (monosaccharides), proteins (amino acids), nucleic acids (nucleotides); lipids are not true polymers.
When discussing proteins, remember that structure determines function — the shape, set by the amino acid sequence, determines what the protein does. This is a recurring theme.
Finally, for enzymes, do not say they are "used up". Enzymes are catalysts and are not consumed; they can be used repeatedly, and extremes of temperature or pH denature them (change their shape).
Exam technique for "Chemistry of Life"
Be ready to link water's life-supporting properties to its polarity and hydrogen bonding, and to give specific examples (cohesion, high specific heat, ice floating, solvent).
Know the four macromolecules, their monomers and functions, and the dehydration synthesis / hydrolysis reactions that build and break them. Emphasise the theme that structure determines function, especially for proteins and enzymes.
AP Biology rewards applying and connecting concepts, so be ready to explain how a molecule's structure suits its role, and to interpret data on enzyme activity. Use precise terms — polarity, hydrogen bond, monomer, polymer, active site, denature — throughout.
Quick revision summary
- Water is polar and forms hydrogen bonds, giving cohesion, adhesion, high specific heat, high heat of vaporisation, floating ice, and solvent properties — all essential to life.
- Carbon forms four bonds, building complex molecules; functional groups give them their properties.
- Polymers are built by dehydration synthesis (remove water) and broken by hydrolysis (add water).
- Four macromolecules: carbohydrates (monosaccharides; energy/structure), lipids (energy, membranes, hormones; not true polymers), proteins (amino acids; huge range of functions), nucleic acids (nucleotides; genetic information).
- Structure determines function — a protein's folded shape (from its amino acid sequence) determines what it does.
- Enzymes are protein catalysts with a specific active site; temperature and pH affect them and extremes denature them.