Chemistry of Life

What this unit covers

The topics below follow the published Biology course framework for Unit 1. This unit is worth 8–11% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Water & Its Properties12 min · 3 objectivesExplain how polarity and hydrogen bonding arise from water’s structure · Connect hydrogen bonding to cohesion, adhesion, and thermal stability · Predict how water behaves as the universal biological solvent
• Carbohydrates & Lipids13 min · 3 objectivesIdentify monomers and polymers of carbohydrates and describe their roles · Explain how dehydration synthesis and hydrolysis build and break polymers · Contrast the structure of saturated and unsaturated lipids and predict their properties
• Proteins & Enzyme Structure14 min · 3 objectivesDescribe how amino acids link into polypeptides via peptide bonds · Distinguish the four levels of protein structure and what stabilizes each · Explain how a protein’s shape determines its function, including enzyme activity
• Nucleic Acids: DNA & RNA Structure12 min · 3 objectivesIdentify the three parts of a nucleotide and how nucleotides link into a strand · Apply complementary base pairing to determine a partner strand · Compare the structures of DNA and RNA and relate antiparallel strands to structure
• Water’s Emergent Properties (Depth)14 min · 3 objectivesTrace each emergent property of water back to hydrogen bonding as a shared cause · Use q = mcΔT to reason quantitatively about water’s high specific heat and heat of vaporization · Explain why ice floats from the geometry of the hydrogen-bonded crystal lattice
• Comparing the Four Macromolecules (Depth)14 min · 3 objectivesMatch each macromolecule to its monomer, characteristic bond, and biological role · Quantify water exchanged in dehydration synthesis and hydrolysis for any polymer or fat · Relate saturation in fats and the four levels of protein structure to physical behavior

Formulas in Unit 1

Every term in Unit 1

All 34 terms we publish for Chemistry of Life, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Hydrogen bonding in water

Weak attraction between the partial positive H of one water molecule and the partial negative O of another. Produces cohesion, adhesion, high specific heat and ice floating.

Cohesion vs adhesion

Cohesion is water sticking to water; adhesion is water sticking to other polar surfaces. Together they pull the transpiration stream up a xylem column.

High specific heat of water

Hydrogen bonds absorb heat before molecules speed up, so water resists temperature change — buffering cells and whole climates.

Evaporative cooling

The most energetic molecules leave first, lowering the average kinetic energy of what remains. Why sweating cools you and why leaves do not overheat.

Why ice floats

Below 4°C hydrogen bonds lock water into an open lattice that is less dense than liquid water, so lakes freeze from the top and life survives beneath.

pH scale

Negative log of hydrogen ion concentration. Each unit is a tenfold change, so pH 5 has ten times the H⁺ of pH 6.

Buffer

A weak acid and its conjugate base that absorb added H⁺ or OH⁻, holding pH nearly constant — the carbonic acid–bicarbonate system does this in blood.

Dehydration synthesis

Monomers join by removing a water molecule, forming a covalent bond. Builds every biological polymer.

Hydrolysis

Adding water breaks a covalent bond between monomers. How digestion and macromolecule turnover work.

Carbohydrate structure and function

C, H and O near 1:2:1. Monosaccharides fuel respiration; polysaccharides store energy (starch, glycogen) or give structure (cellulose, chitin).

Starch vs cellulose

Both are glucose polymers, but starch uses α-glycosidic linkages and cellulose β. Most animals lack the enzyme for β linkages, which is why cellulose is fiber.

Lipid structure

Mostly hydrocarbon, so nonpolar and hydrophobic. Triglycerides store energy, phospholipids build membranes, steroids act as hormones.

Saturated vs unsaturated fat

Saturated chains have no double bonds and pack tightly (solid at room temperature); unsaturated have kinks that keep them fluid.

Phospholipid amphipathicity

A hydrophilic phosphate head and two hydrophobic tails. In water this drives spontaneous bilayer formation — the basis of every membrane.

Protein structure levels

Primary is the amino-acid sequence; secondary is α-helix and β-sheet from backbone hydrogen bonds; tertiary is R-group folding; quaternary is multiple subunits.

Denaturation

Heat or pH change disrupts the weak bonds holding tertiary structure, so shape is lost. Since function follows shape, activity is lost with it.

Nucleic acid directionality

Nucleotides join 5′ phosphate to 3′ hydroxyl, so strands have direction. DNA polymerase can only add to a 3′ end, which forces the leading/lagging asymmetry.

DNA vs RNA

DNA is double-stranded with deoxyribose and thymine; RNA is single-stranded with ribose and uracil. The 2′-OH makes RNA less chemically stable.

Directionality and complementarity

A pairs with T (or U) via two hydrogen bonds, G with C via three. GC-rich regions therefore need more energy to separate.

Polarity of water

Oxygen holds shared electrons more tightly than hydrogen, giving the molecule partial charges. Every other property of water follows from this one fact.

Surface tension

Cohesion at the air-water boundary creates a film strong enough to support small insects; a consequence of hydrogen bonding.

Capillary action

Adhesion to a narrow tube's walls plus cohesion between water molecules pulls a column upward against gravity.

Hydrophilic vs hydrophobic

Polar or charged substances dissolve in water; nonpolar ones are excluded and cluster together, which is what drives membrane formation and protein folding.

Acid and base definitions

An acid donates H⁺ and lowers pH; a base accepts H⁺ or donates OH⁻ and raises it.

Carbonic acid–bicarbonate buffer

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Shifting this equilibrium holds blood pH near 7.4 despite constant acid production.

Functional groups

Hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate and methyl. They determine a molecule's chemical behavior more than its carbon skeleton does.

Isomers

Same molecular formula, different arrangement. Structural, cis-trans and enantiomers — and the difference matters, since only one enantiomer usually fits an active site.

Monomers of the four macromolecules

Monosaccharides build carbohydrates, amino acids build proteins, nucleotides build nucleic acids; lipids are not true polymers.

Peptide bond

Covalent bond between the carboxyl group of one amino acid and the amino group of the next, formed by dehydration synthesis.

R group

The side chain that distinguishes the twenty amino acids. Its polarity, charge and size determine how the chain folds and what the protein can do.

Disulfide bridge

Covalent bond between two cysteine sulfhydryl groups that locks tertiary structure in place — much stronger than the other folding interactions.

Nucleotide structure

A five-carbon sugar, a phosphate group and a nitrogenous base. Purines (A, G) are double-ringed; pyrimidines (C, T, U) are single-ringed.

Antiparallel strands

The two DNA strands run in opposite directions, 5′→3′ against 3′→5′. This is why replication of the two strands cannot be symmetrical.

Chargaff's rules

In double-stranded DNA, %A = %T and %G = %C, because of complementary base pairing. Useful for calculation questions.

What examiners penalize here

• On the AP exam, almost every water question traces back to one root cause: **polarity → hydrogen bonding**. When asked to explain a property, name that chain explicitly rather than just stating the property — the mechanism earns the point.
• The AP throughline for proteins is "**sequence → shape → function**." When a free-response asks why a mutation or a temperature change matters, walk that chain explicitly: the altered structure changes the shape, and the changed shape changes what the protein can do.
• Two facts win most nucleic-acid questions: complementary pairing (**A–T/U, G–C**) lets you fill in any partner strand, and **antiparallel** orientation means you must flip direction when you write it out. State both explicitly on free-response answers.
• Free-response graders reward the **mechanism**, not the label. "Water has a high specific heat" earns less than "heat energy is absorbed to break hydrogen bonds before molecular motion (temperature) increases, so water resists temperature change." Always connect the property back to hydrogen bonding.
• Build a mental four-column table before the exam: **macromolecule | monomer | bond | function**. Carbohydrate | monosaccharide | glycosidic | energy/structure. Protein | amino acid | peptide | enzymes/structure. Nucleic acid | nucleotide | phosphodiester | information. Lipid | (glycerol + fatty acids, not a polymer) | ester | storage/membranes. Most Unit 1 identification questions collapse the moment you can recall this grid.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology