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Unit 1: Chemistry of Life
THE BIG PICTURE. Unit 1 establishes the chemical foundation of life and the structure-function relationship that the rest of AP Biology builds on. The unit weighs 8–11% of the AP exam and connects to Big Idea 2 (Energetics), Big Idea 3 (Information Storage and Transmission), and Big Idea 4 (Systems Interactions). Mastery of monomer-polymer relationships, water chemistry, and protein folding is essential: every later unit assumes this foundation.
STRUCTURE OF WATER AND HYDROGEN BONDING
Model of Hydrogen Bonds in Water Hydrogen bonding among water molecules. Water's polarity, partial negative on oxygen, partial positive on hydrogens, produces the H-bonds that drive cohesion, adhesion, high specific heat, surface tension, and ice's lower density. Every "weird" property of water traces back to this.
Water is a POLAR MOLECULE: oxygen pulls bonding electrons more strongly than hydrogen (electronegativity difference), creating a partial negative charge (δ⁻) on O and partial positive (δ⁺) on each H. This polarity allows HYDROGEN BONDING between water molecules and gives water a remarkable suite of biological properties:
- COHESION (water-water attraction): produces surface tension and supports a continuous water column in xylem for plant transpiration.
- ADHESION (water-other molecules): drives capillary action in narrow plant vessels.
- HIGH SPECIFIC HEAT CAPACITY: water resists temperature change, buffering organisms and oceans against thermal swings.
- HIGH HEAT OF VAPORIZATION: sweating cools mammals as evaporation removes heat.
- LESS DENSE AS A SOLID (ice floats): insulates aquatic life beneath winter ice.
- UNIVERSAL POLAR SOLVENT: dissolves ions and polar molecules; the medium of cellular chemistry.
PRACTICE: WATER PROPERTY, CAUSE, AND BIOLOGICAL EXAMPLE
| Observation | Property of water | Why (hydrogen bonds) |
|---|---|---|
| Water is pulled up a 60 m tree in an unbroken column | Cohesion (plus adhesion to xylem walls) | each molecule is H-bonded to its neighbors |
| A lake's temperature barely changes from day to night | High specific heat | added energy breaks H-bonds before molecules speed up |
| Sweating cools you even on a warm day | High heat of vaporization | escaping molecules carry heat away with them |
| Fish survive under winter ice | Ice is less dense than liquid water | H-bonds lock freezing water into an open lattice |
| Table salt dissolves in blood plasma | Polar solvent | partial charges surround and separate Na⁺ and Cl⁻ |
WORKED EXAMPLE (CONCEPTUAL ANALYSIS): PREDICTING A PROPERTY FROM STRUCTURE
A student compares water with a nonpolar liquid whose molecules are about the same size. Predict which liquid needs more energy to warm by 1 °C, and justify the prediction.
- Structure: water is polar and forms hydrogen bonds; the nonpolar liquid cannot.
- Property: in water, part of the added heat goes into breaking hydrogen bonds instead of speeding up molecules, so the temperature rises more slowly.
- Answer: water needs more energy. It has the higher specific heat.
- Biological consequence: water-rich organisms and large bodies of water resist rapid temperature swings, which keeps enzyme activity steady.
Exam habit: name the structure, then the property, then the consequence for living things.
ELEMENTS OF LIFE
Life is built primarily from six elements (CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur) plus trace amounts of others (Ca, K, Na, Cl, Mg, Fe). CARBON is uniquely central: four valence electrons let it form chains, rings, and branched skeletons of arbitrary complexity. Functional groups (hydroxyl –OH, carbonyl C=O, carboxyl –COOH, amino –NH₂, phosphate –PO₄, sulfhydryl –SH, methyl –CH₃) attach to carbon skeletons and determine reactivity, polarity, and biological behavior.
INTRODUCTION TO MACROMOLECULES
The FOUR MACROMOLECULE CLASSES, carbohydrates, lipids, proteins, nucleic acids, are built from monomers joined by DEHYDRATION SYNTHESIS (releases water; forms covalent bonds) and broken apart by HYDROLYSIS (consumes water; cleaves bonds). Three classes are true polymers built by repeating monomers; lipids are not technically polymers but follow the same dehydration/hydrolysis pattern.
Dehydration synthesis and hydrolysis Building a polymer removes one water molecule for each new bond; digesting it adds one water molecule per bond. The pattern is the same for sugars, amino acids, nucleotides, and lipids.
WORKED EXAMPLE (CALCULATION): COUNTING WATER MOLECULES
A polypeptide of 150 amino acids is built by dehydration synthesis. How many water molecules are released? How many are needed to digest it completely back to amino acids?
- Each new bond joins two monomers and releases one H₂O. A chain of n monomers has n − 1 bonds.
- Building the chain: 150 − 1 = 149 water molecules released.
- Digesting it: hydrolysis must break all 149 bonds, so 149 water molecules are used.
- The same logic works for any polymer: a starch chain of 500 glucose units has 499 glycosidic bonds.
CARBOHYDRATES
Composition (CH₂O)ₙ. MONOSACCHARIDES (single sugars: glucose, fructose, galactose) are immediate energy. Two monosaccharides linked by GLYCOSIDIC BOND form DISACCHARIDES (sucrose, lactose, maltose). POLYSACCHARIDES include:
- STARCH (plants: energy storage; α-glucose, branched amylopectin or unbranched amylose).
- GLYCOGEN (animals: energy storage; α-glucose, highly branched, in liver/muscle).
- CELLULOSE (plants: structure; β-glucose). Most animals cannot directly digest the β-1,4 linkage; many herbivores and detritivores (ruminants, termites, horses, rabbits, some insects) rely on symbiotic gut microbes to break it down.
- CHITIN (arthropod exoskeletons, fungal cell walls).
LIPIDS
A diverse class of mostly HYDROPHOBIC molecules (insoluble in water); some, like phospholipids, are AMPHIPATHIC. Three major types:
- TRIGLYCERIDES (long-term energy storage): glycerol + 3 fatty acids via ester bonds. SATURATED (no C=C double bonds; solid at room temp; animal fats) vs. UNSATURATED (one or more double bonds; liquid; plant oils; double bonds create kinks preventing tight packing).
- PHOSPHOLIPIDS (membrane structure): glycerol + 2 fatty acids + phosphate group. AMPHIPATHIC (hydrophilic head, hydrophobic tail): spontaneously form BILAYERS in water: the basis of every cellular membrane.
- STEROIDS (signaling and structure): four fused carbon rings. CHOLESTEROL stabilizes animal membranes; estrogen, testosterone, cortisol are steroid hormones derived from cholesterol.
NUCLEIC ACIDS
The DNA Double Helix Two antiparallel strands twist around each other; complementary base pairs (A=T, two H-bonds; G≡C, three H-bonds) hold them together. The structure of the molecule directly enables its function: semiconservative replication and templated transcription.
Polymers of NUCLEOTIDES, each containing a 5-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (A, T, G, C for DNA; A, U, G, C for RNA). PHOSPHODIESTER BONDS link the sugar of one nucleotide to the phosphate of the next, creating directional 5'→3' STRANDS. DNA is a DOUBLE HELIX of two ANTIPARALLEL STRANDS held together by complementary base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds). G–C-rich regions are more thermally stable.
WORKED EXAMPLE (CALCULATION): BASE PAIRING AND DNA STABILITY
A double-stranded DNA sample is 18% adenine. (a) Find the percentages of T, G, and C. (b) A second sample is 32% adenine. Which sample needs a higher temperature to separate its two strands?
- (a) A pairs with T, so T = 18%. A + T = 36%, which leaves 64% for G + C, split equally: G = 32% and C = 32%.
- (b) In sample 2, A = T = 32%, so G + C = 36%. Sample 1 is 64% G–C pairs.
- G–C pairs form three hydrogen bonds and A–T pairs form two, so sample 1 needs more heat to separate.
- Watch out: the A = T rule holds only for double-stranded DNA. In single-stranded RNA, the amount of A need not equal the amount of U.
PROTEINS
The Four Levels of Protein Structure Levels:
- 1.Primary: amino acid sequence
- 2.Secondary: α-helices and β-sheets (H-bonds in the backbone)
- 3.Tertiary: overall 3D fold (R-group interactions, disulfide bridges)
- 4.Quaternary: assembly of multiple subunits (e.g., hemoglobin)
Primary determines secondary, which folds into tertiary, which assembles into quaternary. The classic AP Bio causal chain: sequence → shape → function. A single missense substitution can disrupt all four levels (e.g., sickle-cell hemoglobin).
Polymers of AMINO ACIDS (20 standard) joined by PEPTIDE BONDS. Each amino acid has a central α-carbon bonded to: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R-GROUP (side chain) that determines properties (polar, nonpolar, acidic, basic). Function follows from 3D SHAPE:
- PRIMARY: amino acid sequence; encoded by DNA.
- SECONDARY: local folding (α-helix, β-pleated sheet) stabilized by hydrogen bonds in the backbone.
- TERTIARY: overall 3D shape from R-group interactions (hydrophobic core, ionic bonds, disulfide bridges between cysteines).
- QUATERNARY: assembly of multiple polypeptide subunits (e.g., hemoglobin = 4 subunits).
Protein function classes: enzymes, structural (collagen, keratin), transport (hemoglobin), signaling (hormones, receptors), defense (antibodies), motor (myosin). DENATURATION by heat, pH, or chemicals disrupts non-covalent interactions, destroying shape and function.
From one base to one phenotype One base change swaps a charged glutamic acid for a nonpolar valine. At low oxygen, the new hydrophobic patch lets hemoglobin molecules link into fibers that bend red blood cells into sickles.
CASE STUDY: SICKLE-CELL HEMOGLOBIN, THE FIRST "MOLECULAR DISEASE"
In 1949 Linus Pauling and colleagues reported in Science that hemoglobin from people with sickle-cell anemia moves differently in an electric field than normal hemoglobin: evidence that the disease comes from a change in the protein molecule itself. In 1956 and 1957 Vernon Ingram pinpointed the change: a single amino acid, glutamic acid replaced by valine at position 6 of the β-globin chain. When oxygen is low, the nonpolar valine on the protein's surface lets hemoglobin molecules stick together into long fibers that bend red blood cells into a rigid sickle shape.
- Concept: primary structure determines how a protein folds and interacts.
- What it shows: swapping a charged R-group for a nonpolar one can change the behavior of the whole protein and of the cell that carries it.
- Exam link: a common FRQ move is to trace a change from DNA to amino acid to protein shape to phenotype. This allele returns in Unit 7, where carrying one copy protects against malaria.
WORKED EXAMPLE (PREDICT A DISRUPTION): WHERE DOES THE SUBSTITUTION LAND?
An enzyme's hydrophobic core contains a nonpolar leucine. Predict the effect on enzyme activity if the leucine is replaced by (1) valine, another nonpolar amino acid, or (2) aspartic acid, which has a charged R-group.
- Change 1: nonpolar for nonpolar. The hydrophobic interactions in the core are largely kept, so the tertiary structure, and the enzyme's activity, probably change little.
- Change 2: a charged R-group buried among nonpolar ones disrupts the hydrophobic interactions that hold the fold together. The protein is likely to misfold, the active site loses its shape, and activity falls.
- Reasoning chain to write: R-group chemistry → interactions that stabilize tertiary structure → shape of the active site → function.
EXAM CONNECTIONS. FRQs frequently demand structure-to-function reasoning: how water's properties support specific biological processes; how a single amino acid substitution (e.g., sickle-cell hemoglobin: Glu→Val at position 6) alters protein folding and function; how A–T vs. G–C ratios affect DNA thermal stability; why cellulose and starch (both glucose polymers) have such different properties (α vs. β linkage). Expect to predict consequences of denaturation, identify functional groups in molecular diagrams, and explain why monomer concentration, temperature, or pH drives dehydration synthesis vs. hydrolysis.
Key Terms
Polarity
Unequal sharing of electrons in a covalent bond, producing partial positive and partial negative regions on a molecule. Drives water's hydrogen bonding.
Hydrogen Bond
A weak attraction between a partially positive H atom (bonded to N, O, or F) and a lone pair on another electronegative atom. Individually weak, collectively powerful.
Cohesion
Water molecules sticking to other water molecules via H-bonds. Enables surface tension and the unbroken column of water in plant xylem.
Adhesion
Water sticking to other polar surfaces. Helps water climb narrow tubes (capillary action) against gravity.
Dehydration Synthesis
Polymer-building reaction that removes a water molecule to link two monomers via a covalent bond.
Hydrolysis
Polymer-breaking reaction that adds a water molecule to split a covalent bond between monomers.
Primary Structure
The linear amino acid sequence of a protein, determined directly by mRNA codons during translation.
Tertiary Structure
The overall 3D fold of a single polypeptide, stabilized by hydrophobic interactions, disulfide bridges, ionic bonds, and H-bonds among R groups.
Denaturation
Loss of a protein's functional shape due to disruption of bonds (heat, pH change, salt). Sequence is preserved; function is lost.
Exam Tips
- When asked "explain why" water has a property, always start with polarity → H-bonding → the macroscopic property. That causal chain is what a complete explanation needs.
- For protein questions, link sequence → shape → function. Examiners reward the explicit causal chain.
- Memorize H-bond counts: A–T = 2, G–C = 3. A surprising number of questions hinge on this.
- Dehydration vs. hydrolysis: identify the direction of water flow. Out = synthesis; in = hydrolysis.
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