AP Biology Unit 1: Chemistry of Life MCQs – 30 Practice Questions with Answers (College Board CED)

Q1. Equal masses of water and cooking oil are heated with the same energy input, and the water warms far less. Which property of water best explains this result?

• (A) Water molecules are nonpolar, so they absorb heat without changing temperature even as heating continues
• (B) Extensive hydrogen bonding between water molecules absorbs much of the added energy
• (C) Water has a low specific heat, so it resists temperature change
• (D) Dissolved salts in the water prevent its molecules from moving

Q2. A student notes that ice floats on liquid water in a pond, insulating the water below. What molecular explanation fits this observation?

• (A) Ice molecules are heavier than liquid water molecules, so they rise
• (B) Hydrogen bonds break permanently when water freezes, making ice less dense
• (C) Freezing forces water molecules into a compact arrangement with no empty space, so the solid packs more densely than the liquid
• (D) Hydrogen bonds hold water molecules in a lattice that keeps them farther apart than in the liquid

Q3. Water rises through the narrow xylem vessels of a tall plant even though no pump is present. Which combination of forces drives this movement?

• (A) Cohesion between water molecules plus adhesion of water to the vessel walls
• (B) Adhesion between water molecules plus cohesion of water to the vessel walls
• (C) Repulsion between water molecules plus attraction of water to air
• (D) Gravity pulling water down plus evaporation pushing it up from the roots

Q4. Two drops of water placed close together on a waxy leaf merge into a single larger drop. Which property of water does this best demonstrate?

• (A) Water evaporates rapidly at room temperature
• (B) Water dissolves ionic compounds on contact
• (C) Cohesion, because hydrogen bonding pulls water molecules toward one another
• (D) Adhesion, because water is attracted to the waxy leaf surface rather than to itself

Q5. When oil is shaken with water, the two liquids soon separate into layers again. Why does this happen?

• (A) Oil molecules form hydrogen bonds that push water away
• (B) Water molecules hydrogen-bond to each other and exclude the nonpolar oil molecules
• (C) Oil molecules dissolve in water and then sink because they are heavier, forming a separate layer underneath
• (D) Water molecules are repelled by the electrical charge carried by oil

Q6. Table salt dissolves readily in water but not in pure oil. What feature of the water molecule explains the difference?

• (A) Its partial positive and negative charges attract and surround the separated ions
• (B) Its hydrogen bonds break oil molecules into smaller fragments
• (C) Its molecules are large enough to trap salt crystals physically and hold them suspended in solution
• (D) Its nonpolar regions bond directly to sodium and chloride ions

Q7. An experiment heats 100 g of water and 100 g of ethanol by the same amount of energy. The ethanol warms by a larger amount. What is the best conclusion?

• (A) Ethanol molecules are larger, so they store heat more efficiently
• (B) Water evaporates during heating, which raises its measured temperature by removing cooler molecules from the sample
• (C) Ethanol forms stronger hydrogen bonds than water does
• (D) Water has a higher specific heat because more energy goes into disrupting its hydrogen bonds

Q8. Water climbs a few millimetres up a narrow glass capillary tube above the level of the surrounding water. Which explanation is correct?

• (A) Cohesion alone lifts the water, with no role for the glass
• (B) The glass repels water, forcing it upward into the tube
• (C) Adhesion to the glass plus cohesion within the water column draws the water upward
• (D) Surface tension pushes water down, and the tube traps it at a higher level against gravity

Q9. Carbon is the backbone of biological macromolecules. Which property of carbon best accounts for the diversity of these molecules?

• (A) A carbon atom can form four covalent bonds, allowing chains, branches, and rings
• (B) A carbon atom carries a strong negative charge that attracts other elements into long chains
• (C) Carbon bonds only to hydrogen, giving all macromolecules the same shape
• (D) Carbon atoms cannot bond to each other, so each molecule stays small

Q10. Two amino acids join to form a dipeptide, and a small molecule is released. Which reaction has occurred?

• (A) Hydrolysis, because a polymer is being broken apart
• (B) Dehydration synthesis, because water is removed as the monomers are joined
• (C) Neutralisation, because the amino acids cancel each other’s charge and form a neutral salt
• (D) Oxidation, because electrons are transferred to oxygen

Q11. A researcher adds water and a digestive enzyme to a starch sample. Over time, the amount of free glucose rises steadily. Which process best explains the rise?

• (A) Dehydration synthesis joining glucose units into starch
• (B) Active transport moving glucose across a membrane
• (C) Photosynthesis producing new glucose from carbon dioxide in the presence of light
• (D) Hydrolysis breaking glycosidic bonds in starch to release glucose monomers

Q12. A molecule contains a carboxyl functional group. What chemical behaviour is most expected of that group in aqueous solution?

• (A) It accepts protons and always acts as a strong base in aqueous solution
• (B) It forms only nonpolar bonds and repels water
• (C) It can donate a proton, giving the molecule acidic properties
• (D) It stores energy by breaking phosphate bonds

Q13. A student is given four unknown monomers and must identify which one builds proteins. Which monomer should the student select?

• (A) An amino acid, because proteins are polymers of amino acids
• (B) A fatty acid, because proteins are polymers of fatty acids
• (C) A nucleotide, because proteins are polymers of nucleotides
• (D) A monosaccharide, because proteins are polymers of monosaccharides

Q14. Four monomer-polymer pairs are proposed. Which pair is correctly matched?

• (A) Fatty acid and DNA
• (B) Glucose and starch
• (C) Amino acid and cellulose
• (D) Nucleotide and collagen

Q15. Glucose and fructose share the same molecular formula but taste different and are processed differently in cells. What term describes their relationship?

• (A) They are isotopes of the same element
• (B) They are polymers of different monomers
• (C) They are unrelated compounds that happen to contain carbon and share no structural features
• (D) They are structural isomers, with the same formula but a different arrangement of atoms

Q16. Glycogen and cellulose are both polymers of glucose, yet glycogen is a storage molecule and cellulose gives plant walls strength. What best explains this functional difference?

• (A) Glycogen contains nitrogen and cellulose does not
• (B) Cellulose is built from amino acids rather than sugars
• (C) Their glucose units are linked differently and folded into different structures
• (D) Glycogen is found only in plants and cellulose only in animals, so each kingdom uses a different polymer

Q17. A sample of starch is treated with an enzyme that breaks only alpha glycosidic linkages. The sample is digested, but a sample of cellulose treated the same way is not. What does this show?

• (A) Starch and cellulose differ in the type of linkage between their glucose units
• (B) Cellulose contains no glucose at all
• (C) Starch is a lipid and cellulose is a protein
• (D) Enzymes cannot break any bond in a polysaccharide, whatever its linkage type or shape

Q18. A diagram shows a cell membrane as two layers of molecules with round heads facing outward and tails pointing inward. What do the heads and tails represent?

• (A) Heads are hydrophobic fatty acids and tails are hydrophilic sugars
• (B) Heads are hydrophilic phosphate groups facing water and tails are hydrophobic fatty acids shielded from water
• (C) Heads are proteins spanning the membrane from side to side and tails are cholesterol molecules packed between the proteins
• (D) Heads are nucleic acids and tails are amino acids

Q19. Butter is solid at room temperature while olive oil is liquid, although both are fats. Which structural difference best explains this?

• (A) Butter contains more water than olive oil does
• (B) Olive oil contains more protein than butter does
• (C) Butter is made of nucleotides and olive oil is made of sugars, and this difference in building blocks determines their melting points
• (D) Butter’s fatty acids are mostly saturated and pack tightly, while olive oil’s unsaturated fatty acids are kinked by double bonds

Q20. Cholesterol is present in animal cell membranes. What is its most likely effect when temperature falls?

• (A) It helps keep the membrane fluid by preventing phospholipids from packing too tightly
• (B) It dissolves the membrane completely so the cell can divide and reform afterwards into a new membrane
• (C) It converts the phospholipids into proteins
• (D) It blocks all transport across the membrane permanently

Q21. Gram for gram, fats yield more energy than carbohydrates when oxidised. Which compositional fact best explains this?

• (A) Fats contain nitrogen, which releases energy when broken down during oxidation in cells, yielding carbon dioxide and water
• (B) Fats are polymers of glucose and therefore contain more sugar
• (C) Fats contain many carbon-hydrogen bonds whose oxidation releases large amounts of energy
• (D) Fats dissolve in water, making their energy easier to extract

Q22. Chitin is found in fungal cell walls and in the exoskeletons of insects. What category of biological molecule is chitin?

• (A) A storage protein used for energy
• (B) A structural polysaccharide that provides strength and protection
• (C) A steroid hormone that regulates growth
• (D) A nucleic acid that stores genetic information in the nucleus of fungal cells

Q23. A student compares a DNA sample with an RNA sample from the same cell. Which statement correctly distinguishes them?

• (A) DNA is double-stranded, uses deoxyribose, and contains thymine, while RNA is single-stranded, uses ribose, and contains uracil
• (B) DNA is single-stranded, uses ribose, and contains uracil, while RNA is double-stranded, uses deoxyribose, and contains thymine
• (C) DNA and RNA are identical except that DNA is found only in the nucleus
• (D) DNA is built from amino acids and RNA is built from fatty acids

Q24. Analysis of a double-stranded DNA sample shows that 30% of its bases are adenine. What percentage of its bases are guanine?

• (A) 70%, because adenine pairs with guanine
• (B) 30%, because all four bases occur in equal amounts
• (C) 10%, because the remaining bases are split equally between cytosine and guanine after adenine is counted
• (D) 20%, because thymine must also be 30%, leaving 40% shared equally by guanine and cytosine

Q25. A protein’s amino acid sequence is altered by a single substitution, and the protein loses its function even though it still contains the same number of amino acids. What level of structure was directly changed?

• (A) The quaternary structure only, because subunit number changed
• (B) The tertiary structure only, because the sequence itself is unchanged by substitution and only folding shifts
• (C) The primary structure, whose sequence determines how the protein folds into its functional shape
• (D) No level of structure, because function depends only on amino acid number

Q26. A protein is heated gently and loses its activity, but analysis shows its amino acid sequence is intact. What has most likely happened?

• (A) Peptide bonds have been hydrolysed and the chain is cut into fragments that cannot refold into an active enzyme in the cell
• (B) Heat has disrupted hydrogen bonds and other weak interactions, unfolding the protein without breaking its sequence
• (C) The protein has been converted into a nucleic acid
• (D) New amino acids have been added to the chain by dehydration synthesis

Q27. Two amino acids differ only in their side chains: one carries a nonpolar side chain and the other a positively charged side chain. Where will this difference matter most?

• (A) In how each side chain interacts with water and with other side chains during folding
• (B) In the number of peptide bonds each amino acid can form with its neighbours in the chain
• (C) In whether the amino acid contains carbon at all
• (D) In the speed at which the amino acid dissolves in oil

Q28. Haemoglobin consists of four polypeptide subunits working together to carry oxygen. Which level of protein structure does this arrangement describe?

• (A) Primary structure, because it involves the amino acid sequence of each subunit on its own
• (B) Secondary structure, because it involves alpha helices
• (C) Tertiary structure, because it involves a single folded chain
• (D) Quaternary structure, because it involves the assembly of multiple polypeptide subunits

Q29. A mutation replaces a nonpolar amino acid buried inside a protein with a charged amino acid. What is the most reasonable prediction?

• (A) Nothing changes, because buried residues never affect folding
• (B) The protein becomes more stable, because charged residues always strengthen the interior and improve packing throughout the folded protein
• (C) Folding or function may be disrupted, because a charged group in a nonpolar interior is energetically unfavourable
• (D) The protein becomes a nucleic acid, because its monomer type changes

Q30. In a DNA strand, nucleotides are linked by bonds between the phosphate of one nucleotide and the sugar of the next. What does this linkage produce?

• (A) A lipid bilayer that separates the cell from its surroundings and controls what enters and leaves
• (B) A sugar-phosphate backbone running along the strand, with bases projecting inward for pairing
• (C) A polypeptide chain folded into an alpha helix
• (D) A branched storage molecule that can be rapidly broken down