AP Biology Chemistry of Life — Worked Answer Explanations

Unit 1 · 8 questions explained

Below is a complete answer key for our AP Biology Chemistry of Life practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Chemistry of Life practice test and come back here to review, or head back to the Chemistry of Life unit overview.

In-content ad

Question 1 · Easy
Which of the following best explains why water is described as a polar molecule?
- A
  Water has equal numbers of hydrogen and oxygen atoms.
  Why not A: Atomic count doesn't determine polarity.
- B
  Oxygen is more electronegative than hydrogen, creating an uneven distribution of charge.Correct
- C
  Water has a linear molecular geometry.
  Why not C: Water is bent, not linear; if it were linear, the dipoles would cancel.
- D
  The hydrogen atoms are double-bonded to oxygen.
  Why not D: O–H bonds are single covalent bonds.
Explanation
Oxygen pulls bonding electrons more strongly than hydrogen, leaving a partial negative charge on O and partial positives on H. Combined with the bent geometry, this gives water a net dipole.

Key takeaway
Polarity arises from electronegativity differences combined with non-symmetric geometry.
Question 2 · Easy
Which type of bond holds two complementary DNA strands together in the double helix?
- A
  Covalent bonds.
  Why not A: Covalent bonds connect bases to the sugar-phosphate backbone within a strand.
- B
  Hydrogen bonds.Correct
- C
  Ionic bonds.
  Why not C: Ionic bonds aren't characteristic of DNA structure.
- D
  Disulfide bridges.
  Why not D: Found in proteins, not DNA.
Explanation
Complementary bases (A-T, G-C) pair via hydrogen bonds — 2 between A-T, 3 between G-C. The relatively weak hydrogen bonds allow strands to be separated for replication and transcription.

Key takeaway
Hydrogen bonds between bases hold DNA strands; covalent phosphodiester bonds hold a strand together.
Question 3 · Easy
Saturated and unsaturated fatty acids differ in:
- A
  Whether the carbon chain has any double bonds.Correct
- B
  Whether they contain glycerol.
  Why not B: Glycerol is the head group; this is the same in both.
- C
  Whether they are esters or acids.
  Why not C: Both are carboxylic acids.
- D
  Number of carboxyl groups.
  Why not D: Both have one.
Explanation
Saturated fatty acids have only single C-C bonds ("saturated" with hydrogen). Unsaturated have one or more double bonds, introducing kinks that prevent tight packing — explaining why unsaturated fats are typically liquid at room temp.

Key takeaway
Saturated → no C=C; unsaturated → at least one C=C → kinks → fluid at room temp.
Question 4 · Easy
A polysaccharide and a polypeptide both contain monomers linked by which type of reaction?
- A
  Hydrolysis reaction (water added).
  Why not A: Hydrolysis breaks polymers, not builds them.
- B
  Dehydration synthesis (water removed).Correct
- C
  Oxidation reaction.
  Why not C: Not relevant to polymer formation.
- D
  Reduction reaction.
  Why not D: Not relevant to polymer formation.
Explanation
Both polysaccharides (e.g., starch) and polypeptides form by dehydration synthesis: a covalent bond is formed between monomers and a water molecule is released. Hydrolysis is the reverse — used to break polymers.

Key takeaway
Dehydration synthesis builds biopolymers; hydrolysis breaks them down.
Question 5 · Easy
Which level of protein structure is determined by hydrogen bonding between backbone amine and carbonyl groups?
- A
  Primary structure.
  Why not A: Primary is the amino acid sequence (peptide bonds).
- B
  Secondary structure.Correct
- C
  Tertiary structure.
  Why not C: Tertiary involves R-group interactions.
- D
  Quaternary structure.
  Why not D: Quaternary is multi-subunit assembly.
Explanation
Secondary structure (alpha helices and beta sheets) arises from H-bonds between backbone N-H and C=O groups. Tertiary structure involves R-group interactions, and quaternary involves multiple polypeptide subunits.

Key takeaway
Backbone H-bonds → secondary structure; R-group interactions → tertiary.
Question 6 · Easy
Which of the following is an example of a hydrophobic interaction?
- A
  Salt dissolving in water.
  Why not A: Ion-dipole interaction, not hydrophobic.
- B
  Phospholipid tails clustering inward in a bilayer.Correct
- C
  Hydrogen bonding between water and DNA bases.
  Why not C: Hydrogen bonding is the opposite — strong H-bonds with water.
- D
  Ionic attraction between Na $^{+}$ and Cl $^{-}$ .
  Why not D: Ionic interaction.
Explanation
Hydrophobic ('water-fearing') molecules cluster together in aqueous environment to minimize disruption of water's hydrogen bonding. Phospholipid bilayers exploit this property to form membranes.

Key takeaway
Hydrophobic interactions drive membrane formation and protein folding in aqueous environments.
Question 7 · Easy
A buffer maintains stable pH by:
- A
  Reacting with both added acids and bases.Correct
- B
  Preventing all chemical reactions in solution.
  Why not B: Buffers don't stop reactions, they neutralize H $^{+}$ and OH $^{-}$ .
- C
  Producing equal amounts of H $^{+}$ and OH $^{-}$ .
  Why not C: Buffers absorb excess, not produce more.
- D
  Neutralizing only acidic solutions.
  Why not D: Buffers work in both directions.
Explanation
A buffer (a weak acid + its conjugate base) absorbs added H $^{+}$ via the conjugate base or added OH $^{-}$ via the weak acid. The bicarbonate buffer in blood is a key biological example.

Key takeaway
Buffers resist pH change by sponging up both added acid and added base.
Question 8 · Medium
Which functional group is most likely to ionize and donate a proton at physiological pH (~7.4)?
- A
  Hydroxyl ( $- O H$ ).
  Why not A: Stays neutral at physiological pH.
- B
  Methyl ( $- C H_{3}$ ).
  Why not B: Nonpolar, doesn't ionize.
- C
  Carboxyl ( $- C O O H$ ).Correct
- D
  Amino ( $- N H_{2}$ ).
  Why not D: Tends to gain a proton (becomes $- N H_{3}^{+}$ ) at physiological pH.
Explanation
Carboxyl groups have $p K_{a}$ values around $2 - 5$ , so at pH 7.4 they are almost entirely deprotonated as $- C O O^{-}$ . Amino groups, conversely, are usually protonated as $- N H_{3}^{+}$ .

Key takeaway
At physiological pH: $-COOH$ becomes $-COO^-$; $-NH_2$ becomes $-NH_3^+$.