AP Biology Unit 1 review: Chemistry of Life
Unit 1 connects the chemistry of water, carbon compounds and biological structure. It accounts for 8–11% of multiple choice. Expect to use molecular features to explain a result, such as why a membrane stays fluid or why a changed amino acid alters protein activity.
Unit 1 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 8–11% of the multiple-choice section |
| Exam date | Monday, May 3, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 6 questions in 90 minutes, worth 50%. Two long questions worth 9 points each and four short questions worth 4 points each. |
| Calculator | Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027. |
| Class time | About 9–11 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 1 covers
College Board's course description splits the unit into 7 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 1.1 Structure of Water and Hydrogen Bonding | Use partial charges and hydrogen bonds to explain cohesion, adhesion, temperature buffering and the lower density of ice. |
| 1.2 Elements of Life | Trace carbon, nitrogen and phosphorus into biological molecules. Explain why growth requires matter from the environment. |
| 1.3 Introduction to Macromolecules | Connect dehydration synthesis and hydrolysis to the assembly and breakdown of large biological molecules. |
| 1.4 Carbohydrates | Relate sugar linkages and branching to the functions of starch, glycogen and cellulose. |
| 1.5 Lipids | Compare fats, phospholipids and steroids. Predict how fatty-acid saturation affects packing and membrane behavior. |
| 1.6 Nucleic Acids | Identify a nucleotide and compare the sugars, bases and strand structures of DNA and RNA. |
| 1.7 Proteins | Connect amino-acid sequence, side-chain interactions and protein folding to function and the effects of environmental change. |
How molecular structure predicts behavior
Use the structural feature as the reason for a prediction. Include the specific interaction that would change.
| Molecule or feature | Structural evidence | Prediction |
|---|---|---|
| Water | Polar O–H bonds and a bent molecule | Water forms hydrogen bonds with neighboring water molecules and many polar solutes. |
| Starch and glycogen | Glucose units joined in storage polymers | Hydrolysis releases sugar units that cells can use in respiration. |
| Cellulose | Glucose linkages produce straight chains that associate | Fibers resist stretching and support plant cell walls. |
| Unsaturated fatty acid | A cis double bond creates a bend | Tails pack less tightly, helping membranes remain fluid at lower temperatures. |
| Phospholipid | A polar head and nonpolar tails | Bilayers form in water with the tails shielded from water. |
| Protein | Amino-acid side chains interact with water and one another | A replacement that changes charge or polarity can change folding and activity. |
| DNA | Base sequence on a sugar-phosphate backbone | The sequence stores information, while complementary pairing supports copying. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Using similar-sounding words interchangeably. A proton is a subatomic particle. A protein is a chain of amino acids. Name the molecule and interaction precisely. The CED flags terminology errors in Unit 1, printed p. 29.
- Calling every attraction in water a covalent bond. O–H bonds hold each water molecule together. Hydrogen bonds connect neighboring molecules. See CED topic 1.1.
- Treating lipids as repeating monomer chains. A triglyceride contains glycerol and three fatty acids. Its construction differs from a long chain of amino acids or nucleotides. See CED topics 1.3 and 1.5.
- Assuming a change in protein shape proves that its amino-acid sequence changed. Heat or pH can disrupt folding while the peptide backbone remains intact. See CED topic 1.7.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 1.1
A pond develops a layer of ice while liquid water remains below it. Which molecular feature explains why the ice floats?
- AWater molecules become nonpolar during freezing
- BHydrogen bonding creates a more open lattice in ice
- CCovalent bonds within water lengthen during freezing
- DWater molecules lose mass as the liquid becomes solid
Show the answer
B. The open hydrogen-bonded lattice increases volume for the same mass, lowering density. The molecules remain H₂O. Changing the arrangement between molecules can change a bulk property without changing molecular identity.
Question 2 · 1.3
A linear chain is assembled from 14 amino acids by dehydration reactions. How many water molecules are released as its peptide bonds form?
- A7
- B12
- C13
- D14
Show the answer
C. A linear chain of 14 units has 13 connections. Each peptide bond forms with the net removal of one water molecule, so 13 are released. Counting amino acids instead of connections gives 14.
Question 3 · 1.5
A microorganism moves from warm water to colder water. Which change would help its membrane retain fluidity?
- AMore cis-unsaturated fatty-acid tails
- BMore tightly packed saturated tails
- CRemoval of every membrane protein
- DReplacement of phospholipids with cellulose
Show the answer
A. Cis double bonds bend the tails and reduce close packing. This helps the bilayer remain fluid as temperature falls. Saturated tails generally pack more tightly, which would work against the needed adjustment.
Question 4 · 1.6
A double-stranded DNA sample has 28% adenine. What percentage of its bases are guanine?
- A14%
- B28%
- C44%
- D22%
Show the answer
D. Complementary pairing makes thymine 28% too. Adenine and thymine total 56%, leaving 44% for guanine and cytosine together. Their equal shares are 22% each.
Question 5 · 1.7
A protein normally has a charged amino-acid side chain on its water-exposed surface. A mutation replaces it with a nonpolar side chain. Which prediction is best supported?
- AIts peptide backbone gains an extra amino acid
- BIts water and side-chain interactions may change
- CIts covalent backbone must split at every bond
- DIts amino-acid sequence remains unchanged
Show the answer
B. The replacement changes primary structure and changes the chemical interactions available at that position. Folding or activity may change, but the evidence alone does not establish complete loss of function or breakage of the whole chain.
Question 6 · 1.2
A culture has ample carbon and nitrogen but almost no available phosphorus. Synthesis of which pair is most directly limited?
- ACellulose and triglycerides
- BStarch and glycogen
- CDNA and phospholipids
- DAll proteins and all sugars
Show the answer
C. DNA has phosphate groups in its backbone, and phospholipids contain phosphate in their heads. Phosphorus scarcity directly constrains their synthesis. Carbon availability cannot replace the missing element.
How to study Unit 1
Draw one phospholipid, nucleotide and amino acid from memory. Label the parts that interact with water. Then explain what would happen if one charged group were replaced by a nonpolar group. This connects a molecular drawing to a prediction.
Practice identifying the bond or interaction that changes during hydrolysis, freezing and protein denaturation. Write a separate explanation for each. Finish with a short data question that asks how a structural change affects measured activity.
Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
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Starting check
SAT Math · 1 of 5
Calculator allowed
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