Review AP Bio Unit 1 to build the chemical foundation every other unit depends on. From water's hydrogen bonds to the four levels of protein structure, this unit explains how molecular structure drives biological function.
Use the topic guides, key terms, and available practice questions to work through each macromolecule and its role in living systems.
Unit 1 asks one central question: how does molecular structure determine biological function? Every topic in this unit answers that question for a different class of molecule, starting with water and ending with proteins.
Water's polarity and hydrogen bonding produce cohesion, adhesion, surface tension, high specific heat, and high heat of vaporization. These properties directly support homeostasis, transport, and evaporative cooling in living organisms.
Dehydration synthesis links monomers into polymers by removing water; hydrolysis breaks polymers back into monomers by adding water. These two reactions govern the assembly and recycling of carbohydrates, proteins, and nucleic acids.
Each macromolecule class has a specific monomer, bonding pattern, and shape that matches its job. Branched glycogen stores glucose quickly; the antiparallel double helix allows DNA replication; R group chemistry drives protein folding and function.
Every concept in Unit 1 is an example of one AP Biology core principle: the structure of a molecule determines what it can do. Saturated fats pack tightly and store energy; unsaturated fats stay fluid and shape membranes; the sequence of amino acids folds into a protein with a specific active site. Recognizing this pattern lets you reason about any molecule the exam presents, even unfamiliar ones.
Water's polarity and hydrogen bonding produce cohesion, adhesion, surface tension, high specific heat, and high heat of vaporization, all of which support life at the molecular and organismal level.
Carbon, hydrogen, and oxygen build all four macromolecule classes. Nitrogen is in nucleic acids and proteins; phosphorus is in nucleic acids and phospholipids; sulfur is in proteins.
Dehydration synthesis builds polymers from monomers by releasing water; hydrolysis breaks them apart by adding water. These two reactions apply to carbohydrates, proteins, and nucleic acids.
Monosaccharides join via glycosidic bonds to form polysaccharides. Starch and glycogen store energy using alpha-glucose; cellulose provides structural support using beta-glucose.
Lipids are hydrophobic molecules. Triglycerides store energy; phospholipids form membranes; steroids like cholesterol regulate membrane fluidity and act as hormone precursors. Saturation level controls fluidity.
DNA and RNA are built from nucleotide monomers. DNA is an antiparallel double helix with A-T and G-C base pairing that stores hereditary information. RNA is usually single-stranded and carries out information transfer.
Amino acids link via peptide bonds to form polypeptides. Primary sequence drives folding into secondary, tertiary, and quaternary structures. R group chemistry and denaturation connect structure directly to function.
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Review Proteins with attention to how the concept appears in AP-style source and evidence questions.
Review Carbohydrates with attention to how the concept appears in AP-style source and evidence questions.
Review Lipids with attention to how the concept appears in AP-style source and evidence questions.
Review Nucleic Acids with attention to how the concept appears in AP-style source and evidence questions.
Water is polar because oxygen is more electronegative than hydrogen, creating partial negative charge on oxygen and partial positive charges on the two hydrogens. This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules. Those hydrogen bonds produce four biologically critical properties.
| Property | Molecular cause | Biological example |
|---|---|---|
| Cohesion | H-bonds between water molecules | Water column in plant xylem |
| Adhesion | H-bonds to polar surfaces | Capillary action in narrow tubes |
| High specific heat | Energy absorbed breaking H-bonds | Stable ocean and body temperatures |
| High heat of vaporization | Energy needed to vaporize water | Evaporative cooling via sweating |
| Ice floats | Lattice structure less dense than liquid | Aquatic life survives under ice |
Six elements make up the vast majority of biological molecules: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S). Carbon, hydrogen, and oxygen appear in all four macromolecule classes. The other three have more specific roles.
| Element | Macromolecule(s) | Specific role |
|---|---|---|
| C, H, O | All four classes | Carbon backbone; H and O in functional groups |
| Nitrogen (N) | Nucleic acids, proteins | Nitrogenous bases; amino groups |
| Phosphorus (P) | Nucleic acids, lipids | Sugar-phosphate backbone; phospholipid head |
| Sulfur (S) | Proteins | Disulfide bridges in cysteine residues |
Macromolecules are built from monomers through dehydration synthesis and broken down through hydrolysis. These two reactions are opposites and apply to carbohydrates, proteins, and nucleic acids. Lipids use related chemistry but are not true polymers built from identical repeating monomers.
| Reaction | What happens to water | Result |
|---|---|---|
| Dehydration synthesis | Water is released | Monomers join; polymer grows |
| Hydrolysis | Water is added | Bond breaks; monomers released |
Carbohydrates are built from monosaccharide monomers joined by glycosidic bonds through dehydration synthesis. The resulting polysaccharides can be linear or branched, and that structural difference determines whether the carbohydrate stores energy or provides structural support.
| Polysaccharide | Monomer linkage | Structure | Function |
|---|---|---|---|
| Starch | Alpha-glucose | Linear/branched | Energy storage in plants |
| Glycogen | Alpha-glucose | Highly branched | Energy storage in animals |
| Cellulose | Beta-glucose | Linear, H-bonded fibers | Structural support in plant walls |
Lipids are nonpolar, hydrophobic molecules. Their structure is not based on a repeating monomer but on the arrangement of glycerol, fatty acid tails, phosphate groups, or steroid ring systems. The degree of saturation in fatty acids directly controls lipid fluidity and function.
| Lipid type | Key structural feature | Primary function |
|---|---|---|
| Triglyceride | 3 fatty acids + glycerol, ester bonds | Long-term energy storage |
| Phospholipid | 2 fatty acids + phosphate head, amphipathic | Cell membrane bilayer |
| Steroid (cholesterol) | 4 fused carbon rings | Membrane fluidity; hormone precursor |
DNA and RNA store and transmit genetic information using nucleotide monomers. Each nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The sequence of bases encodes biological information.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double-stranded | Usually single-stranded |
| Function | Hereditary information storage | Information transfer and translation |
Proteins are polymers of amino acids linked by peptide bonds. The specific sequence of amino acids determines how the protein folds, and the final shape determines its function. This structure-function relationship is the central theme of Topic 1.7.
| Level | Structural feature | Bonds involved |
|---|---|---|
| Primary | Amino acid sequence | Peptide bonds (covalent) |
| Secondary | Alpha helix or beta sheet | Hydrogen bonds (backbone) |
| Tertiary | Overall 3D shape | H-bonds, ionic, hydrophobic, disulfide bridges |
| Quaternary | Multiple subunits | Same as tertiary, between subunits |
Try stimulus-based AP practice questions and written prompts after you review the notes.
The figure shows DNA melting temperature () plotted against DNA composition for several double-stranded DNA samples.
SDS-PAGE was used to analyze an isolated protein. The untreated protein in lane 2 produced one 40 kDa band, and the same protein treated with a reducing agent in lane 3 produced 15 kDa and 25 kDa bands.
6. The enzyme thermo-stable kinase (TSK) is found in bacteria that live in hot springs. The wild-type (WT) TSK protein contains a tyrosine amino acid at position 105 (Tyr105), which is located in the hydrophobic core of the protein. Tyrosine has a hydroxyl (-OH) group on its R-group that can form hydrogen bonds. Phenylalanine (Phe) has a similar ring structure to tyrosine but lacks the hydroxyl group and cannot form hydrogen bonds. Alanine (Ala) has a small, nonpolar methyl group.
Scientists generated two mutant strains of bacteria: one producing TSK with a phenylalanine at position 105 (Y105F) and one producing TSK with an alanine at position 105 (Y105A). To investigate the role of the Tyr105 R-group in protein stability and function, the scientists purified the WT and mutant enzymes. They measured the relative enzymatic activity of each variant at a physiological temperature of 37°C (Figure 1A) and at a heat-stress temperature of 75°C (Figure 1B).
Figure 1. Relative enzymatic activity of thermo-stable kinase (TSK) variants measured at (A) 37°C and (B) 75°C. Bars show mean relative activity in percent; vertical error bars show ±5 percentage points.
Based on Figure 1A, identify the TSK variant that demonstrates the lowest relative activity at 37°C.
Based on Figure 1B, describe the difference in relative activity between the WT enzyme and the Y105F mutant enzyme at 75°C.
Scientists hypothesize that the hydrogen bond capability of the Tyr105 R-group is essential for maintaining protein stability at high temperatures but is not required for enzymatic function at physiological temperatures. Use the data in Figures 1A and 1B to support the scientists' hypothesis.
Based on the chemical properties of the amino acid R-groups, explain why the Y105A mutant has low activity at 37°C compared to the WT and Y105F variants.
2. Carnivorous plants, such as the pitcher plant Nepenthes, survive in nutrient-poor soils by trapping and digesting insects (see Figure 2). The plants secrete a digestive fluid containing enzymes into a specialized leaf structure called a pitcher. One of the primary enzymes found in this fluid is a protease called Nepenthesin, which breaks down proteins in the prey.
To investigate the environmental conditions required for optimal Nepenthesin activity, researchers extracted the enzyme from the pitcher fluid. They incubated the enzyme with an albumin protein substrate at five different pH levels for 10 minutes at 25°C. The researchers then measured the amount of amino acids released by the breakdown of albumin. The rate of reaction was calculated as micromoles of amino acids released per minute (Table 1).
Nepenthesin is an aspartic protease, meaning its function relies on aspartate amino acid residues in its active site. The researchers proposed a model of the catalytic mechanism in which two specific aspartate residues interact with a water molecule to break the peptide bonds in the substrate (Figure 1). The active site structure is shown in Figure 2.
Figure 1. Model of the Nepenthesin active site showing two catalytic Aspartate residues, a bound water molecule, and an entering polypeptide substrate.
Proteins are complex molecules with specific three-dimensional shapes. Describe how the R-groups of amino acids stabilize the tertiary structure of a protein.
Using the template in the space provided for your response, construct an appropriate type of graph that represents the data in Table 1. Your graph should be appropriately plotted and labeled.
Based on the data in Table 1, determine the pH range in which the enzyme retains at least 50% of its maximum observed activity.
Table 1. Rate of albumin digestion by Nepenthesin at different pH levels (25°C incubation, 10 minutes). Values are mean rate ± 2SE; units are micromoles of amino acids released per minute.
Based on Table 1, identify the pH at which the rate of reaction was lowest.
The researchers created a mutant form of Nepenthesin in which the Aspartate residues shown in Figure 1 were replaced with Alanine, a nonpolar amino acid. Based on Figure 1, predict the effect of this mutation on the enzyme's ability to catalyze the reaction.
The fluid inside the Nepenthes pitcher is maintained at a highly acidic pH by proton pumps in the plant cells lining the pitcher. The researchers claim that this acidification is an essential adaptation that allows the plant to maximize the extraction of nitrogen from captured prey.
Use evidence from the information provided to support the researchers' claim that acidification of the pitcher fluid is essential for nitrogen extraction.
Pepstatin is a molecule that binds tightly to the Aspartate residues in the active site of aspartic proteases. Based on Figure 1, explain how Pepstatin would act as an inhibitor of Nepenthesin.
| Term | Definition |
|---|---|
| Hydrogen Bonds | Weak attractive forces between a slightly positive hydrogen atom and a slightly negative atom (usually oxygen or nitrogen). In water, hydrogen bonds between molecules produce cohesion, surface tension, high specific heat, and high heat of vaporization. |
| Polarity | Uneven distribution of electrons in a molecule creating partial positive and negative regions. Water is polar because oxygen pulls shared electrons more strongly than hydrogen, enabling hydrogen bonding. |
| Evaporative Cooling | Heat loss that occurs when water evaporates from a surface; water's high heat of vaporization means evaporation carries away substantial thermal energy, cooling organisms through sweating or transpiration. |
| Dehydration Synthesis | Reaction that joins two monomers by removing a water molecule and forming a new covalent bond; the mechanism for building polysaccharides, polypeptides, and nucleic acids. |
| Hydrolysis | Reaction that breaks a covalent bond between monomers by adding water; the mechanism for digesting and recycling macromolecules. |
| Monosaccharides | Simple sugar monomers such as glucose (C6H12O6) that join via glycosidic bonds to form polysaccharides like starch, glycogen, and cellulose. |
| Polysaccharides | Large carbohydrate polymers formed by linking monosaccharides; starch and glycogen store energy using alpha-glucose linkages, while cellulose provides structural support using beta-glucose linkages. |
| Fatty Acids | Long hydrocarbon chains with a carboxyl group at one end; saturated fatty acids have only single bonds and pack tightly, while unsaturated fatty acids have double bonds that kink the chain and increase fluidity. |
| Phospholipids | Amphipathic lipid molecules with two fatty acid tails and a phosphate head group; their hydrophilic heads and hydrophobic tails drive spontaneous bilayer formation, creating cell membranes. |
| Nucleotide | Monomer of nucleic acids consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base; nucleotides link via phosphodiester bonds to form DNA and RNA strands. |
| Base pairing | Complementary hydrogen bonding between nitrogenous bases: A-T and G-C in DNA, A-U and G-C in RNA. Base pairing holds the two strands of the DNA double helix together and enables accurate replication and transcription. |
| antiparallel | The orientation of the two DNA strands, where one runs 5' to 3' and the other runs 3' to 5' in the opposite direction; required for complementary base pairing across the double helix. |
| Amino Acid | Monomer of proteins; each has a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable R group that determines the amino acid's chemical properties and role in protein folding. |
| Primary Structure | The specific linear sequence of amino acids in a polypeptide, held together by peptide bonds; this sequence determines all higher levels of protein structure and ultimately protein function. |
| Denaturation | Loss of a protein's three-dimensional structure caused by heat, extreme pH, or chemical agents; disrupts hydrogen bonds, ionic interactions, and hydrophobic interactions without breaking peptide bonds, destroying protein function. |
Hydrogen bonds form between water molecules; the O-H bonds within a single water molecule are polar covalent bonds. Cohesion, adhesion, and specific heat all depend on intermolecular hydrogen bonds, not the intramolecular covalent bonds.
Lipids are not polymers. Triglycerides and phospholipids are assembled through dehydration-like reactions, but they do not have a repeating monomer unit. Do not apply the monomer-polymer framework to lipids the same way you do to carbohydrates, proteins, or nucleic acids.
Both are glucose polysaccharides, but the alpha linkage in starch allows enzymes to break it down for energy, while the beta linkage in cellulose creates rigid fibers most organisms cannot digest. The linkage type, not just the monomer, determines function.
Denaturation disrupts hydrogen bonds, ionic interactions, hydrophobic interactions, and disulfide bridges that maintain secondary, tertiary, and quaternary structure. Peptide bonds in the primary structure remain intact. The protein unfolds but the amino acid sequence does not change.
When writing the complementary strand, the direction matters. If the template strand runs 3' to 5', the new strand runs 5' to 3'. Writing both strands in the same direction is a common error that also affects base-pairing answers.
AP Bio questions frequently present an unfamiliar molecule or a mutation and ask you to predict the effect on function. Unit 1 trains this skill directly: explain why a change in fatty acid saturation alters membrane fluidity, why a single amino acid substitution can denature a protein, or why the beta linkage in cellulose prevents digestion. Practice moving from structural detail to functional consequence in every macromolecule.
Free-response questions in AP Bio often ask you to design an experiment or interpret data involving macromolecules. Unit 1 concepts appear in scenarios such as testing enzyme activity on starch hydrolysis, comparing membrane fluidity across temperatures, or analyzing the effect of pH on protein structure. Be ready to connect experimental observations back to hydrogen bonding, R group chemistry, or lipid saturation.
The chemistry from Unit 1 reappears throughout the course. Phospholipid bilayer structure connects to membrane transport in Unit 2; ATP as a nucleotide connects to cellular energetics in Unit 3; DNA structure connects to replication and gene expression in Units 5 and 6. Exam questions may ask you to apply macromolecule structure or water properties in a context drawn from a later unit, so understanding Unit 1 deeply pays off all year.
Open the individual guides for Unit 1 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 1 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAP Bio Unit 1 covers 7 topics built around the chemistry of life: the properties of water and hydrogen bonding (1.1), elements of life (1.2), introduction to macromolecules (1.3), carbohydrates (1.4), lipids (1.5), nucleic acids (1.6), and proteins (1.7). Together these topics explain how biological molecules are built and how they function in living systems. See all 7 topics with practice on the AP Bio Unit 1 page.
AP Bio Unit 1 makes up 8-11% of the AP exam. That weight covers everything in the Chemistry of Life unit, including carbohydrates, lipids, nucleic acids, proteins, and the properties of water. It's a smaller unit by exam weight, but the macromolecule concepts it introduces show up again in nearly every later unit, so a strong foundation here pays off throughout the course.
The AP Bio Unit 1 progress check includes both MCQ and FRQ parts drawn from all 7 topics in the Chemistry of Life unit. MCQ questions typically test your ability to identify macromolecule structures, compare monomers and polymers, and apply properties of water to biological scenarios. The FRQ portion asks you to explain or analyze concepts like how carbohydrates, lipids, nucleic acids, or proteins relate to biological function. For matched practice questions that mirror the progress check format, visit the AP Bio Unit 1 page.
AP Bio Unit 1 FRQs most often pull from the macromolecule topics: carbohydrates, lipids, nucleic acids, and proteins. Questions typically ask you to describe the relationship between structure and function, explain how monomers and polymers are formed or broken down, or connect a molecule's properties to a biological process. To practice, write out full responses using specific vocabulary, then check that every claim is supported with evidence from the topic. Find Unit 1 FRQ practice on the AP Bio Unit 1 page.
The best place to find AP Bio Unit 1 practice questions, including multiple-choice and practice test sets, is the AP Bio Unit 1 page. It has MCQ and FRQ practice covering all 7 topics, from properties of water and macromolecules to carbohydrates, lipids, nucleic acids, and proteins. Working through topic-by-topic MCQs before attempting a full practice test helps you spot which concepts need more review.
Start AP Bio Unit 1 by locking in the properties of water, since concepts like cohesion, adhesion, and hydrogen bonding reappear throughout the course. Then work through each macromolecule group in order: carbohydrates, lipids, nucleic acids, and proteins. For each one, learn the monomer, the polymer, how they're linked, and what biological role they serve. Drawing out the structures by hand and explaining them out loud helps more than re-reading notes. A practical study sequence: read the topic, do a short MCQ set to check understanding, then try an FRQ response before moving to the next topic. You can find topic-by-topic practice on the AP Bio Unit 1 page.