AP Biology Unit 1 Review: Chemistry of Life Study Guide

Water is polar because oxygen attracts electrons more strongly than hydrogen, creating partial charges on each molecule. Those partial charges let water molecules form hydrogen bonds with neighbors, which explains many life-supporting properties you will see on AP Biology Unit 1 MCQs and FRQs.

Cohesion means water sticks to water; adhesion means water sticks to other surfaces. Together they help explain surface tension, capillary action in plants, and why water climbs narrow tubes. High specific heat means water resists temperature change, helping organisms and ecosystems stay stable when energy input fluctuates.

Water is an excellent solvent for polar and ionic substances because partial charges orient solutes and keep them dispersed. Ice is less dense than liquid water because hydrogen bonds hold molecules slightly farther apart in the solid, so ice floats—a rare trait that protects aquatic life in winter.

On the exam, always connect a named property to a biological effect: cohesion supports transport and surface tension; adhesion supports wetting of surfaces; solvent behavior supports chemistry in cells and blood plasma. If a stem names specific heat or evaporative cooling, trace the explanation back to hydrogen bonding between water molecules.

Unit 1 water questions often appear alongside macromolecule chemistry because polar water interacts with charged and polar biological molecules. Review cohesion versus adhesion carefully—they are related but not interchangeable on FRQs.

Quick review: Water supports life because its polarity and hydrogen bonding create cohesion, adhesion, high specific heat, solvent properties, and ice behavior that help organisms and ecosystems function.

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Living systems rely on CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Hydrogen and oxygen appear constantly in water and organic molecules. Carbon is the backbone of organic diversity because it forms four covalent bonds and can build chains, branches, and rings.

Nitrogen is essential in amino groups of amino acids and in nitrogenous bases of nucleotides. Phosphorus appears in phosphate groups of ATP, nucleic acid backbones, and phospholipid heads—so phosphorus questions often tie energy, genetics, and membranes together.

Sulfur can stabilize protein shape through disulfide bridges in some proteins. When an AP question names an element, ask what class of molecule is being built: carbon skeleton for organic diversity, nitrogen for proteins and nucleic acids, phosphorus for ATP and DNA/RNA.

Element-composition items are fast points if you know which elements appear in which macromolecule class. Carbohydrates are rich in C, H, and O; proteins add N (and sometimes S); nucleic acids add P in phosphate groups; lipids are mostly C and H with variable oxygen.

The elements-of-life lesson walks through carbon skeletons and functional groups in more detail than this review summary allows, but the review table below is your cram-sheet for matching element to molecule.

Quick review: CHNOPS elements build the major biological molecules; carbon provides skeletons, nitrogen and phosphorus anchor proteins and nucleic acids, and sulfur can stabilize some proteins.

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A monomer is a small molecular building block; a polymer is a larger molecule made of linked monomers. Carbohydrates polymerize from monosaccharides, proteins from amino acids, and nucleic acids from nucleotides. Recognizing monomer-polymer pairs is one of the fastest ways to sort AP Biology Unit 1 questions.

Macromolecule is the umbrella term for large biological molecules. Not every macromolecule is a true polymer with repeating identical subunits—lipids are the famous exception discussed later on this review page.

When a stem says building block, subunit, or chain, think monomer and polymer before you guess a specific functional group. Linking monomers usually involves dehydration synthesis; breaking polymers usually involves hydrolysis.

Polymer length and branching change function even when the monomer is the same sugar. That idea previews why starch, glycogen, and cellulose behave differently despite all involving glucose chemistry.

If you can state monomer, bond type, and polymer name for each class, you are already ahead on many comparison charts and FRQ introductions.

Quick review: Monomers are building blocks; polymers are chains of linked monomers; each macromolecule class has a characteristic monomer except lipids.

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Dehydration synthesis joins smaller molecules into larger ones and releases water as a product. Hydrolysis breaks larger molecules into smaller subunits by using water as a reactant. They are opposite directions of the same chemical logic cells use to build and recycle macromolecules.

Dehydration synthesis forms covalent bonds between monomers—peptide bonds in proteins, glycosidic bonds in carbohydrates, phosphodiester bonds in nucleic acids. Hydrolysis cleaves those bonds during digestion and turnover.

On AP-style questions, first decide whether the molecule is being built or broken, then decide whether water is released or consumed. Mixing these up is one of the most common Unit 1 mistakes.

Digestion narratives in later units still depend on hydrolysis vocabulary from Unit 1. Building a polypeptide in a ribosome depends on dehydration synthesis logic even when enzymes have different names.

Write a one-line rule on your notes: build = release water; break = use water. That single line prevents dozens of lost points.

Quick review: Dehydration synthesis builds polymers and releases water; hydrolysis breaks polymers and uses water.

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Carbohydrates include monosaccharides such as glucose and polysaccharides such as starch, glycogen, and cellulose. They are built from CHO-rich subunits linked by glycosidic bonds formed through dehydration synthesis.

Glucose is a central fuel molecule. Starch stores energy in plants; glycogen stores energy in animals; cellulose provides structural support in plant cell walls. All three can involve glucose, but branching and linkage patterns create different functions—an AP trap worth memorizing with structure-function reasoning.

Carbohydrates also appear in recognition and structure roles on membranes in later units, but Unit 1 focuses on energy and plant wall structure.

Ring versus chain forms of sugars matter less at Unit 1 depth than knowing which polysaccharide matches which organism and job. Label diagrams carefully on FRQs: starch in plants, glycogen in animals, cellulose in plant walls.

If a question mentions glycosidic bonds, you are almost certainly in carbohydrate territory—not peptide or phosphodiester chemistry.

Quick review: Carbohydrates are sugars and polysaccharides for energy, storage, and plant structure; know glucose, starch, glycogen, and cellulose.

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Lipids are mostly hydrophobic molecules that store long-term energy, form membranes, and can act as signaling molecules. Triglycerides store energy; phospholipids form bilayers with hydrophilic heads and hydrophobic tails; steroids such as cholesterol influence membrane fluidity and signaling.

Unlike proteins, nucleic acids, and many carbohydrates, lipids are not usually true polymers made of one repeating monomer type. That exception appears on both MCQs and FRQs—do not claim all macromolecules are polymers.

Membrane questions in Unit 1 and later units depend on phospholipid amphipathic structure: heads interact with water; tails avoid water and create bilayers.

Saturated versus unsaturated fatty acids change membrane fluidity—a preview of why diet and temperature matter for cell membranes. Unit 1 may not require drawing fatty acids, but you should recognize hydrophobic tails in diagrams.

When a stem says nonpolar, long-term energy storage, or bilayer, lipids should be your first macromolecule guess.

Quick review: Lipids are hydrophobic, store energy, form membranes with phospholipids, and are not usually true polymers.

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Proteins are polymers of amino acids linked by peptide bonds. The sequence of amino acids (primary structure) folds into secondary, tertiary, and sometimes quaternary structures that determine function. Enzymes, transport proteins, receptors, and structural proteins all depend on shape.

Denaturation changes protein shape—often from pH or temperature—and can reduce function without necessarily breaking peptide bonds. Hydrolysis, by contrast, breaks proteins into smaller parts. Confusing denaturation with hydrolysis is a common Unit 1 mistake.

The AP reasoning chain is amino acid sequence → folding → shape → function. Active sites of enzymes are shape-specific binding regions; if shape changes, catalysis can stop.

Primary structure is still chemistry of life even when textbooks emphasize higher levels later. A single amino acid change can alter folding and cause disease—structure-function at the molecular scale.

If a question mentions R groups, peptide bonds, or active sites, you are in protein territory—not nucleotide chemistry.

Quick review: Proteins fold from amino acids; shape determines function; denaturation alters shape without necessarily digesting the chain.

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Nucleic acids include DNA and RNA, built from nucleotide monomers. Each nucleotide contains a sugar, a phosphate group, and a nitrogenous base. A sugar-phosphate backbone provides structure; the sequence of bases stores genetic information.

DNA usually stores hereditary information with deoxyribose and thymine; RNA helps use genetic information with ribose and uracil. ATP is nucleotide-related and important for energy transfer but is not a hereditary polymer—another favorite AP distinction.

Nucleic acids are made of nucleotides, not amino acids. Base pairing and complementary strands preview replication and gene expression in later units.

Phosphodiester bonds join nucleotides in the backbone; that vocabulary separates nucleic acids from peptide and glycosidic bond questions on the same test.

When a diagram shows double helix, thymine, or sugar-phosphate rails, nucleic acids are the intended answer even if the question also mentions proteins in the same organism.

Quick review: Nucleotides build DNA and RNA; base sequence stores information; ATP is related but not hereditary storage.

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