AP Biology · Lesson 3 of 6 · Unit 1

Carbohydrates and lipids: structure and function

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Carbohydrates and lipids store energy, build cellular boundaries, and transmit chemical signals across every living system. On the AP Biology exam, you will not be asked to memorize chemical formulas of specific sugars or fats; instead, you must connect structural features—such as polysaccharide branching or double bonds in fatty acid tails—to macromolecule properties and membrane fluidity.

By the end you’ll be able to

Describe how monosaccharides are covalently linked to form linear or branched polysaccharides such as starch, glycogen and cellulose.
Describe lipids as typically nonpolar, hydrophobic molecules whose structure and function depend on how their subcomponents are assembled.
Explain how the number of carbon–carbon double bonds in fatty acid tails affects kinking and whether a lipid is liquid at room temperature.

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Core ideas (continued)
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What the exam asks

Structure to function: Explain how covalent bonds between monosaccharides create linear or branched polysaccharides suited for structural support or energy storage.
Saturation and packing: Explain how carbon–carbon double bonds ( $\text{C}=\text{C}$ ) introduce kinks into fatty acid tails and affect whether a lipid is solid or liquid at room temperature.
Membrane predictions: Predict how an organism adjusts its phospholipid saturation or cholesterol content when environmental temperatures shift (frequently tested in Section I stimulus sets and Section II Q4 Conceptual Analysis).
Lipid classes: Identify and describe the roles of fats, phospholipids, steroids, and cholesterol based on diagrams or experimental scenarios.

Core ideas

Carbohydrates: Monosaccharides to Polysaccharides

Carbohydrates consist of carbon, hydrogen, and oxygen. Their monomers are monosaccharides (simple sugars), which are covalently linked via dehydration synthesis (forming glycosidic bonds) into disaccharides and complex polysaccharides.

The orientation of covalent bonds and the degree of chain branching determine a polysaccharide’s function:

Polysaccharide	Structure	Primary Function
Starch	Moderately branched helical chains of glucose monomers	Energy storage in plants
Glycogen	Highly branched chains of glucose monomers	Energy storage in animals (rapidly hydrolyzed at multiple branch ends to release glucose)
Cellulose	Unbranched, linear chains of glucose monomers that form hydrogen bonds with parallel chains	Structural support in plant cell walls (rigid and resistant to breakdown)

Lipids: Nonpolar and Hydrophobic Molecules

Unlike carbohydrates, lipids are not built from repeating monomer chains. Instead, they are diverse, largely nonpolar and hydrophobic molecules constructed from smaller subcomponents. Because their hydrocarbon regions lack charged or polar groups, lipids do not dissolve in water.