AP BiologyUnit 1: Chemistry of Life1.4
AP Biology · Unit 1 · Topic 1.4

Carbohydrates

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Same glucose, different linkage: starch is fuel you digest, cellulose is fiber you can't.

Carbohydrates are sugar units, alone or linked into chains by pulling out water. Link glucose one way and you get starch, fuel you can digest. Link the same glucose another way and you get cellulose, fiber you can't.

Key terms

  • Monosaccharide — one sugar unit, the monomer of carbohydrates. Glucose, fructose, galactose, and ribose are all monosaccharides.
  • Disaccharide — two sugars linked. Sucrose is glucose + fructose, lactose is glucose + galactose, maltose is two glucoses.
  • Polysaccharide — hundreds to thousands of sugar units in one polymer: starch, glycogen, cellulose, chitin.
  • Glycosidic linkage — the covalent bond between two sugars. Not a peptide bond (proteins) or an ester linkage (fats).
  • Dehydration synthesis — joins two sugars and releases one water. Hydrolysis reverses it: add water, break the linkage.
  • Isomers — same formula, different arrangement. Glucose and fructose are both C6H12O6, yet cells treat them as different molecules.

How it works

  1. In water, glucose closes into a ring dotted with –OH groups. They hydrogen bond with water, so simple sugars dissolve easily.
  2. To link two sugars, an –OH from one and an H from the other leave as water. A glycosidic linkage now joins the rings.
  3. Repeat to grow a polysaccharide. Each linkage releases one water, so building a 100-glucose chain releases 99 waters.
  4. Alpha linkages give coiled chains that enzymes break down easily. Starch stores fuel in plants. Glycogen, more branched, stores it in animal liver and muscle.
  5. Beta linkages give straight chains that line up and hydrogen bond into tough fibers: cellulose in plant cell walls, chitin in insect exoskeletons and fungal walls.
  6. Hydrolysis runs it backward. Digestive enzymes add water across each linkage they fit, freeing monomers your cells can use for energy.

Why it matters

Exam questions rarely want a list of sugars. They want structure → function. Why store glucose as glycogen instead of loose glucose? Thousands of linked units count as one dissolved particle, so the cell banks fuel without drawing in much water by osmosis. The branches give many ends, so glucose comes off fast. Why do short sugar chains stick out from cell surfaces? Their pattern is an ID tag other cells read. ABO blood types are that kind of tag.

Exam traps

TrapActually
Glucose and fructose share a formula, so they're the same molecule.They're isomers: same atoms, different arrangement. Different shape means different enzymes recognize them.
Linking sugars uses up water.Linking releases one water per linkage. Hydrolysis, the breakdown, is the step that uses water.
Alpha vs beta explains every polysaccharide difference.Starch and glycogen are both alpha; glycogen is more branched. Cellulose and chitin are both beta; chitin's sugar carries nitrogen.
Starch and cellulose are both glucose, so one enzyme digests both.Enzymes fit a linkage's shape. Amylase fits starch's alpha linkages, not cellulose's beta ones, so cellulose passes through you as fiber.
Worked example

A lab tests an unknown polysaccharide. Complete hydrolysis yields only glucose. The polysaccharide forms tough, insoluble fibers, and amylase, an enzyme that digests starch, leaves it intact. Which polysaccharide is it, and why does amylase fail?

  1. Only glucose comes out. That rules out chitin, whose sugar carries a nitrogen-containing group.
  2. Starch and glycogen are glucose polymers with alpha linkages, the kind amylase cuts. Amylase failed, so rule both out.
  3. Glucose units, tough fibers, and no digestion by amylase all point to cellulose, whose linkages are beta.

Cellulose. Its beta linkages don't fit amylase's active site, which is shaped for alpha linkages.

Starch, glycogen, and cellulose are all pure glucose, so the monomer alone can't decide it. The linkage does: beta linkages make straight chains that pack into fibers and resist amylase. The near-miss is starch, since it also yields only glucose, but amylase would have broken it down.

Now try it on real exam-style questions

Ten questions on Carbohydrates, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 1.4 Carbohydrates

Which of these molecules is a carbohydrate?

  1. A Glycerol
  2. B Glycine
  3. C Glycogen
  4. D Guanine
Show the answer

C — Glycogen is a polysaccharide, a chain of glucose units. Glycerol sounds sugar-like and carries -OH groups, but it is the three-carbon backbone of fats, not a sugar.

Question 2 · 1.4 Carbohydrates

What is the main role of glucose in most cells?

  1. A A fuel broken down to release usable energy
  2. B A catalyst that speeds up chemical reactions
  3. C A molecule that carries hereditary information
  4. D A long-term energy store kept in fat tissue
Show the answer

A — Glucose is the cell's main short-term fuel: cells break it down to release energy they can use. Long-term energy storage in fat tissue is the job of triglycerides, and glucose itself is stored as polysaccharides.

Question 3 · 1.4 Carbohydrates

Which polysaccharide is the primary form in which glucose is stored in animal liver and muscle cells?

  1. A Starch
  2. B Glycogen
  3. C Cellulose
  4. D Chitin
Show the answer

B — Glycogen is a highly branched glucose polymer stored as granules in liver and muscle, where its many branch ends allow rapid mobilization when blood glucose falls or muscles contract.