AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/1/1-4)
Unit 1 · Topic 1.4
1.4 Carbohydrates
Carbohydrates are sugars and the polymers built from them. Monosaccharides like glucose are the monomers, and they link into polysaccharides that can be straight or branched. Starch and glycogen store energy in plants and animals, while cellulose gives plant cell walls their strength.
Key terms
- monosaccharide
- polysaccharide
- starch
- glycogen
- cellulose
Monosaccharides and disaccharides
A monosaccharide is a single sugar unit, often with a formula that's a multiple of CH₂O. Glucose (C₆H₁₂O₆) is the most important one: cells break it down for energy in cellular respiration (3.5) and plants make it in photosynthesis (3.4). Other monosaccharides include fructose and galactose, plus the five-carbon sugars ribose and deoxyribose that are part of RNA and DNA (1.6).
Two monosaccharides joined by dehydration synthesis make a disaccharide. Table sugar, sucrose, is glucose + fructose. Milk sugar, lactose, is glucose + galactose. Maltose is glucose + glucose.
Polysaccharides: storage and structure
Polysaccharides are long chains of monosaccharides joined by covalent bonds. Some chains are linear (unbranched) and some are branched. Three examples are made entirely of glucose, and yet they do very different jobs because of how the glucose units are linked and arranged.
| Polysaccharide | Found in | Shape | Job |
|---|---|---|---|
| Starch | Plants (for example, potatoes and seeds) | Mostly coiled chains, some branched | Stores glucose for later energy use |
| Glycogen | Animals, mainly liver and muscle | Very highly branched | Stores glucose that can be released quickly |
| Cellulose | Plant cell walls | Straight, unbranched chains lying side by side | Gives cell walls strength and rigidity |
How structure fits function
Branching speeds up access. Enzymes release glucose from the ends of chains, so a highly branched molecule like glycogen has many ends to work on at once. That suits animals, which may need a burst of glucose quickly, for example when muscles start working hard.
In cellulose, the glucose units are linked in a different orientation than in starch. That makes cellulose chains straight, so neighboring chains can line up and form many hydrogen bonds with each other. The bundled chains form strong fibers, which is perfect for a cell wall.
The same difference explains digestion. Your enzymes fit the links in starch but not the links in cellulose, so you can digest a potato but not a tree. Cows, sheep and other grazing animals can live on cellulose only because microbes in their guts make enzymes that can hydrolyze it. In your own diet, cellulose passes through as fiber.
Another structural polysaccharide, chitin, builds fungal cell walls and the exoskeletons of insects and crabs. Unlike most carbohydrates, chitin contains nitrogen.
What you won't be tested on
You don't need to memorize the chemical structures of specific carbohydrate polymers, such as ring diagrams or which carbon atoms form each link. Focus instead on monomer → polymer, linear vs. branched, and how each molecule's structure fits its job.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Matching structure to function
Hummingbirds have extremely high energy demands and store glycogen in their muscles and liver. Explain why highly branched glycogen is a better energy store for an animal like this than an unbranched chain of the same number of glucose units would be.
Show the solutionHide the solution
- Step 1: Identify the function: the animal needs to release glucose quickly when its energy demand spikes.
- Step 2: Connect to the mechanism: enzymes remove glucose units one at a time from the ends of the chains.
- Step 3: Compare structures: a highly branched molecule has many more chain ends than an unbranched chain with the same number of glucose units.
- Step 4: Conclude: more ends means many enzymes can release glucose at the same time, so glucose becomes available faster.
Answer: Branching gives glycogen many chain ends where enzymes can work at once, so the animal can release glucose rapidly when it needs energy.
- Example 2
Same monomer, different polymer (classic trap)
Starch and cellulose are both polymers of glucose. A student concludes that any animal that can digest starch should also be able to digest cellulose. Explain why this is wrong.
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- Step 1: Notice the trap: having the same monomer doesn't mean having the same structure.
- Step 2: Describe the difference: the glucose units in cellulose are linked in a different orientation, which makes straight chains that bundle tightly into fibers.
- Step 3: Connect to enzymes: enzymes are specific to the shape of the bond and molecule they act on (3.1). Starch-digesting enzymes don't fit cellulose's links.
- Step 4: Give evidence: animals that live on plant material, like cows, rely on gut microbes that make cellulose-digesting enzymes.
Answer: The links between glucose units differ, so the polymers have different shapes. Enzymes that hydrolyze starch don't fit cellulose, so most animals can't digest it without help from microbes.
Common mistakes
- Saying plants store glycogen or animals store starch. Plants store starch; animals store glycogen.
- Thinking cellulose is an energy store. It's structural: it builds plant cell walls.
- Assuming polymers of the same monomer behave the same. The way monomers are linked changes the shape, and the shape changes the function.
On the exam
- Expect structure-function questions: why is glycogen branched, why is cellulose strong, why can't humans digest cellulose. Always connect the shape to the job.
- If a question gives a diagram of a chain of rings, you won't be asked to name exact links. Focus on whether it's linear or branched and what that means for its function.
Connected topics
Videos
Check yourself
4 questions on 1.4 Carbohydrates. Pick an answer to see if you got it, and why.
Starch and cellulose are both polymers of glucose, yet humans can digest starch but not cellulose. Which of the following best explains this difference?
Glycogen in animal liver cells is much more highly branched than amylose, an unbranched form of starch. Which of the following is the most likely advantage of this branching?
Cellulose is an unbranched glucose polymer whose chains lie side by side and hydrogen bond to one another. Glycogen is a highly branched glucose polymer. Which of the following best explains why cellulose, but not glycogen, is suited to building plant cell walls?
A potato cell stores thousands of glucose units as a few large starch molecules rather than as free glucose. Which of the following is the best explanation of an advantage of this storage form?
0 of 4 answered