Lesson 1.4 · Unit 1 · CED topic 1.4
Carbohydrates
Every cell runs on sugar, and many organisms are built from it. Plants wrap each cell in a wall made of a sugar polymer, and animals store sugar in the liver and muscles for later. All of these are carbohydrates, and the lesson is about a single idea: the same sugar monomer, joined in different patterns, gives molecules with completely different jobs.
Describe the structure and function of carbohydrates. The required statement (EK 1.4.A.1): monosaccharides are the monomers of polysaccharides, joined by covalent bonds, and the polymers may be linear or branched. Examples named by the CED: cellulose, starch, and glycogen.
Monosaccharides and the molecules built from them
A monosaccharide is a simple sugar. Its general formula is \((\mathrm{CH_2O})_n\), which is where the name “carbo-hydrate” (carbon plus water) comes from. Glucose, fructose, and galactose all have the formula C6H12O6 but differ in how the atoms are arranged, so they behave differently. The five-carbon sugars ribose and deoxyribose are part of RNA and DNA (lesson 1.6). Sugar names end in -ose.
- Monosaccharide: one sugar unit (glucose, fructose, galactose).
- Disaccharide: two units joined by dehydration synthesis. Sucrose is glucose + fructose, lactose is glucose + galactose, and maltose is glucose + glucose.
- Polysaccharide: many units joined in a chain. The bond between sugar units is a glycosidic linkage.
Glucose exists mainly as a ring. In the ring form it comes in two versions, α (alpha) and β (beta), that differ only in which way the −OH on carbon 1 points. That small difference changes the shape of any chain built from it, which turns out to separate starch from cellulose. (The CED states that the molecular structure of specific carbohydrate polymers is beyond the scope of the exam. Know the idea, not the drawing.)
Three polysaccharides, one monomer
| Polysaccharide | Found in | Function | Structure that fits the job |
|---|---|---|---|
| Starch | Plants (roots, seeds, tubers) | Stores energy | Coiled chains with only a few branches; easily broken down by enzymes |
| Glycogen | Animals and fungi; stored mainly in the liver and muscle | Stores energy for quick use | Very highly branched (a branch about every 10 units), so there are many free ends for enzymes to work from at once |
| Cellulose | Plant cell walls | Provides structural support | Long straight chains lying side by side and held together by many hydrogen bonds; rigid and strong |
Storage versus structure. Starch and glycogen must be taken apart quickly when the cell needs energy, so they are coiled or branched and easy to reach. Cellulose must not be taken apart, so it is straight, tightly bundled, and hard for most organisms to digest. Humans can digest starch but not cellulose, because our enzymes cannot break the β linkages between cellulose's glucose units. Cellulose passes through us as dietary fiber. Cows can live on cellulose because microbes in their stomachs make an enzyme that can.
Two glucose polymers each contain 10,000 glucose units. Polymer A is unbranched. Polymer B has a branch every 10 units, like glycogen. In a simple model, an enzyme can remove one glucose per second from each free end, and one enzyme can work at each free end at the same time. Compare how long each polymer takes to break down completely.
Count the ends. The unbranched polymer has 1 free end. Each branch point creates one more free end. Polymer B has \(10{,}000 / 10 = 1000\) branch points, so \(1000 + 1 = 1001\) free ends.
Time. A: \(10{,}000\) glucose \(\div\) 1 end \(= 10{,}000\) s, nearly 2.8 hours. B: \(10{,}000 \div 1001 = 9.99\) s.
Conclusion. The branched polymer breaks down about 1000 times faster. This is why animals store glucose as glycogen: when an animal needs energy, many enzymes can release glucose at once.
The model is simplified, since real enzymes are not this tidy, but the relationship between branches, ends, and speed is the point.
Grass is mostly cellulose. Answer in three parts: a claim, the evidence, and the reasoning.
Claim. A cow can obtain energy from cellulose; a human cannot.
Evidence. Cellulose and starch are both polymers of glucose, yet humans digest starch (bread, rice) and cellulose passes through them undigested as fiber.
Reasoning. To use a polysaccharide, an organism must hydrolyze the linkages between its glucose units, and the enzyme that does so is specific to the type of linkage. Starch has α linkages, which human enzymes cleave.
Cellulose has β linkages, which human enzymes cannot cleave. Cows host microbes in their stomachs that make an enzyme (cellulase) that does cleave β linkages, so the cow's microbes release glucose and other products the cow uses.
Answers that only say “cows have four stomachs” do not earn the reasoning point: the explanation must be about the linkage and the enzyme.
- Tie structure to function. Do not just list the three polysaccharides. Say why each shape suits its job (branched for fast release, straight and bundled for strength).
- Starch, glycogen, and cellulose are all made of glucose. The difference is how the units are linked and arranged, not what the units are.
- Bookkeeping: a linear polysaccharide of \(n\) glucose units has the formula C6nH10n+2O5n+1 (the glucose atoms minus \(n-1\) water molecules).
- You are not asked to draw the structure of a specific carbohydrate.
- Starch and cellulose are both polymers of glucose. Explain why a human can use starch but not cellulose as a source of glucose.
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The two polymers differ in the linkage between glucose units: starch has α linkages and cellulose has β linkages. The enzymes that hydrolyze these bonds are specific to the type of linkage. Human digestive enzymes cleave the α linkages of starch but cannot cleave the β linkages of cellulose, so cellulose passes through undigested as fiber.
- Explain how the structure of glycogen makes it well suited to rapid release of glucose.
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Glycogen is highly branched, with a branch about every 10 glucose units. Every branch creates another free end, and enzymes remove glucose from the ends, so a highly branched molecule has many ends where many enzymes can work at once. Glucose can be released very quickly when the cell or animal needs energy.
- A glycogen molecule has 4000 glucose units with a branch every 8 units. In the model of the worked example (one glucose removed per second per free end), how many free ends does it have, and how long would complete breakdown take compared with an unbranched polymer of the same size?
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Branch points: \(4000 / 8 = 500\). Free ends: \(500 + 1 = 501\). Time: \(4000 / 501 = 7.98\) s, compared with \(4000 / 1 = 4000\) s for the unbranched polymer, about 500 times faster.
- A linear polysaccharide is made of 100 glucose units (C6H12O6 each). Write its molecular formula and state how many water molecules are needed to hydrolyze it completely.
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Joining 100 glucose units by 99 dehydration reactions removes 99 H2O. Carbon: \(6 \times 100 = 600\). Hydrogen: \(12 \times 100 - 2 \times 99 = 1002\). Oxygen: \(6 \times 100 - 99 = 501\). The formula is C600H1002O501. Complete hydrolysis needs 99 water molecules.
- A student claims, “Plants use cellulose to store energy, just as animals use glycogen.” Evaluate this claim, and correct it if needed.
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The claim is incorrect. Plants store energy as starch; cellulose is the structural polysaccharide in plant cell walls. Cellulose has β linkages and forms straight, rigid, hydrogen-bonded bundles that give strength, and most organisms cannot digest it, which makes it a poor energy store. Starch is coiled with a few branches and is easily hydrolyzed when the plant needs glucose.