Educerie · AP · Biology
AP Biology 1.4 Carbohydrates
In one breath
- Carbohydrates are sugars and their polymers, made of C, H and O.
- Monosaccharides, such as glucose, are the monomers; two joined by dehydration synthesis make a disaccharide (maltose, sucrose, lactose), many make a polysaccharide, linear or branched, and hydrolysis takes them apart.
- Starch stores glucose in plants; glycogen, more branched, stores it in animal liver and muscle, and its many chain ends let enzymes release glucose fast.
- Cellulose is a linear chain whose straight molecules lie side by side, hydrogen-bonded into the strong fibres of plant cell walls.
- You need the shapes and jobs, not the molecular structures.
What you must be able to do
- Describe monosaccharides as the monomers of polysaccharides"Identify the monomer of glycogen"
- Describe how monosaccharides are joined by covalent bonds into chainsName the reaction that builds a polysaccharide and the one that breaks it
- Describe linear and branched polysaccharides and relate each shape to its functionStarch, glycogen and cellulose: where each is found, what it does, why its shape suits that
Once you have read the note, tick off each one you can do in Check your understanding.
In the exam. Section I multiple choice: naming the monomer of a polysaccharide, matching starch, glycogen and cellulose to their organisms and jobs, and predicting from a description of a branched or unbranched chain how fast glucose can be released; data items compare sugar release from different polysaccharides. In Section II it supplies points in Conceptual analysis (explain how the structure of a storage carbohydrate suits its function), Analyze model or visual representation (describe a drawing of a branched and an unbranched chain), and Interpreting and evaluating experimental results built on enzymes breaking carbohydrates down.
1Monosaccharides: the monomers
A monosaccharide is a single sugar unit, usually with a backbone of three to seven carbon atoms. Monosaccharides are small, polar and hydrophilic, so they dissolve easily and are carried in blood or plant sap. The ones you will meet:
- Glucose, the main fuel for cellular respiration (Unit 3) in almost every organism.
- Fructose and galactose, other common six-carbon sugars.
- Ribose and deoxyribose, five-carbon sugars that form part of every nucleotide (topic 1.6).
Glucose is the monomer of all three polysaccharides in this topic. You are not asked to draw it or any polysaccharide: the CED leaves the detailed molecular structure of particular carbohydrate polymers out of the exam. What you need is the idea of a monomer, the bond, and the shape of the chain.
2Joining sugars: disaccharides and polysaccharides
Two monosaccharides join by dehydration synthesis, releasing one water molecule and forming a covalent bond between them. The product is a disaccharide:
| Disaccharide | Made of | Where you meet it |
|---|---|---|
| Maltose | glucose + glucose | produced when starch is digested |
| Sucrose | glucose + fructose | table sugar; the form plants transport sugar in |
| Lactose | glucose + galactose | the sugar in milk |
Keep adding monosaccharides by dehydration synthesis and you get a polysaccharide, often thousands of units long. Digesting one runs the reaction backwards: hydrolysis adds a water molecule to each bond and frees the monomers.
A polysaccharide is a chain of monosaccharides joined by covalent bonds; whether the chain is linear or branched decides what it is good for.
3Three polysaccharides, three jobs
Starch is the energy store of plants, kept in grains inside cells of seeds, tubers and roots. It is a mixture of unbranched chains and branched chains of glucose. When the plant needs energy, enzymes hydrolyse the starch back to glucose.
Glycogen is the energy store of animals (and fungi), kept mainly in the liver and in muscle. It is a chain of glucose that branches far more often than starch, giving a compact, bushy molecule with a great many chain ends. When your blood glucose falls between meals, liver cells hydrolyse glycogen and release glucose into the blood.
Cellulose is structural, not a store. It is the main material of plant cell walls. Its glucose units are joined in a way that keeps each chain straight and unbranched, so many chains can lie side by side. Hydrogen bonds form between neighbouring chains, and bundles of chains become tough fibres that let a cell wall resist the push of water inside the cell. The bonds in cellulose are a different kind from those in starch, and human digestive enzymes cannot hydrolyse them; cellulose passes through our gut as fibre.
4Why branching matters
The enzymes that take glucose units off glycogen when the body needs them work at the ends of chains, removing units one at a time. A linear chain has only one end to work from. A branched molecule has an end at every branch, so many enzymes can work on it at once and glucose comes off much faster. Branching also coils the store into a compact shape that packs a lot of glucose into a small space.
That fits each store to its owner. A plant stores energy for slow use, in a seed or a tuber. An animal can need glucose fast, for a muscle working hard or to raise blood sugar quickly, and glycogen's many branches let it release glucose at speed.
A linear shape suits cellulose for the opposite reason: straight chains pack tightly and hydrogen-bond to each other along their length, which a branched chain could not do. So the rule runs both ways. Branched for fast release; linear for strength.
5Reading the evidence: a worked example
An invented experiment. Two polysaccharides, P and U, were made up as solutions with the same total number of glucose units. P is highly branched; U is unbranched. An enzyme that removes glucose from chain ends was added to each, and the glucose released in 5 minutes was measured, four trials each.
| Trial | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| P (branched): glucose released / mmol L⁻¹ | 4.2 | 4.8 | 4.5 | 4.5 |
| U (unbranched): glucose released / mmol L⁻¹ | 0.8 | 1.0 | 0.9 | 0.9 |
Means, standard errors (SE = s ÷ √n, with n = 4) and rates:
Claim: glucose is released faster from the branched polysaccharide. Evidence: 0.90 against 0.18 mmol L⁻¹ min⁻¹, five times faster, and the ±2SE ranges are far apart, so the difference is not chance. Reasoning: the enzyme works only at chain ends, and a branched molecule has many more ends than an unbranched one holding the same number of glucose units. The variable held constant (total glucose units) matters: without it, a bigger amount of P could explain the result.
Practice questions
5 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.