Carbohydrates: Study Guide (Biology 1.4)

Sugar as fuel and as building material

Carbohydrates do two big jobs in living things. They are fuel: cells break down glucose to release energy. They are also building material: the walls of plant cells are made mostly of a carbohydrate, cellulose. Both jobs use the same small building block. What changes is how the blocks are joined and arranged. This topic is about that link between structure and function.

The word carbohydrate hints at the elements involved: carbon, hydrogen and oxygen. Many simple sugars have the general formula (CH₂O)ₙ, so glucose, with n = 6, is C₆H₁₂O₆.

Monosaccharides: the monomers

A monosaccharide (simple sugar) is a single sugar unit. It is the monomer of carbohydrates: the small unit that is joined to others to build bigger molecules.

• Glucose (C₆H₁₂O₆) is the main fuel for cellular respiration in most organisms and the monomer of starch, glycogen and cellulose.
• Fructose (fruit sugar) and galactose have the same formula as glucose, C₆H₁₂O₆, but their atoms are arranged differently. Molecules like this are called isomers. A different arrangement gives a different shape, and enzymes respond to shape, so cells treat these sugars differently.
• Ribose and deoxyribose are five-carbon sugars. They form part of the backbone of RNA and DNA, which you meet in Topic 1.6.

Monosaccharides have several polar –OH (hydroxyl) groups. These form hydrogen bonds with water, so simple sugars dissolve easily (Topic 1.1). In water, most glucose molecules close up into a ring. Diagrams often show each sugar as a hexagon for this reason, and so does Figure 1.

Joining sugars: disaccharides and polysaccharides

Two monosaccharides join by dehydration synthesis, the reaction you met in Topic 1.3. An –OH group is removed from one sugar and an H atom from the other. These leave as one molecule of water, and the two sugars become joined by a covalent bond through an oxygen atom. This link between two sugars is called a glycosidic linkage.

Joining two monosaccharides gives a disaccharide. Three common ones:

Disaccharide	Made from	Where you meet it
Maltose	glucose + glucose	formed when starch is digested
Sucrose	glucose + fructose	table sugar; the form in which many plants move sugar around
Lactose	glucose + galactose	the sugar in milk

Keep adding monosaccharides, one dehydration synthesis at a time, and you get a polysaccharide (complex carbohydrate): a polymer that can contain hundreds or thousands of sugar units. Each new link releases one more water molecule. Hydrolysis reverses the process: one water molecule is used to break each link. Digestion of starch in your gut is hydrolysis, speeded up by enzymes.

Linear or branched: shape decides the job

The monomers in a polysaccharide can be joined in a single unbranched chain (linear) or in a chain with side chains coming off it (branched). Figure 2 shows the three polysaccharides you need to know. All three are made only of glucose.

Polysaccharide	Found in	Shape	Function	How the shape fits the function
Starch	plants, for example in seeds and potato tubers	a mix of linear chains and branched chains	energy store	compact and insoluble, so a lot of glucose can be stored without dissolving; enzymes can hydrolyse it back to sugar when the plant needs energy
Glycogen	animals (mainly liver and muscle) and fungi	highly branched	energy store	many branch ends give many places where enzymes can remove glucose at the same time, so glucose can be released quickly, for example during exercise
Cellulose	plant cell walls	linear, unbranched	structure and support	straight chains lie side by side and hydrogen-bond to each other, forming strong fibres that resist stretching

Storage polysaccharides: starch and glycogen

Why store glucose as a polymer at all? A single large polysaccharide molecule takes the place of thousands of separate glucose molecules. Starch is insoluble, and glycogen is packed into granules. Neither behaves like thousands of dissolved glucose molecules, so storage does not upset the water balance of the cell, and the store stays where it is put. When energy is needed, enzymes break the links and free the glucose again.

Animals often need energy fast: to run from a predator, or to keep muscles working. Glycogen’s many branches mean many chain ends. Enzymes that release glucose work at these ends, so more ends means more glucose released per second. Plants are less active, and starch, with fewer branches, suits their slower needs.

Structural polysaccharides: cellulose

In cellulose, the chains are straight and unbranched. Because they are straight, neighbouring chains can lie close together, and their many –OH groups form hydrogen bonds from chain to chain. Thousands of these weak attractions add up, just as they did in water in Topic 1.1, and bundle the chains into tough fibres. Plant cell walls built from these fibres stop cells from bursting and help a plant stand upright.

Another structural polysaccharide, chitin, forms the cell walls of fungi and the outer skeletons of insects and crabs. Its monomer is a modified glucose that carries a nitrogen-containing group.

Background (not examined in detail). In cellulose, each glucose unit is joined in a different orientation from the units in starch and glycogen. That is why the enzymes that digest starch cannot break cellulose. Cows and other grazing animals can use cellulose only because microbes in their gut make the enzymes that can. You do not need to name or draw these linkages: the exam leaves out the detailed molecular structure of specific carbohydrate polymers. For humans, cellulose passes through the gut largely undigested as dietary fibre.

Worked example 1: counting water and mass in dehydration synthesis

Question. (a) Write the formula of maltose, made from two glucose molecules (C₆H₁₂O₆). (b) A short chain of 10 glucose units is built by dehydration synthesis. How many water molecules are released? Give its formula and molar mass. (c) What percentage of the mass of the 10 separate glucose molecules is lost as water? Use C = 12.011, H = 1.008, O = 15.999 g mol⁻¹.

(a) Maltose.

1. Add the two glucose molecules: C₆H₁₂O₆ + C₆H₁₂O₆ gives C₁₂H₂₄O₁₂.
2. Take away one H₂O: C₁₂H₂₂O₁₁. Maltose is C₁₂H₂₂O₁₁.

(b) The 10-unit chain.

1. Count the links: a chain of 10 units has 10 − 1 = 9 links (think of 10 people holding hands in a line: 9 handshakes). Each link releases one water, so 9 water molecules are released.
2. Formula: 10 × C₆H₁₂O₆ = C₆₀H₁₂₀O₆₀. Remove 9 H₂O (18 H, 9 O): C₆₀H₁₀₂O₅₁.
3. Molar mass of glucose = 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g mol⁻¹. Molar mass of water = 2(1.008) + 15.999 = 18.015 g mol⁻¹.
4. Molar mass of the chain = 10 × 180.156 − 9 × 18.015 = 1801.56 − 162.135 = 1639.4 g mol⁻¹.

(c) Mass lost. 162.135 ÷ 1801.56 × 100 = 9.0%.

Check. Working out the molar mass straight from C₆₀H₁₀₂O₅₁ gives the same answer, 1639.4 g mol⁻¹.

Interpretation. The general rule for a chain of n monomers is n − 1 links and n − 1 waters. In a very long chain, each glucose unit adds C₆H₁₀O₅ (162.1 g mol⁻¹), because almost every unit has lost one water. Hydrolysis runs the same numbers in reverse: breaking the 10-unit chain completely uses 9 water molecules and gives back 10 glucose molecules.

Worked example 2: why branching speeds up glucose release

Question. Use a simple model. A storage molecule holds 1200 glucose units. The releasing enzyme can only remove glucose from the free outer end of a chain, and each chain has one free outer end. At each end, the enzyme removes 10 glucose units per second. Compare three forms of the molecule: (i) one linear chain of 1200 units; (ii) a moderately branched form made of chains 25 units long; (iii) a highly branched form made of chains 12 units long. For each, find the number of free ends, the starting rate of release and the time to release the first 600 glucose units. All numbers are fictional.

1. Number of chains (= free ends). (i) 1200 ÷ 1200 = 1. (ii) 1200 ÷ 25 = 48. (iii) 1200 ÷ 12 = 100.
2. Starting rate = ends × 10 units s⁻¹. (i) 1 × 10 = 10 s⁻¹. (ii) 48 × 10 = 480 s⁻¹. (iii) 100 × 10 = 1000 s⁻¹.
3. Time for 600 units = 600 ÷ rate (the model assumes every chain is shortened at the same rate). (i) 600 ÷ 10 = 60 s. (ii) 600 ÷ 480 = 1.25 s. (iii) 600 ÷ 1000 = 0.6 s.

Form	Free ends	Starting rate / glucose s⁻¹	Time for 600 units / s
(i) linear	1	10	60
(ii) moderately branched, like starch	48	480	1.25
(iii) highly branched, like glycogen	100	1000	0.6

Interpretation. Same number of glucose units, same enzyme, but the highly branched form releases glucose 100 times faster at the start than the linear one. This is the structure–function argument in numbers: glycogen’s heavy branching suits animals that may need glucose quickly.

Limits of the model. In a real molecule, inner chains are partly hidden until outer ones are removed, branch points need a second enzyme, and chains are not all the same length. The real difference is smaller than 100 times, but the direction of the effect is the same. A model like this is useful for predicting what happens if branching changes, for example if a mutation lowers the activity of the enzyme that adds branches.

Common misconceptions

• “Carbohydrates are just for energy.” Cellulose and chitin are structural. Ribose and deoxyribose are part of RNA and DNA.
• “Starch, glycogen and cellulose are made of different sugars.” All three are polymers of glucose. They differ in how the units are linked and whether the chains branch.
• “Dehydration synthesis adds water.” It removes water. Hydrolysis adds water to break a link. Think “hydro-lysis”: water splits.
• “A chain of n sugars releases n waters.” It releases n − 1, one per link.
• “The links between sugars are hydrogen bonds.” The links that hold a chain together are covalent bonds. Hydrogen bonds are the weaker attractions between cellulose chains.
• “Branched means bigger.” Branching changes the shape and the number of chain ends, not the number of monomers. A branched and a linear molecule can hold the same number of glucose units.
• “Sugar molecules are stored as free glucose so they are ready to use.” Cells store glucose as large polymers; large amounts of dissolved glucose would draw water into the cell.
• “Humans cannot digest cellulose because it is not made of glucose.” It is made of glucose. Our enzymes do not fit its links.

Where this leads

Next, Topic 1.5: Lipids looks at a group of macromolecules that are mostly nonpolar and are not true polymers. Carbohydrates come back in Unit 3, where glucose is made in photosynthesis and broken down in cellular respiration, and in Unit 2, where sugars attached to membrane proteins help cells recognise each other. If dehydration synthesis and hydrolysis still feel shaky, revisit Topic 1.3. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.