Chemical Basis of Life

Lessons

Senior 4 · Biology

Chemical Basis of Life

5 lessons

Lesson 3 of 5

Carbohydrates: sugars, starch and cellulose

25 min

One sugar, three different jobs

A cassava root, a cotton shirt and your liver have something in common: each holds huge numbers of glucose molecules joined into chains. In the cassava root the chains are starch, a food store. In cotton they are cellulose, so strong that it is spun into thread. In the liver they are glycogen, which can be broken down within minutes when your blood sugar falls. The building block is the same; what differs is the shape of the glucose and the way the units are linked. This lesson explains how.

Definition

Carbohydrate: a compound of carbon, hydrogen and oxygen, usually with the general formula $(\mathrm{CH_2O})_n$ .

Monosaccharide: a single sugar unit, for example glucose. Disaccharide: two monosaccharides joined, for example maltose. Polysaccharide: a polymer of many monosaccharides, for example starch.

Condensation reaction: a reaction that joins two molecules and releases a molecule of water. Hydrolysis: a reaction that splits a molecule by adding water.

Glycosidic bond: the C–O–C link formed between two sugar units by condensation.

Isomers: molecules with the same molecular formula but different arrangements of atoms; $\alpha$ -glucose and $\beta$ -glucose are isomers.

Reducing sugar: a sugar that can donate electrons to (reduce) copper(II) ions in Benedict's reagent; all monosaccharides, and maltose and lactose.

Monosaccharides

Monosaccharides are classed by the number of carbon atoms: trioses (3 C, such as glyceraldehyde, made in photosynthesis), pentoses (5 C, such as ribose in RNA and ATP and deoxyribose in DNA) and hexoses (6 C, such as glucose, fructose and galactose, all $\mathrm{C_6H_{12}O_6}$ ). They are sweet, soluble and small enough to cross cell membranes through carrier proteins, so they are the form in which carbohydrate is absorbed and transported. In water, glucose forms a ring of five carbon atoms and one oxygen atom. The ring can close in two ways, giving two isomers that differ only at carbon 1 (Figure 1).

Figure 1. $\alpha$ -glucose and $\beta$ -glucose differ only in the position of the OH group on carbon 1. This small difference decides whether glucose units make starch or cellulose.

Disaccharides and the glycosidic bond

Two monosaccharides join by a condensation reaction: an OH from one and an H from the OH of the other are removed as water, and the remaining oxygen atom links the two (Figure 2). When carbon 1 of one glucose joins carbon 4 of the next, the link is a 1,4 glycosidic bond.

\mathrm{C_6H_{12}O_6} + \mathrm{C_6H_{12}O_6} \longrightarrow \mathrm{C_{12}H_{22}O_{11}} + \mathrm{H_2O}

Hydrolysis is the reverse: adding water breaks the bond. It happens in digestion (catalysed by enzymes) and in the laboratory when a sugar is boiled with dilute acid.

Figure 2. Condensation of two $\alpha$ -glucose molecules forms maltose, joined by an $\alpha$ -1,4 glycosidic bond. Hydrolysis adds the water back and separates them.

Disaccharide	Made from	Where it is found	Reducing?
Maltose	$\alpha$ -glucose + $\alpha$ -glucose	Germinating seeds (from starch); formed when amylase digests starch	Yes
Sucrose	$\alpha$ -glucose + fructose	Transported in phloem; sugar cane, ripe bananas, table sugar	No
Lactose	$\beta$ -galactose + glucose	Milk	Yes

Polysaccharides: structure fits function

Polysaccharides are large and insoluble, so they do not affect the water potential of a cell and do not diffuse away. That makes them good stores (starch, glycogen) and good building materials (cellulose).

Figure 3. Each hexagon is one glucose unit. A: amylose. B: amylopectin and glycogen. C: cellulose molecules side by side.

Polysaccharide	Structure	How the structure fits the function
Starch (store in plants), a mixture of amylose and amylopectin	Amylose: unbranched chain of $\alpha$ -glucose with 1,4 bonds, coiled into a helix. Amylopectin: 1,4 chains with 1,6 branches	Compact helix packs much glucose into a small space; insoluble, so no osmotic effect; branches give many chain ends where enzymes can release glucose
Glycogen (store in animals and fungi; liver and muscle)	Like amylopectin but much more branched	Very many ends, so glucose can be released very quickly when energy is needed; compact and insoluble
Cellulose (plant cell walls)	Straight, unbranched chains of $\beta$ -glucose with 1,4 bonds; each unit is turned over relative to the next	Straight chains lie side by side and form hydrogen bonds with each other, making microfibrils of great tensile strength; walls resist the pressure of turgid cells

Most animals, including humans, have no enzyme that hydrolyses the $\beta$ -1,4 bonds of cellulose, so in our diet it passes through as fibre (roughage). Cattle and goats can live on grass because microorganisms in their gut make the enzyme cellulase. Chitin, the tough polysaccharide of insect exoskeletons and fungal walls, is built like cellulose but from a glucose unit carrying a nitrogen-containing group.

Tests for carbohydrates

Test for	Method	Positive result
Reducing sugars	Add an equal volume of Benedict's reagent (blue, contains $\mathrm{Cu^{2+}}$ ) to the sample; heat in a boiling water bath for about 5 minutes	A green, yellow, orange or brick-red precipitate of copper(I) oxide: the more reducing sugar, the further along that series
Non-reducing sugars (such as sucrose)	First show that Benedict's stays blue. Then boil a new sample with dilute hydrochloric acid (hydrolysis), cool, neutralise with sodium hydrogencarbonate, and do the Benedict's test	Now a coloured precipitate: hydrolysis released reducing monosaccharides
Starch	Add a few drops of iodine in potassium iodide solution (orange-brown)	Blue-black: iodine fits inside the amylose helix

The Benedict's test can be made semi-quantitative. Either compare the colour with tubes of known glucose concentration, or measure it: filter off the precipitate and place the remaining solution in a colorimeter. The more sugar there was, the more $\mathrm{Cu^{2+}}$ has been used up, so the paler blue the solution and the lower its absorbance.

Worked example 1: a calibration curve

Standard glucose solutions were tested with excess Benedict's reagent, the precipitate filtered off, and the absorbance of the blue filtrate measured (illustrative data):

Glucose concentration / %	0.0	0.2	0.4	0.6	0.8	1.0
Absorbance	1.00	0.84	0.68	0.52	0.36	0.20

A sample of urine treated the same way gave an absorbance of 0.60. Find its glucose concentration.

The absorbance falls by $0.16$ for every $0.2 %$ of glucose, a straight line: absorbance $= 1.00 - 0.80 c$ , where $c$ is the concentration in %.
$0.60 = 1.00 - 0.80 c$ , so $c = \frac{1.00 - 0.60}{0.80} = 0.50\,\%$ .
On a graph you would read the same value from the calibration line. Glucose in urine is a sign that blood glucose is too high, as in uncontrolled diabetes; a doctor would confirm it with a blood test.

Worked example 2: identifying three sugars

Three colourless solutions, X, Y and Z, each contain one carbohydrate: glucose, sucrose or starch. Results: X gives a brick-red precipitate with Benedict's. Y stays blue with Benedict's, but gives a brick-red precipitate after boiling with acid and neutralising. Z stays blue with Benedict's even after acid hydrolysis for one minute, and turns blue-black with iodine. Identify each.

X is a reducing sugar: glucose.
Y is not reducing until its glycosidic bond is hydrolysed: sucrose, a non-reducing disaccharide.
Z gives the iodine test: starch. A long polysaccharide is hydrolysed only slowly by brief boiling with acid, so little reducing sugar is released.

Worked example 3: the mass of a glycogen molecule

A glycogen molecule contains 30 000 glucose units ( $M_{r}$ of glucose = 180; of water = 18). Every unit except the first is joined by one glycosidic bond, so there are 29 999 condensations. Calculate the relative molecular mass.

Mass of the glucose units: $30\,000 \times 180 = 5\,400\,000$ .
Water removed: $29\,999 \times 18 = 539\,982$ .
$M_r = 5\,400\,000 - 539\,982 = 4\,860\,018 \approx 4.86 \times 10^{6}$ . Each unit in the chain adds $180 - 18 = 162$ , which is why polysaccharides are often written $(\mathrm{C_6H_{10}O_5})_n$ .

A seed split open after its seed coat (left) was peeled off, with a drop of iodine solution on the cut surface: where the iodine has soaked in, the stored food has stained dark blue-black, showing starch. Photo: Biology Big Brother, CC BY 2.0, via Wikimedia Commons

CrashCourse: "Biological Molecules - You Are What You Eat" (Crash Course Biology #3). The first part is on carbohydrates: watch for condensation, glycosidic bonds and why we cannot digest cellulose. Lipids and proteins follow in the next two lessons.

Warning

Common mistakes and safety

Writing that hydrolysis "uses up" water and condensation "uses" water. Condensation releases water; hydrolysis adds it.
Saying cellulose is made of $\alpha$ -glucose. Starch and glycogen are $\alpha$ ; cellulose is $\beta$ .
Calling sucrose a reducing sugar. It gives a positive Benedict's result only after hydrolysis.
Forgetting to neutralise after acid hydrolysis: Benedict's reagent works only in alkaline conditions.
Safety: wear eye protection; heat tubes in a water bath, not directly in a flame; hold them with a test-tube holder and point them away from people. Dilute hydrochloric acid irritates the skin and eyes.

Tip

Memory hook. "Alpha for All stores, Beta for Building." Starch and glycogen ( $\alpha$ ) store energy; cellulose ( $\beta$ ) builds walls. In an exam answer about any polysaccharide, give three things: the monomer, the bond (1,4 and/or 1,6) and the shape (helix, branched or straight), then link the shape to the job.

✦

Practice: carbohydrates

Name the monosaccharides that make up (a) maltose, (b) sucrose, (c) lactose.
Write the equation for the formation of sucrose from its monosaccharides, using molecular formulae.
Describe how you would show that a fruit juice contains a non-reducing sugar as well as a reducing sugar.
Using the calibration data in worked example 1, find the glucose concentration of a sample with an absorbance of 0.44.
Explain two ways in which the structure of glycogen suits it to be an energy store in muscle.
Explain why cellulose is strong but starch is not, although both are made of glucose.
A pupil says: "Cows can digest grass because cellulose is a reducing sugar." Evaluate this statement.

Show answersHide answers

(a) Glucose + glucose. (b) Glucose + fructose. (c) Galactose + glucose.
$\mathrm{C_6H_{12}O_6} + \mathrm{C_6H_{12}O_6} \longrightarrow \mathrm{C_{12}H_{22}O_{11}} + \mathrm{H_2O}$ (glucose + fructose → sucrose + water).
Do the Benedict's test on one sample: a coloured precipitate shows reducing sugar. To test for non-reducing sugar, boil another sample with dilute HCl, cool, neutralise with sodium hydrogencarbonate and repeat the Benedict's test; a heavier precipitate (further towards brick-red) than in the first test shows that a non-reducing sugar was present and has been hydrolysed.
$c = \frac{1.00 - 0.44}{0.80} = \frac{0.56}{0.80} = 0.70\,\%$ .
Any two: it is highly branched, so it has many ends where enzymes can remove glucose quickly; it is insoluble, so it has no osmotic effect and stays in the cell; it is compact, so a lot is stored in a small space.
Cellulose is made of $\beta$ -glucose; its 1,4 bonds with alternate units turned over give straight chains that lie side by side and are cross-linked by many hydrogen bonds into microfibrils. Starch is $\alpha$ -glucose, coiled or branched, so its chains cannot pack and bond together in the same way.
Wrong. Cellulose is a polysaccharide, not a reducing sugar. Cows digest it because microorganisms in their gut make cellulase, which hydrolyses the $\beta$ -1,4 glycosidic bonds to glucose.

Quick check

Put the steps of the test for a non-reducing sugar in order.

Tap the cards in the right order.

Key points

Monosaccharides ( $(\mathrm{CH_2O})_n$ ) join by condensation to form glycosidic bonds; hydrolysis breaks them.
$\alpha$ -glucose builds starch (amylose helix, branched amylopectin) and glycogen (very branched): compact, insoluble stores. $\beta$ -glucose builds cellulose: straight, hydrogen-bonded chains with great tensile strength.
Benedict's reagent detects reducing sugars and can be made semi-quantitative; non-reducing sugars need acid hydrolysis first; iodine detects starch.

Sources and credits

1.3.2: Synthesis of Biological Macromolecules and 1.3.3: Carbohydrates, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for condensation and hydrolysis and the structure of mono-, di- and polysaccharides. Adapted by Ganzaa.
Figures 1–3, the tables, worked examples (the absorbance data are illustrative), practice and quick check: written by Ganzaa.
Photos: Denis kasozi, CC BY-SA 4.0; HunterRSC, CC0; Biology Big Brother, CC BY 2.0; all via Wikimedia Commons.
Video: "Biological Molecules - You Are What You Eat: Crash Course Biology #3", CrashCourse, embedded from YouTube.

1One sugar, three different jobs

2Monosaccharides

3Disaccharides and the glycosidic bond

4Polysaccharides: structure fits function

5Tests for carbohydrates

One sugar, three different jobs

Monosaccharides

Disaccharides and the glycosidic bond

Polysaccharides: structure fits function

Tests for carbohydrates