Starch, glycogen and cellulose are all made of nothing but glucose, yet one is a plant energy store, one is an animal energy store and one holds up a tree. The difference starts with the direction of one OH group on carbon 1.
Carbohydrates are built from monosaccharides such as glucose (a hexose) and ribose (a pentose). Glucose exists as α-glucose (OH on carbon 1 below the ring) and β-glucose (OH on carbon 1 above). Two monosaccharides join by condensation, losing water and forming a covalent glycosidic bond; hydrolysis adds water to break it. Chains of α-glucose coil into starch and glycogen, compact insoluble energy stores; chains of β-glucose, with alternate units inverted, lie straight and hydrogen-bond into the microfibrils of cellulose.
Fig. 1: ring forms of α-glucose and β-glucose. The only difference is the position of the OH group on carbon 1
Section 01
Monomers, polymers and ring sugars.
Rotate α-glucose, β-glucose and ribose in the simulator, with the carbon numbers switched on. Find carbon 1 in each glucose and see which way its OH points relative to the CH2OH on carbon 5.
Monomer: a small molecule that can be joined to others like it to form a polymer (glucose, amino acids, nucleotides).
Polymer: a large molecule made of many repeating monomers joined by covalent bonds. A macromolecule is any very large molecule, such as a polysaccharide, protein or nucleic acid.
Monosaccharide: a single sugar unit, general formula (CH2O)n: glucose and fructose C6H12O6 (hexoses), ribose C5H10O5 (a pentose).
Disaccharide: two monosaccharides joined by a glycosidic bond (maltose, sucrose, lactose). Polysaccharide: many monosaccharides joined (starch, glycogen, cellulose).
Reducing or not: glucose, fructose and maltose are reducing sugars; sucrose is a non-reducing sugar.
Section 02
Condensation and hydrolysis.
Choose the glycosidic reaction and press Condense, then Hydrolyse. Watch where the water molecule comes from and where it goes back.
Disaccharide
Monomers
Bond
Reducing?
maltose
α-glucose + α-glucose
α-1,4 glycosidic
yes
sucrose
α-glucose + fructose
1,2 glycosidic
no
lactose
galactose + glucose
1,4 glycosidic
yes
Condensation: an OH on carbon 1 of one sugar and an OH on the next lose H2O; the remaining O bridges the two rings. C6H12O6 + C6H12O6 → C12H22O11 + H2O.
Hydrolysis: water is added across the glycosidic bond to split it. In cells enzymes such as amylase and maltase do this; in the lab, boiling with dilute acid does it, which is the basis of the non-reducing sugar test.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the 3D models, how does β-glucose differ from α-glucose?
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The simulator labels ribose as which type of sugar?
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When two α-glucose molecules condense in the simulator, what forms?
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Which of these is a non-reducing sugar?
Section 03
Starch, glycogen and cellulose.
Switch between the four polysaccharides and compare the monomer, the bonds and the overall shape.
Amylose
Amylopectin
Glycogen
Cellulose
monomer
α-glucose
α-glucose
α-glucose
β-glucose
bonds
1,4 only
1,4 + 1,6 branches
1,4 + more 1,6 branches
1,4, alternate units inverted
shape
unbranched helix
branched
very branched, compact
straight chains
role
energy store in plants (starch grains)
energy store in animals (liver, muscle)
plant cell walls
Storage polysaccharides are insoluble, so they do not lower the water potential of the cell or diffuse out. Coiling and branching make them compact, and the many branch ends of amylopectin and glycogen let enzymes release glucose quickly.
Cellulose: each β-glucose is turned 180° relative to its neighbour, giving long straight chains. Hydrogen bonds between OH groups of neighbouring parallel chains bundle them into microfibrils, which group into fibres laid down in layers at different angles. The wall has high tensile strength, so the cell can become turgid without bursting, yet it stays freely permeable.
Examiner trap
Each hydrogen bond in cellulose is weak; the strength comes from the very large number of them between many parallel chains. And cellulose is not coiled: only α-glucose chains form helices.
Stage 2 · Exam
Exam-style questions
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Glycogen is more highly branched than amylopectin. What is the advantage of this for an animal?
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Why do cellulose molecules form straight chains, while amylose forms a helix?
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Which statement about the hydrolysis of a glycosidic bond is correct?
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Which feature of starch makes it suitable as a storage molecule in a plant cell?
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Carbohydrates, mastered.
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