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CarbohydratesEdexcel A-Level Biology B: Flashcards

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What is the difference between a monosaccharide, disaccharide and polysaccharide?

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What is the difference between a monosaccharide, disaccharide and polysaccharide?
A monosaccharide is one sugar unit, a disaccharide is two joined, and a polysaccharide is many joined into a polymer.
How do α-glucose and β-glucose differ?
The OH on carbon 1 is below the ring in α-glucose and above the ring in β-glucose.
What is ribose?
A pentose (five-carbon) monosaccharide found in RNA.
What is a condensation reaction?
A reaction that joins two molecules by removing a molecule of water, forming a new bond.
What is hydrolysis?
A reaction in which a bond is broken by adding a molecule of water.
Which bond joins monosaccharides?
A glycosidic bond.
Name the monomers of maltose, sucrose and lactose.
Maltose: glucose + glucose. Sucrose: glucose + fructose. Lactose: glucose + galactose.
What are the two polysaccharides in starch?
Amylose (unbranched, coiled) and amylopectin (branched), both made of α-glucose.
Why is starch a good storage molecule?
It is compact and insoluble, so it stores lots of glucose and does not affect water potential.
Why can glycogen be hydrolysed faster than starch?
It is more highly branched, so it has more free ends for enzymes to act on.
Why does cellulose have high tensile strength?
Straight, unbranched chains lie side by side and are held by very many hydrogen bonds in microfibrils.
Why is every other β-glucose inverted in cellulose?
So that the OH groups line up to form glycosidic bonds between neighbouring units.

Exam questions on Carbohydrates

  1. A brewing technician is studying how barley grains are converted into fermentable sugars. Barley grains store starch. When the grains germinate, enzymes break the starch down into maltose, which is then broken down further so that yeast can ferment the sugars.
    Describe what happens when a molecule of maltose is hydrolysed.2 marks
  2. A biochemistry student is drawing the structures of three monosaccharides. She draws α-glucose, β-glucose and ribose, the sugar found in RNA.
    Ribose, maltose and glycogen are all carbohydrates. Classify each as a monosaccharide, a disaccharide or a polysaccharide, and state how these three groups differ in structure.2 marks
  3. A student is investigating milk. Lactose is the main carbohydrate in milk. A dairy company treats milk with an enzyme that hydrolyses lactose, producing 'lactose-free' milk for customers who cannot digest lactose.
    Describe how a molecule of lactose is formed from its monomers.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).