What is a monosaccharide?

A single sugar unit, e.g. glucose, fructose or galactose.

Name three disaccharides and their monosaccharides.

Maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose).

What is a polysaccharide?

A polymer made of many monosaccharides joined by glycosidic bonds, e.g. starch or glycogen.

Describe the structure of amylose.

An unbranched chain of glucose joined by 1,4 glycosidic bonds, coiled into a compact helix.

Describe the structure of amylopectin.

A branched chain of glucose, with 1,4 bonds in the chains and 1,6 bonds at the branch points.

Where is glycogen stored and what is its structure?

In liver and muscle cells; a highly branched polymer of glucose.

Why is glycogen's extra branching useful in animals?

More ends for enzymes, so glucose is released very quickly to meet a high metabolic rate.

Why is an insoluble store better than soluble glucose?

It does not affect the water potential of the cell and does not diffuse out, so no osmotic problems.

What does Benedict's reagent test for, and what is a positive result?

Reducing sugars; blue changes to green, yellow, orange or brick-red on heating.

How is Benedict's reagent used semi-quantitatively?

Compare the colour of the test solution with colour standards of known glucose concentration, using the same volumes, temperature and time.

What does iodine solution test for?

Starch; orange-brown turns blue-black.

Why is the colour-standard method only semi-quantitative?

Colour matching by eye is subjective and gives a range, not an exact concentration.

Water and carbohydratesEdexcel International A Level Biology: Flashcards

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Why is water a good solvent?

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Why is water a good solvent?: It is a dipole (δ− oxygen, δ+ hydrogens), so water molecules are attracted to ions and polar molecules and surround them.
What is a monosaccharide?: A single sugar unit, e.g. glucose, fructose or galactose.
Name three disaccharides and their monosaccharides.: Maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose).
What is a polysaccharide?: A polymer made of many monosaccharides joined by glycosidic bonds, e.g. starch or glycogen.
Name the two polysaccharides in starch.: Amylose and amylopectin.
Describe the structure of amylose.: An unbranched chain of glucose joined by 1,4 glycosidic bonds, coiled into a compact helix.
Describe the structure of amylopectin.: A branched chain of glucose, with 1,4 bonds in the chains and 1,6 bonds at the branch points.
Where is glycogen stored and what is its structure?: In liver and muscle cells; a highly branched polymer of glucose.
Why is glycogen's extra branching useful in animals?: More ends for enzymes, so glucose is released very quickly to meet a high metabolic rate.
Why is an insoluble store better than soluble glucose?: It does not affect the water potential of the cell and does not diffuse out, so no osmotic problems.
What does Benedict's reagent test for, and what is a positive result?: Reducing sugars; blue changes to green, yellow, orange or brick-red on heating.
How is Benedict's reagent used semi-quantitatively?: Compare the colour of the test solution with colour standards of known glucose concentration, using the same volumes, temperature and time.
What does iodine solution test for?: Starch; orange-brown turns blue-black.
Why is the colour-standard method only semi-quantitative?: Colour matching by eye is subjective and gives a range, not an exact concentration.

Exam questions on Water and carbohydrates

Blood plasma is about 90% water. It carries dissolved glucose, amino acids and mineral ions such as hydrogencarbonate ions (HCO₃⁻) around the body of a mammal.
Explain how the structure of glucose allows it to dissolve in blood plasma so that it can be transported.2 marks
A marathon runner stores energy as glycogen in her liver and muscle cells. During a race, the glycogen is broken down to provide glucose for respiration.
Explain why glycogen is a suitable energy store for the runner's muscle cells.2 marks
Potato tuber cells store starch over the winter. In spring the tuber must release glucose quickly to supply energy for the growth of new shoots. Starch is a mixture of two polysaccharides, amylose and amylopectin.
Compare the structures of amylose and amylopectin.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).