Water, carbohydrates and lipids — Edexcel International A Level Biology
Water as a polar solvent, the structure of sugars, starch and glycogen, glycosidic and ester bonds, and the colour tests for sugars and starch.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Water is an effective solvent because its molecules are dipoles: oxygen is slightly negative and each hydrogen slightly positive. Water molecules cluster around ions and polar molecules, separating and dispersing them. This is why dissolved substances can be transported in blood plasma, xylem sap and phloem sap, while non-polar lipids cannot dissolve.
Monosaccharides are single sugar units, disaccharides are two joined together and polysaccharides are long chains of glucose. Starch (amylose and amylopectin) stores energy in plants and glycogen stores it in animals. Their compact, branched, insoluble structures store a lot of glucose without affecting water potential and can be hydrolysed quickly.
A condensation reaction joins two monosaccharides by forming a glycosidic bond and releasing one molecule of water. Repeating it builds disaccharides (maltose, sucrose, lactose) and polysaccharides (glycogen, amylose, amylopectin). A hydrolysis reaction adds water to break a glycosidic bond and release the monosaccharides again.
A semi-quantitative method estimates concentration by matching a colour to standards of known concentration. Benedict’s reagent, heated with a reducing sugar, changes from blue through green, yellow and orange to brick-red as concentration rises. Iodine solution changes from yellow-brown to blue-black as starch concentration rises. Volumes, temperature and timing must be kept the same.
A triglyceride is formed when glycerol joins to three fatty acids. Each of the three condensation reactions links an –OH group of glycerol to the carboxyl group (–COOH) of a fatty acid, forming an ester bond and releasing a molecule of water. Hydrolysis of the ester bonds releases glycerol and fatty acids.
Saturated lipids contain fatty acids with no carbon-to-carbon double bonds, so their straight chains pack closely and they are usually solid at room temperature. Unsaturated lipids contain one or more C=C double bonds, which bend the chain, so they pack less closely and are usually liquid oils. Diets high in saturated fat are linked with raised LDL cholesterol.
Exam questions
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Quick check questions
What is a dipole?
Show answer
A molecule with a slightly positive end and a slightly negative end because its electrons are shared unevenly.
Name the two types of bond in amylopectin.
Show answer
α-1,4 glycosidic bonds (main chain) and α-1,6 glycosidic bonds (branch points).
Which two monosaccharides join to make sucrose?
Show answer
Glucose and fructose.
How many water molecules are released when a triglyceride forms?
Show answer
Three, one for each ester bond formed.
What colour change shows starch with iodine solution?
Show answer
Yellow-brown to blue-black.
Frequently asked questions
Why is water a good solvent?
Water is a good solvent because its molecules are dipoles, with a slightly negative oxygen end and slightly positive hydrogen ends. These charged ends are attracted to ions and polar molecules, surround them and keep them apart, so they dissolve. This lets blood plasma and sap carry substances.
What is the difference between starch and glycogen?
Both are polysaccharides made of α-glucose, but starch is the energy store in plants and glycogen is the store in animals. Glycogen is more highly branched, with α-1,6 branches every 8 to 12 units, so it has more free ends and releases glucose faster.
What is a condensation reaction?
A condensation reaction joins two molecules together and releases a molecule of water. When monosaccharides join, a glycosidic bond forms. When glycerol and fatty acids join, ester bonds form. Hydrolysis is the reverse, as water is added to break the bond.
What is the difference between saturated and unsaturated fats?
Saturated fats have fatty acids with no C=C double bonds, so their chains are straight, pack closely and are usually solid at room temperature. Unsaturated fats have one or more C=C double bonds that kink the chains, so they pack less closely and are usually liquid.
How does the Benedict’s test show how much sugar there is?
Heated with a reducing sugar, Benedict’s reagent changes from blue through green, yellow and orange to brick-red as the concentration rises. Comparing the colour of an unknown with colour standards of known concentration gives a semi-quantitative estimate of the concentration.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026