Syllabus coverage
- List the chemical elements that make up carbohydrates, fats and proteins
- State that large molecules are made from smaller molecules, limited to: (a) starch, glycogen and cellulose from glucose; (b) proteins from amino acids; (c) fats and oils from fatty acids and glycerol
- Describe the use of the iodine solution test for starch
- Describe the use of the Benedict's solution test for reducing sugars
- Describe the use of the biuret test for proteins
- Describe the use of the ethanol emulsion test for fats and oils
- Describe the use of the DCPIP test for vitamin C
- Describe the structure of a DNA molecule: two strands coiled together to form a double helix; each strand contains chemicals called bases; bonds between pairs of bases hold the strands together; the bases always pair up in the same way: A with T, and C with G
Part of the IGCSE Co-ordinated Sciences syllabus 0654 / 0973.
The elements that make up biological molecules
Every biological molecule is built from a small number of chemical elements. The syllabus asks you to list the elements in the three main groups: carbohydrates, fats and proteins. Learn the list for each group as a set, because element questions are quick marks.
Carbohydrates contain carbon, hydrogen and oxygen only. Fats and oils (together called lipids) also contain carbon, hydrogen and oxygen only, but they contain far less oxygen in proportion to hydrogen than carbohydrates do. Proteins contain carbon, hydrogen and oxygen and nitrogen; some proteins also contain sulphur.
The examinable odd one out is nitrogen. It is present in proteins and absent from carbohydrates and fats. If a question gives you a chemical analysis of an unknown food and asks which type of molecule it contains, the presence of nitrogen points to protein.
| Molecule group | Elements present |
|---|---|
| Carbohydrates (for example starch, glucose) | Carbon, hydrogen, oxygen |
| Fats and oils (lipids) | Carbon, hydrogen, oxygen |
| Proteins | Carbon, hydrogen, oxygen, nitrogen |
Large molecules are built from smaller ones
Most biological molecules are polymers: large molecules made by joining many small, similar molecules together. The syllabus limits the examples you need to know. Starch, glycogen and cellulose are all made from glucose; proteins are made from amino acids; and fats and oils are made from fatty acids and glycerol.
When two small molecules join, a condensation reaction takes place. A bond forms between them and a molecule of water is released. Repeating this many times builds the large molecule from its small units.
The reverse reaction is hydrolysis. A molecule of water is used to break the bond, splitting the large molecule back into its smaller units. This is exactly what happens during digestion, when enzymes such as carbohydrases and proteases hydrolyse the polymers in food back into glucose and amino acids so that they can be absorbed.
| Large molecule (polymer) | Built from (smaller molecules) |
|---|---|
| Starch | Glucose |
| Glycogen | Glucose |
| Cellulose | Glucose |
| Protein | Amino acids |
| Fat or oil (lipid) | Fatty acids and glycerol |
Carbohydrates: starch, glycogen and cellulose
All three of these carbohydrates are polymers of glucose, but the glucose units are joined in different arrangements. That difference in arrangement is the whole reason they behave differently in the body.
Starch is the storage carbohydrate of plants, found in seeds, roots and tubers such as potato. Its glucose units are joined in a way that human enzymes can hydrolyse, so we can digest it. Glycogen is the equivalent storage carbohydrate in animals, stored in the liver and muscles; it is more highly branched than starch, which lets it be broken down quickly when energy is needed.
Cellulose is also a polymer of glucose, but the glucose units are joined in a different arrangement and the long chains are held together in straight, rigid bundles. Human digestive enzymes cannot hydrolyse these bonds, so cellulose passes through the gut undigested as dietary fibre. It is still essential: the fibre adds bulk to the contents of the gut and helps keep them moving, which prevents constipation.
| Carbohydrate | Made from | Where it is found | Can humans digest it? |
|---|---|---|---|
| Starch | Glucose | Plant storage organs: seeds, roots, tubers | Yes - hydrolysed by carbohydrase to glucose |
| Glycogen | Glucose | Animal liver and muscle | Yes - hydrolysed to glucose |
| Cellulose | Glucose | Plant cell walls | No - human enzymes cannot break the bonds, so it passes through as dietary fibre |
Proteins and lipids: two more families of molecules
Proteins are polymers of amino acids. About twenty different amino acids occur in living things, and the order in which they are joined determines the shape, and therefore the function, of the protein. Proteins are used for growth and repair of tissues, and to make enzymes, hormones and antibodies. Every amino acid contains nitrogen, which is why nitrogen appears in the element list for proteins but not for carbohydrates or fats.
Fats and oils (together called lipids) are built from fatty acids and glycerol. One glycerol molecule joins to three fatty acid molecules in condensation reactions, releasing water. Fats are solid at room temperature and oils are liquid, but chemically they are the same kind of molecule. Both are used for long-term energy storage and insulation, and as a source of the fatty acids the body needs to build cell membranes.
Notice that a lipid is not a long chain of many identical units in the way that starch is. The syllabus wording is 'fats and oils from fatty acids and glycerol', so use those exact terms in your answer rather than inventing a different description.
The structure of DNA (Extended)
DNA is the molecule that carries genetic information from one generation to the next. For the Extended paper you must be able to describe its structure, and the syllabus lists exactly which features to include.
A DNA molecule is made of two strands that are coiled around each other to form a double helix. Each strand contains chemicals called bases. Bonds between pairs of bases hold the two strands together, rather like the rungs of a twisted ladder.
The bases always pair up in the same way: A pairs with T, and C pairs with G. This complementary base pairing is what allows a DNA molecule to be copied accurately before a cell divides. You do not need to give the full names of the bases.
Testing foods for biological molecules
You must be able to describe the use of each food test. That means the reagent, the conditions, and the positive result. Learn them as a set, because a common question gives you several unknown samples and asks you to identify what each one contains.
Iodine solution tests for starch. Add a few drops to the sample at room temperature; a positive result is a colour change from yellow-brown to blue-black. Benedict's solution tests for reducing sugars such as glucose. Add it to the sample and heat the mixture, for example in a water bath; a positive result is a colour change from blue to a brick-red precipitate. The colour you see depends on how much reducing sugar is present - green, then yellow, then orange, then brick-red as the concentration rises.
Biuret solution tests for protein. Add it at room temperature and a positive result is a colour change from blue to purple (violet). Ethanol tests for fats and oils: add ethanol to the sample, shake, then pour the mixture into water; a positive result is a white emulsion, which looks cloudy. DCPIP tests for vitamin C: the solution is blue when added and turns colourless when vitamin C is present.
| Test for | Reagent | Conditions | Positive result |
|---|---|---|---|
| Starch | Iodine solution | Room temperature | Yellow-brown to blue-black |
| Reducing sugar | Benedict's solution | Heat in a water bath | Blue to brick-red precipitate |
| Protein | Biuret solution | Room temperature | Blue to purple (violet) |
| Fat or oil | Ethanol, then water | Room temperature | White emulsion (cloudy) |
| Vitamin C | DCPIP | Room temperature | Blue to colourless |
Water as a solvent
Water is not a polymer like starch or protein, but it is essential to life and it must be supplied in the diet. Its most important property for biology is that it is an excellent solvent.
Because water is a good solvent, substances dissolve in it and can then be transported. In blood plasma, glucose, amino acids, mineral ions and urea are all dissolved in water, which is what allows them to be carried around the body. In plants, mineral ions dissolve in the water moving up the xylem. Digestion also depends on water, because the enzymes that hydrolyse food work in solution.
A balanced diet must supply carbohydrates and fats for energy, proteins for growth and repair, vitamins and minerals, water, and dietary fibre. Being able to test a food for the biological molecules it contains is how a dietician checks that a person's diet is supplying what the body needs.
Key terms you must be able to define
- Carbohydrate
- A biological molecule containing carbon, hydrogen and oxygen only, for example glucose, starch, glycogen and cellulose.
- Protein
- A polymer of amino acids containing carbon, hydrogen, oxygen and nitrogen, used for growth and repair and to make enzymes, hormones and antibodies.
- Lipid (fat or oil)
- A biological molecule made from fatty acids and glycerol, containing carbon, hydrogen and oxygen only, used for long-term energy storage and insulation.
- Glucose
- The small sugar molecule from which the carbohydrates starch, glycogen and cellulose are all built.
- Amino acid
- The small molecule containing nitrogen from which proteins are built; about twenty different amino acids occur in living things.
- Condensation reaction
- A reaction in which two smaller molecules join together to form a larger molecule and a molecule of water is released.
- Hydrolysis
- A reaction in which a molecule of water is used to break a bond, splitting a large molecule back into its smaller units, as happens in digestion.
- Reducing sugar
- A sugar such as glucose that gives a positive result with Benedict's solution, turning it from blue to a brick-red precipitate when heated.
Applied to the real world
Cambridge sets Co-ordinated Sciences in unfamiliar contexts - a hospital, a power station, a farm, a laboratory. Use these as models for how to apply the science, then write your own.
A technician is given an unlabelled sample of a food and must work out what it contains. A few drops of iodine solution turning blue-black shows starch; biuret solution turning purple shows protein; shaking the sample with ethanol and then adding water to form a white emulsion shows a fat or oil; and Benedict's solution turning brick-red on heating shows a reducing sugar.
A dietician needs to know which nutrients a patient's diet supplies. A patient recovering from surgery needs plenty of protein, because amino acids are used for growth and repair of tissues, while a patient who needs more energy needs carbohydrates and fats. Testing foods, or reading their composition, lets the dietician build a diet that supplies the right molecules in the right amounts.
Cellulose is a polymer of glucose just like starch, but human digestive enzymes cannot hydrolyse the bonds between its glucose units, so it cannot be digested. It is still essential as dietary fibre: it adds bulk to the contents of the gut and helps keep them moving, which prevents constipation.
Starch is used as a thickener in sauces and gravies. When starch is heated in water the long polymer chains swell and trap water, thickening the liquid. The same glucose polymer that stores energy in a potato is therefore also a useful ingredient in cooking.
Flashcards
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Exam-style questions
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Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- Learn the exact wording of every positive result: blue-black for starch, brick-red precipitate for reducing sugar, purple (violet) for protein, white emulsion for fat, and colourless for vitamin C. Vague colours such as 'dark' or 'orangey-red' do not score.
- Only the Benedict's test needs heat, and you must say so. 'Add Benedict's solution and heat in a water bath' earns both marks; 'add Benedict's solution' alone earns one.
- In the ethanol emulsion test, state the order clearly: add ethanol to the sample and shake, then add the mixture to water. If you leave out the water there is no white emulsion and no mark.
- For element questions, remember that fats contain carbon, hydrogen and oxygen only. Only proteins contain nitrogen - that single difference answers most of these questions.
- When a question asks about condensation or hydrolysis, always say what happens to water: condensation releases a water molecule, hydrolysis uses one.
- For the DNA question, keep to the four syllabus features: two strands, coiled into a double helix, bases held together by bonds between the strands, and the pairing A with T and C with G.
Why students lose marks here
Where this fits in IGCSE Co-ordinated Sciences
This page is one topic of Cambridge IGCSE Co-ordinated Sciences 0654 / 0973. IGCSE Co-ordinated Sciences is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Co-ordinated Sciences syllabus page.