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2.3Structures and Functions in Living Organisms

Biological Molecules — Edexcel Linear - OL Revision Notes & Topical Questions

10 min read 17 parts Lesson 18 of 25 1 quiz

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Lesson summary

Structure of Biological Molecules and Food Tests — EDEXCEL LINEAR - OL notes: Describe the elements and building blocks of carbohydrates, proteins and lipids and carry out food tests for starch, reducing sugar, protein and lipid.Most of the molecules in living organisms are carbo…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Worked example 5
  • Benedict’s solution

Part 1 of 5Carbohydrates, proteins and lipids

Most of the molecules in living organisms are carbohydrates, proteins and lipids (fats and oils). All contain carbon, so they are described as organic molecules.

MoleculeElementsSmaller unitsExamples
CarbohydrateC, H, OSimple sugars such as glucoseStarch, glycogen, cellulose
ProteinC, H, O, N (and sometimes S)Amino acidsEnzymes, haemoglobin, keratin
LipidC, H, OGlycerol and fatty acidsTriglycerides (fats and oils)

Starch, glycogen and cellulose are all made from many glucose molecules joined together.

1
Plants store glucose as starch, animals as glycogen (found, for example, in muscle cells); take care not to confuse the two.
2
You do not need the terms monosaccharide or polysaccharide.
3
A triglyceride is formed from one glycerol and three fatty acids.
4
These facts link to digestion, as all large molecules must be broken into their smaller units by enzymes so they can be absorbed.

Part 2 of 5Practical: food tests

First prepare solid foodcrush it with a pestle and mortar, add distilled water, mix and filter the mixture to give a liquid sample.

TestReagent and methodPositive result
StarchAdd a few drops of iodine solution to the sampleOrange-brown to blue-black
Reducing sugars (e.g. glucose)Add Benedict’s solution and heat in a hot water bath (about 80 °C) for a few minutesLight blue to green, yellow, orange or brick red (red/orange/yellow for a positive result)
ProteinAdd Biuret reagent (sodium hydroxide followed by dilute copper sulfate)Blue to purple / lilac
LipidsMix with ethanol, then pour into water (emulsion test)A cloudy white emulsion forms

A negative test gives no colour change: iodine stays orange-brown, Benedict’s stays blue, Biuret stays blue.

Part 3 of 5Safety and answering test questions

Iodine solution is an irritant to the eyes.

1
Sodium hydroxide in Biuret is corrosive; if it gets onto the skin wash it off immediately.
2
Ethanol is highly flammable and must be kept away from the Bunsen burner, which should be turned off when not in use.
3
Wear eye protection.
4
When asked to describe a test, always name the reagent and the colour change; for glucose, say you would use Benedict’s solution, heat it,
  • and expect a colour change to red, orange or yellow for a positive result.

Use these tests to identify the nutrients in a food: a food that turns blue-black with iodine and purple with Biuret contains starch and protein. Note that Benedict’s detects only reducing sugars such as glucose; starch is not a reducing sugar and gives a negative result.

Part 4 of 5Why the food tests work and how to record results

Each reagent reacts with a particular chemical group.

Why the food tests work and how to record results
Iodine forms a complex with starch molecules, Benedict’s solution is reduced by reducing sugars to an orange-red precipitate, Biuret detects the peptide bonds in proteins,
  • and the ethanol test works because lipids dissolve in ethanol but come out of solution as tiny droplets when water is added.
Record results in a table with the food, the test and the colour observed, then give your conclusion.
Use the same volume of sample and reagent each time and compare against a control (distilled water), which should give no change.

Foods can contain more than one nutrient: milk contains protein, lipid and a reducing sugar (lactose), while bread gives a positive starch test.

Part 5 of 5Worked examples

  1. State the elements in a protein molecule. [2]2 marks

  2. Describe how to test a food for glucose. [3]3 marks

  3. What does a purple colour with Biuret reagent show? [1]1 mark

  4. Name the smaller units that make a lipid. [2]2 marks

  5. State the difference between how plants and animals store glucose. [2]2 marks

Key terms

TermMeaning
CarbohydrateA molecule made of C, H and O, such as starch.
ProteinA molecule made of amino acids.
LipidA fat or oil made of glycerol and fatty acids.
Benedict’s solutionReagent to test for reducing sugars on heating.
Biuret reagentReagent that turns purple with protein.
GlycogenStorage carbohydrate in animals.

Avoid these mistakes

1
Saying iodine turns blue-black for sugar — it is for starch.
2
Not heating Benedict’s solution.
3
Mixing up starch (plants) and glycogen (animals).
4
Giving only the reagent without the colour change.

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