Biological molecules and food tests Cambridge IGCSE Biology revision
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In plain words
Your food is mostly three kinds of big molecule, each built from small units like beads on a string: carbohydrates from sugars, proteins from amino acids, fats from fatty acids and glycerol. Simple chemical tests show which ones a food contains.
Three things to know
- Carbohydrates and fats contain carbon, hydrogen and oxygen. Proteins contain those and nitrogen. Starch and glycogen are made of glucose; proteins of amino acids; fats and oils of fatty acids and glycerol.
- Food tests: iodine solution turns blue-black with starch. Benedict's solution, heated, turns from blue to brick red with reducing sugars such as glucose. Biuret solution turns from blue to purple with protein.
- The ethanol emulsion test gives a cloudy white emulsion with fats. DNA is two strands twisted into a double helix, held together by paired bases: A with T, C with G.
Tips and tricks
- Always give the colour change from and to: "from blue to purple", not just "purple".
- Benedict's is the only one of these tests that needs heating.
It lands in your notebook with its questions as flashcards.
Biological molecules and food tests: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which element is found in proteins but not in carbohydrates?
Carbohydrates and fats contain only carbon, hydrogen and oxygen.
What colour does iodine solution turn when starch is present?
Without starch it stays yellow-brown.
Which test needs heating?
Benedict's solution only changes colour with reducing sugar when it is heated.
What are proteins made of?
Long chains of amino acids fold up to make a protein.
In DNA, which base pairs with A?
A pairs with T, and C pairs with G.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Biological molecules and food tests
Cambridge IGCSE Biology 0610 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe the test for starch and the result if starch is present.[2]
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Add iodine solution. It changes from brown (yellow-brown) to blue-black.
Describe how to test a food for reducing sugars.[3]
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Add Benedict's solution and heat in a water bath. If reducing sugar is present the colour changes from blue to green, yellow, orange or brick red.
Name the smaller molecules that make up (a) proteins and (b) fats.[2]
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(a) Amino acids. (b) Fatty acids and glycerol.
Answers: Biological molecules and food tests
- 1. Add iodine solution. It changes from brown (yellow-brown) to blue-black.
- 2. Add Benedict's solution and heat in a water bath. If reducing sugar is present the colour changes from blue to green, yellow, orange or brick red.
- 3. (a) Amino acids. (b) Fatty acids and glycerol.