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Cambridge IGCSE0610

Biological molecules

Biology 0610 Chapter Notes

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Biological molecules
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In this chapter

  1. 1Elements in carbohydrates, fats and proteins
  2. 2Large molecules from small molecules
  3. 3Iodine and Benedict’s tests
  4. 4Biuret and ethanol emulsion tests
  5. 5DCPIP test for vitamin C
  6. 6Structure of DNA

1. Elements in carbohydrates, fats and proteins

Carbohydrates and fats are made from the elements carbon, hydrogen and oxygen.

Proteins contain those three elements and also nitrogen.

Examiner insight

“Carbon, hydrogen and oxygen were well known to more able candidates, although carbon dioxide was stated fairly frequently instead of carbon.” “Some candidates had difficulty naming the elements present in carbohydrates and often answered by naming different examples of carbohydrates.”

Worked example 13 marks

All biological molecules contain elements.

Complete Table 1.1 by placing ticks in the boxes to show the elements that each biological molecule contains.

biological moleculecarbonhydrogennitrogenoxygen
carbohydrate
fat
protein
  1. 1

    carbohydrate: carbon, hydrogen and oxygen [1]

  2. 2

    fat: carbon, hydrogen and oxygen [1]

  3. 3

    protein: carbon, hydrogen, nitrogen and oxygen [1]

2. Large molecules from small molecules

Large molecules are made from smaller molecules joined together.

Starch, glycogen and cellulose are made from glucose. Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol.

Examiner insight

“Most candidates correctly linked the large molecule with the small molecule it is made from, but a few incorrectly thought that fat is made from glycogen.”

Worked example 11 mark

Which smaller molecules is cellulose made from?

A amino acids · B fatty acids · C glucose · D glycerol

  1. 1

    C: glucose. [1]

3. Iodine and Benedict’s tests

Test for starch: add iodine solution to the sample. A positive result is blue-black. A negative result stays yellow-brown.

Test for reducing sugars: add Benedict’s solution, which is blue, and heat the mixture. A positive result is a brick-red precipitate. Orange, yellow or green precipitates are also credited. A negative result stays blue.

Examiner insight

“A common error was to state the colour as blue. This was considered ambiguous as a negative result for other food tests, such as Benedict’s reagent or biuret solution, would be blue.” “Almost all candidates were familiar with the Benedict’s test, but a number forgot the heating stage and others referred to a water-bath without clarifying that heat was used.”

Worked example 11 mark

Chilli pepper fruits contain reducing sugar and non-reducing sugar. A student tests a sample of chilli pepper fruits with Benedict’s solution to show that they contain reducing sugar.

State the result of a positive test.

  1. 1

    (brick) red / orange / yellow / green precipitate [1]

4. Biuret and ethanol emulsion tests

Test for protein: add biuret solution. The solution turns from blue to purple.

Test for fats and oils: mix the sample with ethanol, then add the mixture to water. A positive result is an emulsion.

Examiner insight

“The most common error was to give Benedict’s as the name of the test.” “Many candidates understood that the food sample had to be first mixed with ethanol, but very few went on to describe how it was then added to water to form an emulsion.”

Worked example 11 mark

A test was carried out on a solution containing protein.

Which row shows the test solution used and the result of the test?

test solutionresult
ABenedict’sbrick-red
BBenedict’spurple
Cbiuretbrick-red
Dbiuretpurple
  1. 1

    D: biuret solution, purple. [1]

5. DCPIP test for vitamin C

Test for vitamin C: add the sample to DCPIP solution, which is blue.

If vitamin C is present, the DCPIP turns from blue to colourless.

Examiner insight

“The test reagent for vitamin C is DCPIP. If vitamin C is present, DCPIP turns from blue to colourless.” “Some were unable to identify the positive result for the DCPIP test and some incorrectly thought that Benedict’s solution is used to test for protein.”

Worked example 11 mark

Potatoes are good sources of starch and vitamin C.

State the name of the solution you would use to test a food sample for vitamin C.

  1. 1

    DCPIP [1]

6. Structure of DNA

A DNA molecule has 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. Full names are not required.

Examiner insight

“It was common for bases to be incorrectly labelled as genes or chromosomes.” “Some candidates incorrectly selected option B, each strand contains a sequence of amino acids.”

Worked example 14 marks

Describe the structure of a DNA molecule.

  1. 1

    two strands [1]

  2. 2

    coiled together to form a double helix [1]

  3. 3

    each strand contains bases, and bonds between pairs of bases hold the strands together [1]

  4. 4

    the bases always pair up in the same way: A with T, and C with G [1]

Recap

  • Carbohydrates and fats contain carbon, hydrogen and oxygen. Proteins also contain nitrogen.
  • Starch, glycogen and cellulose are made from glucose. Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol.
  • Iodine solution: starch gives blue-black.
  • Benedict’s solution, heated: reducing sugars give a brick-red precipitate.
  • Biuret solution: protein gives purple.
  • Ethanol emulsion test: fats and oils give an emulsion. DCPIP: vitamin C turns it from blue to colourless.
  • Extended: DNA is two strands coiled into a double helix. Bonds between pairs of bases hold the strands together: A with T, and C with G.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. (a) List the chemical elements that make up a fat. (b) Proteins contain the same elements as fats and one more. State this other element.4 marks
  2. Name the smaller molecules that make up (a) a protein, (b) glycogen, (c) a fat.3 marks
  3. Describe how to test a food sample for starch.2 marks
  4. Describe how to test a food sample for reducing sugars.3 marks
  5. A student tests four foods, P, Q, R and S, and records the colour of each test mixture. (a) Name the foods that contain protein. (b) State which food contains starch but no reducing sugar. (c) State which food contains a reducing sugar but no starch. (d) State which food contains starch, reducing sugar and protein. | food | iodine solution | Benedict’s solution | biuret solution | | --- | --- | --- | --- | | P | blue-black | blue | blue | | Q | yellow-brown | brick-red | blue | | R | yellow-brown | blue | purple | | S | blue-black | brick-red | purple |4 marks
  6. A student wants to find out whether orange juice contains vitamin C and whether a sausage contains fat. Describe how to carry out each test and state its positive result.4 marks
  7. A milk drink may contain protein and reducing sugar. Describe how to show that the drink contains both. Include the positive results.5 marks
  8. One strand of a DNA molecule has the base sequence A C C G T A. (a) Write the base sequence of the strand paired with it. (b) State what holds the two strands together.3 marks
  9. A short piece of DNA has two strands, each with 20 bases. Strand 1 contains 8 A, 4 T, 5 C and 3 G. (a) State the number of each base in strand 2. (b) Calculate the percentage of all the bases in this piece of DNA that are G.5 marks
  10. Scientists measure the percentage of each base in DNA from two sources. In DNA from source X, 31% of the bases are A. In DNA from source Y, 24% are A, 31% are T, 20% are C and 25% are G. (a) The DNA from X is a double helix. Calculate the percentage of its bases that are G. (b) One DNA molecule from X contains 6000 bases. Calculate the number of pairs of bases in it in which A is paired with T. (c) The scientists conclude that the DNA from Y is not made of two strands paired together. Use the data to explain why.7 marks

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