Lipids are tested for using the Emulsion test
Proteins are tested for using the Biuret test
Starch is tested for using Iodine solution – turns blue black colour
Glucose is tested for using Benedict’s test – turns red/orange colour
Test for reducing sugars
- The test for reducing sugars is known as the Benedict’s test.
- The main reagent is Benedict’s solution which contains copper(II) sulfate.
- Reducing sugars can reduce copper(II) ions in Benedict’s solution to copper(I) in the form of copper(I) oxide, a brick-red precipitate.
- Reducing sugars are glucose, fructose, galactose, maltose and lactose. Sucrose is not a reducing sugar.
- Procedure: Add 2 cm3 of Benedict’s solution to 2 cm3 of sample solution and mix the contents thoroughly. Heat the test tube in a boiling water bath for 5 minutes. If the sample is an insoluble solid, crush it or cut it into small pieces before adding 2 cm3 of water and 2 cm3 of Benedict’s solution.
- The colour of the solution changes from green to orange to brick-red with increasing amounts of reducing sugars present.
Test for starch
- The test for starch is called the iodine test. Iodine is added to the sample and the colour change (if any) is observed.
- Procedure: Add a few drops of iodine solution to the sample. If the sample contains starch, it will turn blue-black in colour.
Test for fats
- The test for fats is known as the ethanol emulsion test.
- Ethanol is added to the sample to allow the fats present in it to dissolve. Water is then added to the ethanolic mixture. Since fats do not dissolve in water, they precipitate out of the solution to give a cloudy white emulsion.
- Procedure: Add 2 cm3 of ethanol to the sample in a test tube and shake the contents thoroughly. Add 2 cm3 of water and mix the contents. If fats are present, a white emulsion will be observed.
Test for proteins
- The test for protein is known as the biuret test.
- The main reagents are sodium hydroxide and copper(II) sulfate.
- Procedure: Add 1 cm3 of sodium hydroxide solution to 1 cm3 of sample solution in a test tube and mix thoroughly. Add a few drops of 1% copper(II) sulfate solution dropwise into the mixture, shaking after each drop. Allow the mixture to stand for 5 minutes.
- If proteins are present, a violet colouration will be observed.
- Test for starch
- Shake a little starch powder in a test-tube with some warm water to make a suspension.
- Add 3 or 4 drops of iodine solution. A dark blue colour should be produced.
Note: it is also possible to use iodine solution to test for starch in leaves but a different procedure is used
- Test for reducing sugar
- Heat a little glucose solution with an equal volume of Benedict’s solution in a test-tube. The heating is done by placing the test-tube in a beaker of boiling water see Figure 4.1, or warming it gently over a blue Bunsen flame. However, if this second technique is used, the test-tube should be moved constantly in and out of the Bunsen flame to prevent the liquid boiling and shooting out of the tube. The solution will change from clear blue to cloudy green, then yellow and finally to a red precipitate (deposit) of copper(I) oxide.
- Test for protein (Biuret test)
- To a 1% solution of albumen (the protein of egg-white) add 5 cm3 dilute sodium hydroxide (CARE: this solution is caustic), followed by 5 cm3 1% copper sulfate solution. A purple colour indicates protein. If the copper sulfate is run into the food solution without mixing, a violet halo appears where the two liquids come into contact.
- Test for fat
- Shake two drops of cooking oil with about 5 cm3 ethanol in a dry test-tube until the fat dissolves.
- Pour this solution into a test-tube containing a few cm3 water. A milky white emulsion will form. This shows that the solution contained some fat or oil.
- Test for vitamin C
- Draw up 2 cm3 fresh lemon juice into a plastic syringe.
- Add this juice drop by drop to 2 cm3 of a 0.1% solution of DCPIP (a blue dye) in a test-tube. The DCPIP will become colourless quite suddenly as the juice is added. The amount of juice added form the syringe should be noted down.
- Repeat the experiment but with orange juice in the syringe. If it takes more orange juice than lemon juice to decolourise the DCPIP, the orange juice must contain less vitamin C.
Application of the food tests
The tests can be used on samples of food such as milk, potato, raisins, onion, beans, egg-yolk or peanuts to find out what food materials are present. The solid samples are crushed in a mortar and shaken with warm water to extract the soluble products. Separate samples of the watery mixture of crushed food are tested for starch, glucose or protein as described above. To test for fats, the food must be crushed in ethanol, not water, and then filtered. The clear filtrate is poured into water to see if it goes cloudy, indicating the presence of fats.
To describe simple tests for the molecules of living organisms
Scientists often need to know whether or not a particular type of molecule is present in a solution. For example, a doctor might try to detect glucose in a urine sample (glucose in the urine suggests that patient has diabetes), or an environmental scientist might test for starch in the outflow from a food factory. There are a number of simple chemical tests that can be carried out on biological solutions.
A special test for lipids
An important feature of fats and oils is that they they are insoluble in water. This means that you cannot make an aqueous solution of a fat or oil on which to carry out a biochemical test. However, the fact that lipids are insoluble forms the basis of a physical test. This is known as the emulsion test:
- 2 cm3 of ethanol are added to the unknown solution, and the mixture is gently shaken.
- The mixture is poured into a test tube containing an equal volume of distilled water.
- If a lipid is present, a milky-white emulsion is formed.
To test for protein, a few drops of Biuret reagent are added to 2 cm3 of the unknown solution, and the mixture is gently shaken. A mauve/purple colour is a positive result (protein is present)
To test for starch, a few drops of iodine solution are added to 2 cm3 of the unkown solution, and the mixture is gently shaken. A deep blue – black colour is a positive result (starch is present)
To test for glucose (a reducing sugar), 2 cm3 of Benedict’s reagent are added to 2 cm3 of the unknown solution, and the mixture is heated in a boiling water bath for 2-3 minutes. An orange/brick-red colour is a positive result (glucose is present).
A control is needed to make sure that results are valid
- To show that the test is working properly, a solution that is known to contain the substance is tested (for example, the Biuret reagent is used with a solution known to contain protein). This should give a positive result.
- To show that the test solutions are not contaminated, each test should be carried out on a sample of water. This should give a negative result.
When making comparisons between different solutions – for example, to compare the glucose content of different urine samples – it is important to carry out all tests under the same conditions. For example, a series of Benedict’s tests should be performed:
- on equal volumes of unknown solutions
- using equal volumes of Benedict’s solution
- with all mixtures heated to the same temperature
- for the same length of time
Structure of DNA
A DNA molecule is made up of long chains of nucleotides, formed into two strands. A nucleotide is a 5-carbon sugar molecule joined to a phosphate group (-PO3) and an organic base Figure 4.2. In DNA the sugar is deoxyribose and the organic base is either adenine (A), thymine (T), cytosine (C) or guanine (G).
Note: for exam purposes, it is only necessary to be able to state the letters, not the names of these bases.
The nucleotides are joined by their phosphate groups to form a long chain, often thousands of nucleotides long. The phosphate and sugar molecules are the same all the way down the chain but the bases may be any one of the four listed above Figure 4.3.
The DNA in a chromosome consists of two strands (chains of nucleotides) held together by chemical bonds between the bases. The size of the molecules ensures that A (adenine) always pairs with T (thymine) and C (cytosine) pairs with G (guanine). The double strand is twisted to form a helix (like a twisted rope ladder with the base pairs representing the rungs).
DNA (deoxyribonucleic acid) molecules make up the genetic material of living organisms. DNA is an extremely long molecule but, like proteins and carbohydrates, it is built up of many subunits. The subunits of DNA are called nucleotides.
Each nucleotide consists of three parts – a sugar (deoxyribose), a phosphate group and a nitrogenous base. DNA contains four different bases: adenine, guanine, cytosine and thymine. These are usually known by their letters: A, G, C and T Figure 4.4.
To form a DNA molecule, nucleotides are linked together. The phosphate group of one nucleotide links to the deoxyribose of the next molecule to form a chain of nucleotides, as shown in Figure 4.5. The sugar and phosphate groups are identical all the way along the chain and form the backbone of the DNA molecule. The sequence of bases in the chain will vary and it is this sequence that forms the genetic code determining the characteristics of an organism.
Two strands of nucleotides are linked by hydrogen bonds that form between the bases and this double strand makes up the double helix of a complete DNA molecule Figure 4.5. Adenine always pairs with thymine and is bonded with two hydrogen bonds, while cytosine is paired with guanine by three hydrogen bonds. The arrangement is known as complementary base pairing. Notice that two DNA chains run in opposite directions and are said to be antiparallel.
You can imagine the molecule rather like a rope ladder with the sugar-phosphate backbone being the sides of the ladder and the rungs being formed by the hydrogen-bonded base pairs. To form the characteristic double helix of a DNA molecule, the ladder must be twisted to resemble a spiral staircase.