Question 1

Four solutions were tested to see whether they contained protein, starch or glucose.The colours of the solutions after the tests are recorded in the table.Which solution contained protein and glucose but not starch?

Accepted Answer

The biuret test is a chemical test that can be used to check for the presence of peptide bonds in a given analyte. A positive test is indicated by a purple color due to the copper ion complex with the amide group of the protein. Hence solution C and D have proteins.Now in the next step iodine test is conducted, Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. As shown in the table, solution  B and D turned blue-black, therefore we can conclude that solution A and C does not have starch. Benedict's test is a chemical test that can be used to check for the presence of reducing sugars (such as glucose) in a given analyte. Therefore, when reducing sugars are present in the analyte, the cupric ions (Cu2+) in Benedict's reagent are reduced to cuprous ions (Cu+). These cuprous ions form copper(I) oxide with the reaction mixture and precipitate out as a brick-red/orange coloured compound.Hence, it is quite evident that amongst the four solutions, solution C has proteins as well as glucose but it lacks starch.

Question 2

Small molecules are used as the basic units in the synthesis of large food molecules.Which statement is correct?

Accepted Answer

- Amino acids are the basic units of proteins, not carbohydrates.- Fatty acids are not the basic units of glycogen; glucose molecules are the basic units of glycogen.- Glycerol is a component of lipids (fats and oils), and it can combine with fatty acids to form triglycerides, which are the main constituents of oils.- Simple sugars (monosaccharides) are the basic units of carbohydrates, not proteins.So, the correct answer is that glycerol is a basic unit of oils.

Question 3

Which row shows the chemical elements contained in fats?

Accepted Answer

Fat is a class of macro nutrients containing fatty acids and glycerol. Chemical elements that are present in a fat molecule are carbon, oxygen and hydrogen. Nitrogen is an important constituent of amino acids or proteins but is absent in fats.

Question 4

Small molecules are used as the basic units in the synthesis of large food molecules. Which statement is correct?

Accepted Answer

Large food molecules (carbohydrates, proteins and fats) are complex structures or polymers made up of number of monomers. 
Carbohydrates are polysaccharides made up of monosaccharaides or sugars. For instance, Glycogen is a polysaccharide with glucose as its monomer. 
Proteins are polypeptide chains made up of amino acids.
Fats are made up of fatty acids and glycerol. For instance, Oils are triglycerides containing fatty acids and glycerol.

Question 5

Which reagent is used when testing a food for vitamin C?

Accepted Answer

DCPIP (dichlorophenolindophenol) solution can be used to test for the presence of vitamin C in food. It changes from blue to red with acids but loses its colour in the presence of certain chemicals, one of which is ascorbic acid (vitamin C). 
Benedict's solution is used to test the presence of reducing sugars.
Ethanol is used to dissolve lipids or fats present in any sample.
Iodine solution is used to detect the presence of starch.

Question 6

Small molecules are used as the basic units in the synthesis of large food molecules. Which statement is correct?

Accepted Answer

Large food molecules (carbohydrates, proteins and fats) are complex structures or polymers made up of number of monomers. 
Carbohydrates are polysaccharides made up of monosaccharaides or sugars. For instance, Glycogen is a polysaccharide with glucose as its monomer. 
Proteins are polypeptide chains made up of amino acids.
Fats are made up of fatty acids and glycerol. For instance, Oils are triglycerides containing fatty acids and glycerol.

Question 7

A student investigates the breakdown of fats in milk by lipase. Four test-tubes labelled A to D are set up. The table shows the contents of each test-tube. In which test-tube will the contents become acidic most quickly?

Accepted Answer

Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases. Lipase break down fats into monoglyceride and diglycerides (fatty acid and glycerol). Test tube A has milk, bile and lipase. So, the fat in the milk will be broken by lipase and bile, imparting acidity to the solution.Test tube B does not contain lipase so there will be no breakdown of fats.Test tube C does not have bile as well as lipase, again there will be no breakdown of fats.Test tube D contains lipase so it will break down fats into smaller subunits, imparting acidity. However due to absence of bile further breakdown of fat subunits is not possible. Therefore, this solution will be less acidic than the solution in test tube A.Hence, in test-tube A the contents will become acidic most quickly.

Question 8

Which row shows the chemical elements contained in fats?

Accepted Answer

. Fats  are organic compounds, like carbohydrates, are composed of the elements carbon (C), hydrogen (H), and oxygen (O), arranged to form molecules.. Sugar is a compound which contains carbon, hydrogen, nitrogen and oxygen.. Ammonia is the chemical compound which contains nitrogen and hydrogen.

Question 9

Enzymes are proteins. State the chemical elements that enzymes are made from. .............................................................................................................................................

Accepted Answer

All known enzymes are proteins. They are high molecular weight compounds made up principally of chains of amino acids linked together by peptide bonds. Amino acids are small organic molecules that consist of an alpha (central) carbon atom linked to an amino group, a carboxyl group, a hydrogen atom, and a variable component called a side chain.  Therefore the chemical elements that enzymes are made from are- Carbon (C), hydrogen (H), oxygen (O) and nitrogen (N).

Question 10

During germination the starch within seeds is broken down to form reducing sugars.Describe how you could test a sample of germinating seeds to find out if reducing sugars are present....................................................................................................................................................

Accepted Answer

In order to test the presence of reducing sugars in a sample of germinating seeds, benedict's reagent can be used. 
If to a mixture(paste) of germinating seeds benedict's reagent is added and then the mixture is heated, reddish orange colour will appear if there will be the presence of reducing sugars.

Question 11

Figure below shows a section through an unfertilised chicken’s egg.The egg is made up of the outer shell, inner yellow yolk and albumen (egg white).The albumen and yolk are composed of different substances, including fats.Describe how ethanol can be used to test a sample of food for the presence of fat. Include the result for a positive test....................................................................................................................................................................................................................................................................................................... You should use the safety equipment provided while you are carrying out the practical work.

Accepted Answer

To test for fats one can carry out the ethanol emulsion test. This involves adding ethanol to a food sample in a test tube then shaking the mixture. Deionized water is then added. If no emulsion forms and the solution remains colourless then no fats are present. If emulsion is observed or a cloudy suspension is seen, presence of fats is confirmed.

Question 12

You are provided with a sample of albumen suspension in a beaker, labelled A.You are going to test the albumen suspension for the presence of protein. - Use a syringe to put 2 cm3  of albumen suspension A  into a test-tube.- 2 cm3  of biuret solution from the beaker labelled biuret.- Mix thoroughly by gently shaking the test-tube.Describe your observations and state your conclusion. observations .............................................................................................................................conclusion .................................................................................................................................

Accepted Answer

Observations: The sample albumen suspension in the test tube changes the colour from blue to lilac. 
Conclusion: The changing of colour confirms the presence of proteins in the sample albumen suspension.

Question 13

DCPIP can be used to test for vitamin C in food. Lemon juice contains vitamin C.When lemon juice is tested what are the results?

Accepted Answer

When lemon juice is tested using DCPIP, the results are: **starts blue, finishes colourless**.DCPIP (2,6-dichlorophenolindophenol) is a common reagent used to test for the presence of vitamin C (ascorbic acid) in food. As vitamin C is an antioxidant, it can reduce DCPIP from its blue oxidized state to a colorless reduced state. When vitamin C is present in the lemon juice, it will act as a reducing agent and cause the DCPIP solution to change color from blue to colorless as it undergoes reduction. This change in color indicates the presence of vitamin C in the lemon juice.

Question 14

Which identifies the chemical elements found in proteins?

Accepted Answer

. Proteins are one of the primary constituents of living matter. They consist of long chains of amino acids, which are bonded together by peptide linkages and are thus called polypeptides. There are about 20 amino acids, and the atoms most prevalent in these are carbon, hydrogen, oxygen, nitrogen, and sulfur. These four elements are the primary constituents of amino acids, which are the building blocks of proteins. . Carbohydrates are compounds made up of carbon, hydrogen and oxygen.

Question 15

A student carries out a test to see whether the solution in a test-tube contains protein. Which row shows the correct food test and a positive result?

Accepted Answer

. Benedict's test is a chemical test that can be used to check for the presence of reducing sugars in a given analyte.. Biuret test is the simplest and a rapid way to detect protein in a sample. Steps of Biuret method :. A liquid solution of a sample is treated with sodium or potassium hydroxide to make the solution alkaline. A few drops of copper (II) sulfate solution (which is blue) is added to the sample. Biuret ‘reagent’ contains an alkali and copper (II) sulfate. If a colour change is observed from blue to lilac/purple, then protein is present.. If no colour change is observed, no protein is present.Hence, the row D shows  the correct food test and a positive result.

Question 16

Amino acids are also transported through plants.State the name of the mineral ion that is used to make amino acids..............................................................................................................................................

Accepted Answer

Nitrate ions are required for producing amino acids which are then used to form proteins.

Question 17

Chloroplasts can contain starch.State the name of the substance that would be used to test for the presence of starch..............................................................................................................................................

Accepted Answer

Iodine would be used to test for the presence of starch in chloroplasts.

Question 18

You will investigate the effect of the enzyme amylase (A) on the breakdown of starch (S). The starch suspension S will be placed inside a bag made from dialysis tubing.Dialysis tubing is made from a type of membrane that is partially permeable. Only small molecules can pass through this membrane. Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results.You should use the gloves and eye protection provided while you are carrying out the practical work.You are supplied with two dialysis tubing bags, amylase solution A and starch suspension S.Step 1 Label two large test-tubes, E and W, and place them in a test-tube rack. Step 2 Use a syringe to put 20cm3  of distilled water into each of the labelled test-tubes. Step 3 Use a syringe to draw up 3cm3  of starch suspension S. Step 4 Use another syringe to draw up 3cm3  of amylase solution A. Step 5 One end of each of the pieces of dialysis tubing has been knotted to form a bag. Open the other end of one of the dialysis tubing bags. You may need to rub the tubing between your fingers to open it. Step 6 Put starch suspension S and amylase solution A from steps 3 and 4 into the open dialysis tubing bag. Step 7 Rinse the outside of the dialysis tubing bag by dipping it into the water for washing. Keep the open end of the bag above the water. Step 8 Carefully lower the dialysis tubing bag into the large test-tube labelled E. Fold the top of the bag over the outside of the test-tube. Use an elastic band to hold the dialysis tubing in place as shown in figure given below.Step 9 Repeat steps 3 to 7, using 3cm3  of distilled water instead of amylase solution A in step 4. Step 10 Carefully lower the dialysis tubing bag into the large test-tube labelled W. Fold the top of the dialysis tubing bag over the outside of the test-tube. Use an elastic band to hold the dialysis tubing in place as shown in above given figure. Step 11 Prepare a water-bath by putting approximately 100cm3  of hot water into the beaker labelled water-bath. Raise your hand when you are ready for a supply of hot water for your water-bath. Step 12 Put both large test-tubes into the water-bath and leave them for 10 minutes. Continue with the other questions while you are waiting. Step 13 After 10 minutes carefully remove the dialysis tubing bag from large test-tube E and pour the contents of the dialysis tubing bag into the beaker labelled E2. Step 14 Carefully remove the dialysis tubing bag from large test-tube W and pour the contents of the dialysis tubing bag into the beaker labelled W2. You are going to test the contents of the two large test-tubes (E and W) and the two beakers (E2 and W2) for starch. Step 15 Use a clean pipette to place a drop of the solution from large test-tube E onto a white tile and add one drop of iodine solution. Record the colour in the table you have prepared.Step 16 Repeat step 15 using the solution from large test-tube W. Step 17 Repeat step 15 using the solution from beaker E2. Step 18 Repeat step 15 using the solution from beaker W2. A student carried out the same investigation but also tested the solutions for reducing sugars. Describe how the student would find out if the solutions contained reducing sugars.

Accepted Answer

Benedict’s test is a chemical test that can be used to check for the presence of reducing sugars in a given analyte. The test is based on Benedict’s reagent (also known as Benedict’s solution), which is a complex mixture of sodium citrate, sodium carbonate, and the pentahydrate of copper(II) sulfate.When exposed to reducing sugars, the reactions undergone by Benedict’s reagent result in the formation of a brick-red precipitate, which indicates a positive Benedict’s test.

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