Format
Until 2008, NECTA biology practicals contained three questions. Question 1 was required, and was a food test. Students then chose to answer either question 2 or question 3.
One of these questions was usually classification. The format changed in 2008. Now, the practical contains two questions, and both are required.
Food test and classification remain the most common questions, but sometimes only one of these two topics is on a given exam.
The second question may cover one of a variety of topics, including respiration, transport, coordination, photosynthesis, and movement.
Each question is worth 25 marks.
Common Practicals
<> Food test: students must test a solution for starch, sugars, fats, and protein
<> Classification: students must name and classify specimens, then answer questions about their characteristics
<> Respiration: students use lime water to test air from the lungs for carbon dioxide
<> Transport: students investigate osmosis by placing leaf petioles or pieces of raw potato in solutions of different solute concentrations
<> Photosynthesis: students test a variegated leaf for starch to prove that chlorophyll is necessary for photosynthesis
<>Coordination: students look at themselves in the mirror and answer questions about the sense organs they see
Note: These are the most common practicals, but they are not necessarily the only practicals that can occur on the national exam. Biology practicals frequently change, and it is possible that a given exam will contain a new kind of question. Look through past NECTA practicals yourself to get an idea of the kind of questions that can occur.
Food Test Biology Practical Questions And Answers
Food Tests
In this practical, students test a solution of unknown food substances for starch, protein, reducing sugars, non- reducing sugars, and fats/oils. They record their procedure, observation, and conclusions, then answer questions about nutrition and the digestive system.
This section contains the following:
<> How to carry out food tests
<> How to write a report
<> Sample practical with solutions
How to Carry Out Food Tests
Starch
Add a few drops of iodine to the solution and shake well. A blue-black color forms if starch is present
Lipids
Add a few drops of iodine to the solution and shake well. A red ring will form at the top of the test tube is lipids are present.
You can also have your students do the grease spot test { rub a drop of solution onto a piece of paper, and let dry. A translucent spot forms if fat is present. This test is great for its simplicity, but is not used on national exams.
Protein (Biuret test)
Add a few drops of 1 M NaOH to the solution and shake well. Then add a few drops of 1% CuSO4 solution and shake. A violet color forms if protein is present. Sometimes the color takes a minute or two to appear.
Some textbooks may recommend using Millon's reagent to test for protein.
This reagent contains mercury, which is extremely poisonous and should never be handled by students.
The purple colour from a positive test is the result of a complex between four nitrogen atoms and the copper (II) ion. Specifically, these nitrogen atoms are all part of peptide bonds. These peptide bonds are adjacent on a protein, either two from one protein and two from another, or two from one part of a protein and two from another part of the same protein.
Reducing sugar
Place some food solution in a test tube, and add an equal volume of Benedict's solution. Heat to boiling, then let cool. A brick red or orange precipitate forms if a reducing sugar is present.
Benedict's solution contains aqueous copper (II) sulphate, sodium carbonate, and sodium citrate. The citrate ions in Benedict's solution complex the copper (II) ions to prevent the formation of insoluble copper (II) carbonate.
In the presence of a reducing sugar, however, the copper (II) ions are reduced to copper (I) ions which form a brick red precipitate of copper (I) oxide. The oxygen in the copper (I) oxide come from hydroxide; the purpose of the sodium carbonate is to provide this hydroxide by creating an alkaline environment.
Normally, sugar molecules form five or six member rings and have no reducing properties. In water, however, the rings of some sugar molecules can open to form a linear structure, often with an aldehyde group at one end. These aldehyde groups react with copper (II) to reduce it to copper
Sugars that do not have an aldehyde group in the linear structure or that are not able to open are not able to reduce copper (II) ions and are thus called non-reducing sugars. Students do not need to understand this chemistry for their exam, but they may ask about what is happening in the reaction.
Non-reducing sugar
Do the test for a reducing sugar using Benedict's solution. Notice that no reaction occurs. Add a few drops of citric acid solution to the solution, then heat to boiling. Let solution cool. Add a few drops of 1 M NaOH, and shake well.
Then, add some Benedict's solution (equal in volume to the liquid in the test tube). Boil the solution, and let it cool. A brick red or orange precipitate forms if a non-reducing sugar is present.
This experiment will also test positive for all reducing sugars. Therefore it is important to first perform the test for reducing sugars before considering this test. If the test for reducing sugars is positive, there is no reason to perform the test for non-reducing sugars - the conclusion will be invalid.
Non-reducing sugars are a misnomer, that is, their name is incorrect. This test does not test for any sugar that is not reducing. Rather, this is a test for any molecule made of multiple reducing sugars bound together, such as sucrose or starch.
When these polysaccharides are heated in the presence of acid, they hydrolyse and release monosaccharides. The presence of these monosaccharides is then identified with Benedict's solution.
The purpose of the sodium hydroxide is to neutralize the citric acid added for hydrolysis. If the citric acid is not hydrolysed, it will react with the sodium carbonate in Benedict's solution, possibly making the solution ineffective.
How to Write a Report
Food test data is reported in a table containing four columns: test for, procedure, observation, and inference. With the exception of the `test for' column, data should be reported in full sentences written in past tense.
The procedure should also be in passive voice. No, this is not the way professional scientists write. However, students here must use passive voice to get marks on the national exam.
Note that every column is worth marks on the exam. Even if students fail to do the food tests correctly, they can still get marks for writing what they are testing for and what the procedure should be.
See the sample practical below for an example of a report.
Sample Food Test Practicals
Practical 01
You have been provided with Solution K. Carry out food test experiments to identify the food substances present in the solution.
Record your experimental work as shown in the table below.
Suggest two natural food substances from which solution K might have been prepared.
What is the function of each of the food substances in solution K to human beings?
For each food substance identified, name the enzyme and end product of digestion taking place in the:
- Stomach
- Duodenum
What deficiency diseases are caused by a lack of the identified food substances?
(Assume Solution K contains protein and starch.)
The results were as follows
Solution K could have been prepared from egg and maize. (Note: Any non-processed food containing protein or starch is correct here.)
Starch provides energy to the body. Proteins are used in growth and tissue repair.
A deficiency of protein causes kwashiorkor. A deficiency of starch causes marasmus.
You are provided with food sample A. By using scientific procedures, taste what sample A contained.
TABLE OF RESULTS
FOOD TASTED | PROCEDURES | OBSERVATION | INFERENCE |
STARCH | A small portion of food solution A was kept in a test tube then a few drops of iodine solution were added and was shaken. |
| |
REDUCING SUGAR | A small portion of food solution A was kept in a test tube then an equal amount of Benedict’s solution was added and the mixture was heated to boil. |
|
|
PROTEIN | A small portion of food solution A was kept in a test tube then a few drops of Na OH and Cu SO 4 solution was added and shaken. |
|
|
NON- REDUCING SUGAR | A small portion of food solution A was kept in a test tube then a few drops of dill HCl was added heated and then cooled then a few drops of Na OH was added and equal amount of Benedicts solution was added and the mixture was heated to boil. |
| |
FATSTOLS | A small portion of food solution A was kept in a test tube then few drops of sudden III solution were added and were shaken strongly and leave it to settle. |
|
|
Solution
By using the table below
TABLE OF RESULTS
FOODTASTED | PROCEDURES | OBSERVATION | INFERENCE |
STARCH | A small portion of food solution A was kept in a test tube then a few drops of iodine solution were added and was shaken. | The colour of solution A changed from white to black | Starch is present |
REDUCING SUGAR | A small portion of food solution A was kept in a test tube then an equal amount of Benedict’s solution was added and the mixture was heated to boil. | Series of colour change from blue to green to yellow to Orange PPts | Food solution A contained reducing sugar |
PROTEIN | A small portion of food solution A was kept in a test tube then a few drops of Na OH and Cu SO4 solution was added and shaken. | Food sample A retained light blue colour of Cu S04 | Protein is absent |
NON- REDUCING SUGAR | A small portion of food solution A was kept in a test tube then a few drops of dill HCl was added heated and then cooled then a few drops of Na OH was added and equal amount of Benedicts solution was added and the mixture was heated to boil. | Food solution A retained blue colour of Benedict’s solution | Non-reducing sugar is absent |
FATS/OILS | A small portion of food solution A was kept in a test tube then few drops of sudden III solution were added and were shaken strongly and leave it to settle. | Food solution A retained red colour of Sudan III solution | Fats. Oils is absent |
Hence
The food solution A contained
- Reducing sugar
- Starch
From the samples
- Sample A…………………….
- Sample B…………………….
TABLE OF RESULTS
FOOD TEST | PROCEDURES | OBSERVATION | INFERENCE |
Starch | A small portion of food solution A and B were kept in two different test tubes then few drops of iodine solution were added to each test tube and were shaken |
|
|
Reducing sugar | A small portion of food solution A and B were kept in two different test tubes then on equal amount of Benedicts solution was added in each test tube and the mixture of each was heated to boil. |
|
|
Non-reducing sugar | A small portion of food solution A and B were kept in two different test tubes then a few drops of dill HCl were added; heated and then cooled; then a few drops of NaOH were added and equal amount of benedicts solution were addled and the mixture were heated to boil for all solution A and B. |
| |
Protein | A small portion of food solution A and B were kept in two different test tubes then a few drops of NaOH and CuS04 solution were added to both solution and shaken |
|
|
Lipids ( Fat and Oil) | A small protein of food solution A and B were kept in two different test tubes then a few drops of Sudan III solution were added and were shaken: strongly and leave to settle to both two test tubes |
|
Solution
From the samples
- Sample A – Tomato
- Sample B - Onion
TABLE OF RESULTS
FOOD TEST | PROCEDURES | OBSERVATION | INFERENCE |
Starch | A small portion of food solution A and B were kept in two different test tubes then few drops of iodine solution were added to each test tube and were shaken | Food sample A and B solutions rationed brownish yellow color of iodine | Starch was absent in both sample A and B |
Reducing sugar | A small portion of food solution A and B were kept in two different test tubes then on equal amount of Benedicts solution was added in each test tube and the mixture of each was heated to boil. | Series of colors change from blue to green to yellow to change PPts (Brick red PPts) to both food solution A and B | Reducing sugar is present to both food samples |
Non-reducing sugar | A small portion of food solution A and B were kept in two different test tubes then a few drops of dill HCl were added; heated and then cooled; then a few drops of NaOH were added and equal amount of benedicts solution were addled and the mixture were heated to boil for all solution A and B. | Both solution A and B rationed black color of Benedict its solution | Non-reducing sugar to both sample food A and B |
Protein | A small portion of food solution A and B were kept in two different test tubes then a few drops of NaOH and CuS04 solution were added to both solution and shaken | Food solution A and B rationed; light blue color of CuS04 | Protein is absent in all food sample A and B |
Lipids ( Fat and Oil) | A small protein of food solution A and B were kept in two different test tubes then a few drops of Sudan III solution were added and were shaken: strongly and leave to settle to both two test tubes | Food solution A and B rationed red color of Sudan III solution | Lipids is absent in all food sample A and B |
Practical 04
You are provided with solution S
- Carry out experiments to identify the food substances present in solution S
- Record your experimental work as shown in table 1 below Table 1
Test for | Procedure | Observations | Inference |
|
|
|
- Solution S contains…………………….
- Suggest one storage organ in a plant from which solution S might have been prepared.
- For each food substance identified in (a) (ii) above, name its end product(s) of digestion.
- Which of the identified food substance is mostly needed by small children?
Solution
i) Experimental work to identify food substance(s) present in solution S.
TEST FOR | PROCEDURE | OBSERVATION | INFERENCE |
Starch | To 2 cm3 of a sample solution S, few drops of iodine solution was added. | Yellow- brown color was observed i.e. iodine color was retained. | Starch was absent. |
Reducing sugar | To 2 cm3 of a sample solution S, equal volume of Benedict’s solution was added and boiled for few minutes. | The blue color of Benedict’s retained (observed). | Reducing sugar was not present |
Non reducing sugar | To 2 cm3 of a sample solution S, few drops of Hcl solution was added and heated followed by cooling. After cooling few drops of NaoH/NaHC03 were added, followed by Benedicts solution + boiling | The blue - color of Benedict’s solution was retained (observed.) | No reducing sugar was not present |
Protein | To 2 cm3 a sample solutions, few drops of sodium hydroxide solution was added followed by addition 1% copper II sulphate solution drop vise white shaking | Purple color was formed | Protein was present. |
Lipids (Fats and oil) | To 2 cm3 a sample solution s, few drops of Sudan III dye solution was added followed by addition of few mills of distilled water and then the mixture was shaken Vigorously and left to stand for about 5min. | Red stained food droplets were found at the upper layer of the test tube | Lipid (oil) was present |
(ii) Solution S contains protein and lipid (oil).
Storage organ in a plant from which solution S might have been prepared might be root tuber (bulb - like tuber (which is ground nut
Protein end product of digestion was amino acid(s)
Lipid (Fat and) oil end product of digestion was Fatty acids and glycerol.
Food substance in solution S identified which is mostly needed by small children is protein.
In a practical lesson, a candidates has been provided with a sweet potato and a ginger.
Explain how the candidates should prepare these organs for investigation of stored foods and suggest the experiment (s) he/she would carry out and his her observations
Record the information as shown in the table below
Test for | Producer | Observation | Inference |
|
|
|
(i) State the nature of the stored food substances identified in each storage organ.
(ii) Name the plant from which each storage organ develops.
Solution
a) Preparation for sweet
- Clean your sweet potatoes under normal tap running water
- Peel off your sweet potatoes using normal kitchen knife
- Cut it into small slices or pieces using kitchen knife
- Grind it either by using mortar and pestle or blender machine.
- Slurry or porridge of sweet potato is now read for laboratory food test - procedures.
(b) Preparation for a ginger
- Clean your ginger preferably under normal running tap water.
- Peel off your ginger using normal kitchen knife.
- Cut into small slices or pieces using kitchen knife
- Grind it by using either mortar and pestle or blender machine
- Slurry or porridge of ginger is now ready for laboratory food test procedures.
TEST FOR | PROCEDURE | OBSERVATION | INFERENCE |
Starch | To 2mls of a sample (potato slurry), a few drop of iodine solution was added. | Blue - black coloration was observed | Indicates that starch was present in the sample. |
Reducing sugar | To 2cm3 of a sample in a test tube, equal volume of Benedict’s solution was added and the mixture was shaken. The mixture was then boiled for about 2-5 min. | The series of color changes was then observed from blue to green, yellow, orange and finally - brick -red coloration | This shows that reducing sugar was present in the food sample (sweet potato) |
Non reducing sugar | To 2 cm3 of a food sample in a test tube, 1 cm3 of HCl was added to the sample and the mixture was then boiled for about one minute. After cooling the mixture, sodium hydrogen carbonate solution was then added to the mixture. Equal volume of Benedict’s solution was added into the mixture and boiled again. | The series of color changes was observed from blue to green, yellow, orange and finally brick – red coloration. | Non reducing sugar was present |
Protein | To 2 cm3 of potato sample in a test tube, a little solution of NaOH was added, followed by solution of Copper II Sulphate drop wise | Purple color was observed | Protein was present |
Lipids (fats & oil) | To 2 cm3 of sweet potato sample, a little amount of water was placed, followed by addition of few drops of Sudan III dye solution and then, the mixture was shaken. After five (5 min) the observation was done. | There was no any food droplets trapped by the Sudan III solution at the upper part (top) of the test tube. | Lipids was absent |
(i) The nature of the stored food substances identified in each storage organ.
In sweet potato: (as storage organ)
Starch found in sweet potato remain as it is i.e. is stored as starch.
Protein found in sweet potato are stored in form of amino acids
In ginger (as a storage organ
Starch found in sweet potato is stored as starch (it remains as it was).
Reducing sugar and non reducing sugar are both stored as starch since the storage of glucose is plants is starch.
Protein however small is it; it is stored in form of amino acids.
The plant part from which each storage organ develops
For sweet potato was root tuber
For ginger is a rhizome (modified plant stem.)
Practical 06
From two students of Salma Kikwete secondary school were asked to carry out an experiment by their biology teacher the experiment was conducted as follows
Three test tubes labeled A, B and C were set as shown in figure 4 below. Each of the three test tubes contained 1 ml saliva and 1 ml water. The three test tubes wee heated in water bath at different temperature for 30 minutes.
Another set of three test tubes also labeled A, B, and C each containing 1 mil starch solution was heated for the same duration in water bath as shown in figure 5 below.
The contents of the test tubes in the corresponding water bath of figure 4 and figure 5 was mixed and heated further for 30 minutes.
The contents of each test tube was then tested for starch using iodine solution Study the above procedures carefully and then answer the following questions
1. What was the aim of the experiment?
2. Why was it necessary to heat the tubes for 30 minutes before mixing their contents?
4. Account for the expected observations
Solution
The aim of the experiment was to find out the effect of temperature on salivary amylases or to study the effect of temperature on enzyme salivary amylase) activity
It was necessary to heat the tubes for 30 minutes before mixing their content in order to obtain optimal temperature for enzymes (salivary amylase) to work properly.
After adding iodine solution, the following color change were observed
Test tube labeled A: in this case, blue- black colouration was observed
Test tube labeled B: in this case, yellowish -brown colouration was observed Test tube labeled C: blue - black colouration was observed as well.
Accounting for the expected observations.
For the test tube labeled A blue- black colouration was observed to indicate that starch is still present because the temperature 5 is not suitable temperature for the enzyme to act on substrate (starch)
For the test tube labeled B, the colour of iodine (yellowish - brown) was retained because all the substrate (starch) has been reduced into simple sugar by the action of enzymes (salivary amylase) due to the availability of favourable or suitable temperature for such enzyme to act on the substrate
For test tube labeled C, blue - black colouration was observed to indicate; substrate (starch) was still present because the temperature 7 is not optimal temperature for the enzymes (salivary- amylase) to act on substrate (starch).