Biological Molecules: Carbohydrates, Proteins, Lipids, DNA and Food Tests

1. A student tests a food sample.
   The sample gives:

• Benedict’s test after heating: remains blue

• iodine test: blue-black

• Biuret test: purple

• ethanol emulsion test: cloudy white

Which molecules are present?

A starch, protein and lipid only
B reducing sugar, starch and protein only
C starch and reducing sugar only
D protein and lipid only

2. Which row correctly matches the molecule with its smaller building units?

A starch → fatty acids and glycerol
B protein → amino acids
C lipid → glucose molecules
D DNA → amino acids

3. A food sample is boiled with dilute hydrochloric acid, neutralised, then heated with Benedict’s solution. A brick-red precipitate forms.
   Before boiling with acid, the sample gave a negative Benedict’s test.
   Which substance was most likely present in the original sample?

A reducing sugar
B non-reducing sugar
C protein
D lipid

4. Which molecule contains carbon, hydrogen and oxygen only?

A protein
B DNA
C carbohydrate
D enzyme

5. Which statement about glycogen is correct?

A It is a protein used to build muscles.
B It is a storage carbohydrate in animals.
C It is a structural carbohydrate in plant cell walls.
D It is a lipid used for insulation.

6. Which result would show a positive test for protein?

A iodine turns blue-black
B Benedict’s solution forms brick-red precipitate
C Biuret reagent turns purple
D ethanol produces a cloudy white emulsion

7. A student crushes a peanut with ethanol and then pours the liquid into water. The mixture turns cloudy white.
   What does this show?

A lipid is present
B protein is present
C reducing sugar is present
D starch is present

8. Which molecule is correctly matched with its function?

A cellulose: energy storage in animals
B starch: structural support in plant cell walls
C lipid: insulation and energy storage
D protein: genetic material

9. A sample gives a positive iodine test but a negative Benedict’s test.
   Which conclusion is most valid?

A starch is present but reducing sugar is absent
B glucose is present but starch is absent
C protein is present but lipid is absent
D reducing sugar is present but starch is absent

10. Which food molecule is made from glucose molecules joined together?

A starch
B protein
C lipid
D DNA

11. Which statement about enzymes is correct in relation to biological molecules?

A all enzymes are carbohydrates
B enzymes are proteins
C enzymes are lipids
D enzymes are DNA molecules

12. Which row correctly shows the test and positive result?

A starch: Benedict’s solution turns brick-red
B lipid: iodine turns blue-black
C protein: Biuret reagent turns purple
D reducing sugar: ethanol forms cloudy white emulsion

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Which molecule contains nitrogen?

A glucose
B starch
C protein
D lipid

14. A student tests four food samples using iodine solution.

Sample 1: brown
Sample 2: blue-black
Sample 3: orange
Sample 4: purple

Which sample contains starch?

A sample 1
B sample 2
C sample 3
D sample 4

15. Which substance is most suitable for long-term energy storage in animals?

A DNA
B lipid
C water
D enzyme

16. Which statement about cellulose is correct?

A It is made from amino acids.
B It is found in animal cell membranes.
C It strengthens plant cell walls.
D It is tested using Biuret reagent.

17. A solution is heated with Benedict’s reagent and changes from blue to green.
    Which conclusion is correct?

A no reducing sugar is present
B a low concentration of reducing sugar is present
C protein is present
D starch is present

18. Which row correctly compares starch and glucose?

A starch is soluble; glucose is insoluble
B starch is made of amino acids; glucose is made of fatty acids
C starch is a large molecule; glucose is a small molecule
D starch can pass through cell membranes more easily than glucose

19. Which molecule is made from nucleotides?

A starch
B DNA
C protein
D lipid

20. Which statement about DNA is correct?

A It is made from glucose molecules.
B It contains genetic information.
C It is the main molecule used for insulation.
D It is broken down by Benedict’s solution.

21. A food sample gives:

• iodine: brown

• Benedict’s after heating: orange

• Biuret: blue

• ethanol emulsion: colourless clear mixture

Which molecule is definitely present?

A starch
B reducing sugar
C protein
D lipid

22. Which combination of elements is found in proteins but not in carbohydrates?

A carbon
B hydrogen
C oxygen
D nitrogen

23. Which test requires heating?

A iodine test for starch
B Benedict’s test for reducing sugar
C Biuret test for protein
D ethanol emulsion test for lipid

24. A student tests a solution with Benedict’s reagent but forgets to heat it. The solution remains blue.
    Which conclusion is safest?

A reducing sugar is definitely absent
B reducing sugar may be present, but the test was not completed correctly
C starch is definitely present
D protein is definitely absent

25. Which molecule is insoluble in water and forms a cloudy white emulsion with ethanol and water?

A glucose
B amino acid
C lipid
D starch

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Which row correctly matches carbohydrate type and example?

A monosaccharide: starch
B disaccharide: glycogen
C polysaccharide: cellulose
D amino acid: glucose

27. A student tests a food sample. Iodine turns blue-black and Benedict’s turns brick-red after heating.
    Which conclusion is correct?

A starch only is present
B reducing sugar only is present
C both starch and reducing sugar are present
D protein and lipid are present

28. Which statement about lipids is correct?

A They are made from amino acids.
B They are made from fatty acids and glycerol.
C They are made from glucose molecules only.
D They contain genetic instructions.

29. Which substance is a storage carbohydrate in plants?

A cellulose
B starch
C glycogen
D lipid

30. Which molecule is most directly involved in carrying genetic instructions from parents to offspring?

A DNA
B starch
C triglyceride
D cellulose

31. Which result would be expected if a food sample contains protein but no starch, reducing sugar or lipid?

A iodine blue-black, Benedict’s blue, Biuret blue, ethanol cloudy
B iodine brown, Benedict’s blue, Biuret purple, ethanol clear
C iodine brown, Benedict’s brick-red, Biuret blue, ethanol clear
D iodine blue-black, Benedict’s brick-red, Biuret purple, ethanol cloudy

32. Which food test result is incorrectly matched?

A starch present: iodine turns blue-black
B protein present: Biuret turns purple
C lipid present: ethanol emulsion becomes cloudy white
D reducing sugar present: Benedict’s remains blue after heating

33. Which molecule is used to make cell membranes?

A phospholipid
B starch
C cellulose only
D glycogen

34. A sample contains sucrose but no glucose.
    What is expected in Benedict’s test before hydrolysis?

A brick-red precipitate
B orange precipitate
C green precipitate
D remains blue

35. Which statement about amino acids is correct?

A They join to form proteins.
B They join to form starch.
C They are made from glycerol and fatty acids.
D They are the monomers of DNA.

36. A food sample is mixed with iodine solution and remains brown. It is then tested with Biuret reagent and turns purple.
    Which conclusion is correct?

A starch and protein are present
B starch is absent and protein is present
C reducing sugar and lipid are present
D protein is absent and starch is present

37. Which biological molecule has the highest energy value per gram?

A carbohydrate
B protein
C lipid
D water

38. A molecule is described as large, insoluble, made from glucose units, and used for storage in plants.
    Which molecule is it?

A glucose
B starch
C cellulose
D glycogen

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. Which molecule is a carbohydrate but is not used mainly for energy storage?

A starch
B glycogen
C cellulose
D glucose

40. Which statement about food tests is correct?

A all positive food tests require heating
B colour change intensity can sometimes suggest concentration
C ethanol emulsion test detects protein
D iodine test detects reducing sugar

41. A student obtains a purple colour in the Biuret test.
    Which bonds or molecules are most likely present?

A peptide bonds in protein
B glycosidic bonds in starch only
C ester bonds in lipid
D hydrogen bonds in water only

42. Which molecule is formed from three fatty acids joined to glycerol?

A triglyceride
B cellulose
C glycogen
D DNA

43. Which statement correctly compares starch and cellulose?

A both are proteins made from amino acids
B both are carbohydrates made from glucose units
C starch is found in animal cell walls, cellulose in blood plasma
D cellulose is a reducing sugar, starch is a lipid

44. A student adds Benedict’s reagent to a solution and heats it. The final colour is yellow.
    What is the best interpretation?

A no reducing sugar is present
B reducing sugar is present at a moderate concentration
C protein is present
D lipid is present

45. Which row correctly identifies the main molecule in each structure?

A plant cell wall: cellulose; chromosomes: DNA
B plant cell wall: glycogen; chromosomes: lipid
C plant cell wall: protein; chromosomes: starch
D plant cell wall: DNA; chromosomes: cellulose

46. Which substance would not give a positive Benedict’s test unless first broken down?

A glucose
B maltose
C sucrose
D fructose

47. Which statement explains why starch is useful for storage in plant cells?

A it is soluble, so it diffuses quickly out of cells
B it is insoluble, so it does not affect water potential greatly
C it is a protein, so it can catalyse reactions
D it is made of fatty acids, so it stores genetic code

48. Which food sample contains all four major tested food groups: starch, reducing sugar, protein and lipid?

A iodine brown; Benedict’s blue; Biuret purple; ethanol cloudy
B iodine blue-black; Benedict’s brick-red; Biuret purple; ethanol cloudy
C iodine blue-black; Benedict’s blue; Biuret blue; ethanol clear
D iodine brown; Benedict’s brick-red; Biuret blue; ethanol clear

49. Which molecule is correctly matched with its chemical test?

A DNA: iodine solution
B lipid: ethanol emulsion test
C protein: Benedict’s solution
D starch: Biuret reagent

50. A student claims, “All carbohydrates taste sweet and all sweet foods are reducing sugars.”
    Which statement best evaluates the claim?

A correct, because all carbohydrates are small soluble sugars
B incorrect, because some carbohydrates such as starch and cellulose are not sweet reducing sugars
C correct, because starch gives a positive Benedict’s test
D incorrect, because reducing sugars are proteins

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. Answer: A
   A is correct because iodine blue-black shows starch, Biuret purple shows protein, and cloudy white emulsion shows lipid. Benedict’s remaining blue shows no reducing sugar.
   B is wrong because Benedict’s did not show reducing sugar.
   C is wrong because protein and lipid are also present.
   D is wrong because starch is also present.

2. Answer: B
   A is wrong because starch is made from glucose, not fatty acids and glycerol.
   B is correct because proteins are made from amino acids.
   C is wrong because lipids are made from fatty acids and glycerol.
   D is wrong because DNA is made from nucleotides.

3. Answer: B
   A is wrong because a reducing sugar would have given a positive Benedict’s test before hydrolysis.
   B is correct because non-reducing sugar can be broken down by acid into reducing sugars, giving a positive Benedict’s test after neutralisation and heating.
   C is wrong because protein gives a positive Biuret test, not this sequence.
   D is wrong because lipid gives an ethanol emulsion test.

4. Answer: C
   A is wrong because proteins contain nitrogen.
   B is wrong because DNA contains nitrogen and phosphorus.
   C is correct because carbohydrates contain carbon, hydrogen and oxygen.
   D is wrong because enzymes are proteins, so they contain nitrogen.

5. Answer: B
   A is wrong because glycogen is not a protein.
   B is correct because glycogen is a storage carbohydrate in animals.
   C is wrong because cellulose is the structural carbohydrate in plant cell walls.
   D is wrong because glycogen is not a lipid.

6. Answer: C
   A is wrong because iodine blue-black shows starch.
   B is wrong because Benedict’s brick-red shows reducing sugar.
   C is correct because Biuret reagent turns purple when protein is present.
   D is wrong because ethanol cloudy white emulsion shows lipid.

7. Answer: A
   A is correct because a cloudy white emulsion after ethanol and water shows lipid.
   B is wrong because protein is tested with Biuret reagent.
   C is wrong because reducing sugar is tested with Benedict’s solution and heat.
   D is wrong because starch is tested with iodine.

8. Answer: C
   A is wrong because cellulose gives structural support in plant cell walls, not animal energy storage.
   B is wrong because starch is storage in plants, not structural support.
   C is correct because lipids are used for insulation and energy storage.
   D is wrong because DNA, not protein, is genetic material.

9. Answer: A
   A is correct because iodine positive shows starch, while Benedict’s negative shows no reducing sugar.
   B is wrong because glucose would give a positive Benedict’s test.
   C is wrong because these tests do not show protein or lipid.
   D is wrong because reducing sugar is absent.

10. Answer: A
    A is correct because starch is made from many glucose molecules joined together.
    B is wrong because proteins are made from amino acids.
    C is wrong because lipids are made from fatty acids and glycerol.
    D is wrong because DNA is made from nucleotides.

11. Answer: B
    A is wrong because enzymes are not carbohydrates.
    B is correct because enzymes are proteins.
    C is wrong because enzymes are not lipids.
    D is wrong because enzymes are not DNA molecules.

12. Answer: C
    A is wrong because Benedict’s tests for reducing sugar, not starch.
    B is wrong because iodine tests for starch, not lipid.
    C is correct because Biuret reagent turns purple with protein.
    D is wrong because ethanol emulsion tests for lipid, not reducing sugar.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Answer: C
    A is wrong because glucose contains carbon, hydrogen and oxygen only.
    B is wrong because starch contains carbon, hydrogen and oxygen only.
    C is correct because proteins contain nitrogen.
    D is wrong because lipids usually contain carbon, hydrogen and oxygen.

14. Answer: B
    A is wrong because brown iodine means starch is absent.
    B is correct because blue-black iodine shows starch is present.
    C is wrong because orange is not the iodine result for starch.
    D is wrong because purple is associated with the Biuret test for protein.

15. Answer: B
    A is wrong because DNA stores genetic information, not long-term energy.
    B is correct because lipids are suitable for long-term energy storage.
    C is wrong because water does not store energy.
    D is wrong because enzymes catalyse reactions; they are not energy stores.

16. Answer: C
    A is wrong because cellulose is made from glucose units, not amino acids.
    B is wrong because cellulose is found in plant cell walls.
    C is correct because cellulose strengthens plant cell walls.
    D is wrong because Biuret tests for protein.

17. Answer: B
    A is wrong because green is a positive Benedict’s result, not negative.
    B is correct because green shows a low concentration of reducing sugar.
    C is wrong because protein is tested using Biuret reagent.
    D is wrong because starch is tested using iodine.

18. Answer: C
    A is wrong because glucose is soluble and starch is relatively insoluble.
    B is wrong because starch and glucose are carbohydrates, not amino acids or fatty acids.
    C is correct because starch is a large molecule and glucose is a small molecule.
    D is wrong because glucose passes through membranes more easily than starch.

19. Answer: B
    A is wrong because starch is made from glucose units.
    B is correct because DNA is made from nucleotides.
    C is wrong because proteins are made from amino acids.
    D is wrong because lipids are made from fatty acids and glycerol.

20. Answer: B
    A is wrong because DNA is not made from glucose.
    B is correct because DNA contains genetic information.
    C is wrong because lipids are mainly used for insulation.
    D is wrong because Benedict’s solution tests reducing sugars, not DNA.

21. Answer: B
    A is wrong because iodine stayed brown, so starch is absent.
    B is correct because Benedict’s orange after heating shows reducing sugar is present.
    C is wrong because Biuret stayed blue, so protein is absent.
    D is wrong because ethanol stayed clear, so lipid is absent.

22. Answer: D
    A is wrong because carbohydrates and proteins both contain carbon.
    B is wrong because both contain hydrogen.
    C is wrong because both contain oxygen.
    D is correct because proteins contain nitrogen, while carbohydrates do not.

23. Answer: B
    A is wrong because the iodine test does not require heating.
    B is correct because Benedict’s test requires heating.
    C is wrong because Biuret test does not require heating.
    D is wrong because ethanol emulsion test does not require heating.

24. Answer: B
    A is wrong because the test was incomplete, so reducing sugar cannot be ruled out.
    B is correct because Benedict’s test must be heated for a valid result.
    C is wrong because Benedict’s does not test for starch.
    D is wrong because Benedict’s does not test for protein.

25. Answer: C
    A is wrong because glucose does not form a cloudy white emulsion.
    B is wrong because amino acids do not form a cloudy white emulsion.
    C is correct because lipids are insoluble in water and form a cloudy white emulsion with ethanol and water.
    D is wrong because starch gives blue-black with iodine.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Answer: C
    A is wrong because starch is a polysaccharide, not a monosaccharide.
    B is wrong because glycogen is a polysaccharide, not a disaccharide.
    C is correct because cellulose is a polysaccharide.
    D is wrong because glucose is not an amino acid.

27. Answer: C
    A is wrong because Benedict’s positive also shows reducing sugar.
    B is wrong because iodine positive also shows starch.
    C is correct because iodine blue-black shows starch and Benedict’s brick-red shows reducing sugar.
    D is wrong because these tests do not show protein or lipid.

28. Answer: B
    A is wrong because amino acids make proteins.
    B is correct because lipids are made from fatty acids and glycerol.
    C is wrong because glucose molecules make carbohydrates such as starch and glycogen.
    D is wrong because DNA contains genetic instructions.

29. Answer: B
    A is wrong because cellulose is structural, not storage.
    B is correct because starch is the storage carbohydrate in plants.
    C is wrong because glycogen is the storage carbohydrate in animals.
    D is wrong because lipid is not a carbohydrate.

30. Answer: A
    A is correct because DNA carries genetic instructions from parents to offspring.
    B is wrong because starch stores energy in plants.
    C is wrong because triglycerides are lipids.
    D is wrong because cellulose supports plant cell walls.

31. Answer: B
    A is wrong because iodine blue-black and ethanol cloudy would show starch and lipid.
    B is correct because iodine brown means no starch, Benedict’s blue means no reducing sugar, Biuret purple means protein, and ethanol clear means no lipid.
    C is wrong because Benedict’s brick-red shows reducing sugar.
    D is wrong because all tests are positive.

32. Answer: D
    A is correct matching, so it is not the answer.
    B is correct matching, so it is not the answer.
    C is correct matching, so it is not the answer.
    D is incorrectly matched because reducing sugar makes Benedict’s turn green/yellow/orange/brick-red after heating, not remain blue.

33. Answer: A
    A is correct because phospholipids are major components of cell membranes.
    B is wrong because starch is a plant storage carbohydrate.
    C is wrong because cellulose is found in plant cell walls.
    D is wrong because glycogen is storage carbohydrate in animals.

34. Answer: D
    A is wrong because sucrose is non-reducing and would not give brick-red before hydrolysis.
    B is wrong because orange is also a positive reducing sugar result.
    C is wrong because green is also a positive reducing sugar result.
    D is correct because sucrose gives a negative Benedict’s test before hydrolysis, so it remains blue.

35. Answer: A
    A is correct because amino acids join to form proteins.
    B is wrong because glucose units join to form starch.
    C is wrong because fatty acids and glycerol form lipids.
    D is wrong because nucleotides are the monomers of DNA.

36. Answer: B
    A is wrong because iodine remained brown, so starch is absent.
    B is correct because iodine brown shows no starch, while Biuret purple shows protein.
    C is wrong because no Benedict’s or ethanol result is given.
    D is wrong because protein is present and starch is absent.

37. Answer: C
    A is wrong because carbohydrates have less energy per gram than lipids.
    B is wrong because protein is not the highest energy value per gram.
    C is correct because lipids have the highest energy value per gram.
    D is wrong because water has no energy value.

38. Answer: B
    A is wrong because glucose is small and soluble.
    B is correct because starch is large, insoluble, made from glucose units and stored in plants.
    C is wrong because cellulose is structural, not storage.
    D is wrong because glycogen is storage in animals.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. Answer: C
    A is wrong because starch is mainly used for energy storage.
    B is wrong because glycogen is mainly used for energy storage in animals.
    C is correct because cellulose is a carbohydrate used mainly for structural support, not energy storage.
    D is wrong because glucose is used in respiration for energy release.

40. Answer: B
    A is wrong because iodine, Biuret and ethanol tests do not all require heating.
    B is correct because stronger colour changes can sometimes suggest higher concentration, especially in Benedict’s test.
    C is wrong because ethanol emulsion test detects lipid.
    D is wrong because iodine detects starch.

41. Answer: A
    A is correct because Biuret detects peptide bonds in proteins.
    B is wrong because starch does not give a purple Biuret result.
    C is wrong because ester bonds in lipids are not detected by Biuret.
    D is wrong because water does not give a positive Biuret test.

42. Answer: A
    A is correct because a triglyceride is formed from three fatty acids joined to glycerol.
    B is wrong because cellulose is made from glucose units.
    C is wrong because glycogen is made from glucose units.
    D is wrong because DNA is made from nucleotides.

43. Answer: B
    A is wrong because starch and cellulose are not proteins.
    B is correct because both starch and cellulose are carbohydrates made from glucose units.
    C is wrong because starch is not in animal cell walls, and cellulose is not in blood plasma.
    D is wrong because cellulose is not a reducing sugar and starch is not a lipid.

44. Answer: B
    A is wrong because yellow is a positive Benedict’s result.
    B is correct because yellow indicates reducing sugar at a moderate concentration.
    C is wrong because Benedict’s does not test for protein.
    D is wrong because Benedict’s does not test for lipid.

45. Answer: A
    A is correct because plant cell walls mainly contain cellulose, and chromosomes contain DNA.
    B is wrong because plant cell walls are not glycogen and chromosomes are not lipid.
    C is wrong because plant cell walls are not mainly protein and chromosomes are not starch.
    D is wrong because plant cell walls are not DNA and chromosomes are not cellulose.

46. Answer: C
    A is wrong because glucose is a reducing sugar.
    B is wrong because maltose is a reducing sugar.
    C is correct because sucrose is non-reducing and must first be broken down before giving a positive Benedict’s test.
    D is wrong because fructose is a reducing sugar.

47. Answer: B
    A is wrong because starch is insoluble, so it does not diffuse quickly out of cells.
    B is correct because starch is insoluble and does not greatly affect water potential, making it useful for storage.
    C is wrong because starch is not a protein or catalyst.
    D is wrong because starch is not made of fatty acids and does not store genetic code.

48. Answer: B
    A is wrong because iodine brown and Benedict’s blue show no starch or reducing sugar.
    B is correct because all four tests are positive: iodine blue-black, Benedict’s brick-red, Biuret purple, ethanol cloudy.
    C is wrong because Benedict’s, Biuret and ethanol are negative.
    D is wrong because starch, protein and lipid are absent.

49. Answer: B
    A is wrong because iodine tests for starch, not DNA.
    B is correct because lipid is tested using the ethanol emulsion test.
    C is wrong because Benedict’s tests for reducing sugar, not protein.
    D is wrong because Biuret tests for protein, not starch.

50. Answer: B
    A is wrong because not all carbohydrates are small soluble sweet sugars.
    B is correct because starch and cellulose are carbohydrates but are not sweet reducing sugars.
    C is wrong because starch does not give a positive Benedict’s test unless broken down into reducing sugars.
    D is wrong because reducing sugars are carbohydrates, not proteins.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.