Biological Molecules

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Lesson notes

Chemical Elements in Living Organisms

Most biological molecules are organic, meaning they contain carbon.
The three main groups are carbohydrates, proteins, and lipids.
These molecules are built from smaller subunits joined together.

Carbohydrates

Carbohydrates are long chains of simple sugars (monosaccharides).
Glucose is a monosaccharide; two glucose molecules join to form maltose (a disaccharide).
Many glucose molecules join to form starch, glycogen, or cellulose (polysaccharides).
Starch, glycogen, and cellulose are all made from glucose.

Glycogen, cellulose and starch are all made from glucose molecules

Lipids (Fats and Oils)

Most lipids are triglycerides, made of 1 glycerol molecule bonded to 3 fatty acid chains.
Fatty acids vary in size and structure.
Fats are solid at room temperature; oils are liquid at room temperature.

Structure of a triglyceride: one glycerol molecule bonded to three fatty acid chains.

Proteins

Proteins are long chains of amino acids (about 20 different types).
All amino acids have the same basic structure but differ in their R group.
The sequence of amino acids determines the protein; even a small change produces a different protein.

General structure of an amino acid: central carbon bonded to amino group, carboxyl group, hydrogen, and R group.

Food Tests

Benedict's test for reducing sugars: add Benedict's solution, heat at 60–70°C for 5 min; positive: blue → orange/brick red.
Iodine test for starch: add iodine solution; positive: orange-brown → blue-black.
Biuret test for protein: add Biuret solution; positive: blue → violet/purple.
Emulsion test for lipids: mix sample with ethanol, then add cold water; positive: cloudy emulsion forms.
DCPIP test for vitamin C: add sample to DCPIP solution; positive: blue colour disappears.

The Benedict's test for glucose

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Practice questions

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1.Which elements are present in all carbohydrates, proteins, and lipids?
Easy
- ACarbon, hydrogen, oxygen
- BCarbon, hydrogen, oxygen, nitrogen
- CCarbon, hydrogen, oxygen, nitrogen, sulfur
- DCarbon, hydrogen, oxygen, phosphorus
2.What is the basic structural unit of a carbohydrate?
Easy
- AAmino acid
- BSimple sugar (monosaccharide)
- CGlycerol
- DFatty acid
3.Which test is used to detect the presence of starch?
Easy
- ABenedict's test
- BIodine test
- CBiuret test
- DEmulsion test
4.What colour change indicates a positive result for the Benedict's test for reducing sugars?
Easy
- ABlue to violet
- BOrange-brown to blue-black
- CBlue to orange/brick red
- DColourless to cloudy white
5.A student tests a food sample with Biuret solution and observes a colour change from blue to violet. Which nutrient is present?
Easy
- AStarch
- BGlucose
- CProtein
- DLipid
6.Which of the following is a disaccharide formed when two glucose molecules join together?
Easy
- AStarch
- BGlycogen
- CMaltose
- DCellulose
7.What are the building blocks of proteins?
Easy
- AMonosaccharides
- BGlycerol and fatty acids
- CNucleotides
- DAmino acids
8.A student adds DCPIP solution to a fruit juice sample. The blue colour disappears. What does this indicate?
Medium
- AThe juice contains reducing sugar
- BThe juice contains starch
- CThe juice contains vitamin C
- DThe juice contains protein

More questions

9.Which of the following is NOT a polysaccharide?
Easy
- AStarch
- BGlycogen
- CMaltose
- DCellulose
10.A food sample is tested with Benedict's solution and heated. The solution remains blue. Which conclusion is correct?
Medium
- AThe sample contains starch
- BThe sample contains no reducing sugar
- CThe sample contains protein
- DThe sample contains lipid
11.Which statement about lipids is correct?
Easy
- ALipids are made of amino acids
- BLipids are solid at room temperature
- CLipids are made of glycerol and fatty acids
- DLipids are polysaccharides
12.A student performs the emulsion test for lipids. What is the correct procedure and positive result?
Medium
- AAdd iodine solution; blue-black colour
- BAdd ethanol, then water; cloudy white emulsion
- CAdd Benedict's solution and heat; orange colour
- DAdd Biuret solution; violet colour
13.Which of the following correctly describes the structure of a triglyceride?
Medium
- AOne glycerol molecule bonded to three fatty acid chains
- BThree glycerol molecules bonded to one fatty acid chain
- COne glucose molecule bonded to three amino acids
- DThree glucose molecules bonded to one fatty acid
14.Which part of an amino acid varies between different amino acids?
Easy
- AThe amino group
- BThe carboxyl group
- CThe R group
- DThe central carbon atom
15.Which food test would you use to distinguish between a solution of glucose and a solution of starch?
Medium
- ABiuret test
- BBenedict's test
- CEmulsion test
- DDCPIP test
16.A student carries out the iodine test on a piece of bread and observes a blue-black colour. What does this indicate?
Easy
- AThe bread contains protein
- BThe bread contains starch
- CThe bread contains reducing sugar
- DThe bread contains lipid
17.A student tested a food sample with iodine solution. The colour changed from orange-brown to blue-black. Which nutrient is present?
Easy
- AStarch
- BGlucose
- CProtein
- DLipid
18.A student tested a food sample with the emulsion test and observed a cloudy white layer. Which nutrient is present?
Easy
- AProtein
- BLipid
- CStarch
- DReducing sugar
19.A student carries out the DCPIP test on three fruit juices: A, B, and C. The student adds 1 cm³ of DCPIP to each test tube, then adds 1 cm³ of fruit juice. The time taken for the blue colour to disappear is recorded. Fruit juice A: 30 seconds, B: 60 seconds, C: 120 seconds. Which fruit juice contains the highest concentration of vitamin C?
Medium
- AA
- BB
- CC
- DIt cannot be determined from the data
20.In a food test experiment, a student is asked to test a sample for the presence of reducing sugars. The student heats the sample with Benedict's solution for 5 minutes but observes no colour change from blue. Which of the following could explain this result?
Medium
- AThe sample contains a non-reducing sugar such as sucrose
- BThe sample contains starch
- CThe sample was heated too long
- DThe sample was not heated enough
21.Which food test involves mixing the sample with ethanol, then adding cold water, and observing a cloudy white emulsion?
Easy
- ATest for lipids
- BTest for starch
- CTest for protein
- DTest for reducing sugars
22.A student tests a food sample with Biuret solution. The colour remains blue. What does this indicate?
Easy
- AProtein is absent
- BProtein is present
- CStarch is absent
- DLipids are present
23.Which of the following is a correct description of a triglyceride molecule?
Easy
- AOne glycerol molecule bonded to three fatty acid chains
- BThree glycerol molecules bonded to one fatty acid chain
- COne glycerol molecule bonded to two fatty acid chains
- DThree glucose molecules bonded together
24.A student is testing a colourless solution for the presence of reducing sugars. After adding Benedict's solution and heating, the solution turns orange. Which of the following statements is correct?
Medium
- AThe solution contains a reducing sugar
- BThe solution contains starch
- CThe solution contains protein
- DThe solution contains lipid
25.In an experiment, a student uses the DCPIP test to compare the vitamin C content of two fruit juices. 1 cm³ of DCPIP is added to each test tube. Juice X is added dropwise until the blue colour disappears. Juice Y requires 15 drops, while Juice X requires 10 drops. Which juice has the higher concentration of vitamin C?
Medium
- AJuice X
- BJuice Y
- CBoth have the same concentration
- DIt cannot be determined
26.Which of the following contains only carbon, hydrogen and oxygen?
Medium
- ACarbohydrates and lipids
- BProteins and carbohydrates
- CProteins and lipids
- DCarbohydrates, proteins and lipids
27.What is the basic unit (monomer) of proteins?
Easy
- AAmino acids
- BGlucose
- CFatty acids
- DGlycerol
28.In a food test, a student is asked to distinguish between two solutions: one contains glucose and the other contains sucrose. Which test would allow the student to distinguish between them?
Hard
- ABenedict's test after heating with dilute hydrochloric acid and neutralising
- BIodine test
- CBiuret test
- DEmulsion test
29.Which of the following is a polysaccharide?
Easy
- AStarch
- BMaltose
- CGlucose
- DSucrose
30.A student performs the emulsion test for lipids. The correct procedure is:
Medium
- AMix the sample with ethanol, shake, then add to water and shake again
- BMix the sample with water, shake, then add ethanol and shake
- CAdd ethanol to the sample, then heat gently
- DAdd water to the sample, then add iodine solution
31.What colour change indicates a positive result for the Biuret test?
Easy
- ABlue to violet/purple
- BBlue to orange/brick red
- COrange-brown to blue-black
- DBlue to colourless
32.A student is given three food samples: X, Y, and Z. The student tests each sample with iodine solution. X turns blue-black, Y remains orange-brown, Z turns blue-black. Which of the following is a correct conclusion?
Medium
- AX and Z contain starch, Y does not
- BY contains starch, X and Z do not
- CAll three contain starch
- DNone contain starch
33.Which elements are always present in carbohydrates?
Easy
- Acarbon, hydrogen, oxygen
- Bcarbon, hydrogen, oxygen, nitrogen
- Ccarbon, hydrogen, oxygen, nitrogen, sulfur
- Dcarbon, hydrogen
34.What is the monomer of proteins?
Easy
- Aamino acids
- Bnucleotides
- Cmonosaccharides
- Dglycerol and fatty acids
35.In a triglyceride molecule, one glycerol molecule is bonded to three fatty acid chains by ester bonds. Which part forms the central backbone of the molecule?
Easy
- AThe glycerol molecule
- BThe fatty acid chains
- CThe ester bonds
- DThe hydrogen atoms
36.All organic molecules in living organisms contain carbon.
Easy
True or false?
37.Which of the following is a monosaccharide?
Easy
- AGlucose
- BMaltose
- CStarch
- DGlycogen
38.Match each food test to the nutrient it detects.
Medium
- Benedict's test
- Biuret test
- Emulsion test
- Lipid
- Reducing sugar
- Protein
39.A triglyceride molecule is composed of:
Easy
- AOne glycerol and three fatty acids
- BThree glycerols and one fatty acid
- COne glycerol and one fatty acid
- DThree glycerols and three fatty acids
40.Proteins are made up of long chains of amino acids.
Easy
True or false?
41.Which of the following elements are found in all carbohydrates, proteins, and lipids? (Select all that apply)
Medium
- ACarbon
- BHydrogen
- COxygen
- DNitrogen
- EPhosphorus
42.Arrange the following in order of increasing complexity: amino acid, protein, dipeptide, polypeptide.
Medium
- amino acid
- dipeptide
- polypeptide
- protein
43.Match each biomolecule to its correct description.
Medium
- Carbohydrate
- Lipid
- Protein
- Long chain of amino acids
- Long chain of simple sugars
- Made of glycerol and fatty acids
44.A student tests an unknown food sample with iodine solution and observes a blue-black colour. Which of the following is most likely present in the sample?
Easy
- AGlucose
- BStarch
- CProtein
- DLipid
45.Complete the sentence.
Easy
The test for starch uses ____ solution, which changes from orange-brown to ____ in the presence of starch.
46.State one safety precaution that should be taken when carrying out the Benedict's test and explain why.
Medium
47.Complete the sentence:
Easy
A triglyceride molecule is composed of one molecule of ____ bonded to three molecules of ____.
48.A student tests a food sample with Biuret solution and observes a colour change from blue to violet. The student then heats the same sample with Benedict's solution and observes a colour change from blue to orange. Describe the food tests and state which nutrients are present in the sample.
Medium

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