Biological Molecules

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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.

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.

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.

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**.

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

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

Benedict's test for reducing sugars: colour change from blue to orange/brick red indicates a positive result.

Iodine test for starch: colour change from orange-brown to blue-black indicates a positive result.

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?
Medium
- AStarch
- BGlucose
- CProtein
- DLipid
6.Which of the following is a disaccharide formed when two glucose molecules join together?
Medium
- AStarch
- BGlycogen
- CMaltose
- DCellulose
7.What are the building blocks of proteins?
Medium
- 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?
Medium
- AStarch
- BGlycogen
- CMaltose
- DCellulose
10.A food sample is tested with Benedict's solution and heated. The solution remains blue. Which conclusion is correct?
Hard
- 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?
Hard
- 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?
Hard
- 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?
Hard
- 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.The diagram shows the structure of an amino acid. Which part of the amino acid varies between different amino acids?
Medium
- 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?
Easy
- 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?
Medium
- 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?
Medium
- AStarch
- BGlucose
- CProtein
- DLipid
18.Which test would you use to distinguish between a solution of glucose and a solution of starch?
Easy
- ABenedict's test
- BIodine test
- CBiuret test
- DEmulsion test
19.A student tested a food sample with Biuret solution and observed a colour change from blue to violet. Which nutrient is present?
Medium
- AProtein
- BStarch
- CGlucose
- DLipid
20.A student tests a food sample with Benedict's solution and heats it. The solution remains blue. Which conclusion is correct?
Hard
- AThe sample contains no reducing sugar
- BThe sample contains starch
- CThe sample contains protein
- DThe sample contains lipid
21.A student tested a food sample with the emulsion test and observed a cloudy white layer. Which nutrient is present?
Hard
- AProtein
- BLipid
- CStarch
- DReducing sugar
22.A student carries out the DCPIP test on three fruit juices: A, B, and C. The student adds $1 cm^{3}$ of DCPIP to each test tube, then adds $1 cm^{3}$ 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?
Hard
- AA
- BB
- CC
- DIt cannot be determined from the data
23.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?
Hard
- 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
24.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
25.A student tests a food sample with Biuret solution. The colour remains blue. What does this indicate?
Medium
- AProtein is absent
- BProtein is present
- CStarch is absent
- DLipids are present
26.Which of the following is a correct description of a triglyceride molecule?
Medium
- 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
27.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
28.In an experiment, a student uses the DCPIP test to compare the vitamin C content of two fruit juices. $1 cm^{3}$ 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?
Hard
- AJuice X
- BJuice Y
- CBoth have the same concentration
- DIt cannot be determined
29.Which of the following contains only carbon, hydrogen and oxygen?
Medium
- ACarbohydrates and lipids
- BProteins and carbohydrates
- CProteins and lipids
- DCarbohydrates, proteins and lipids
30.What is the basic unit (monomer) of proteins?
Easy
- AAmino acids
- BGlucose
- CFatty acids
- DGlycerol
31.A student tests a food sample with iodine solution. The colour changes from orange-brown to blue-black. Which nutrient is present?
Medium
- AStarch
- BProtein
- CLipid
- DReducing sugar
32.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
33.The diagram shows the structure of an amino acid. Which part of the molecule varies between different amino acids?
Hard
- AThe R group
- BThe amino group
- CThe carboxyl group
- DThe central carbon atom
34.Which of the following is a polysaccharide?
Medium
- AStarch
- BMaltose
- CGlucose
- DSucrose
35.A student performs the emulsion test for lipids. The correct procedure is:
Hard
- 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
36.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
37.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
38.Which elements are always present in carbohydrates?
Easy
- Acarbon, hydrogen, oxygen
- Bcarbon, hydrogen, oxygen, nitrogen
- Ccarbon, hydrogen, oxygen, nitrogen, sulfur
- Dcarbon, hydrogen
39.What is the monomer of proteins?
Easy
- Aamino acids
- Bnucleotides
- Cmonosaccharides
- Dglycerol and fatty acids
40.The diagram shows the structure of a triglyceride. Which part of the molecule is the glycerol backbone?
Medium
- AA
- BB
- CC
- DD

Slides

Biological Molecules: Carbohydrates, Proteins & Lipids

Most molecules in living organisms are carbohydrates, proteins or lipids.
These are organic molecules because they contain carbon.
Large molecules are made from smaller building blocks.

Carbohydrates

Carbohydrates are long chains of simple sugars.
Glucose is a monosaccharide (simple sugar).
Two glucose molecules join to form maltose (a disaccharide).
Many glucose molecules join to form starch, glycogen or cellulose (polysaccharides).

Which of the following is a polysaccharide?

Glucose
Maltose
Starch
Glycerol

Lipids (Fats and Oils)

Most lipids in the body are triglycerides.
A triglyceride is made of one glycerol molecule bonded to three fatty acid chains.
Fats are solid at room temperature; oils are liquid.

A triglyceride is made of $one ____$ molecule and three ____ chains.

Proteins

Proteins are long chains of amino acids.
There are about 20 different amino acids.
All amino acids have the same basic structure but differ in the 'R' group.
The order of amino acids determines the protein's identity.

Which of these are true about proteins?

Proteins are made of amino acids.
There are only 5 different amino acids.
The 'R' group is the same in all amino acids.
A small change in amino acid order can create a different protein.

Food Tests: Glucose (Reducing Sugar)

Add Benedict's solution to the sample and heat at 60–70°C for 5 minutes.
A positive result: colour change from blue to orange or brick red.

What colour indicates a positive Benedict's test for glucose?

Blue
Orange/brick red
Blue-black
Purple

Food Tests: Starch

Add iodine solution to the food sample.
A positive result: colour change from orange-brown to blue-black.

Iodine solution turns from orange-brown $to ____ in$ the presence of starch.

Food Tests: Protein

Add Biuret solution to the food sample.
A positive result: colour change from blue to violet/purple.

What colour change shows a positive Biuret test?

Blue to orange
Orange-brown to blue-black
Blue to violet/purple
Blue to colourless

Food Tests: Lipids

Mix the food sample with $2 cm^{3}$ of ethanol and shake.
Add the ethanol to an equal volume of cold water.
A positive result: a cloudy white emulsion forms.

Which of these are correct about the lipid (ethanol) test?

A cloudy emulsion indicates a positive result.
The test uses Benedict's solution.
The sample is mixed with ethanol first.
The final mixture is heated.

Food Tests: Vitamin C

Add $1 cm^{3}$ of DCPIP solution to a test tube.
Add a small amount of food sample (as a solution).
A positive result: the blue colour of DCPIP disappears.

In the DCPIP test, a positive result is shown by $the ____$ colour disappearing.

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