Biological molecules
Chemical Composition of Biological Molecules
Biological molecules are the building blocks of life. To understand them, we must first identify the chemical elements that make them up. All biological molecules are organic, meaning they contain carbon. However, different types of molecules require different additional elements.
Carbohydrates and Fats: Both carbohydrates (like starch) and fats (lipids) are made from only three elements: carbon, hydrogen, and oxygen. This is a key similarity between them.
Proteins: Proteins contain the same three elements as carbohydrates and fats (carbon, hydrogen, oxygen), but they also always contain nitrogen. This is the defining difference. If a question asks for an element found in proteins but not in carbohydrates or fats, the answer is nitrogen.
Monomers and Polymers
Large biological molecules (polymers) are constructed from smaller building blocks called monomers. This process is known as polymerization. Understanding which monomer makes up which large molecule is essential.
| Monomer (Building Block) |
|---|
| Glucose |
| Amino acids |
| Fatty acids and Glycerol |
Note that fats are slightly different because they are made from two types of monomers: fatty acids and glycerol, whereas starch and proteins are made from just one type of repeating monomer.
Food Tests for Biological Molecules
We use specific chemical reagents to detect the presence of biological molecules in food samples. Each test has a distinct positive result colour or appearance.
| Positive Result |
|---|
| Blue-black colour |
| Green → Yellow → Orange → Brick-red precipitate (depends on concentration) |
| Lilac / Purple colour |
| Cloudy white emulsion |
| Decolourises (turns from blue to colourless) |
Important Note on Benedict’s Test: Unlike the other tests which are simple mixtures, the test for reducing sugars requires heating. You must add Benedict’s solution and then heat the mixture in a water bath to see the colour change.
Ethanol Emulsion Test for Fats
Fats are insoluble in water but soluble in ethanol. The test relies on this property.
Procedure:
- Add ethanol to the food sample and shake vigorously to dissolve any fats.
- Pour this solution into a test tube containing water.
- Observe the result.
Positive Result: A cloudy white emulsion forms. The cloudiness is caused by tiny fat droplets suspended in the water, which scatter light. This is distinct from the colour changes seen in other tests.
DNA Structure
DNA (Deoxyribonucleic acid) is a large molecule that carries genetic information. Its structure is specific and must be described accurately.
Double Helix: DNA consists of two strands coiled together to form a twisted ladder shape called a double helix.
The Backbone: Each strand is made of alternating sugar and phosphate groups. This forms the 'sides' or backbone of the ladder.
The Bases: Attached to each sugar molecule is a chemical called a base. These bases project inwards from the backbone, like the rungs of a ladder.
Base Pairing: The two strands are held together by bonds between the bases. The bases always pair up in a specific complementary way:
- A pairs with T
- C pairs with G
This means if one strand has an 'A', the opposite strand must have a 'T'. This rule is constant for all DNA.
⚠︎ Confusing Elements and Monomers
Error: Students often think that proteins are made from glucose or that fats are made from amino acids.
Correction: Remember the specific building blocks:
- Proteins are made from amino acids (which contain nitrogen).
- Carbohydrates (starch/glycogen) are made from glucose.
- Fats are made from fatty acids and glycerol.
Also, do not confuse the element nitrogen with other elements like iron or sulfur. Nitrogen is the key element distinguishing proteins from carbohydrates and fats.
⚠︎ Misremembering Food Test Results
Error: Students frequently confuse the positive results of different tests. For example, stating that Biuret turns blue or that Iodine turns orange.
Correction:
- Iodine: Blue-black (starch). Negative is orange-brown.
- Benedict’s: Brick-red precipitate (reducing sugar). Negative is blue.
- Biuret: Lilac/Purple (protein). Negative is blue.
- Ethanol: Cloudy white emulsion (fats). Negative is clear/colourless.
- DCPIP: Colourless (Vitamin C). Negative is blue.
Describing the Ethanol Test Correctly
When asked to describe the test for fats, you must mention both ethanol and water. Simply saying 'add ethanol' is incomplete because the emulsion only forms when the ethanol-fat mixture is poured into water.
Why this matters: Examiners look for the specific phrase 'cloudy white emulsion' or 'white precipitate'. Do not use colour change terms like 'turns purple' or 'turns blue-black' for fats, as these are incorrect. The cloudiness is due to light scattering by fat droplets.
Describing DNA Base Pairing
When describing DNA structure, you must specify that bases pair in a complementary manner. You cannot just say 'bases pair up'.
Correct phrasing: 'Adenine (A) always pairs with Thymine (T), and Cytosine (C) always pairs with Guanine (G).'
Why this matters: This specific pairing rule is fundamental to DNA replication. If you write 'A pairs with C', it is scientifically incorrect and will receive no marks. Note that full names are not required; the letters A, T, C, G are sufficient.
Identifying Elements and Monomers
Q:
Which chemical element is found in proteins but NOT in carbohydrates or fats?
A:
Nitrogen
Q:
Name the monomer that makes up starch.
A:
Glucose
Q:
What are the two components that make up a fat molecule?
A:
Fatty acids and glycerol
Food Test Results and Procedures
Q:
Describe the test for reducing sugars. Include the reagent used and the required condition.
A:
Add Benedict’s solution to the sample and heat (or warm) it.
Q:
What is the positive result for a test using Biuret solution?
A:
Lilac or purple colour.
Q:
A student adds DCPIP to a fruit juice and the blue solution turns colourless. What does this indicate?
A:
Vitamin C is present.
Q:
Describe how you would test a food sample for the presence of fats.
A:
- Add ethanol to the sample and shake.
- Pour the mixture into water.
- A cloudy white emulsion indicates a positive result.
DNA Structure Description
Q:
Describe the structure of a DNA molecule.
A:
DNA consists of two strands coiled together to form a double helix. Each strand has a backbone of alternating sugar and phosphate groups. The strands are held together by bonds between bases. The bases always pair up: A with T, and C with G.
Q:
If one strand of DNA has the sequence A-C-G-T, what is the sequence of the opposite strand?
A:
T-G-C-A