biological molecules

Question 1

Why is cellulose a suitable material for forming plant cell walls?
(Explain using its structure and how that relates to its function.)

Answer 1

Cellulose is made of β-glucose (not α-glucose — this is super important).

The β-glucose monomers are joined by 1,4-glycosidic bonds, and every other monomer is flipped 180° to allow bonding.

This creates long, straight, unbranched chains.

Hydrogen bonds form between adjacent chains, holding them together.

These chains bundle into microfibrils, which are strong, rigid structures.

The strength and rigidity of microfibrils make cellulose ideal for providing structural support in plant cell walls.

Insoluble in water, so it doesn’t dissolve and maintains its function.

Question 2

How would you test a sample for the presence of lipids, and what result would indicate a positive test?

Answer 2

Test for Lipids (Emulsion Test)
Add ethanol to the sample and shake well (this dissolves any lipids present).

Then add water to the solution.

Observe the result.

Question 3

You’re given a sample of an unknown sugar. Describe how you would test for the presence of a reducing sugar and state what a positive result would look like.

Answer 3

Test for Reducing Sugars (e.g. glucose, maltose, galactose)
Add Benedict’s reagent (blue solution) to the sample.

Heat the mixture in a water bath at around 80°C for 2–5 minutes.

Observe the colour change.

🔍 Positive result:
The solution will change from blue to:
🟠 green → yellow → orange → brick-red (depending on how much reducing sugar is present — the more sugar, the redder the precipitate).

Question 4

Sucrose is a non-reducing sugar.
Explain how you would test for a non-reducing sugar in a sample.
What result would show it’s present?

Answer 4

Test for Non-Reducing Sugars (e.g. sucrose)
First, do the Benedict’s test as normal. If no colour change (stays blue), then no reducing sugars are present.

Add dilute hydrochloric acid (HCl) to a fresh sample – this hydrolyses the non-reducing sugar (e.g. sucrose) into its monosaccharides (e.g. glucose + fructose).

Boil in a water bath for a couple of minutes.

Neutralise the acid with sodium hydrogen carbonate (NaHCO₃), or test with pH paper until neutral.

Then add Benedict’s reagent and reheat.

If a non-reducing sugar was present, you will now get a colour change from blue to brick-red (positive result for reducing sugars after hydrolysis).

Question 5

Compare and contrast the structure and function of triglycerides and phospholipids.

Answer 5

Triglycerides:
Structure:

1 glycerol molecule (hydrophilic) bonded to 3 fatty acid chains (hydrophobic).

The fatty acid chains are long hydrocarbons, which make them hydrophobic (water-repelling).

Function:

Energy storage in animals and plants.

Acts as an insulator and protective padding around organs.

Compact, insoluble in water, making them ideal for storage in the body.

Phospholipids:Structure:

1 glycerol molecule (hydrophilic) bonded to 2 fatty acid chains (hydrophobic).

The third group on the glycerol is a phosphate group (hydrophilic), making the molecule amphipathic (having both hydrophobic and hydrophilic regions).

Function:

Found in cell membranes where they form a bilayer.

The hydrophobic tails face inward, away from water, and the hydrophilic heads face outward toward the water, creating a selective barrier.

This structure allows membrane fluidity and enables cells to maintain their shape and control what enters/exits.