2. disaccharides and polysaccharides

flashcard notes

What type of reaction joins monosaccharides?

Condensation reaction (removes H and OH to form water)

What bond is formed when two monosaccharides join?

A glycosidic bond.

What reaction breaks a glycosidic bond?

Hydrolysis (uses water to add back H and OH).

Define a disaccharide.

Two monosaccharides joined by a glycosidic bond through condensation.

Which monosaccharides form these disaccharides?

• Maltose = glucose + glucose

• Sucrose = glucose + fructose

• Lactose = glucose + galactose

What disaccharide is formed from two glucose molecules?

Maltose.

How are polysaccharides formed?

By condensation of many monosaccharides.

Name three important polysaccharides.

Starch, glycogen, cellulose.

Which glucose isomer forms starch and glycogen?

α-glucose.

Which glucose isomer forms cellulose?

β-glucose.

Why is starch a good storage molecule in plants?

• Insoluble → doesn’t affect water potential.

• Too large to leave cells.

• Can be hydrolysed to glucose for respiration.

What are the two components of starch?

Amylose (coiled, unbranched) and amylopectin (branched).

Why is glycogen suited for energy storage in animals?

Compact, insoluble, and highly branched → glucose released rapidly for respiration.

What is cellulose made of?

β-glucose molecules joined to form long straight chains.

How is cellulose strengthened?

Chains held by hydrogen bonds → form microfibrils → strong structural support in plant cell walls.

How do you test for non-reducing sugars?

1. Add dilute HCl and heat → hydrolyse disaccharide/polysaccharide.

2. Neutralise with alkali (e.g., NaOH).

3. Add Benedict’s reagent and heat → brick-red precipitate if sugar present.

Why is this test needed?

Some sugars (e.g., sucrose) cannot reduce Benedict’s directly.

What is the test for starch?

Add iodine solution → orange-brown turns blue-black if starch is present.

What type of result does Benedict’s/iodine test give: qualitative or quantitative?

Qualitative (based on colour change).

Suggest a way to make Benedict’s test quantitative.

Use a colorimeter to measure absorbance → gives numerical data.