Carbohydrates

Question 1

Disaccharide

Answer 1

2 monosaccharides joined together by a glycosidic bond in a condensation reaction
e.g…
Glucose + Glucose = Maltose
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose

Question 1

Monosaccharide

Answer 1

Single sugar monomer e.g Glucose and Ribose

Question 2

Polysaccharide

Answer 2

A polymer formed by many monosaccharides joined together by glycosidic bonds in a condensation reaction e.g Cellulose, Starch and Glycogen

Question 3

Reducing sugars (and test)

Answer 3

Reducing sugars can donate electrons (they’re oxidised)
- Benedict’s test…
1) Add 2cm3 of food sample (liquid)
2) Add 2cm3 of Benedict’s reagent
3) Heat mixture in a water bath for 5 minutes
4) Orange-brown solution = positive result (reducing sugar) - Blue = negative result

Question 4

Non-reducing sugars (and test)

Answer 4

Non-reducing sugars cannot donate electrons (can’t be oxidised)
- Benedict’s test…
1) Repeat steps 1-4 from reducing sugar test
2) If solution is blue i.e negative result…
3) Add 2cm3 of same sample and 2cm3 of HCl, then place in a water bath for 5 mins
4) Then add sodium hydrogencarbonate
5) Place in water bath for another 5 mins
6) Blue to brick red = non-reducing sugar

Question 5

Hydrolysis reaction

Answer 5

The breaking down of polymers by the addition of a water molecule

Question 6

Condensation reaction

Answer 6

The formation of polymers by polymerisation (removes a water molecule)

Question 7

Isomers of Glucose

Answer 7

• Alpha glucose = both OH groups on the bottom
• Beta glucose = one OH on the top and one OH on the bottom

Question 8

Starch

Answer 8

• Mixture of 2 different polysaccharides - Amylose and Amylopectin
• Starch is stored as it is compact and insoluble so it doesn’t affect the water potential
• Large molecule so doesn’t diffuse out of cells

Question 9

Amylose

Answer 9

An unbranched chain of alpha-glucose
- Has a coil structure due to the angle of the glycosidic bonds

Question 10

Amylopectin

Answer 10

A branched chain of alpha-glucose
- Side branches can easily be broken down by enzymes

Question 11

Glycogen

Answer 11

Branched chain of alpha-glucose
- Side chains are same as Amylopectin
- Glycogen is stored as it is compact and insoluble so it doesn’t affect the water potential
- Large molecule so doesn’t diffuse out of cells

Question 12

Cellulose

Answer 12

Made up of monomers of beta-glucose
- Has straight, unbranched chains that run parallel to each other
- The glycosidic bonds are formed between 2 monomers of beta-glucose so positions of the glycosidic bonds alternate with each monomer added to the polysaccharide chain
- Beta-glucose and inverted beta-glucose
- Cellulose chains are linked together by hydrogen bonds which form microfibrils which provide structural support