Carbohydrates

Question 1

Monosaccharides

Answer 1

single unit sugars e.g. glucose, fructose, galactose.
- source of energy / aerobic respiration / generate ATP
- building blocks (monomers) for polysaccharides
- building DNA / RNA nucleotides
- chemical intermediates

Question 2

Disaccharides

Answer 2

double unit sugars e.g. maltose, sucrose, lactose

Question 3

Polysaccharides

Answer 3

large, insoluble polymers e.g. starch (amylose), glycogen, cellulose

-Starch / glycogen act as energy stores in plants / animals respectively

• Cellulose makes plant cell walls / structural role

Question 4

What is the empirical formula for carbohydrates?

Answer 4

CH2O(n) where n = the number of carbon atoms

Question 5

How are alpha and beta glucose similar and different

Answer 5

Glucose is an ISOMER. Both alpha and beta forms are the same molecule but different arrangement.
Alpha glucose, the -OH (HYDROXYL groups) on CARBON 1 and 4 both point BELOW the plane of the ring. Beta glucose the -OH group on carbon 1 is ABOVE the plane of the ring.

Question 6

Benedict test for reducing sugars

Answer 6

Add 2 cm3 Benedict’s solution to the sample

Heat in water bath, 90oC for 5 mins

Colour change from light blue to brick red. ( semi-quantitative test)

Question 7

Explain the formation of maltose.

Answer 7

2 alpha glucose molecules

Condensation reaction

A water molecule is released

Forming a C1 - C4 glycosidic bond

Question 8

Sucrose ( Disaccharide)

Answer 8

Glucose and fructose (Monosaccharides)
Non-reducing

Question 9

Lactose (Disaccharide)

Answer 9

Glucose and galactose (Monosaccharides)
Reducing

Question 10

Maltose ( Disaccharide)

Answer 10

x2 alpha glucose (Monosaccharides)
Reducing

Question 11

Benedicts test for non-reducing sugars

Answer 11

Complete Benedict’s test and if it stays blue / negative

BOIL with hydrochloric acids then neutralise with alkali

Repeat Benedict’s test by heating 90oC 5 mins

Colour change from light blue to brick red

Question 12

formation of starch

Answer 12

Condensation reaction between MANY alpha glucose monomers.
Water released.
Forms C1-C4 glycosidic bonds.
Amylose - tightly coiled / compact, insoluble polysaccharide.
Amylopectin - branched polysaccharide.

Question 13

The role of starch in plants

Answer 13

STARCH consists mainly of the polysaccharide AMYLOSE.

It a a STORAGE polysaccharide for energy.

Amylose is COMPACT, so a plant cell can store a lot of energy in a small space.

Amylose is coiled by HYDROGEN bonding within the chain.

It is INSOLUBLE, so cell water potential / enzymes are not affected and it CANNOT diffuse out of cells.

HYDROLYSIS of starch releases glucose for use in RESPIRATION to make ATP.

Question 14

GLYCOGEN

Answer 14

Condensation reaction between MANY alpha glucose monomers.
Water released.

Forms C1-C4 and C1-C6 glycosidic bonds.

It is a branched polyssachride so more “terminal ends” allow more rapid hydrolysis / release of glucose for respiration

Question 15

Food test for starch

Answer 15

Add 2cm3 of iodine in a potassium iodide solution.
Colour change from orange to blue / black.

Question 16

Formation of cellulose

Answer 16

Polymer of beta glucose (monomer).
Condensation reaction between monomers forming
C1 - C4 glycosidic bonds.
Every 2nd beta glucose rotates 180o.
Forms a straight, cellulose strand.

Multiple parallel strands, bonded by extensive hydrogen bonding.
Forming microfibrils

Many microfibrils combine to form macro fibres.
Microfibres combine to form cellulose.

Question 17

Relate the structure of cellulose to its function with plant cells.

Answer 17

Fibrous polysaccharide.
Tensile.
Withstand osmotic forces when cells absorb water by osmosis and become turgid.
Spaces between cellulose strands allow material to pass through freely to cell surface membrane.