3.1.2 Carbohydrates

Question 1

what are monosaccharides?

Answer 1

the monomers from which larger carbohydrates are made.
glucose, fructose and galactose are the most common monosaccharides

Question 2

what type of reaction creates a bond between two monosaccharides?

Answer 2

condensation reaction

Question 3

name the type of bond formed when monosaccharides react.

Answer 3

1,4 or 1,6 glycosidic bond

Question 4

what is a disaccharide?

Answer 4

formed by the condensation of two monomers (monosaccharides) to form one chemical bond (glycosidic)

Question 5

what is formed by the condensation of multiple monosaccharides?

Answer 5

polysaccharides

Question 6

name 3 disaccharides. how do they form?

Answer 6

maltose - glucose + glucose
sucrose - glucose + fructose
lactose - glucose + galactose
condensation reaction

Question 7

describe the structure and functions of starch.

Answer 7

storage polymer of alpha glucose in plant cells
- insoluble: no effect on water potential
- large: does not diffuse out of cells
made from amylose:
1,4 glycosidic bonds
helix with intermolecular H bonds = compact
and amylopectin
1,4 and 1,6 glycodsidic bonds
branched = many terminal ends for hydrolysis into glucose

Question 8

describe the structure and functions of glycogen.

Answer 8

main storage polymer of alpha glucose in animal cells (also found in plant cells)
- 1,4 and 1,6 glycosidic bonds
- branched = many terminal ends for hydrolysis
- insoluble = no effect on water potential and does not diffuse out
- compact

Question 9

describe the structure and functions of cellulose.

Answer 9

polymer of alpha glucose
gives rigidity to the plant cell wall
- 1,4 glycosidic bonds
- straight chain, unbranched molecule
- alternate glucose molecules are rotated 180 degrees
- H-bond crosslinks between parallel strands to form microfibrils = high tensile strength

Question 10

describe the Benedict’s test for reducing sugars.

Answer 10

1. add equal volume of Benedict’s reagent to a sample
2. Heat the mixture in an electric water bath (at 100 degrees) for 5 mins
3. Positive result: colour change from blue to orange & brick red ppt. forms

Question 11

describe the Benedict’s test for non-reducing sugars.

Answer 11

1. Negative result: Benedict’s reagent remains blue
2. Hydrolyse non-reducing sugars into their monomers by adding 1cm of HCl
3. Neutralise the mixture using NaCO3 (sodium carbonate) solution
4. Proceed with the Benedict’s test as usual.

Question 12

describe the test for starch.

Answer 12

1. Add iodine solution
2. Positive result: colour change from orange to blue/black

Question 13

Outline how colorimetry could be used to give qualitative results for the presence of sugars and starch.

Answer 13

1. Make standard solutions with known concentrations. Record absorbance or % transmission values.
2. Plot calibration curve: absorbance or % transmission (y-axis), concentration (x-axis)
3. Record absorbance or % transmission values of unknown samples. Use calibration curve to read off the concentration.