Biological molecules: Carbohydrates

Question 1

What are biological molecules?

Answer 1

Substances where a C-H bond is present

Question 2

What is a monomer?

Answer 2

The individual molecules that make up polymers

Question 3

Why is carbon dioxide an inorganic molecule?

Answer 3

It doesn’t have the C-H bond

Question 4

What chemical elements are polymers usually based on?

Answer 4

Carbon, hydrogen, oxygen and nitrogen

Question 5

What is a monosaccharide?

Answer 5

The basic monomer unit of carbohydrates

Question 6

What is a disaccharide?

Answer 6

2 monosaccharide units joined together to form a glycosidic bond in a condensation reaction

Question 7

What is a polysaccharide?

Answer 7

A long chain of monosaccharides

Question 8

Give the four monomers you need to know and their corresponding polymer

Answer 8

Amino acids - proteins
Nucleotides - nucleic acids (DNA, RNA)
glycerol and fatty acids - triglyceride fats
Monosaccharides - polysaccharides (these are under carbohydrates which refers to both monomer and polymer)

Question 9

What is the general formula of a monosaccharide?

Answer 9

(CH ₂O)n where n can be any number from 3 to 7

Question 10

Give three examples of monosaccharides

Answer 10

Glucose, fructose and galactose

Question 11

What kind of sugar is glucose?

Answer 11

A hexose sugar because it had 6 carbons

Question 12

What are the two isomers of glucose and draw them.

Answer 12

Alpha glucose and beta glucose ( drawing in camera roll)

Question 13

What is a reducing sugar?

Answer 13

A sugar that can donate electrons to (or reduce) another chemical (in this case that chemical is benedicts solution)

Question 14

What is reduction?

Answer 14

A chemical reaction involving the gain of electrons or hydrogen

Question 15

What is the test for reducing sugars and how do you carry it out?

Answer 15

Benedict’s test:

1. Add 2cm of the food sample to be tested in a test tube and it should be in liquid for, by grinding it up in water
2. Add an equal volume of benedict’s reagent
3. Heat the mixture in a gently boiling water bath for 5 minutes. If reducing sugar is present, solution turns orange-brown

Question 16

Why is benedict’s test semi quantitative?

Answer 16

Because there are differences in the colour in the result

Question 17

What are the three disaccharides you need to know and what monosaccharides form them?

Answer 17

Maltose = glucose + glucose
Sucrose = glucose + fructose
Lactose = glucose + galactose

Question 18

Why is it when monomers become polymers they go through a condensation reaction?

Answer 18

Because for each monomer that bonds, a molecule of water is released

Question 19

What kind of reaction happens when a disaccharide breaks down into a monosaccharide?

Answer 19

A hydrolysis reaction because water is added to the disaccharide because water is added to the disaccharide breaking the glycosidic bond and releasing the constituent monosaccharide

Question 20

What are all monosaccharides?

Answer 20

Reducing sugars

Question 21

What must you do if you want to detect a non reducing sugar? Give the steps to the process

Answer 21

By hydrolysing it into its monosaccharide components via hydrolysis:

1. If not already make the substance liquid form by grinding it with water
2. Add 2cm of the food sample being tested to 2cm of benedicts solution in a test tube and filter
3. Place the test tube in a gently boiling water bath for 5 mins (if no colour change is present and solution stays blue then there is no reducing sugar present)
4. Add another 2cm of the food sample to 2cm of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for 5 minutes. The dilute hcl will hydrolyse any disaccharide present into its constituent monosaccharides
5. Slowly add some sodium hydrogen carbonate solution to the test tube to neutralise the hcl as benedicts solution doesn’t work in acid and check its alkaline with pH paper
6. Retest the solution with 2cm of benedicts solution and leave in a gently boiling water bath for 5 minutes
7. If non reducing sugar was present in the original sample, the benedicts reagent will now turn orange brown due to the reducing sugars that were reduced by the hydrolysis of the non reducing sugar

Question 22

Give an example of a disaccharide that is a reducing sugar and one that’s not

Answer 22

Maltose is a reducing sugar and sucrose is not

Question 23

Why are polysaccharides insoluble? What does this make them suitable for?

Answer 23

Because they are very large molecules, making them suitable for storage

Question 24

Give an example of a polysaccharide in plants that isn’t used for storage and why its useful

Answer 24

The polysaccharide cellulose is useful as it gives structural support to the plant cell

Question 25

Where is the polysaccharide starch found?

Answer 25

In many parts of plants in the form of small granules or grains for example starch grains in chloroplasts

Question 26

How is the polysaccharide starch formed?

Answer 26

By the joining of between 200- 100000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions

Question 27

What are the steps to carrying out the test for starch?

Answer 27

1. Place 2cm of the sample being tested into a test tube
2. Add 2 drops of potassium iodide solution and shake or stir
3. The presence of starch is indicated when the iodine solution goes from yellow to blue-black

Question 28

Why does benedicts reagent turn red when heated with a reducing sugar?

Answer 28

Benedicts reagent turns red when heated with a reducing sugar because it has formed in insoluble layer of copper (I) oxide where electrons have been donated from the reducing sugar to the copper