2.1.2 Carbohydrates

Question 1

What is a monomer?

Answer 1

A small, basic, individual, molecular unit

Question 2

What is a polymer?

Answer 2

Large, complex molecule, made up of long chains of monomers joined together

Question 3

What is a dimer?

Answer 3

When 2 molecular units are joined together

Question 4

What is a condensation reaction?

Answer 4

A reaction that occurs when 2 molecules are JOINED together, and so water is formed and removed

Question 5

What is a hydrolysis reaction?

Answer 5

A reaction that uses water to split 2 molecules apart

Question 6

What elements do all carbohydrates contain?

Answer 6

Carbon, Hydrogen, and oxygen

Question 7

What is the general formula for carbohydrates?

Answer 7

C x (H2) Oy

Question 8

What are the main monosaccharides of carbohydrates?

Answer 8

• glucose
• fructose
• ribose
• galactose
• deoxyribose

Question 9

What are the main disaccharides of carbohydrates and what monomers are they made up of?

Answer 9

• maltose = glucose + glucose
• lactose = glucose + galactose
• sucrose = glucose + fructose

Question 10

What are isomers?

Answer 10

Molecules with the same chemical formula, but different structures/ arrangements of atoms

Question 11

Why is glucose an important molecule?

Answer 11

• It’s the main source of energy in respiration

- It’s the building block for larger carbohydrates

Question 12

How is the structure/ properties of glucose related to it’s function/role?

Answer 12

• Small= easily transported in and out of cells through carrier proteins
• Soluble=easily transported around an organism
• Less reactive than other monosaccharides= breakdown must be catalysed and controlled by enzymes

Question 13

For the polysaccharide ‘Glycogen’ explain what it is, its structure, and how this helps it to carry out its function.

Answer 13

Glycogen:
-Is the energy storage polysaccharide in animals

-Made up of all alpha glucose molecules,1-4 and 1-6 glycosidic bonded chains= very branched structure

• stores excess glucose as glycogen in liver and muscle cells
• multiple branches/ end points = rapid release of energy and storage of energy=used as fuel for cells when glucose supply is low
• insoluble= does not affect water potential in cells
• large molecule can be hydrolysed to release large quantities of glucose for respiration

Question 14

For the polysaccharide ‘Starch’ explain what it is, its structure and the 2 structures it is made up of, and how this helps it to carry out its function.

Answer 14

STARCH:
-made up of both amylose and amylopectin (large molecule)and is the plants store of energy

AMYLOSE:
-made up of alpha glucose molecules, has 1-4 glycosidic bonded chains= long chains which coil into springs (through shape of the glucose molecules and glycosidic bonding) =compact structure

• function is to act as plants energy storage and reserve food supply
• only having 2 ends (rather than being branched) doesn’t give it as quick a release of energy, but it is not needed as much in plants as it is in animals
• coiled/compact structure= takes up little space in plant cells
• insoluble=doesn’t interfere with water potential of cell

AMYLOPECTIN:
-made up of alpha glucose molecules, has 1-4 and 1-6 glycosidic bonds= branched structure (less branched then glycogen) =compact molecule

• multiple end points= faster release of glucose
• compact= can store more glucose
• insoluble= doesn’t interfere with cell water potential

Question 15

For the polysaccharide ‘Cellulose’ explain what it is, its structure, and how this helps it to carry out its function.

Answer 15

-long, straight chains of entirely beta glucose monomers= unbranched shape, held together by 1-4 glycosidic chains, every other glucose molecule is flipped 180 degrees to allow bonding of hydroxyl groups, 60-70 cellulose molecules are held together by hydrogen bonds= a microfibrel. Many microfibrels are held together= macrofibrils

• function is to provide structural support for cells, for e.g in a plants cell wall
• insoluble in water

Question 16

What is the definition of a reducing sugar? Give examples of reducing sugars.

Answer 16

One of the two types of sugars which can donate an electron to another molecule.

• All monosaccharides e.g glucose
• Some disaccharides e.g maltose, lactose

Question 17

What is the method for testing if a solution is a reducing sugar?

Answer 17

1) Add 2cm3 of solution to be tested in test tube
2) Add 2cm3 of benedicts solution into test tube
3) Put test tube into boiling water bath (above 85 degrees) for 5 minutes
4) If solution remains blue, it does not contain any reducing sugar
5) If there is any colour change, then an amount of reducing sugar is present. Brick red indicating a high quantity of reducing sugar, which green indicating lower amounts

Question 18

Why may the benedicts test be known as semi-quantitative?

Answer 18

It only gives a very approximate idea of the amount of reducing sugar because it only shows a narrow range of colour changes, and all humans perceive colours slightly differently

Question 19

What is the method of testing for a non reducing sugar?

Answer 19

1) Take small amount of solution being tested and carry out the benedicts test, noting down any colour change that occurs
2) Add 2cm3 of solution being tested and put into a boiling tube along with 2cm3 of dilute hydrochloric acid
3) Gently boil test tube in a hot water bath for 5 minutes
4) At this point, if a non reducing sugar is present, then the acid hydrolyses the glycosidic bonds, releasing the monosaccharides
5) Add 2cm3 of dilute sodium hydroxide solution, and then use pH paper to check solution is an alkaline (Benedicts test can’t work under acidic conditions)
6) Add 2cm3 of benedicts solution, and then heat in hot water bath for 5 minutes, noting down any colour change

Question 20

Instead of testing for non reducing sugars directly, what do we do instead?

Answer 20

We need to break the glycosidic bond between the 2 monosaccharides, making up the disaccharide (e.g sucrose made up of glucose + fructose) and then test for these monosaccharides using the benedicts test.