Carbohydrates

Question 0

What is a carbohydrate?

Answer 0

A carbohydrate is a molecule which contains carbon, hydrogen and oxygen only.

Question 1

What is a organic molecule?

Answer 1

An organic molecule is a molecule containing two or more carbon atoms.

Question 2

What are the main functions of a carbohydrate? Give examples.

Answer 2

To provide energy to do work in a cell. e.g glucose

To act as an energy store for later use. e.g starch (plants) and glycogen (animals)

Structural functions. E.g cellulose in plant cell walls and chitin in the exoskeletons of an insect.

Question 3

What are the three structural levels of carbohydrates?

Answer 3

Monosaccharides.

Disaccharides.

Polysaccharides.

Question 4

What are monosaccharides? Give an example.

Answer 4

These are simple sugars such as glucose.

Question 5

What are disaccharides? Give an example.

Answer 5

These consist of two monosaccharides covalantly bonded together. E.g maltose.

Question 6

What are polysaccharides? Give examples.

Answer 6

Also known as complex carbohydrates these consist of large numbers of monosaccharides bonded together such as starch and cellulose.

Question 8

Give the name of each monosaccharide according to the number of Carbon atoms from three to six. Giving examples and formulas where appropriate.

Answer 8

3 - Triose’s : C3-H6-O3

4 - Tetrose’s : C4-H8-O4

5 - Pentose’s : C5-H10-O5 e.g Ribose sugar in RNA not DNA

6 - Hexose’s : C6-H12- O6 e.g Glucose

Question 9

What are the general characteristics of monosaccharides?

Answer 9

Water soluble.

Are white crystals in a solid state.

Are sweet tasting.

Reduce Benedicts reagent (the copper sulphate component) to form a brick red precipitate.

Question 10

What is the general formula for a hexose monosaccharide and where are they most common?

Answer 10

C6-H12-O6

These are the most common sugars in biological molecules.

Question 11

There are many structural isomers of hexose sugars, what are the two simplest isomers?

Answer 11

The two types of Glucose, alpha and Beta.

Question 12

How do alpha and beta glucose differ from each other?

Answer 12

In alpha the hydrogen on the C1 is above the hydroxyl group.

In Beta the hydrogen is below.

Question 13

Name the three most common hexose sugars in biological molecules.

Answer 13

Glucose.

Fructose.

Galactose.

Question 14

Outline the structure and features of galactose.

Answer 14

Is a hexose sugar with an almost identical structure to beta glucose apart from the hydrogen on the C4 carbon is also below the hydroxyl group.

Question 15

Outline the structure and features of fructose.

Answer 15

Is a hexose sugar with a four carbon atoms in its ring and two outside it one group attached to the C1 carbon and another on the C5 carbon.

Question 20

The formulas for monosaccharides are part of a homologous series. What is this formula?

Answer 20

(C-H2-O)n

Where “n” is the number of carbon atoms.

Question 21

What are D and L isomers?

Answer 21

D and L carbohydrate isomers are mirror images of each other and are also known as optical isomers or enantiomers as they have the ability to bend light.

Question 22

What is the Valancy of an element?

Answer 22

The number of bonds an atom can make.

Question 23

What are the characteristics of disaccharides?

Answer 23

The same as monosaccharides except they don’t reduce Benedicts reagent apart from maltose.

Question 24

How are disaccharides formed?

Answer 24

Through a condensation reaction between two hydroxy groups, one on the C1 carbon and one on the C4 of the other monosaccharide. Forming a 1-4 glycosidic link and producing a single molecule of water.

Question 25

Explain how are disaccharides broken apart, giving an example.

Answer 25

The reverse reaction is called hydrolysis whereby the monosaccharides are separated through the addition of water. For example the hydrolase enzyme amylase in the mouth breaks down starch into units of maltose in the first stage of its metabolic pathway.

Question 26

What are hexose disaccharides in relation to each other?

Answer 26

Structural isomers with the general formula C12-H22-O11. (2(C6-H12-O6)-(H2-O))

Question 27

What is maltose constructed out of?

Answer 27

A combination of two glucose molecules.

Question 28

What is sucrose constructed out of?

Answer 28

Glucose and fructose.

Question 29

What is lactose constructed of?

Answer 29

Glucose and galactose.

Question 30

Why do monosaccharides and disaccharides dissolve in water?

Answer 30

This is because their free hydroxyl groups are able to form hydrogen bonds with water.

Question 31

What are the characteristics of polysaccharides?

Answer 31

They are insoluble, do not taste sweet, do not reduce Benedicts solution and are insoluble.

Question 32

What are the two types of starch molecule?

Answer 32

Amylose (20%) and Amylopectin (80%).

Question 33

What is the structure of amylose?

Answer 33

Amylose is constructed of between 200 and 5000 alpha glucose molecules which form an unbranched helical chain whose turns are bonded together by hydrogen bonds. (6 alpha glucose molecules to a turn) which is tightly coiled making it difficult for hydrolase enzymes to break it down as a result amylose releases its energy slowly. The resulting tube has closed ends with the hydroxyl groups inside making it inert and insoluble in water.

Question 34

Give two examples of where starch is found?

Answer 34

Plant tubers and in seed endosperm where it is found as granules.

Question 35

Describe the structure of amylopectin.

Answer 35

Is constructed of between 5000 and 100000 alpha glucose molecules which form a branched helical chain due to the presence of both 1-4 and 1-6 glycosidic links. Again this is an inert and insoluble molecule.

Question 36

Describe the structure of glycogen.

Answer 36

It is very similar to amylopectin apart from it has looser coils making it easier to access it’s energy and making the glycogen granules smaller than starch granules as they don’t stick together as well. As the coil isn’t as tight it is partially soluble in water.

Question 37

What is the reason for forming a insoluble energy store?

Answer 37

To conserve energy because if the glucose was left in its soluble form energy would be needed to stop it leaving by diffusion and to throw water out to prevent osmotic bursting.

Question 38

Describe the structure of a cellulose molecule.

Answer 38

This is constructed of about 15000 Beta glucose so the glycosidic links alternate with each other rather than forming a linear plane Like starch and glycogen.

Question 39

Describe the structure of a microfibril.

Answer 39

This consists of about 2000 linear cellulose molecules held together by very large numbers of hydrogen bonds and are about 20nm wide.

Question 40

Describe the properties of cellulose.

Answer 40

It has high tensile strength

Is insoluble due to hydrogen bonds between its hydroxyl groups.

Question 41

Describe the structure of a cell wall.

Answer 41

This consists of a middle lamella made of pectic compounds such as pectin which is a polymer of about 200 galacturonic acid molecules on top of which the cellulose microfibrils are mounted in a gel-like matrix. There is then a secondary layered wall made of cellulose, hemicellulose and lignin.

Question 42

What is the function of a cell wall?

Answer 42

To protect the cell inside and to help it resist osmotic bursting when placed in pure water. Also to give support to plant cells and therefore the whole plant this is especially important for structures like leaves which would fold in on themselves if these walls weren’t present.

Question 43

How can cellulose be broken down.

Answer 43

Cellulose can be broken down by cellulase enzymes in ruminating herbivores (lateral chewing) such as cows.