Carbohydrates

Question 1

State the elements present in carbohydrates

Answer 1

Carbon, Hydrogen, Oxygen

Question 2

State the general formula of carbohydrates

Answer 2

(CH20)x

Question 3

Define the term monsaccharide

Answer 3

A monomer of carbohydrates. A single sugar

Question 4

Define the term disaccharide

Answer 4

A molecule comprised of two monosaccharides, joined together by glycosidic bonds.

Question 5

Define the term polysaccharide

Answer 5

A polymer made up of many sugar monomers.

Question 6

Define the term pentose sugar

Answer 6

A monosaccharide composed of 5 carbons

Question 7

Define the term hexose sugar

Answer 7

A monosaccharide composed of 6 carbons

Question 8

Define the term triose sugar

Answer 8

A monosaccharied containing 3 carbon atoms e.g. L-glyceraldehyde and D- glyceraldehyde

Question 9

Draw a molecule of alpha glucose, beta glucose and ribose

Answer 9

Draw a diagram

Question 10

Define the term isomer

Answer 10

Isomers are molecules with the same moleclear formula but are arranged differently

Question 11

Describe the difference between alpha- and beta-glucose.

Answer 11

Alpha glucose has an -OH hydroxyl group that points downwards, away from the ring.
Beta glucose has an -OH hydroxyl group that points upwards above the ring.

Question 12

Draw a table listing the similarities and differences between alpha glucose and ribose

Answer 12

Similarities:

• both carbohydrates
• both monosaccharides
• they both have the same general formula
• they both form rings or chains
• they are both soluable in water

Differences:

• Glucose is a hexose sugar and ribose is a pentose sugar
• Glucose has 6 carbons and ribose has 5
• Glucose’s main function is the production of ATP during aerobic resp
• Ribose forms a component of nucleotide DNA and RNA

Question 13

List 3 examples of disaccharides and for each state which monosaccharides they are composed of.

Answer 13

Sucrose - formed from a glucose and a fructose monosaccharide
Lactose - a glucose and a galactose
Maltose - two alpha glucose monosaccharides

Question 14

State the properties and functions of glucose

Answer 14

• Is a hexose sugar and can either be alpha or beta glucose which both a ring structure
• Its structure makes it soluble so it can be easily transported. It is polar which means it is hydrophylic and so hydrogen bonds form between the hydroxyl group and water molecules
• It’s chemical bonds contain a lot of energy, this relates to its function as the main energy source in animals and plants

Question 15

State the functions and properties of fructose

Answer 15

• A hexose monosaccharide.

- It is the suagr found in fruit. It is used as a sugar substitute for diabetics

Question 16

State the functions and properties of galactose

Answer 16

• A hexose monosaccharide.

- It is used to make lactose (the sugar in milk)

Question 17

State the properties and functions of maltose

Answer 17

• A dissacharide made of two alpha glucose molecules

- Found in intenstines and in germinating seeds, is an intermediate in the hydrolysis of starch

Question 18

State the properties and functions of sucrose

Answer 18

• A dissacharide made of alpha glucose and fructose

- In plants carbohydrate is moved from one region to another as sucrose in the plant phloem

Question 19

State the properties and functions of lactose

Answer 19

• A dissacharide made from alpha glucose and galactose
• The main sugar in milk
• It is difficult to hydrolyse, providing a slow release of sugars to baby.

Question 20

Draw a labelled diagram demonstrating how two molecules of glucose form a disaccharide in a condensation reaction, showing the location of a 1,4-glycosidic bond and from where a water molecule is generated.

Answer 20

Draw a diagram

Question 21

Describe, using a diagram, how hydrolysis of maltose occurs and why water is needed.

Answer 21

It occurs when a water molecule is added to the maltose dissacharide. The addition of the water molecule results in the breakdown of maltose into two alpha glucose monosaccharides. The two hydrogen atoms and the oxygen atom from the water molecule bond with the oxygen joining the monosaccharides together to form a hydroxyl group on each monosaccharide.

Question 22

Explain why alpha-glucose links together to form starch whereas beta-glucose links together to form cellulose.

Answer 22

Beta glucose has a hydroxyl group pointing upwards, this means that the hydroxyl on carbon-1 and carbon-4 are be too far away to react. As a result, the only way that beta glucose molecules can join together is if each monomer rotates 180 degrees to join to its neighbour. This means that the polysaccharide is a straight chain so it is unable to coil or form branches. Cellulose is a straight chain molecule.
However, alpha glucose polysaccharides can form branches and coils e.g. amylopectin forms 1-6 glycosidic bonds as well as 1-4.

Question 23

List the two different polysaccharides that make up starch.

Answer 23

Amylose and Amylopectin

Question 24

Explain with the use of diagrams why glycosidic bonds are called 1,4 or 1,6.

Answer 24

Because the glycosidic bond either happens between the hydroxyl group on the 1st carbon and the 4th carbon or between the hydroxyl group on the 1st carbon and 6th carbon

Question 25

Explain how to calculate the number of glycosidic bonds in a carbohydrate given the number of monosaccharides it is made up from.

Answer 25

Number of monosaccharides - 1 = number of glycosidic bonds

Question 26

Explain how to calculate the chemical formula for a carbohydrate given the number of hexose sugars it is made up from.

Answer 26

(CH20)x General formula multiplied by x, minus h20 multiplied by x-1

Question 27

Describe the structure of a cellulose fibre.

Answer 27

Cellulose molecules make hydrogen bonds with eachother, forming microfibrils. These microfibrils join together to make macrofibrils which combine to produce fibres. These fibres are strong and insolubable and are used to make cell walls. Cellulose is an important part of our diet, it is very hard to breakdown and form monomers and so it forms the fibre neccessary for a healthy digestive system. It is a polymer of beta glucose.

Question 28

State the properties and functions of cellulose

Answer 28

It is made of long, unbranched chains of beta glucose. Because the chains are straight they can lie parallel to eachother. Numerous hydrogen bonds link parallel chains, producing strength. This means it is difficult to hydrolyse so it makes a good cell wall. The cell wall is fully permeable due to gaps in the fibres

Question 29

State properties and functions of starch

Answer 29

- A polsaccharide of alpha glucose. It is the main energy storage material in plants and animals. It is insoluable in water so it can be stored and it doesn't cause water to enter the cell by osmosis (that would make the cell swell) - Amylose is a long unbranched chain of alpha glucose, the angles of the glycosidic bonds give it a coil structure making it compact so it is very good for storage as it can fit a lot in a small space. - Amylopectin is a long branched chain of alpha glucose, its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily and release glucose quickly

Question 30

State the properties and functions of glycogen

Answer 30

A polysaccharide of alpha glucose. Its bonding is very similar to amylopectin except it has a lot more side branches linked by 1-6 glycosidic bonds. A lot of branches means that stored glucose can be released quickly which is important for energy release in animals. It is also a very compact molecule so it is good for storage.

Question 31

What is a reducing sugar?

Answer 31

A sugar that can donate electrons.

Question 32

How to carry out Benedict's test for reducing sugars?

Answer 32

1) Place sample to be tested in a boiling tube, either ground up or in liquid form. 2) Add equal volume in Benedict's reagant (a solution of copper sulfate) 3) Heat the mixture gently in a boiling water bath for five minutes

Question 33

Describe and explain the results of the Benedict's test for reducing sugars,

Answer 33

If reducing sugars are present, the blue solution will turn brick red. This is because the blue Cu3+ ions that were in the Benedict's reagent were reduced by the reducing sugar to Cu+ ions, which are brick red.