3.3 Carbohydrates

Question 1

What are carbohydrates?

Answer 1

Carbohydrates are molecules that only contain the elements carbon, hydrogen and oxygen.

Question 2

What is glucose?

Answer 2

Glucose is a monosaccharide with six carbons and therefore is a hexose monosaccharide.

Question 3

What are the two forms of glucose? What are the differences between them?

Answer 3

Alpha glucose and beta glucose.
Alpha glucose has H on the top and OH on the bottom on the carbon-1, whereas beta glucose has OH at the top and H at the bottom.

Question 4

Is glucose soluble?

Answer 4

Yes.

1) Its solubility in water is important because it means glucose is dissolved in the cytosol of the cell.
2) It is also easily transported, and its chemical bonds contain lots of energy relating to its function as the main energy source in animals and plants.

Question 5

What bonds join monosaccharides together?

Answer 5

Glycosidic bonds.

Question 6

What is a condensation reaction?

Answer 6

When a molecule of water is released due to a hydrogen atom on one monosaccharide bonding to a hydroxyl group on the other.

Question 7

What is the reverse of synthesis?

Answer 7

Hydrolysis. A molecule of water reacts with the glycosidic bond, breaking it apart.

Question 8

What is a monosaccharide?

Answer 8

A single sugar unit is called a monosaccharide. E.g glucose, fructose and galactose

Question 9

How is a disaccharide formed?

Answer 9

It is formed when two monosacchrides link together.

E.g two a-glucose molecules are joined together by a 1-4 glycosidic bond to form maltose.

Question 10

Other disaccharides & their formation.

Answer 10

1) Sucrose is a disaccharide formed when a-glucose and fructose join together.
2) Lactose is a disaccharide formed by the joining together of either a-glucose or b-glucose and galactose.
Fructose is sweeter than glucose and glucose is sweeter than galactose.

Question 11

What are pentose monosaccharides?

Answer 11

Monosaccharides that contain five carbon atoms.
E.g ribosome- the sugar present in RNA nucleotides
Deoxyribose- The sugar present in DNA nucleotide

Question 12

How are polysaccharides formed?

Answer 12

A polysaccharide is formed when more than two monosaccharides join together e.g starch, glycogen and cellulose

Question 13

What is starch?

Answer 13

1) Starch is the main energy storage material in plants.
2) Cells get energy from glucose. Plants store excess glucose as starch and when a plant needs more glucose for energy it breaks down starch to release the glucose.

Question 14

Which two polysaccharides is starch a mixture of?

Answer 14

Starch is a mixture of two polysaccharides of alpha-glucose - amylose and amylopectin.

Question 15

What is amylose?

Answer 15

1) A long, unbranched chain of a-glucose.
2) Amylose is formed by alpha glucose molecules joined together only by 1-4 glycosidic bonds.
3) The angles of the glycosidic bonds give it a coiled structure making it compact and good for storage.

Question 16

What is amylopectin?

Answer 16

1) A long, branched chain of a-glucose.
2) It is also made by 1-4 glycosidic bonds between alpha glucose molecules, but in amylopectin there are also some 1-6 glycosidic bonds.
3) Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly.

Question 17

Is starch soluble?

Answer 17

No, it is insoluble in water so it doesn’t cause the water to enter cells by osmosis as it would make them swell. This makes it good for storage.

Question 18

What is glycogen?

Answer 18

1) Glycogen is the main energy storage material in animals.
2) Animals store excess glucose as glycogen.
3) Its structure is very similar to amylopectin, yet glycogen forms more branches than amylopectin. This means it is more compact and less space is needed for it to be stored.
4) More branches means that stored glucose can be released quickly, which is important for energy release in animals.

Question 19

What is cellulose?

Answer 19

1) Cellulose is the major component of cell walls in plants.
2) Cellulose is made of long, unbranched chains of beta-glucose.
3) When beta-glucose molecules bond, they form straight cellulose chains.
4) Beta glucose molecules are unable to join together in the same way that alpha glucose molecules can as the hydroxyl groups on carbon 1 and 4 are too far from each other to react. So an alternate beta glucose molecule is turned upside down.
5) The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose provides structural support for cells.