Carbohydrates

Question 1

What is the general formula for carbohydrates?

Answer 1

(CH2O)x

Question 2

Why are monosaccharides soluble in water?

Answer 2

• polar hydroxyl groups form hydrogen bonds with water making them soluble

Question 3

What are ‘hexose’ and ‘pentose’ sugars?

Answer 3

• hexose: 6 carbons

- pentose: 5 carbons

Question 4

How is a disaccharide formed from 2 monosaccharides?

Answer 4

• join together during a condensation reaction
• forms a 1,4 glycosidic bond
• water molecule produced
• hydroxyl group from one joins to a hydrogen from the other

Question 5

How is the disaccharide sucrose formed?

Answer 5

glucose + fructose

Question 6

How is the disaccharide lactose formed?

Answer 6

galactose + glucose

Question 7

How is the disaccharide maltose formed?

Answer 7

glucose + glucose

Question 8

How is glucose made available for respiration?

Answer 8

• 1,4 glycosidic bond is broken using a water molecule
• in a hydrolysis reaction
• catalysed by an enzyme

Question 9

Describe the difference between alpha and beta glucose

Answer 9

• alpha glucose on carbon 1 has the hydroxyl group below the ring
• beta glucose on carbon 1 has the hydroxyl group above the ring

Question 10

What is amylose?

Answer 10

• compact, insoluble
• angle of 1,4 glycosidic bond cause it to twist into helix
• made of alpha glucose

Question 11

What is amylopectin?

Answer 11

• every 25 glucose units, a condensation reaction occurs between a C1 and a C6 of two glucose molecules
• made of alpha glucose

Question 12

What is the structure of glycogen?

Answer 12

• every 6 glucose units glycogen branches due to 1,6 glycosidic bond

Question 13

What impact does glycogen having more branches than amylopectin have?

Answer 13

• glycogen has more ‘free ends’ where hydrolysis of glucose takes place
• so more glucose released per unit time for use in respiration

Question 14

What are the roles of glucose and how do its properties enable this?

Answer 14

• respiratory substrate
• building blocks for larger molecules, like starch, glycogen and cellulose
• polar so soluble in water (and cell cytoplasm) transport medium
• energy trapped within the C-H bonds
• hydroxyl groups can form glycosidic bonds with other monosaccharides

Question 15

What are the roles of starch and glycogen and how do its properties enable this?

Answer 15

• chemical energy store in plants (starch), animal and fungi (glycogen)
• helical structure (stabilised by H bonds) makes it compact (less space needed for it to be stored)
• insoluble (no osmotic effect)
• (amylopectin in starch) is branched so has many ‘free ends’ for adding or removing glucose (respiration)

Question 16

How is it possible for beta glucose to from 1,4 glycosidic bonds and what is the effect on the cellulose molecule?

Answer 16

• flipping every other glucose 180oC ensures OH groups are close enough to form bond
• effect is chain is straight and not coiled

Question 17

Where is cellulose found?

Answer 17

Plant cell walls

Question 18

Why is cellulose so strong?

Answer 18

• crosslinks (hydrogen bonds) between the cellulose molecules
• staggered ends offer more strength as there’s no weak points

Question 19

What makes up cellulose?

Answer 19

• lots of cellulose molecules makes the microfibril
• lots of microfibril makes the macrofibril
• lots of macrofibril make the cellulose fibres

Question 20

Explain how the structure of cellulose is related to its function?

Answer 20

• straight chain molecule
• many H bonds between individual chains
• staggered ends that give strength to fibres
• all make it good for strength in a plant cell wall

Question 21

Explain why beta glucose, when polymerised, leads to the production of cellulose instead of starch

Answer 21

• in beta glucose OH group at carbon 1 is above the ring
• so alternate molecules rotate 180
• this means OH groups are close enough to react
• in a condensation reaction forming a glycosidic bond
• produces a straight chain molecule: cellulose