Carbohydrates

Question 1

Reducing sugars

Answer 1

can donate electrons

Question 2

Non-reducing sugars

Answer 2

cannot donate electrons

Question 3

Glucose exists in two structurally different forms

Answer 3

alpha (α) glucose and beta (β) glucose and is therefore known as an isomer

Question 4

What is the difference between the two glucoses?

Answer 4

structural variety results in different functions between carbohydrates

Question 5

Define the term isomer?

Answer 5

Organic molecules that have the same chemical formula but different structural formula

Question 6

Disaccharides and polysaccharides are formed

Answer 6

When two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond called the glycosidic bond

Question 7

Why Form Glycosidic Bonds

Answer 7

-More suitable for transport & storage
-Has less influence on a cells’ osmolality

Question 8

What happens when forming glycosidic bonds?

Answer 8

-Every glycosidic bond results in one water molecule being removed, thus glycosidic bonds are formed by condensation

-Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting

Question 9

Common examples of disaccharides include:

Answer 9

Maltose
Sucrose
Lactose

Question 10

Forming Maltose

Answer 10

Two molecules of glucose form a glycosidic bond by condensation to form maltose (a reducing sugar)

Question 13

Forming lactose

Answer 13

One molecule of glucose and one molecule of galactose form a glycosidic bond by condensation to form lactose (a reducing sugar)

Question 14

Polysaccharides

Answer 14

Polysaccharides are macromolecules that are polymers formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form chains

Question 15

Polysaccharides chains can be

Answer 15

Branches or unbranched
Folded - ideal for compact molecules - storage
Straight or coiled - structural

Question 16

Example of Polysaccharides

Answer 16

Starch, Glycogen & Cellulose

Question 17

Starch and glycogen are storage polysaccharides because they are

Answer 17

Compact (so large quantities can be stored)
Insoluble (so will have no osmotic effect)*

Question 18

Glycogen

Answer 18

-Glycogen is the storage polysaccharide of animals & fungi

-It is branched & not coiled - more compact
Often stored as granules in liver and muscle cells

-Branching allows glucose molecules to be easily added or removed

Question 19

Difference between structure of starch and glycogen?

Answer 19

Both made of the same monomer
both are branched molecules
Both contain 1,4 and 1,6 glycosidic bonds

Starch is made of amylose and amylopectin
Glycogen is more highly branched

Question 20

Cellulose: Structure

Answer 20

Cellulose is a structural polysaccharide

It’s structure: long chains of β-glucose joined together by 1,4 glycosidic bonds

Consecutive β-glucose molecules are inverted

Question 22

Due to the inversion of the β-glucose molecules

Answer 22

Many hydrogen bonds form between the long chains giving cellulose it’s strength

Question 23

Cellulose: Function

Answer 23

-The main structural component of cell walls

-Very strong & insoluble (due to the many H bonds between cellulose fibres)

-Cell walls are able to withstand turgor pressure

-Lignin further increases the strength & support to the plant

-Cellulose fibres are freely permeable

Question 24

The Benedict’s test for reducing sugars

Answer 24

The Benedict’s test for reducing sugars produces a colour change from blue towards brick-red if a reducing sugar is present

Question 25

Amylopectin

Answer 25

Branched - easily broken down for use in respiration

Question 26

Amylose

Answer 26

Unbranched & helix-shaped - more compact

Question 27

The Benedict’s test for non-reducing sugars

Answer 27

1)Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil

2)addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules

3)Neutralise with sodium hydrogencarbonate

4) Carry out the Benedict’s test as normal

Question 28

The iodine test for starch

Answer 28

1)add a few drops of orange/brown iodine in potassium iodide solution to the sample

2) Starch is present? - iodine ions in the solution interact with starch molecules → form a blue-black colour