Biological Molecules - Carbohydrates

Question 1

What is a polymer?

Answer 1

A large, complex molecule composed of long chains of monomers joined together.

Question 2

What is a monomer?

Answer 2

A small, basic molecular unit.

Question 3

What’s an example of a monomer?

Answer 3

Monosaccharides, amino acids, nucleotides,…

Question 4

Carbohydrates are made from monosaccharides.

All carbohydrates contain the elements C, H and O.

The monomers they’re made from are monosaccharides.
Give 3 examples of these monosaccharides.

Answer 4

Glucose, fructose, galactose…

Question 5

What is a monosaccharide with six carbon atoms called?

Answer 5

A hexose sugar.

Question 6

What are the two types of glucose?

Answer 6

Alpha (α) and beta (β).

Question 7

What are isomers?

Answer 7

Molecules with the same molecular formula as each other, but with the atoms connected in a different way.

Question 8

What’s the difference between alpha (α) glucose and beta (β) glucose?

Answer 8

The H and the OH are reversed.

H is at the top for alpha (α) and at the bottom for beta (β).

Question 9

What is a condensation reaction?

Answer 9

When two molecules join together with the formation of a new chemical bond, and a water molecule is released when the bond is formed.

Question 10

Monosaccharides are joined together by condensation reactions.

What bond is formed?
What is released?

Answer 10

A glycosidic bond is formed.

Water is released.

Question 11

Sucrose is a disaccharide. What is it made up of?

Answer 11

Glucose and fructose.

Question 12

Lactose is a disaccharide. What is it made up of?

Answer 12

Glucose and galactose.

Question 13

How are polymers broken down into monomers?

Answer 13

A hydrolysis reaction breaks the chemical bond between monomers using a water molecule.

For example, carbohydrates can be broken down into their constituent monosaccharides by hydrolysis reactions.

Question 14

Sugar is a general term for monosaccharides and disaccharides.
All sugars can be classified as reducing or non-reducing.

What is the test for reducing sugars?

Answer 14

Benedict’s Test:

• You add Benedict’s reagent (which is blue) to a sample and heat it in a water bath that’s been brought to the boil.
• If the test is positive it will form a coloured precipitate (solid particles suspended in the solution).

- The colour of the precipitate ranges from:
               blue - none
             green - trace
            yellow - low
           orange - moderate / medium
         brick red - high

• The higher the concentration of reducing sugar, the further the colour change goes – you can use this to compare the amount of reducing sugar in different solutions. A more accurate way of doing this is to filter the solution and way the precipitate.

Question 15

What is the main energy storage material in plants?

Answer 15

Starch

Question 16

What is starch made up of?

Answer 16

Two polysaccharides of alpha-glucose (amylose) and amylopectin.

Question 17

What is amylose?

Answer 17

A long, unbranched chain of a-glucose.

The angles of the glycosidic bonds
give it a coiled structure, almost like a cylinder.
This makes it compact,
so it’s really good for storage
because you can fit more in a small space.

Question 18

What is amylopectin?

Answer 18

A long, branched chain of a-glucose.

Its side branches
allow the enzymes that break down the molecule
to get at the glycosidic bonds easily.

Question 19

Why is starch good for storage?

Answer 19

Starch is insoluble in water
and doesn’t affect water potential
so it doesn’t cause water to enter cells by osmosis
which would make them swell.

Question 20

How do you test for starch?

Answer 20

Iodine test for starch:

Add iodine dissolved in potassium iodide solution to the test sample.
If there is starch present, the sample changes from a browny-orange to a dark, blue-black colour.

Question 21

What is the main storage material in animals?

Answer 21

Glycogen.

Animals get energy from glucose too,
but animals store excess glucose as glycogen
– another polysaccharide of alpha-glucose.

Question 22

What is the structure of glycogen?

Answer 22

Its structure is very similar to amylopectin,
except that it has loads more side branches coming off of it.
Loads of branches means that stored glucose can be released quickly,
which is important for energy release in animals.

It’s also a very compact molecule,
so it’s good for storage.

Question 23

What is the major component of cell walls in plants?

Answer 23

Cellulose.

Question 24

What is cellulose made up of?

Answer 24

Long, unbranched chains of beta-glucose.

Question 25

Why is cellulose good for cell walls?

Answer 25

When beta-glucose molecules bond,
they form straight cellulose chains.

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 (e.g. in plant cell walls).