Carbohydrates

Question 1

Give examples of Monomers

Answer 1

monosaccharides,
amino acids
and
nucleotides

Question 2

What is a saccharide?

Answer 2

In carbohydrates, the basic monomer unit is a sugar, otherwise known as a saccharide.

Question 3

What is a monosaccharide?

Answer 3

A single monomer is called a monosaccharide

Monosaccharides are sweet-tasting, soluble substances that have the general formula (CHO), where n can be any number from three to seven.

Question 4

What is a disaccharide?

Answer 4

A pair of monosaccharides can be combined to form a disaccharide.

Question 5

How can Polysaccharides be made?

Answer 5

Monosaccharides can also be combined in much larger numbers to form polysaccharides.

Question 6

Give examples of monosaccharides

Answer 6

glucose, galactose and fructose

Question 7

What is glucose?

Answer 7

Glucose is a hexose (6-carbon) sugar and has the formula C6H12O6.

glucose has two isomers - a-glucose and ẞ-glucose.

Question 8

Give examples of reducing sugars

Answer 8

All monosaccharides and some disaccharides (e.g., maltose) are reducing sugars

Question 9

What is reduction (in terms of electrons), and what does it mean in the context of reducing sugar?

(so what is a reducing sugar?)

Answer 9

Reduction is a chemical reaction involving the gain of electrons or hydrogen.

A reducing sugar is therefore a sugar that can donate electrons to (or reduce) another chemical, in this case Benedict’s reagent.

The test for a reducing sugar is therefore known as the Benedict’s test.

Question 10

What is Benedicts reagent? (the chemical)

Answer 10

Benedict’s reagent is an alkaline solution of copper(II) sulfate

Question 11

What happens whena reducing sugar is heated with Benedict’s reagent?

Answer 11

When a reducing sugar is heated with Benedict’s reagent it forms an insoluble red precipitate of copper(I) oxide

Question 12

Describe the process of testing for reducing sugars.

Answer 12

• Add 2 cm3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up in water.
• Add an equal volume of Benedict’s reagent.
• Heat the mixture in a gently boiling water bath for five minutes.

Question 13

Maltose

Answer 13

Glucose + Glucose

Question 14

Sucrose

Answer 14

Glucose + Fructose

Question 15

Lactose

Answer 15

Glucose + Galactose

Question 16

Give 3 examples of Disaccharides

Answer 16

Maltose
Sucrose
Lactose

Question 17

What happens when monosaccharides join?

Answer 17

A molecule of water is removed, which is called condensation reaction.

The bond that is formed is called glycosidic bond

Question 18

What does hydrolysis do to a disaccharide?

Answer 18

When water is added, the glycosidic bond breaks releasing the constituent monosaccharides

Question 19

What is hydrolysis?

Answer 19

Additional of water that causes a breakdown

Question 20

What are polysaccharides?

Answer 20

Polymers formed by combining many monosaccharides.

These are joined by glycosidic bonds which were formed in condensation reactions.

Question 21

Why are polysaccharides suitable for storage?

Answer 21

Polysaccharides are very large molecules, which makes them insoluble.

This feature makes them suitable for storage.

Question 22

What happens when polysaccharides are hydrolysed?

Answer 22

They break down into disaccharides or monosaccharides

Question 23

Name a polysaccharide that is not used to storage but another function.

Hint: it’s in plants

Answer 23

Cellulose - to give structural strength/support in plant cells

Question 24

What is starch?

Answer 24

A polysaccharide, found in the form of starch granules/grains

Eg starch grains in chloroplasts

It formed by joining 200 - 100,000 a-glucose molecules by glycosidic bonds in a series of condensation reactions

Question 25

Some disaccharides (e.g. maltose) are reducing sugars. To detect these we use the Benedict’s test.

Other disaccharides, such as sucrose, are known as non-reducing sugars because they do not change the colour of Benedict’s reagent when they are heated with it.

In order to detect a non-reducing sugar it must first be hydrolysed into its monosaccharide components by hydrolysis.

Describe this process.

Answer 25

• If the sample is not already in liquid form, it must first be ground up in water.
• Add 2 cm3 of the food sample being tested to 2 cm3 of Benedict’s reagent in a test tube and filter.
• Place the test tube in a gently boiling water bath for 5 minutes. If the Benedict’s reagent does not change colour (the solution remains blue), then a reducing sugar is not present.
• Add another 2 cm3 of the food sample to 2 cm3 of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for five minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides.

-Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. (Benedict’s reagent will not work in acidic conditions.) Test with pH paper to check that the solution is alkaline.

• Re-test the resulting solution by heating it with 2 cm3 of Benedict’s reagent in a gently boiling water bath for five minutes.
• If a non-reducing sugar was present in the original sample, the Benedict’s reagent will now turn orange-brown. This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.

Question 26

Starch is easily detected by its ability to change the colour of the iodine
in potassium iodide solution from yellow to blue-black.

Explain the test for starch.

Answer 26

The test is carried out at room temperature.

Steps:

• Place 2 cm3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile).
• Add two drops of iodine solution and shake or stir.
• The presence of starch is indicated by a blue-black coloration.

Question 27

Where does starch occur mainly and why is it important?

Answer 27

Starch is a polysaccharide that is found in many parts of a plant in the form of small grains.

Especially large amounts occur in seeds and storage organs, such as potato tubers.

It forms an important component of food and is the major energy source in most diets.

Starch is never found in animal cells.

Question 28

Describe the structure of the polysaccharide Starch.

Answer 28

Starch is made up of chains of a-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.

The chains may be branched or unbranched.

The unbranched chain is wound into a tight coil that makes the molecule very compact.

Question 29

The main role of starch is energy storage.

Describe how starch is structure is suited for this.

Answer 29

• it is insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis
• being large and insoluble, it does not diffuse out of cells
• it is compact, so a lot of it can be stored in a small space
• when hydrolysed it forms a-glucose, which is both easily transported and readily used in respiration
• the branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly.

Question 30

Where is Glycogen found?

What is it’s structure like?

Answer 30

Glycogen is found ONLY in animals and bacteria

• shorter chains (than starch)
• more highly branched (than starch)

Question 31

Where is glycogen STORED in animals?

Answer 31

In animals it is stored as small granules mainly in the muscles and the liver.

The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals.

Question 32

How is glycogen structure suited for storage?

Answer 32

• it is insoluble and therefore does not tend to draw water into the cells by osmosis
• being insoluble, it does not diffuse out of cells
• it is compact, so a lot of it can be stored in a small space
• It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active.

Question 33

How is cellulose different for starch/glycogen?

Answer 33

Cellulose differs from starch and glycogen in one major respect: it is made of monomers of ẞ-glucose rather than a-glucose.

This seemingly small variation produces fundamental differences in the structure and function of this polysaccharide.

Question 34

Describe the structure of a Cellulose molecule

Answer 34

• has straight, unbranched chains.
• These run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains.

While each individual hydrogen bond adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose, making it the valuable structural material that it is.

Question 35

What are microfibrils?

Answer 35

The cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres

Question 36

Why is cellulose important in plant cells?

Answer 36

Cellulose is a major component of plant cell walls and provides rigidity to the plant cell.

The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It does this by exerting an inward pressure that stops any further influx of water.

As a result, living plant cells are turgid and push against one another, making non-woody parts of the plant semi-rigid.

This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.

Question 37

The structure of cellulose is suited to its function of providing support and rigidity because:

Answer 37

-cellulose molecules are made up of B-glucose and so form long straight, unbranched chains

• these cellulose molecular chains run parallel to each other and are crossed linked by hydrogen bonds which add collective strength
• these molecules are grouped to form microfibrils which in turn are grouped to form fibres all of which provides yet more strength.