1.1 Biological Molecules- Carbohydrates — AQA A Level Biology 7402 Revision Notes & Topical Questions

Section 1 of 28Monomers and polymers

There is great variety of life within and between organisms but the biochemical basis is similar for all living things.

Monomers and polymers

The key molecules needed to build structures and enable organisms to work are carbohydrates, lipids, proteins and nucleic acids.

Monomers make up polymers, which make up living organisms.

When discussing them, give the definition and name examples, e.g. a nucleic acid is a polymer made of nucleotide monomers.

Section 2 of 28Condensation and hydrolysis

Many biological reactions involve forming covalent bonds, which add strength and structure to a molecule.

Two reactions are important.

Condensation reaction

In a condensation reaction a covalent bond forms between two molecules with the loss of a water molecule.

Hydrolysis

In hydrolysis ('hydro' = with water, 'lyse' = to break) water is added to break a covalent bond.

You must be able to recognise and identify the location of covalent bonds in molecules and draw them, even if the molecules are unfamiliar.

Section 3 of 28Carbohydrates and sugars

Carbohydrates are one of the main carbon-based compounds in living organisms.

All contain carbon, hydrogen and oxygen.

Glucose

Monosaccharides are the simplest sugars; the most familiar monomer is glucose, molecular formula C₆H₁₂O₆.

Glucose has two isomers, α-glucose and β-glucose, which differ in the position of the OH group on carbon 1 (below the ring in α, above in β).

Different polysaccharides are formed from the two isomers, and you must be able to recognise and draw both.

Section 4 of 28Reducing and non-reducing sugars

Sugars are classed as reducing or non-reducing according to whether they can donate electrons.

Reducing and non-reducing sugars

Reducing sugars (all monosaccharides and some disaccharides such as maltose and lactose) can be oxidised and therefore give a positive Benedict's test.

Non-reducing sugars (for example sucrose) cannot donate electrons so cannot be oxidised.

To detect them, the disaccharide must first be hydrolysed to its two monosaccharides (by boiling with dilute acid,

then neutralising),
and then Benedict's reagent is used.

Section 5 of 28The glycosidic bond

Disaccharides and polysaccharides form when two hydroxyl (-OH) groups on different saccharides interact to create a strong covalent glycosidic bond, with the loss of water. Each glycosidic bond is catalysed by enzymes specific to the OH groups involved. Sucrose is a disaccharide formed by condensation of a glucose molecule and a fructose molecule.

Section 6 of 28Worked examples

Define the terms monomer and polymer, giving one example of each pair. [3]3 marks
State two differences between α-glucose and β-glucose. [2]2 marks
Explain why sucrose gives a negative Benedict's test but a positive result after boiling with acid. [3]3 marks

Section 7 of 28Forming disaccharides

Monosaccharides join via condensation reactions to form disaccharides; the new bond between them is a glycosidic bond. Examples: sucrose is formed by condensation of α-glucose and β-fructose; lactose by condensation of glucose and galactose; maltose from two glucose units. Make sure you can identify where the glycosidic bond is in a carbohydrate for these specific examples.

Section 8 of 28Forming polysaccharides

Polysaccharides are formed by the condensation of many glucose units. Glycogen and starch are formed by condensation of α-glucose, giving glycosidic bonds between carbons 1 and 4 (and, at branch points, 1 and 6), as in amylopectin.

Section 9 of 28Hydrolysis of glycosidic bonds

The glycosidic bond is broken when water is added in a hydrolysis reaction. One molecule of sucrose hydrolyses to one glucose and one fructose molecule. Disaccharides hydrolyse to two monosaccharides, whereas polysaccharides must undergo many hydrolysis reactions until they form monosaccharides.

Section 10 of 28Paper chromatography of monosaccharides

Chromatography separates a mixture into its components using differences in the solubility of the different chemicals (solutes) in the solvent.

The paper is the stationary phase and the solvent is the mobile phase.

A spot of the mixture is placed on a pencil origin line above the solvent; as the solvent rises, the more soluble solutes travel further.

Chromatogram

Paper chromatography is the overall technique; a chromatogram is the visual output, the paper with visibly separated components after the run.

A mixture of monosaccharides can be separated,

and individual sugars identified by comparing their R_f values with known standards.
- (distance moved by the spot divided by the distance moved by the solvent front)

The sugars are colourless, so the paper must be developed with a locating agent.

Section 11 of 28Starch and glycogen

Starch and glycogen are polysaccharides that store glucose as energy: starch in plants, glycogen in animals and fungi. Both are made of α-glucose and are compact and insoluble, so they do not affect water potential or diffuse out of cells.

Section 12 of 28Why condensation and hydrolysis matter in digestion

In digestion, enzymes catalyse the hydrolysis of glycosidic bonds so large insoluble carbohydrates can be absorbed.

1

Different enzymes are specific to different bonds: amylase hydrolyses the 1,4 bonds in starch; maltase hydrolyses maltose into two glucose molecules; sucrase hydrolyses sucrose; lactase hydrolyses lactose to glucose and galactose.

2

Lactose intolerance results when lactase is lacking.

3

The reverse process, condensation, occurs when cells build storage polysaccharides from glucose, and each step releases one water molecule.

4

In an equation, n glucose molecules joined as a chain release (n − 1) water molecules.

Section 13 of 28Worked examples

Name the monosaccharides in sucrose and lactose. [2]2 marks
Describe how a mixture of monosaccharides is separated and identified using paper chromatography. [4]4 marks
Explain why a polysaccharide needs many hydrolysis reactions to produce monosaccharides. [2]2 marks

Section 14 of 28Starch

Starch is the storage polysaccharide of plants and is made of two polysaccharides of α-glucose. Amylose comprises 10-30% of starch; it is an unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules. The helix makes it compact and therefore more resistant to digestion. Amylopectin is branched, with 1,4 bonds in chains and 1,6 bonds at branch points, so the many ends allow enzymes to hydrolyse it rapidly.

Section 15 of 28Glycogen

Glycogen is the storage polysaccharide of animals and fungi. It is highly branched and not coiled, more branched than amylopectin. Liver and muscle cells have a high concentration of glycogen, present as visible granules, because the cellular respiration rate is high in these cells (animals are mobile). Its many branches give many ends from which glucose is quickly released.

Section 16 of 28Cellulose

Cellulose is a polysaccharide found in plant cell walls.

1

It consists of long, straight, unbranched chains of β-glucose joined by 1,4 glycosidic bonds.

2

Since β-glucose is an isomer of α-glucose, consecutive β-glucose molecules must be rotated 180° to each other to form the 1,4 bonds.

3

Parallel chains are held together by many hydrogen bonds into microfibrils,

which give cellulose high tensile strength: it can be stretched without breaking,
so cell walls can withstand turgor pressure.

4

The fibres and other molecules such as lignin form a matrix that increases strength and gives the plant support.

5

Its strength and insolubility make it suitable for building cell walls.

Section 17 of 28Comparison

Feature	Starch	Glycogen	Cellulose
Monomer	α-glucose	α-glucose	β-glucose
Bonds	1,4 (and 1,6 in amylopectin)	1,4 and many 1,6	1,4 only
Shape	Helix / branched	Highly branched	Straight chains in microfibrils
Found in	Plants	Animals and fungi	Plant cell walls
Function	Energy storage	Energy storage	Structural support

Section 18 of 28Biochemical tests for sugars and starch

These tests are qualitative.

1

For reducing sugars add Benedict's reagent and heat: blue changes towards red (green, yellow, orange, brick red) as the concentration increases.

2

Non-reducing sugars lack the groups needed and cannot react directly with the oxidising agent, so they must be hydrolysed first.

3

For starch add a few drops of orange/brown iodine in potassium iodide solution (iodine is insoluble in water); iodide ions interact with the centre of the starch molecules and give a blue-black complex.

4

This test is useful to show that starch has been digested by enzymes.

Section 19 of 28Iodine test mechanism and digestion

In the iodine test the amylose helix traps iodine molecules/iodide ions in its centre, which is why a deep blue-black colour forms; branched amylopectin and glycogen give weaker red-brown or purple colours.

Iodine test mechanism and digestion

Starch is digested to maltose by amylase, and the disappearance of the blue-black colour shows this.

Because starch is insoluble it does not alter the water potential of cells, and because it is compact large amounts can be stored in a small space,

whereas the branched structure of glycogen means more glucose can be released rapidly, matching the faster metabolic rate of animals.

Section 20 of 28Worked examples

Explain how the structure of glycogen makes it suitable as an energy store. [3]3 marks
Explain why cellulose is a good structural molecule. [3]3 marks
State what a positive iodine test shows and describe the colour change. [2]2 marks

Section 21 of 28Semi-quantitative Benedict's test

Benedict's reagent identifies reducing sugars such as glucose in a semi-quantitative way.

Semi-quantitative Benedict's test

Reducing sugars reduce the blue copper(II) sulfate in the reagent to a brick-red precipitate of copper(I) oxide; the more reducing sugar, the more precipitate and the colour moves from blue through green, yellow and orange to brick red.

Judging colour by eye is subjective, which is a source of error.

Section 22 of 28Improving accuracy with a colorimeter

To avoid human interpretation of colour, a colorimeter gives a quantitative measure of colour intensity.

Improving accuracy with a colorimeter

It passes light of a specific wavelength (for example blue light for a blue solution or a suitable filter) through a solution of known concentration and measures the absorbance or the percentage transmission of light.

After the test the precipitate is filtered or centrifuged and the colour of the remaining solution is measured; the more reducing sugar was present, the less blue is left,

so absorbance falls and transmission rises.

Section 23 of 28Producing a calibration curve

1

Prepare a range of standard glucose solutions of known concentration by serial dilution from a stock solution.

2

Carry out the Benedict's test on each standard under identical conditions (same volume, temperature and time).

3

Measure the absorbance (or transmission) of each standard and the unknown sample, using distilled water as the blank to set the colorimeter to zero.

4

Plot a calibration curve of absorbance or transmission against known glucose concentrations.

5

Locate the unknown's reading on the curve and read off the corresponding concentration. This is called interpolation.

Serial dilutions prepare standard solutions for comparison, for example diluting by a factor of ten at each step.

Section 24 of 28Why this is better

This method minimises subjective interpretation of colour changes and gives more reliable, quantitative data.

Why this is better

Repeats allow means and the identification of anomalies.

Improvements: use the same volumes, control time and heating temperature, use a water bath rather than a flame, and use a wider range of standards.

The calibration curve must cover the unknown's value; extrapolating beyond the curve is unreliable.

Section 25 of 28Worked interpretation of a calibration curve

Suppose standards of 0, 2, 4, 6 and 8 mmol dm^-3 give transmission of 40, 52, 64, 76 and 88%.

1

The graph is a straight line with increasing transmission as glucose concentration increases.

2

An unknown with 58% transmission lies halfway between 2 and 4 mmol dm^-3, so its concentration is about 3 mmol dm^-3.

3

State your answer with units and note that the estimate is only valid if the unknown reading lies within the range of the standards.

4

To prepare standards by serial dilution, take 10 cm³ of stock solution and add 90 cm³ of distilled water for a tenfold dilution,

then repeat on the diluted solution for each further tenfold step.

Section 26 of 28Worked examples

Describe how you would estimate the glucose concentration of an unknown sample. [5]5 marks
As glucose concentration increases, what happens to the absorbance of the filtrate? Explain. [2]2 marks
Give one advantage of a colorimeter over visual comparison. [1]1 mark

Section 27 of 28Key terms

Term	Meaning
Condensation reaction	Reaction joining molecules with loss of water
Hydrolysis	Reaction using water to break a bond
Glycosidic bond	Covalent bond between two saccharides
Isomer	Molecule with the same formula but a different arrangement
Non-reducing sugar	Sugar that cannot donate electrons, for example sucrose

Term	Meaning
Disaccharide	Two monosaccharides joined by a glycosidic bond
Polysaccharide	Many monosaccharides joined by glycosidic bonds
Chromatogram	Paper showing the separated components after chromatography
R_f value	Distance moved by a solute divided by distance moved by the solvent
Sucrose	Disaccharide of glucose and fructose

Term	Meaning
Amylose	Unbranched helical polymer of α-glucose in starch
Amylopectin	Branched polymer of α-glucose in starch
Glycogen	Highly branched storage polysaccharide of animals and fungi
Cellulose	Polymer of β-glucose forming plant cell walls
Microfibril	Bundle of parallel cellulose chains

Term	Meaning
Colorimeter	Instrument measuring absorbance or transmission of light
Calibration curve	Graph of readings for standards of known concentration
Interpolation	Estimating an unknown value from within the curve
Serial dilution	Repeated dilution by a constant factor
Semi-quantitative	Giving an approximate rather than exact amount

Section 28 of 28Exam technique

1

Saying hydrolysis removes water; it adds water.

2

Writing the formula of glucose as C₆H₁₂O₁₂.

3

Thinking all disaccharides are non-reducing.

4

Placing the OH on the wrong side of carbon 1 in the isomer drawings.

1

Drawing the origin line in ink; it dissolves in the solvent.

2

Letting the solvent level cover the spots.

3

Saying one hydrolysis reaction breaks down a polysaccharide completely.

4

Confusing the chromatogram with the technique.

1

Saying cellulose is made of α-glucose.

2

Describing amylose as branched.

3

Writing that glycogen is found in plants.

4

Forgetting the rotation of alternate β-glucose units by 180°.

1

Reading the unknown's concentration by extrapolation outside the standards.

2

Using different volumes or heating times in the standards.

3

Saying more reducing sugar gives a bluer solution.

4

Forgetting a blank or zero reading for the colorimeter.