AQA A Level Biology: Topic 1 Biological Molecules and Enzyme Reactions

Biological Molecules Overview

AQA A Level Biology
Topic 1
Biological molecules
Model answer notes by @biologywitholivia
Topic
Understand
Memorise
Practise
....

• 1 Monomers & polymers
• 2 Carbohydrates
• 3 Lipids
• 4.1 General properties of proteins
• 4.2 Many proteins are enzymes

Required practical 1

• 5.1 Structure of DNA and RNA
• 5.2 DNA replication
• 6 ATP
• 7 Water
• 8 Inorganic ions

Monomers and Polymers

AQA A Level Biology Topic 1 Biological molecules
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Defining Monomers and Polymers

What are monomers and polymers?

• Monomers - smaller / repeating molecules from which larger molecules / polymers are made
• Polymer - molecule made up of many identical / similar molecules / monomers

Condensation and Hydrolysis Reactions

What happens in condensation and hydrolysis reactions?

Condensation
reaction

• 2 molecules join together
• Forming a chemical bond
• Releasing a water molecule

Hydrolysis
reaction

• 2 molecules separated
• Breaking a chemical bond
• Using a water molecule

Examples of Polymers and Monomers

Give examples of polymers and the monomers from which they're made

Monomer
Polymer
Nucleotide
Condensation
Hydrolysis
Polynucleotide
(DNA or RNA)
Condensation
Monosaccharide
eg. glucose
Hydrolysis
Polysaccharide
eg. starch
Condensation
Amino acid
Hydrolysis
Polypeptide
(protein)

Common Mistakes in Monomers and Polymers

Exam insight: common mistakes

Mistake
Explanation

• Mixing up hydrolysis and
  condensation reactions .

'C' for condensation; 'C' for connecting molecules. Imagine
condensation on a window to remember water is released.

• Forgetting to include H2O in diagrams of
  condensation and hydrolysis reactions .

1 H2O molecule is released for every condensation reaction
and 1 H2O molecule is used for every hydrolysis reaction.

"Lipids are polymers."
Lipids are not made from repeating monomers.

"A polymer is made of two
or more monomers."
'Poly' means many. Two monomers joined
together is a dimer.

Carbohydrates

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Monosaccharides

What are monosaccharides? Give 3 common examples

• Monomers from which larger carbohydrates are made
• Glucose, fructose, galactose

Alpha-Glucose Structure

Describe the structure of a-glucose

CH,OH
H
C-
-O
H
6
H
C
.C.
4
OH
H
1
C.
C
OH
3
1 2
H
OH

• Left - full structure, carbon
  atoms labelled
• Right - simplified structure
  as in the specification to
  be memorised for exam

Alpha and Beta Glucose Differences

Describe the difference between the structure of a-glucose and @-glucose

OH group is below carbon 1 in a-glucose
but above carbon 1 in @-glucose

Alpha & beta glucose are isomers ->
same molecular formula, differently arranged atoms

Disaccharides Formation

What are disaccharides and how are they formed?

• Two monosaccharides joined together with a glycosidic bond
• Formed by a condensation reaction, releasing a water molecule

Common Disaccharides and Monosaccharides

List 3 common disaccharides & monosaccharides from which they're made

Disaccharide
Monosaccharides
Maltose
Glucose + glucose
Sucrose
Glucose + fructose
Lactose
Glucose + galactose

Joining Monosaccharides Diagram

Draw a diagram to show how two monosaccharides are joined together

Alpha glucose
Alpha glucose
Maltose
H
C
I
H
O
/H
H
-0
HH
-O
,H
+
+ H2O
Hydrolysis
O
HO
OH HO
OH
HO
OH
Glycosidic bond

Polysaccharides Formation

What are polysaccharides and how are they formed?

• Many monosaccharides joined together with glycosidic bonds
• Formed by many condensation reactions, releasing water molecules

3
H
-0
H
HO
HO
OH
AQA A Level Biology Topic 1 Biological molecules
Condensation

Starch and Glycogen

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Function and Structure of Starch and Glycogen

Describe the basic function and structure of starch and glycogen

Starch
Energy store in
plant cells

• Polysaccharide of a-glucose
• Amylose - 1,4-glycosidic bonds -> unbranched
• Amylopectin - 1,4- and 1,6-glycosidic bonds -> branched

Glycogen
Energy store in
animal cells

• Polysaccharide made of a-glucose
• 1,4- and 1,6-glycosidic bonds -> branched

Starch - amylose
Glycogen
4
HO
O
Starch - amylopectin
H H
0
0
1,4-glycosidic
bond
1,6-glycosidic
bond
0
0
H
H
H
H H
H H
H
HO
0
0
0
OH

Structure-Function Relationship of Starch and Glycogen

Explain how the structures of starch and glycogen relate to their functions

Starch
(amylose)

• Helical -> compact for storage in cell
• Large, insoluble polysaccharide molecule -> can't leave cell / cross cell membrane
• Insoluble in water -> water potential of cell not affected (no osmotic effect)

Glycogen
(and starch
amylopectin)

• Branched -> compact / fit more molecules in small area
• Branched -> more ends for faster hydrolysis -> release glucose for respiration to
  make ATP for energy release
• Large, insoluble polysaccharide molecule -> can't leave cell / cross cell membrane
• Insoluble in water -> water potential of cell not affected (no osmotic effect)

Cellulose

Function and Structure of Cellulose

Describe the basic function and structure of cellulose

Function

• Provides strength and structural support to plant / algal cell walls

Structure

• Polysaccharide of @-glucose
• 1,4-glycosidic bond -> straight, unbranched chains
• Chains linked in parallel by hydrogen bonds forming microfibrils

Structure-Function Relationship of Cellulose

Explain how the structure of cellulose relates to its function

• Every other ß-glucose molecule is inverted in a
  long, straight, unbranched chain
• Many hydrogen bonds link parallel strands
  (crosslinks) to form microfibrils (strong fibres)
• Hydrogen bonds are strong in high numbers
• So provides strength to plant cell walls

B-glucose 1,4-glycosidic bond
Hydrogen bond
4
O
O
0
AQA A Level Biology Topic 1 Biological molecules

Biochemical Tests for Sugars

AQA A Level Biology Topic 1 Biological molecules
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Test for Reducing Sugars

Describe the test for reducing sugars
Reducing sugars = monosaccharides, maltose, lactose

1. Add Benedict's solution (blue) to sample
2. Heat in a boiling water bath
3. Positive result = green / yellow / orange / red precipitate

Test for Non-Reducing Sugars

Describe the test for non-reducing sugars
Non-reducing sugars = sucrose

1. Do Benedict's test and stays blue / negative
2. Heat in a boiling water bath with acid (to hydrolyse into reducing sugars)
3. Neutralise with alkali (eg. sodium bicarbonate)
4. Heat in a boiling water bath with Benedict's solution
5. Positive result = green / yellow / orange / red precipitate

Measuring Sugar Quantity: Precipitation Method

Suggest a method to measure the quantity of sugar in a solution

• Carry out Benedict's test as above, then filter and dry precipitate
• Find mass / weight

Measuring Sugar Quantity: Colorimetry Method

Suggest another method to measure the quantity of sugar in a solution

1. Make sugar solutions of known concentrations
   (eg. dilution series)
2. Heat a set volume of each sample with a set
   volume of Benedict's solution for same time
3. Use colorimeter to measure absorbance (of
   light) of each known concentration
4. Plot calibration curve - concentration on x axis,
   absorbance on y axis and draw line of best fit
5. Repeat Benedict's test with unknown sample and
   measure absorbance
6. Read off calibration curve to find concentration
   associated with unknown sample's absorbance

1.0
Absorbance of
light / abritrary units
Absorbance of
unknown solution
0.8
0.6
0.4
×
0.2
Concentration
of unknown
solution
0
0
1
2
3
4
5
Concentration of sugar / mol dm -3

Biochemical Test for Starch

Describe the biochemical test for starch

1. Add iodine dissolved in potassium iodide (orange / brown) and shake / stir
2. Positive result = blue-black

None
Increasing quantity of sugar
5

Common Mistakes in Carbohydrate Biology

AQA A Level Biology Topic 1 Biological molecules
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Exam Insight: Carbohydrate Mistakes

Exam insight: common mistakes

Mistake
Explanation

• Referring to alpha glucose
  as 'a glucose' .

This is not an equivalent of writing 'alpha glucose' or
'a-glucose' and will usually be rejected.

"Glycogen and starch are energy stores as
hydrolysis of glycosidic bonds releases
energy."
Glycogen and starch are energy stores as they are made of
a-glucose, which is the substrate for respiration in cells. This
produces ATP for energy release.

• Comparing and contrasting
  polysaccharides by making a list about
  each .

This will achieve no marks. In these questions, each statement
needs to make a clear comparison. Aim to use a word like
'whereas' to ensure you're covering both sides.

When chains of beta glucose are linked by
hydrogen bonds, myofibrils form."
Myofibrils are found in muscle fibres and are completely
different. Microfibrils are found in cellulose cell walls.

"Cellulose is strong because of
hydrogen bonds."
Hydrogen bonds are weak individually, but strong in high
numbers. You need to say that there are many hydrogen bonds.

"Use the same amount of Benedict's
solution on each sample when comparing
the quantity of reducing sugar."
Amount is too vague. You need to use the
term volume to get the mark.

Lipids

Groups of Lipids

Name two groups of lipid
Triglycerides and phospholipids

Saturated fatty acid
Unsaturated fatty acid
H H H H H H H H H H H H H H H
O = C - ¿ - c - c - c - c - c - c - c - C - C - C - C - C - C - C - H
H-O-CES-
ИННИННИН
Н Н Н Н НАН
. { C . c . c . c . c - C - C ' C - C - C - C - C - C - C - C - H
Н Н Н Н Н НА
H-O-
Double bond

Fatty Acid Structure

Describe the structure of a fatty acid (RCOOH)

• Variable R-group - hydrocarbon chain (saturated or unsaturated)
• -COOH = carboxyl group

Saturated vs. Unsaturated Fatty Acids

Describe the difference between saturated and unsaturated fatty acids

• Saturated: no C=C double bonds in hydrocarbon chain; all carbons fully saturated with hydrogen
• Unsaturated: one or more C=C double bond in hydrocarbon chain (creating bend / kink)

Triglyceride Formation

Describe how triglycerides form

• 1 glycerol molecule and 3 fatty acids
• Condensation reaction
• Removing 3 water molecules
• Forming 3 ester bonds

Glycerol
3 fatty acids
Triglyceride
H
0
H -C
OH
HO
c
I
O-C
0
+ H2O
H -
OH
HO
c
H -C
0-C
O
O
H -C
OH
HO
C
H -C
0-C
H
H Ester bond
O
+ H2O
I-0-
O
+ H2O
6

Triglycerides and Phospholipids

AQA A Level Biology Topic 1 Biological molecules
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Triglyceride Properties and Structure

Explain how the properties of triglycerides are related to their structure

Function: energy storage

• High ratio of C-H bonds to carbon atoms in hydrocarbon chain
• So used in respiration to release more energy than same mass of carbohydrates
• Hydrophobic / non-polar fatty acids so insoluble in water (clump together as droplets)
• So no effect on water potential of cell (or can be used for waterproofing)

Triglyceride and Phospholipid Structure Differences

Describe the difference between the structure
of triglycerides and phospholipids

Triglyceride
Glycerol
Glycerol
Fatty acid
Fatty acid
Fatty acid
Fatty acid
Fatty acid
Phospholipid
Phosphate
group
One of the fatty acids of a triglyceride is substituted
by a phosphate-containing group

Phospholipid Properties and Structure

Describe how the properties of
phospholipids relate to their structure

Function: form a bilayer in cell membrane, allowing diffusion of lipid-soluble (non-polar) or very small
substances and restricting movement of water-soluble (polar) or larger substances

• Phosphate heads are hydrophilic
• Attracted to water so point to water (aqueous environment) either side of membrane
• Fatty acid tails are hydrophobic
• Repelled by water so point away from water / to interior of membrane

Test for Lipids

Describe the test for lipids

1. Add ethanol, shake (to dissolve lipids), then add water
2. Positive = milky white emulsion

Common Mistakes in Lipid Biology

Exam insight: common mistakes

Mistake
Explanation

"Phospholipids have a phosphorus group."
Phospholipids have a phosphate group (PO23-).

"Phospholipids don't contain glycerol."
Both phospholipids and triglycerides contain glycerol.

"To test for lipids, add water then ethanol."
Ethanol is added first and this is crucial to pick up the mark.

"A positive test for lipids is cloudy."
This is too vague. You need to use the term 'emulsion'.

"Phospholipids have phosphodiester bonds."
Phospholipids have ester bonds. Phosphodiester bonds are
found in nucleic acids such as DNA and RNA.

"In the test for lipid, ethanol is heated."
Only Benedict's solution (for sugars) requires heating.
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