AQA A level Biology Paper 1-2-3
,AQA A level Biology Paper 1-2-3
What are the useful properties of water and why ?
1) Water is a metabolite - in many metabolic reactions including condensation and hydrolysis
2) Water is a solvent - medium in which enzyme catalysed reactions take place (metabolic reactions )
and readily dissolves substances like inorganic ions ( as water is dipolar so totally surrounds the ions) ,
excretory products (urea/ammonia) , respiratory gases and small hydrophilic molecules ( amino acids ,
etc)
3) Water has a high latent heat of vaporisation (lots of energy required to break hydrogen bonds
between water molecules so a lot of energy is used up when water evaporates ) so helps with
temperature control as evaporation of sweat cools humans down
4) Water has a high specific heat capacity ( to separate the many hydrogen bonds requires a lot of
energy) so acts as a buffer and resists temperature changes to maintain a constant internal body
temperature
5) Water has large cohesive forces ( as hydrogen bonds stick together) so allows transportation of
substances( water travelling in columns in xylem) and means water has a high surface tension when it
comes into contact with air ( allowing pond skaters to walk on its surface)
Why is water dipolar ? - As water is made up of two hydrogen atoms and one oxygen atom , the
hydrogen atoms have a slightly positive pole and the oxygen atom has a slightly negative pole making
water dipolar.
Hydrogen bonds - Slightly negatively charged oxygen atom is attracted to the slightly positively
charged hydrogen atom of other water molecules. The attraction between opposite charges is called
hydrogen bonding, which are weak bonds but strong in large numbers. This makes water a major
component in cells
Hydrolysis - Breaks a bond between two molecules by the addition of a molecule of water
Condensation - Joins two molecules together with the formation of a bond by the removal of a
molecule of water
Polymer - A molecule ( carbohydrate, nucleotide or protein) made up of many monomers ( small
repeating units) joined together by condensation reactions and broken down my hydrolysis
,AQA A level Biology Paper 1-2-3
Monosaccharides - Are the monomers from which larger carbohydrates are made and contain the
elements C H and O. They are sweet and soluble and there are three types : fructose , glucose and
galactose. Hexose sugars (glucose) have the formula C6H12O6
Glucose is the product of photosynthesis and a major substrate of respiration , draw the two
structural isomers of glucose in long and short hand and state the difference between them - On
carbon 1 in beta glucose the hydroxyl group (OH) points upwards and in alpha glucose the hydroxyl
group points down on carbon 1
Sucrose - Glucose and fructose
Maltose - Glucose and glucose
Lactose - Glucose and galactose
How do monosaccharides join together - By condensation to form a glycosidic bond between two
monosaccharides and a disaccharide and a molecule of water are formed
Disaccharide - Two monosaccharides joined together by condensation forming a glycosidic bond.
There are three types : maltose, sucrose and lactose. They have the chemical formula C12H22O11
Draw the hydrolysis of maltose in structural formula and short hand formula -
Test for starch - Add iodine solution to the test sample , if starch is present colour changes from
yellow to a blue-black colour.
Test for a reducing sugar - Heat with Benedict's reagent ( blue) and if reducing sugar is present a brick
red precipitate will form
Issues with the Benedict's test ? - 1) It is non specific - doesn't tell which reducing sugar is present , a
biosensor can be used to detect for specific sugars
2) It's qualitative so colour change is being measured so can't obtain concentration of reducing sugar
but allows an estimate for how much is present in the strength of the colour
, AQA A level Biology Paper 1-2-3
3) Is subjective
How to quantify results on the concentration of unknown glucose concentration? - Use a colorimeter,
as a serial dilution of known glucose concentrations can be measured producing a set of solutions
with known concentrations . The Benedict's test can be performed with each of these and the colour
intensity measured. A CALIBRATION CURVE can be produced from the results and used to read iff the
concentration of glucose in an unknown solution.
Test for a non-reducing sugar ? - Heat the sample with Benedict's reagent and if there is no colour
change a reducing sugar is not present but a non-reducing sugar could be.
Heat a fresh sample with dilute acid ( which will hydrolyse the non reducing sugar into its constituent
monosaccharides).
Neutralise with alkali and carry out the Benedict's test again
If the test is positive for a non-reducing sugar it will form a brick red precipitate
Polysaccharide - Three main types - starch , glycogen and cellulose which are chains of many
monomers of glucose formed in condensation reactions.
Starch - Starch is used for energy storage in plants and is made from condensation of alpha glucose.
Amylose- compact helical chains so store lots of glucose in a small space
Amylopectin - branched chains which allow many ends to be quickly hydrolysed by amylase enzymes
to release glucose
Insoluble and large so no osmotic effects , inert and cannot diffuse out of the cells which they are
stored in .
Cellulose - Major component of plant cell walls made up of beta glucose monomer in a condensation
reaction.
Chains of beta glucose molecules form long straight unbranched chains.
These chains run parallel to each other and hydrogen bonds form cross-linkages between chains.
These many hydrogen bonds provide high tensile strength which makes the cell wall rigid and
prevents osmotic lysis
Hydrogen bonds also form strong fibres called microfibrils which are grouped into fibres which
provide structural support.
,AQA A level Biology Paper 1-2-3
What are the useful properties of water and why ?
1) Water is a metabolite - in many metabolic reactions including condensation and hydrolysis
2) Water is a solvent - medium in which enzyme catalysed reactions take place (metabolic reactions )
and readily dissolves substances like inorganic ions ( as water is dipolar so totally surrounds the ions) ,
excretory products (urea/ammonia) , respiratory gases and small hydrophilic molecules ( amino acids ,
etc)
3) Water has a high latent heat of vaporisation (lots of energy required to break hydrogen bonds
between water molecules so a lot of energy is used up when water evaporates ) so helps with
temperature control as evaporation of sweat cools humans down
4) Water has a high specific heat capacity ( to separate the many hydrogen bonds requires a lot of
energy) so acts as a buffer and resists temperature changes to maintain a constant internal body
temperature
5) Water has large cohesive forces ( as hydrogen bonds stick together) so allows transportation of
substances( water travelling in columns in xylem) and means water has a high surface tension when it
comes into contact with air ( allowing pond skaters to walk on its surface)
Why is water dipolar ? - As water is made up of two hydrogen atoms and one oxygen atom , the
hydrogen atoms have a slightly positive pole and the oxygen atom has a slightly negative pole making
water dipolar.
Hydrogen bonds - Slightly negatively charged oxygen atom is attracted to the slightly positively
charged hydrogen atom of other water molecules. The attraction between opposite charges is called
hydrogen bonding, which are weak bonds but strong in large numbers. This makes water a major
component in cells
Hydrolysis - Breaks a bond between two molecules by the addition of a molecule of water
Condensation - Joins two molecules together with the formation of a bond by the removal of a
molecule of water
Polymer - A molecule ( carbohydrate, nucleotide or protein) made up of many monomers ( small
repeating units) joined together by condensation reactions and broken down my hydrolysis
,AQA A level Biology Paper 1-2-3
Monosaccharides - Are the monomers from which larger carbohydrates are made and contain the
elements C H and O. They are sweet and soluble and there are three types : fructose , glucose and
galactose. Hexose sugars (glucose) have the formula C6H12O6
Glucose is the product of photosynthesis and a major substrate of respiration , draw the two
structural isomers of glucose in long and short hand and state the difference between them - On
carbon 1 in beta glucose the hydroxyl group (OH) points upwards and in alpha glucose the hydroxyl
group points down on carbon 1
Sucrose - Glucose and fructose
Maltose - Glucose and glucose
Lactose - Glucose and galactose
How do monosaccharides join together - By condensation to form a glycosidic bond between two
monosaccharides and a disaccharide and a molecule of water are formed
Disaccharide - Two monosaccharides joined together by condensation forming a glycosidic bond.
There are three types : maltose, sucrose and lactose. They have the chemical formula C12H22O11
Draw the hydrolysis of maltose in structural formula and short hand formula -
Test for starch - Add iodine solution to the test sample , if starch is present colour changes from
yellow to a blue-black colour.
Test for a reducing sugar - Heat with Benedict's reagent ( blue) and if reducing sugar is present a brick
red precipitate will form
Issues with the Benedict's test ? - 1) It is non specific - doesn't tell which reducing sugar is present , a
biosensor can be used to detect for specific sugars
2) It's qualitative so colour change is being measured so can't obtain concentration of reducing sugar
but allows an estimate for how much is present in the strength of the colour
, AQA A level Biology Paper 1-2-3
3) Is subjective
How to quantify results on the concentration of unknown glucose concentration? - Use a colorimeter,
as a serial dilution of known glucose concentrations can be measured producing a set of solutions
with known concentrations . The Benedict's test can be performed with each of these and the colour
intensity measured. A CALIBRATION CURVE can be produced from the results and used to read iff the
concentration of glucose in an unknown solution.
Test for a non-reducing sugar ? - Heat the sample with Benedict's reagent and if there is no colour
change a reducing sugar is not present but a non-reducing sugar could be.
Heat a fresh sample with dilute acid ( which will hydrolyse the non reducing sugar into its constituent
monosaccharides).
Neutralise with alkali and carry out the Benedict's test again
If the test is positive for a non-reducing sugar it will form a brick red precipitate
Polysaccharide - Three main types - starch , glycogen and cellulose which are chains of many
monomers of glucose formed in condensation reactions.
Starch - Starch is used for energy storage in plants and is made from condensation of alpha glucose.
Amylose- compact helical chains so store lots of glucose in a small space
Amylopectin - branched chains which allow many ends to be quickly hydrolysed by amylase enzymes
to release glucose
Insoluble and large so no osmotic effects , inert and cannot diffuse out of the cells which they are
stored in .
Cellulose - Major component of plant cell walls made up of beta glucose monomer in a condensation
reaction.
Chains of beta glucose molecules form long straight unbranched chains.
These chains run parallel to each other and hydrogen bonds form cross-linkages between chains.
These many hydrogen bonds provide high tensile strength which makes the cell wall rigid and
prevents osmotic lysis
Hydrogen bonds also form strong fibres called microfibrils which are grouped into fibres which
provide structural support.
