A LEVEL BIOLOGY 7402 PAPER 3 2026
COMPLETE QUESTIONS AND ANSWERS
GRADED A+
◉ Why is water dipolar ? Answer: 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 Answer: 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 Answer: Breaks a bond between two molecules by the
addition of a molecule of water
◉ Condensation Answer: Joins two molecules together with the
formation of a bond by the removal of a molecule of water
◉ Polymer Answer: A molecule ( carbohydrate, nucleotide or
protein) made up of many monomers ( small repeating units) joined
together by condensation reactions and broken down my hydrolysis
,◉ Monosaccharides Answer: 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 Answer: 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 Answer: Glucose and fructose
◉ Maltose Answer: Glucose and glucose
◉ Lactose Answer: Glucose and galactose
◉ How do monosaccharides join together Answer: By condensation
to form a glycosidic bond between two monosaccharides and a
disaccharide and a molecule of water are formed
◉ Disaccharide Answer: 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 Answer:
◉ Test for starch Answer: 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 Answer: Heat with Benedict's reagent (
blue) and if reducing sugar is present a brick red precipitate will
form
◉ Issues with the Benedict's test ? Answer: 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
3) Is subjective
◉ How to quantify results on the concentration of unknown glucose
concentration? Answer: 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 ? Answer: 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 Answer: Three main types - starch , glycogen and
cellulose which are chains of many monomers of glucose formed in
condensation reactions.
◉ Starch Answer: 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 .
COMPLETE QUESTIONS AND ANSWERS
GRADED A+
◉ Why is water dipolar ? Answer: 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 Answer: 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 Answer: Breaks a bond between two molecules by the
addition of a molecule of water
◉ Condensation Answer: Joins two molecules together with the
formation of a bond by the removal of a molecule of water
◉ Polymer Answer: A molecule ( carbohydrate, nucleotide or
protein) made up of many monomers ( small repeating units) joined
together by condensation reactions and broken down my hydrolysis
,◉ Monosaccharides Answer: 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 Answer: 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 Answer: Glucose and fructose
◉ Maltose Answer: Glucose and glucose
◉ Lactose Answer: Glucose and galactose
◉ How do monosaccharides join together Answer: By condensation
to form a glycosidic bond between two monosaccharides and a
disaccharide and a molecule of water are formed
◉ Disaccharide Answer: 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 Answer:
◉ Test for starch Answer: 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 Answer: Heat with Benedict's reagent (
blue) and if reducing sugar is present a brick red precipitate will
form
◉ Issues with the Benedict's test ? Answer: 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
3) Is subjective
◉ How to quantify results on the concentration of unknown glucose
concentration? Answer: 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 ? Answer: 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 Answer: Three main types - starch , glycogen and
cellulose which are chains of many monomers of glucose formed in
condensation reactions.
◉ Starch Answer: 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 .
