2025 aqa as biology exam MS
Describe a biochemical test to show that raffinose solution contains a non-reducing
sugar. - answers First heat the sample with an acid in a water bath and neutralise, then
heat with Benedict solution in a water bath for five minutes. A positive test should
produce a red precipitate.
Explain the arrangement of phospholipids in a cell-surface membrane. - answers
Phospholipids are arranged in a phospholipid bilayer. The hydrophobic fatty acid tails
point away from the water and the hydrophilic heads attract the water.
Describe how an ester bond is formed in a phospholipid molecule. - answers An ester
bond is formed between a glycerol and a fatty acid, producing a water molecule in a
condensation reaction.
State and explain the property of water that helps to prevent temperature increase in a
cell. - answers Water has a high specific heat capacity. This buffers changes in
temperature, so lots of energy is required to change the temperature.
Describe how a phagocyte destroys a pathogen present in the blood. - answers The
phagocyte is attracted to the pathogen by chemoattractants. The phagocyte binds to the
pathogen. The pathogen is engulfed and lysosomes migrate to the phagosome. The
lysosomes then release lytic enzymes which break down the pathogen. The breakdown
products are then absorbed by the phagocyte.
Give two types of cell, other than pathogens, that can stimulate an immune response. -
answers Cancer cells and virus infected cells.
What is the role of a disulphide bridge in forming the quaternary structure of an
antibody. - answers It joins two polypeptide chains
Outline the role of organelles in the production, transport and release of proteins from
Eukaryotic cells.
Do not include details of transcription or translation in your answer. - answers DNA in
the nucleus codes for the amino acids which make up proteins. Ribosomes on the RER
then synthesise these proteins. Mitochondria produce ATP for use in protein synthesis.
The Golgi apparatus then packages and modifies proteins which are transported to the
cell surface membrane in vesicles where they fuse.
Give one advantage of viewing a biological specimen using a transmission electron
microscope compared with using a scanning electron microscope. - answers There is a
higher resolution / view internal structures
Describe and explain the mechanism that causes lungs to fill with air. - answers The
diaphragm muscle contracts and external intercostal muscles contract. This causes an
,increase in volume and decrease in pressure. This causes air to move down the
pressure gradient and into the lungs.
Two solutions often used to stain tissues are haematoxylin solution and iodine solution.
• Haematoxylin solution stains DNA a blue colour. • Iodine solution stains starch a blue-
black colour.
The scientist used haematoxylin solution and not iodine solution to stain the lung tissue.
Suggest why. - answers The tissue doesn't contain starch and it makes the nucleus,
which contains DNA, visible.
Describe how mRNA is produced from an exposed template strand of DNA. Do not
include DNA helices or splicing in your answer. - answers Free RNA nucleotides form
complementary base pairs. RNA polymerase then forms phosphodiester bonds.
Define the term exon. - answers A triplet base sequence which codes for a sequence of
amino acids.
Describe binary fission in bacteria. - answers Circular DNA and plasmids are replicated,
tenth cytoplasm divides producing two daughter cells.
Suggest and explain how two environmental variables could be changed to increase the
growth rate of cells. - answers Increase the concentration of glucose and oxygen to
increase the rate of respiration, so growth rate increases.
Increasing the temperature increases enzyme activity.
Suggest and explain one advantage and one disadvantage of increased stomatal
density on the growth of tobacco plant leaves. - answers There is more carbon dioxide
uptake therefore the rate of photosynthesis increases resulting in more growth.
There is also more water loss via transpiration through the stomata, decreasing
photosynthesis, slowing growth.
The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.
- answers In chloroplasts DNA is shorter and circular rather than linear. Introns are also
absent in chloroplasts but present in DNA.
Some DNA nucleotides have the organic base thymine, but RNA nucleotides do not
have thymine. RNA nucleotides have uracil instead of thymine.
Give one other difference between the structure of a DNA nucleotide and the structure
of an RNA nucleotide. - answers DNA is deoxyribose and RNA is ribose.
Not all mutations in the nucleotide sequence of a gene cause a change in the structure
of a polypeptide.
Give two reasons why. - answers Some triplets code for the same amino acid and also
if mutations occur in introns and non-coding sequences.
, What is a monoclonal antibody? - answers Antibodies with the same tertiary structure.
Give one example of using monoclonal antibodies. - answers Used in medicine to carry
drugs to specific cells.
Describe the role of monoclonal antibodies in producing a positive ELISA test result. -
answers First the antibody binds to the complementary antigen. A second antibody with
an enzyme attached to it is attached. A solution is added resulting in a colour change.
A student carried out the Benedicts test. Suggest a method, other than using a
colorimeter, that this student could use to measure the quantity of reducing sugar in a
solution. - answers Filter and dry the precipitate and find a mass.
Use of a colorimeter in this investigation would improve the repeatability of the student's
results. - answers It is quantitive rather than colour change which is qualitative therefore
subjective. This standardises the method.
Explain why it is more useful to calculate an index of diversity than to record species
richness. - answers Index of diversity also measures the abundance of each species. It
is useful as there may be many of some species.
Suggest how the scientist measured the rate of water flow in the river. - answers
Measured the movement of a floating object down river over a period of time.
Use information in Table 2 and Table 3 to suggest and explain a reason for the
difference in the numbers of Slate Drake mayfly at these sites in this river. - answers
There is less food at the site so the may flies die.
It was important that the sampling procedure was standardised when collecting the
Slate Drake mayflies from the two sites.
Give one way in which the sampling procedure could be standardised. - answers Take
the samples on the same day or time of day and make sure the sample area is the
same size throughout.
Name an organelle found in both a chloroplast and a prokaryotic cell. - answers
Ribosome
A biologist separated cell components to investigate organelle activity. She prepared a
suspension of the organelles in a solution that prevented damage to the organelles.
Describe three properties of this solution and explain how each property prevented
damage to the organelles. - answers The solution must be ice cold to reduce enzyme
activity.
It must be buffered to maintain the pH and stop enzymes denaturing.
The solution must have the same water potential as the cell to stop the cell bursting or
shrinking.
Describe a biochemical test to show that raffinose solution contains a non-reducing
sugar. - answers First heat the sample with an acid in a water bath and neutralise, then
heat with Benedict solution in a water bath for five minutes. A positive test should
produce a red precipitate.
Explain the arrangement of phospholipids in a cell-surface membrane. - answers
Phospholipids are arranged in a phospholipid bilayer. The hydrophobic fatty acid tails
point away from the water and the hydrophilic heads attract the water.
Describe how an ester bond is formed in a phospholipid molecule. - answers An ester
bond is formed between a glycerol and a fatty acid, producing a water molecule in a
condensation reaction.
State and explain the property of water that helps to prevent temperature increase in a
cell. - answers Water has a high specific heat capacity. This buffers changes in
temperature, so lots of energy is required to change the temperature.
Describe how a phagocyte destroys a pathogen present in the blood. - answers The
phagocyte is attracted to the pathogen by chemoattractants. The phagocyte binds to the
pathogen. The pathogen is engulfed and lysosomes migrate to the phagosome. The
lysosomes then release lytic enzymes which break down the pathogen. The breakdown
products are then absorbed by the phagocyte.
Give two types of cell, other than pathogens, that can stimulate an immune response. -
answers Cancer cells and virus infected cells.
What is the role of a disulphide bridge in forming the quaternary structure of an
antibody. - answers It joins two polypeptide chains
Outline the role of organelles in the production, transport and release of proteins from
Eukaryotic cells.
Do not include details of transcription or translation in your answer. - answers DNA in
the nucleus codes for the amino acids which make up proteins. Ribosomes on the RER
then synthesise these proteins. Mitochondria produce ATP for use in protein synthesis.
The Golgi apparatus then packages and modifies proteins which are transported to the
cell surface membrane in vesicles where they fuse.
Give one advantage of viewing a biological specimen using a transmission electron
microscope compared with using a scanning electron microscope. - answers There is a
higher resolution / view internal structures
Describe and explain the mechanism that causes lungs to fill with air. - answers The
diaphragm muscle contracts and external intercostal muscles contract. This causes an
,increase in volume and decrease in pressure. This causes air to move down the
pressure gradient and into the lungs.
Two solutions often used to stain tissues are haematoxylin solution and iodine solution.
• Haematoxylin solution stains DNA a blue colour. • Iodine solution stains starch a blue-
black colour.
The scientist used haematoxylin solution and not iodine solution to stain the lung tissue.
Suggest why. - answers The tissue doesn't contain starch and it makes the nucleus,
which contains DNA, visible.
Describe how mRNA is produced from an exposed template strand of DNA. Do not
include DNA helices or splicing in your answer. - answers Free RNA nucleotides form
complementary base pairs. RNA polymerase then forms phosphodiester bonds.
Define the term exon. - answers A triplet base sequence which codes for a sequence of
amino acids.
Describe binary fission in bacteria. - answers Circular DNA and plasmids are replicated,
tenth cytoplasm divides producing two daughter cells.
Suggest and explain how two environmental variables could be changed to increase the
growth rate of cells. - answers Increase the concentration of glucose and oxygen to
increase the rate of respiration, so growth rate increases.
Increasing the temperature increases enzyme activity.
Suggest and explain one advantage and one disadvantage of increased stomatal
density on the growth of tobacco plant leaves. - answers There is more carbon dioxide
uptake therefore the rate of photosynthesis increases resulting in more growth.
There is also more water loss via transpiration through the stomata, decreasing
photosynthesis, slowing growth.
The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.
- answers In chloroplasts DNA is shorter and circular rather than linear. Introns are also
absent in chloroplasts but present in DNA.
Some DNA nucleotides have the organic base thymine, but RNA nucleotides do not
have thymine. RNA nucleotides have uracil instead of thymine.
Give one other difference between the structure of a DNA nucleotide and the structure
of an RNA nucleotide. - answers DNA is deoxyribose and RNA is ribose.
Not all mutations in the nucleotide sequence of a gene cause a change in the structure
of a polypeptide.
Give two reasons why. - answers Some triplets code for the same amino acid and also
if mutations occur in introns and non-coding sequences.
, What is a monoclonal antibody? - answers Antibodies with the same tertiary structure.
Give one example of using monoclonal antibodies. - answers Used in medicine to carry
drugs to specific cells.
Describe the role of monoclonal antibodies in producing a positive ELISA test result. -
answers First the antibody binds to the complementary antigen. A second antibody with
an enzyme attached to it is attached. A solution is added resulting in a colour change.
A student carried out the Benedicts test. Suggest a method, other than using a
colorimeter, that this student could use to measure the quantity of reducing sugar in a
solution. - answers Filter and dry the precipitate and find a mass.
Use of a colorimeter in this investigation would improve the repeatability of the student's
results. - answers It is quantitive rather than colour change which is qualitative therefore
subjective. This standardises the method.
Explain why it is more useful to calculate an index of diversity than to record species
richness. - answers Index of diversity also measures the abundance of each species. It
is useful as there may be many of some species.
Suggest how the scientist measured the rate of water flow in the river. - answers
Measured the movement of a floating object down river over a period of time.
Use information in Table 2 and Table 3 to suggest and explain a reason for the
difference in the numbers of Slate Drake mayfly at these sites in this river. - answers
There is less food at the site so the may flies die.
It was important that the sampling procedure was standardised when collecting the
Slate Drake mayflies from the two sites.
Give one way in which the sampling procedure could be standardised. - answers Take
the samples on the same day or time of day and make sure the sample area is the
same size throughout.
Name an organelle found in both a chloroplast and a prokaryotic cell. - answers
Ribosome
A biologist separated cell components to investigate organelle activity. She prepared a
suspension of the organelles in a solution that prevented damage to the organelles.
Describe three properties of this solution and explain how each property prevented
damage to the organelles. - answers The solution must be ice cold to reduce enzyme
activity.
It must be buffered to maintain the pH and stop enzymes denaturing.
The solution must have the same water potential as the cell to stop the cell bursting or
shrinking.
