AQA BIOLOGY PAPER 3 COMPREHENSIVE
REVIEW 2026 QUESTIONS AND DETAILED
SOLUTIONS
◉ What is dynamic equilibrium? Answer: Same number of particles
diffusing in each direction
◉ How is rate of diffusion increased ? Answer: Larger surface area
Larger concentration gradient
Thinner exchange surface( short diffusion pathway )
◉ Facilitated diffusion Answer: Passive process where larger or
polar molecules are transported across the membrane through
either carrier or channel proteins from a high concentration to a low
concentration down a concentration gradient.
Each protein transports a specific molecule due to a specific tertiary
structure.
◉ Osmosis Answer: Passive process of the diffusion of water
through a partially permeable membrane from a high water
potential to a low water potential down a water potential gradient.
,◉ What happens if animal cells are placed in hypertonic solution ?
Answer: Water will diffuse out of the cell by osmosis from a high
water potential gradient to a low water potential gradient causing
the cell to crenate / shrink
◉ What happens if animal cells are placed in a hypotonic solution ?
Answer: Water will enter the cell by osmosis from a high to low
water potential gradient causing the cell to swell and burst ( osmotic
lysis)
◉ What happens to an animal cell if placed in isotonic solution ?
Answer: No net osmotic movement as isotonic solution has and
water potential as cell
◉ What happens to a plant cell if placed in hypertonic solution ?
Answer: Water leaves the cell and vacuole by osmosis from a high to
low water potential causing the cytoplasm to shrink away from cell
wall leaving gaps between cell membrane and cell wall which is
filled with salt solution, which is known as plasmolysis
◉ What happens to a plant cell in a hypotonic solution? Answer:
Water enters the cell by osmosis from a high to low water potential
gradient but due to high tensile strength of cell wall the cell doesn't
burst and becomes turgid
,◉ What happens to a plant cell in isotonic solution ? Answer: No net
osmotic movement as same water potential
◉ Active transport Answer: Requires energy from ATP by a
hydrolysis reaction converting ATP into ADP and Pi which releases
energy. Movement of molecules from a low to high concentration
against a concentration gradient using a specific carrier protein. If
any process reduces respiration , rate of active transport will be
reduced as less ATP will be produced.
◉ Cotransport of sodium and glucose or amino acids in the lumen of
ileum to epithelial cells Answer: Sodium ions are actively
transported out of the epithelial cells into the blood steam via
sodium-potassium pump , using ATP, which creates a low
concentration gradient so sodium ions can diffuse into the cell by
facilitated diffusion as there is a higher concentration of sodium in
the lumen of the ileum than in the epithelial cells. Sodium ions and
glucose molecules enter epithelial cells from the gut lumen via a
cotransport protein( as they bind and change the shape of the
protein) bringing glucose into the epithelial cell against its
concentration gradient using hydrolysis of ATP. This causes the
concentration of glucose inside the epithelial cell to increase so
glucose is transported into the blood stream by facilitated diffusion
through a specific carrier protein. Circulation of blood maintains
concentration gradient of glucose into blood stream as it removes
blood with a high glucose concentration and replaces it with blood
with a low glucose concentration.
, ◉ Exocytosis Answer: Removes materials in vesicles to leave
through the cell membrane
◉ Endocytosis Answer: Imports materials into the cell by forming a
vesicle
◉ What are the three things a pathogen can be and how do they
cause disease ? Answer: Bacteria , viruses and fungi cause disease by
damaging body cells or releasing toxins
◉ What is an antigen ? Answer: A protein or glycoprotein often on a
cell membrane that can cause an immune response, involving
antibodies
◉ Each type of cell has what on its surface that can be used in cell
recognition ? Answer: Specific antigens
◉ What foreign antigens can the immune system identify ? Answer:
Pathogens , abnormal body cells , toxins and cells from organisms of
same species ( during transplants)
◉ What's the effect of antigen variability on disease and disease
prevention ? Answer: Antigen variability is where the antigens on a
strain of pathogen are different and can change their surface
antigens due to changes in the genes of a pathogen. So when you're
REVIEW 2026 QUESTIONS AND DETAILED
SOLUTIONS
◉ What is dynamic equilibrium? Answer: Same number of particles
diffusing in each direction
◉ How is rate of diffusion increased ? Answer: Larger surface area
Larger concentration gradient
Thinner exchange surface( short diffusion pathway )
◉ Facilitated diffusion Answer: Passive process where larger or
polar molecules are transported across the membrane through
either carrier or channel proteins from a high concentration to a low
concentration down a concentration gradient.
Each protein transports a specific molecule due to a specific tertiary
structure.
◉ Osmosis Answer: Passive process of the diffusion of water
through a partially permeable membrane from a high water
potential to a low water potential down a water potential gradient.
,◉ What happens if animal cells are placed in hypertonic solution ?
Answer: Water will diffuse out of the cell by osmosis from a high
water potential gradient to a low water potential gradient causing
the cell to crenate / shrink
◉ What happens if animal cells are placed in a hypotonic solution ?
Answer: Water will enter the cell by osmosis from a high to low
water potential gradient causing the cell to swell and burst ( osmotic
lysis)
◉ What happens to an animal cell if placed in isotonic solution ?
Answer: No net osmotic movement as isotonic solution has and
water potential as cell
◉ What happens to a plant cell if placed in hypertonic solution ?
Answer: Water leaves the cell and vacuole by osmosis from a high to
low water potential causing the cytoplasm to shrink away from cell
wall leaving gaps between cell membrane and cell wall which is
filled with salt solution, which is known as plasmolysis
◉ What happens to a plant cell in a hypotonic solution? Answer:
Water enters the cell by osmosis from a high to low water potential
gradient but due to high tensile strength of cell wall the cell doesn't
burst and becomes turgid
,◉ What happens to a plant cell in isotonic solution ? Answer: No net
osmotic movement as same water potential
◉ Active transport Answer: Requires energy from ATP by a
hydrolysis reaction converting ATP into ADP and Pi which releases
energy. Movement of molecules from a low to high concentration
against a concentration gradient using a specific carrier protein. If
any process reduces respiration , rate of active transport will be
reduced as less ATP will be produced.
◉ Cotransport of sodium and glucose or amino acids in the lumen of
ileum to epithelial cells Answer: Sodium ions are actively
transported out of the epithelial cells into the blood steam via
sodium-potassium pump , using ATP, which creates a low
concentration gradient so sodium ions can diffuse into the cell by
facilitated diffusion as there is a higher concentration of sodium in
the lumen of the ileum than in the epithelial cells. Sodium ions and
glucose molecules enter epithelial cells from the gut lumen via a
cotransport protein( as they bind and change the shape of the
protein) bringing glucose into the epithelial cell against its
concentration gradient using hydrolysis of ATP. This causes the
concentration of glucose inside the epithelial cell to increase so
glucose is transported into the blood stream by facilitated diffusion
through a specific carrier protein. Circulation of blood maintains
concentration gradient of glucose into blood stream as it removes
blood with a high glucose concentration and replaces it with blood
with a low glucose concentration.
, ◉ Exocytosis Answer: Removes materials in vesicles to leave
through the cell membrane
◉ Endocytosis Answer: Imports materials into the cell by forming a
vesicle
◉ What are the three things a pathogen can be and how do they
cause disease ? Answer: Bacteria , viruses and fungi cause disease by
damaging body cells or releasing toxins
◉ What is an antigen ? Answer: A protein or glycoprotein often on a
cell membrane that can cause an immune response, involving
antibodies
◉ Each type of cell has what on its surface that can be used in cell
recognition ? Answer: Specific antigens
◉ What foreign antigens can the immune system identify ? Answer:
Pathogens , abnormal body cells , toxins and cells from organisms of
same species ( during transplants)
◉ What's the effect of antigen variability on disease and disease
prevention ? Answer: Antigen variability is where the antigens on a
strain of pathogen are different and can change their surface
antigens due to changes in the genes of a pathogen. So when you're
