A LEVEL BIOLOGY 7402 PAPER 3 2026 TEST
PAPER SOLVED QUESTIONS AND SOLUTIONS
◉ Phospholipids Answer: Contain a glycerol molecule combined
with two fatty acid chains by two condensation reactions forming
ester bonds. However they have a phosphors group attached to the
glycerol instead of an extra fatty acid in triglycerides. They are
composed of a hydrophobic non polar fatty acid tail which repels
water and a hydrophilic polar head which attracts water. This forms
a bilayer where the head points outwards and the gals point
inwards.
◉ Test for lipids ? Answer: Shake with ethanol then shake gently
with water and if a lipid is present a milky white emulsion will be
present.
◉ 3 functions of a cell membrane Answer: 1) control transport of
substances in an out of the cell as the membrane is partially
permeable
2) acts as a receptor site
3) separates cell and organelles from each other
◉ What is the fluid mosaic model? Answer: Fluid as different
molecules are constantly moving around
,Mosaic as many different types of molecules are present and the
proteins and carbohydrates float around in the bilayer
◉ How does the structure of a phospholipid relate to its function ?
Answer: Hydrophobic tails point towards each other away from
water and hydrophilic heads point towards water which arranges
the the phospholipids in a bilayer which allows non polar, lipid
soluble and small molecules to pass through the membrane and acts
as a barrier to polar, water soluble and large molecules.
◉ Role of intrinsic proteins in the cell membrane ? Answer:
Transport polar and large molecules through channels and carriers (
or enzymes )
◉ Role of extrinsic proteins in cell membrane ? Answer: Enzymes ,
act as receptors for binding to hormones as have a specific receptor
site and antigens for cell recognition
◉ Role of glycoproteins in the cell membrane? Answer: A protein
bonded to a carbohydrate which acts as a receptor for hormones and
neurotransmitters or antigens for cell recognition
◉ Role of cholesterol in the cell membrane ? Answer: Is a type of
lipid that increases strength and stability of membranes by
restricting movement to make the membranes less flexible/ fluid
,and more rigid. Helps prevent loss of water as helps maintain the
shape of the cell
◉ Role of glycolipids in the cell membrane? Answer: A protein
bonded to a lipid which acts as a recognition site and help main
stability of membrane
◉ Fick's Law of Diffusion Answer: rate of diffusion = surface area x
difference in concentration gradient over the thickness of the
exchange surface
SAD TOES
◉ Simple diffusion Answer: Is a passive process where molecules
move from an area of high concentration to an area of low
concentration down a concentration gradient directly across the cell
membrane as the molecules are small ( oxygen, carbon dioxide and
water ).
◉ 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
PAPER SOLVED QUESTIONS AND SOLUTIONS
◉ Phospholipids Answer: Contain a glycerol molecule combined
with two fatty acid chains by two condensation reactions forming
ester bonds. However they have a phosphors group attached to the
glycerol instead of an extra fatty acid in triglycerides. They are
composed of a hydrophobic non polar fatty acid tail which repels
water and a hydrophilic polar head which attracts water. This forms
a bilayer where the head points outwards and the gals point
inwards.
◉ Test for lipids ? Answer: Shake with ethanol then shake gently
with water and if a lipid is present a milky white emulsion will be
present.
◉ 3 functions of a cell membrane Answer: 1) control transport of
substances in an out of the cell as the membrane is partially
permeable
2) acts as a receptor site
3) separates cell and organelles from each other
◉ What is the fluid mosaic model? Answer: Fluid as different
molecules are constantly moving around
,Mosaic as many different types of molecules are present and the
proteins and carbohydrates float around in the bilayer
◉ How does the structure of a phospholipid relate to its function ?
Answer: Hydrophobic tails point towards each other away from
water and hydrophilic heads point towards water which arranges
the the phospholipids in a bilayer which allows non polar, lipid
soluble and small molecules to pass through the membrane and acts
as a barrier to polar, water soluble and large molecules.
◉ Role of intrinsic proteins in the cell membrane ? Answer:
Transport polar and large molecules through channels and carriers (
or enzymes )
◉ Role of extrinsic proteins in cell membrane ? Answer: Enzymes ,
act as receptors for binding to hormones as have a specific receptor
site and antigens for cell recognition
◉ Role of glycoproteins in the cell membrane? Answer: A protein
bonded to a carbohydrate which acts as a receptor for hormones and
neurotransmitters or antigens for cell recognition
◉ Role of cholesterol in the cell membrane ? Answer: Is a type of
lipid that increases strength and stability of membranes by
restricting movement to make the membranes less flexible/ fluid
,and more rigid. Helps prevent loss of water as helps maintain the
shape of the cell
◉ Role of glycolipids in the cell membrane? Answer: A protein
bonded to a lipid which acts as a recognition site and help main
stability of membrane
◉ Fick's Law of Diffusion Answer: rate of diffusion = surface area x
difference in concentration gradient over the thickness of the
exchange surface
SAD TOES
◉ Simple diffusion Answer: Is a passive process where molecules
move from an area of high concentration to an area of low
concentration down a concentration gradient directly across the cell
membrane as the molecules are small ( oxygen, carbon dioxide and
water ).
◉ 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
