Paper 1 AQA A level Biology 2025
1. Explain why plasma membranes are said to have a fluid mosaic
structure.: 1. Fluid because the phospholipids are constantly moving. 2. Mosaic
because the proteins are scattered throughout the membrane-like tiles in a mosaic.
2. What effect does cholesterol have on the cell membrane?: 1.Cholesterol
molecules fit between the phospholipids and bind to the hydrophobic tails of the
phospholipids. 2.Causes them to pack together more closely, restricting the
movement of other molecules in the membrane. 3.Cholesterol makes the
membrane less fluid and more rigid. 4.Cholesterol helps to maintain the shape of
animal cells, which is important because animal cells don't have cell walls
(especially important for cells which are not supported by other cells e.g. red blood
cells).
3. State 3 functions of glycolipids in the cell membrane.: 1. Act as recognition
sites. 2. Maintain the stability of the membrane. 3. Help cells attach to one another
and so form tissues.
4. State 3 functions of glycoproteins in the cell membrane.: 1.Act as
recognition sites.2.Help cells to attach to one another so form tissues.3.Allow cells
to recognise one another e.g. lymphocytes can recognise an organisms's own cells.
5. Describe the functions of membranes within cells.: 1.Control the entry and
exit of materials in membrane-bound organelles such as mitochondria and
chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic
reactions can take place.3.Provide an internal transport system e.g. endoplasmic
reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide
surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough
ER.
6. Suggest reasons why most molecules do not diffuse freely through the
cell membrane.: 1.They're not lipid soluble.2.Too large to pass through protein
channels.3.Have the same charge as the charge on the protein channels, so they
are repelled even if they are small enough to pass through the channel.1
7. State two factors which affect the permeability of cell membranes:
.1.Temperature2.Solvent concentration
8. How can substances move across a cell surface membrane?: 1.Simple
diffusion (passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active
transport
(active)5.Co-transport (active)
9. Describe how substances can move across a cell surface membrane.:
1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a
concentration gradient.2.Large/polar molecules - pass through channel
proteins/carrier proteins, by facilitated diffusion, down a concentration
, Paper 1 AQA A level Biology 2025
gradient.3.Water - moves by osmosis from an area of high water potential to an
area of lower water potential.4.Active transport - moves substances against a
concentration gradient, involves protein carriers and requires ATP.5.Glucose/amino
acids can be co-transported across a cell membrane alongside sodium ions.*
DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS' OR 'ALONG' A
CONCENTRATION GRADIENT - MAKE
SURE YOU SAY 'UP' OR 'DOWN' OR 'AGAINST'
10. Define diffusion.: The net movement of molecules or ions from an area of
higher concentration to an area of lower concentration until evenly
distributed.Define osmosis.The movement of water from a region of higher water
potential to a region of lower water potential across a partially permeable
membrane.
11. Define active transport.: The movement of molecules from an area of lower
concentration to an area of higher concentration using ATP and carrier proteins.
12. State factors which affect the rate of diffusion.: 1.Concentration
gradient2.Area over which diffusion takes place3.Thickness of exchange
surface4.Temperature5.Size of molecules (smaller the molecule, the faster the rate
of diffusion).2
13. What is meant by the term simple diffusion: .The diffusion of molecules
directly through a cell membrane.(most molecules don't easily pass across the cell
surface membrane - those that do are small, non-polar molecules).
14. What is Ficks Law?: The rate of diffusion is proportional to : (Surface Area x
Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The
higher the concentration gradient, the faster the rate of diffusion.3.The thinner the
exchange surface, the faster the rate of diffusion.
15. What is meant by the term passive process.: No energy is needed for it to
happen.
16. Why are oxygen and carbon dioxide able to diffuse across cell
membranes?: 1.They're small, so they can pass through the spaces between the
phospholipids.2.They're non-polar, so they are lipid-soluble and so can dissolve in
the phospholipid bilayer.
17. Describe and explain the effect of an increase in temperature on the rate
of diffusion.: 1.Rate of diffusion increase as temperature rises2.because the
molecules have more kinetic energy and so move faster.
18. Describe the process of active transport.: 1.Molecules in an area of lower
concentration bind to carrier proteins.2.ATP breaks down to ADP, Pi and
energy.3.The Pi binds to the carrier protein, causing it to change shape. The energy
is used in the transport of the molecules.4.This moves the molecule across the
1. Explain why plasma membranes are said to have a fluid mosaic
structure.: 1. Fluid because the phospholipids are constantly moving. 2. Mosaic
because the proteins are scattered throughout the membrane-like tiles in a mosaic.
2. What effect does cholesterol have on the cell membrane?: 1.Cholesterol
molecules fit between the phospholipids and bind to the hydrophobic tails of the
phospholipids. 2.Causes them to pack together more closely, restricting the
movement of other molecules in the membrane. 3.Cholesterol makes the
membrane less fluid and more rigid. 4.Cholesterol helps to maintain the shape of
animal cells, which is important because animal cells don't have cell walls
(especially important for cells which are not supported by other cells e.g. red blood
cells).
3. State 3 functions of glycolipids in the cell membrane.: 1. Act as recognition
sites. 2. Maintain the stability of the membrane. 3. Help cells attach to one another
and so form tissues.
4. State 3 functions of glycoproteins in the cell membrane.: 1.Act as
recognition sites.2.Help cells to attach to one another so form tissues.3.Allow cells
to recognise one another e.g. lymphocytes can recognise an organisms's own cells.
5. Describe the functions of membranes within cells.: 1.Control the entry and
exit of materials in membrane-bound organelles such as mitochondria and
chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic
reactions can take place.3.Provide an internal transport system e.g. endoplasmic
reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide
surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough
ER.
6. Suggest reasons why most molecules do not diffuse freely through the
cell membrane.: 1.They're not lipid soluble.2.Too large to pass through protein
channels.3.Have the same charge as the charge on the protein channels, so they
are repelled even if they are small enough to pass through the channel.1
7. State two factors which affect the permeability of cell membranes:
.1.Temperature2.Solvent concentration
8. How can substances move across a cell surface membrane?: 1.Simple
diffusion (passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active
transport
(active)5.Co-transport (active)
9. Describe how substances can move across a cell surface membrane.:
1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a
concentration gradient.2.Large/polar molecules - pass through channel
proteins/carrier proteins, by facilitated diffusion, down a concentration
, Paper 1 AQA A level Biology 2025
gradient.3.Water - moves by osmosis from an area of high water potential to an
area of lower water potential.4.Active transport - moves substances against a
concentration gradient, involves protein carriers and requires ATP.5.Glucose/amino
acids can be co-transported across a cell membrane alongside sodium ions.*
DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS' OR 'ALONG' A
CONCENTRATION GRADIENT - MAKE
SURE YOU SAY 'UP' OR 'DOWN' OR 'AGAINST'
10. Define diffusion.: The net movement of molecules or ions from an area of
higher concentration to an area of lower concentration until evenly
distributed.Define osmosis.The movement of water from a region of higher water
potential to a region of lower water potential across a partially permeable
membrane.
11. Define active transport.: The movement of molecules from an area of lower
concentration to an area of higher concentration using ATP and carrier proteins.
12. State factors which affect the rate of diffusion.: 1.Concentration
gradient2.Area over which diffusion takes place3.Thickness of exchange
surface4.Temperature5.Size of molecules (smaller the molecule, the faster the rate
of diffusion).2
13. What is meant by the term simple diffusion: .The diffusion of molecules
directly through a cell membrane.(most molecules don't easily pass across the cell
surface membrane - those that do are small, non-polar molecules).
14. What is Ficks Law?: The rate of diffusion is proportional to : (Surface Area x
Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The
higher the concentration gradient, the faster the rate of diffusion.3.The thinner the
exchange surface, the faster the rate of diffusion.
15. What is meant by the term passive process.: No energy is needed for it to
happen.
16. Why are oxygen and carbon dioxide able to diffuse across cell
membranes?: 1.They're small, so they can pass through the spaces between the
phospholipids.2.They're non-polar, so they are lipid-soluble and so can dissolve in
the phospholipid bilayer.
17. Describe and explain the effect of an increase in temperature on the rate
of diffusion.: 1.Rate of diffusion increase as temperature rises2.because the
molecules have more kinetic energy and so move faster.
18. Describe the process of active transport.: 1.Molecules in an area of lower
concentration bind to carrier proteins.2.ATP breaks down to ADP, Pi and
energy.3.The Pi binds to the carrier protein, causing it to change shape. The energy
is used in the transport of the molecules.4.This moves the molecule across the
