AS Level Biology Notes for Topic 2 – Unit 1 ; Membranes, Proteins, DNA & Gene Expression
2A Membranes & Transport —-
Cell Membranes
Membranes:
➢ Surrounds the organelles.
➢ Controls what enters and leaves the cell.
○ Acts as a barrier/compartmentalization of other fluids.
Plasma:
➢ Holds the cell together and gives it shape.
➢ Controls the entry/exit of substances.
➢ A site for biochemical reactions.
Phospholipids:
➢ In non-aqueous environments, they form a monolayer,
with the phosphate heads in water and the fatty tails
in the air.
➢ In aqueous environments, they form a bilayer – with
the heads facing towards the water and the fatty tails
facing another pair. They are 7 nanometers.
○ This provides a barrier to large, polar molecules.
○ Fat-soluble or small molecules can pass through.
➢ Amphipathic → contains both hydrophilic & hydrophobic regions.
➢ Polar → an imbalance in charges.
➢ Charged → either gained/lost an electron.
Membrane Fluidity:
➢ Affected by the proportion of unsaturated hydrocarbons in the bilayer.
○ More saturated tails = viscous layer.
○ More unsaturated tails = fluid layer.
Cholesterol:
➢ Stabilises the membrane.
➢ Regulates fluidity by binding to the phospholipids.
➢ Reduces the permeability to charged particles.
,AS Level Biology Notes for Topic 2 – Unit 1 ; Membranes, Proteins, DNA & Gene Expression
The Fluid Mosaic Model:
➢ “Fluid” because the phospholipids are constantly moving & “mosaic” because
the proteins are scattered through the bilayer, similarly to the tiles in a
mosaic.
➢ Functions of the plasma cell membrane;
○ Hold the cell together.
○ Controls entry/exit of substances (selectively permeable barrier).
○ Allows the cell to recognise other cells & to be recognised.
○ Bind to other cells and molecules.
○ A site for biochemical reactions.
○ Allows the cell to change shape.
○ Transport & packaging within a cell.
➢ Peripheral proteins are bound to the surface of the membrane.
○ Used as cell markers (antigens) for cell recognition.
➢ Channel proteins are integral proteins which are attached to the membrane
(transmembrane – span across the bilayer).
○ Used in facilitated diffusion – doesn’t require ATP.
○ Has a hydrophilic core.
○ Highly selective.
➢ Carrier proteins are a type of transmembrane integral protein.
○ Used in facilitated diffusion – doesn’t require ATP.
○ Allows substances to move across the membrane by changing shape.
➢ Protein pumps are a type of transmembrane integral protein too.
○ Requires ATP – moves substances from a low concentration to a high
concentration.
, AS Level Biology Notes for Topic 2 – Unit 1 ; Membranes, Proteins, DNA & Gene Expression
➢ Transport proteins — used for transport of small molecules through integral
proteins (includes facilitated & active transport).
➢ Receptor proteins — found on the outside surface of the cell membrane; it has a
specific binding site for hormones/chemicals to bind to, and this triggers a
series of events inside the cell.
➢ Enzyme proteins — catalyses reactions in the cytoplasm/outside the cell,
breaking down substrates into products.
➢ Recognition proteins — often are glycoproteins which are involved in cell
recognition.
➢ Structural proteins — found on the inside surface of the cell membrane & are
attached to the cytoskeleton; involved in maintaining the cell's shape (motility).
Cell Transport & Diffusion
Passive Transport:
➢ Passive processes don’t require energy.
➢ Properties of molecules;
○ Size of the molecule.
○ Solubility in lipids or water.
○ Presence or absence of a charge.
Lipid Diffusion:
➢ Cannot be switched on or off by a cell.
○ This continues to happen as long as there’s a concentration gradient AND
the particles have appropriate properties.
○ e.g. CO2, O2, H2O, vitamin K (lipid soluble), and steroids.
2A Membranes & Transport —-
Cell Membranes
Membranes:
➢ Surrounds the organelles.
➢ Controls what enters and leaves the cell.
○ Acts as a barrier/compartmentalization of other fluids.
Plasma:
➢ Holds the cell together and gives it shape.
➢ Controls the entry/exit of substances.
➢ A site for biochemical reactions.
Phospholipids:
➢ In non-aqueous environments, they form a monolayer,
with the phosphate heads in water and the fatty tails
in the air.
➢ In aqueous environments, they form a bilayer – with
the heads facing towards the water and the fatty tails
facing another pair. They are 7 nanometers.
○ This provides a barrier to large, polar molecules.
○ Fat-soluble or small molecules can pass through.
➢ Amphipathic → contains both hydrophilic & hydrophobic regions.
➢ Polar → an imbalance in charges.
➢ Charged → either gained/lost an electron.
Membrane Fluidity:
➢ Affected by the proportion of unsaturated hydrocarbons in the bilayer.
○ More saturated tails = viscous layer.
○ More unsaturated tails = fluid layer.
Cholesterol:
➢ Stabilises the membrane.
➢ Regulates fluidity by binding to the phospholipids.
➢ Reduces the permeability to charged particles.
,AS Level Biology Notes for Topic 2 – Unit 1 ; Membranes, Proteins, DNA & Gene Expression
The Fluid Mosaic Model:
➢ “Fluid” because the phospholipids are constantly moving & “mosaic” because
the proteins are scattered through the bilayer, similarly to the tiles in a
mosaic.
➢ Functions of the plasma cell membrane;
○ Hold the cell together.
○ Controls entry/exit of substances (selectively permeable barrier).
○ Allows the cell to recognise other cells & to be recognised.
○ Bind to other cells and molecules.
○ A site for biochemical reactions.
○ Allows the cell to change shape.
○ Transport & packaging within a cell.
➢ Peripheral proteins are bound to the surface of the membrane.
○ Used as cell markers (antigens) for cell recognition.
➢ Channel proteins are integral proteins which are attached to the membrane
(transmembrane – span across the bilayer).
○ Used in facilitated diffusion – doesn’t require ATP.
○ Has a hydrophilic core.
○ Highly selective.
➢ Carrier proteins are a type of transmembrane integral protein.
○ Used in facilitated diffusion – doesn’t require ATP.
○ Allows substances to move across the membrane by changing shape.
➢ Protein pumps are a type of transmembrane integral protein too.
○ Requires ATP – moves substances from a low concentration to a high
concentration.
, AS Level Biology Notes for Topic 2 – Unit 1 ; Membranes, Proteins, DNA & Gene Expression
➢ Transport proteins — used for transport of small molecules through integral
proteins (includes facilitated & active transport).
➢ Receptor proteins — found on the outside surface of the cell membrane; it has a
specific binding site for hormones/chemicals to bind to, and this triggers a
series of events inside the cell.
➢ Enzyme proteins — catalyses reactions in the cytoplasm/outside the cell,
breaking down substrates into products.
➢ Recognition proteins — often are glycoproteins which are involved in cell
recognition.
➢ Structural proteins — found on the inside surface of the cell membrane & are
attached to the cytoskeleton; involved in maintaining the cell's shape (motility).
Cell Transport & Diffusion
Passive Transport:
➢ Passive processes don’t require energy.
➢ Properties of molecules;
○ Size of the molecule.
○ Solubility in lipids or water.
○ Presence or absence of a charge.
Lipid Diffusion:
➢ Cannot be switched on or off by a cell.
○ This continues to happen as long as there’s a concentration gradient AND
the particles have appropriate properties.
○ e.g. CO2, O2, H2O, vitamin K (lipid soluble), and steroids.
