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Biology OCR A level Biological membranes Summary Notes
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Biology OCR A level Biological membranes Summary Notes
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1 of 5B5- Biological membranes
2.1.5 Biological membranes
(a) The roles of membranes within cells and at the surface of cells.
Key words:
Partially permeable- membranes that allows some substances to cross but not others.
Roles of membranes:
• Partially permeable barriers between the cell and its environment, between organelles and the
cytoplasm and within organelles.
• Sites of chemical reactions.
• Sites of cell communication and recognition (cell signalling).
• Regulating the transport of chemicals into and out of cells.
• Provide a surface for metabolic pathways.
(b) The fluid mosaic model of membrane structure and the roles of its components.
Key words:
Compartmentalisation- the separation of cellular activities into various compartments of
organelles inside the cell.
Plasma membrane- a selectively-permeable phospholipid bilayer forming the boundary of the
cells.
Phospholipid bilayer- a double layer of phospholipids that makes up plasma and organelle
membranes, about 7nm thick.
Cholesterol- a lipid that forms an essential component of animal cell membranes and acts as a
precursor molecule for the synthesis of other biologically important steroids.
Intrinsic/integral proteins- proteins of the cell-surface membrane that completely span the
phospholipid bilayer from one side to the other.
Extrinsic/peripheral proteins- are proteins which are confined to the outer or inner surface of the
membrane.
Glycolipids- are lipids with attached carbohydrate chains, they are antigens.
Glycoproteins- are intrinsic proteins that are embedded in the cell-surface membrane with
attached carbohydrate chains of varying lengths and shapes. They play a role in cell adhesion and
stabilise the membrane, along with acting as receptors for chemical signals.
Cell signalling/communication- when chemicals bind to the receptor, causing the cell to
respond directly or set off a cascade of events inside the cell.
Membrane proteins- embedded proteins that perform specific functions for the cell membrane.
Channel proteins- proteins that provide passageways through the membrane for certain
hydrophilic substances.
Carrier proteins- proteins that bind solutes and transport them across the plasma membrane.
Fluid mosaic model structure + cell membrane theory
• Plasma membranes separate the contents of cells from its external environment.
• Composed of lipids, proteins and carbohydrates.
• Discovered in 1972, American scientist Singer and Nicolson proposed a model, in which
proteins occupy various positions in the membrane. Known as the fluid mosaic model as the
phospholipids are free to move within the layer relative to each other, giving the membrane
flexibility. Mosaic structure as protein molecules are scattered through the bilayers like tiles in a
mosaic.
• Phospholipids have a phosphate group head which is hydrophilic joined to two fatty acids tails
which are hydrophobic.
• When phospholipids come into contact with water they orientate so the heads are in the water
with the tail pointing out. The arranged double layer forms a hydrophobic core.
• Centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances
through it, but fat-soluble substances can dissolve in the bilayer.
• Images taken in the 1950s showed two black parallel lines supporting the theory of a lipid
bilayer.
• Membrane bound receptors are a site where hormones and drugs bind.
, 2 of 5
Cholesterol Glycolipids Glycoproteins Extrinsic proteins Intrinsic proteins
• A lipid with a • Lipids with • Intrinsic • Present in one • Transmembrane
hydrophilic attached proteins side of the proteins that are
head and a carbohydrate embedded in bilayer. embedded
hydrophobic tail chains. the cell-surface • Normally have through both
which regulates • They are membrane with hydrophilic R- layers of a
the fluidity of antigens, cell attached groups on their membrane.
membranes. surface carbohydrate outer surfaces • Have amino acids
• Positioned molecules chains of and interact with with hydrophobic
between involved in the varying lengths the polar heads R-groups on their
phospholipids, immune and shapes. of the external surfaces,
they bind to the response. • Play a role in phospholipids or which interact
hydrophobic • Stabilise the cell adhesion, with intrinsic with the
tails of the membrane by stabilise the proteins. hydrophobic
phospholipids, forming membrane and • Present in either core.
causing them to hydrogen act as a layer and some • Channel and
pack more bonds with receptor for cell move between carrier proteins
closely surrounding signalling. layers. are both involved
together. water • Sites where in transport
• Cholesterol molecules. drugs, across the
molecules • Act as hormones and membrane.
prevent the receptors for antibodies bind.
membranes cell signalling.
becoming too
solid by
stopping the
phospholipid
molecules from
grouping too
closely and
crystallising.
(c) (i) Factors affecting membrane structure and permeability.
Temperature
• Phospholipids in a cell membrane are constantly moving.
• Increased temperature gives phospholipids more kinetic energy so they move more.
• Cells break down completely if temperature keeps rising.
• Loss of structure increases permeability of membrane.
• High temperatures denature carrier and channel proteins.
Solvents
• Water is needed for bilayers to form.
• Organic solvents will dissolve membranes, disrupting cells. Alcohols dissolve the membranes of
bacteria in a wound killing them and reducing the risk of infection.
• When the membrane is disrupted it becomes more fluid and more permeable.
• Surrounding cells in a solvent increases the permeability of their cell membranes. This is
because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure.
(ii) Practical investigations into factors affecting membrane structure and permeability.
(d) (i) The movement of molecules across membranes.
Key words:
Passive transport- the movement of materials through a cell membrane without using energy.
Diffusion- movement of molecules from an area of higher concentration to an area of lower
concentration.
Facilitated diffusion- the transport of substances through a cell membrane along a concentration
gradient with the aid of carrier/channel proteins, selectively permeable.
Concentration gradient- the path from an area of higher concentration to an area of lower
concentration, down.
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