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Biological Membranes and Cell Division
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This is a document that consists of notes for the topic, Biological Membranes and Cell Division, as part of the AS/A level OCR Biology A course
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Cells, and many organelles, are surrounded by membranesMembranes at surface of cells (Plasma Membranes):
● Barrier between cell and its environment, controlling which substances enter/leave
cell
● Partially permeable - let some molecules in and out
● Substances move across by diffusion, osmosis or active transport
● Allow recognition by other cells (e.g. cells of immune system
● Allow cell communication (cell signalling)
Membranes within cells:
● Membranes around organelles divide cell into different compartments - act as barrier
between organelle and cytoplasm - makes different functions more efficient (e.g.
substances needed for respiration, like enzymes, kept together inside mitochondria)
● Form vesicles - transport substances between different areas of cell
● Control which substances enter/leave the organelle (RNA leaves nucleus via nuclear
membrane) - Also partially permeable
● Also get membranes within organelles - act as barriers between membrane contents
and rest of organelle (e.g. thylakoid membranes in chloroplasts)
● Membranes within cells can be site of chemical reactions (inner membrane of
mitochondria contains enzymes for respiration)
Fluid Mosaic Structure/Model:
● Structure of all membranes - lipids (phospholipids), proteins and carbohydrates
(usually attached to proteins or lipids)
● Phospholipid molecules form continuous, double layer (bilayer)
● Bilayer is ‘fluid’ - phospholipids constantly moving
● Cholesterol molecules present within bilayer
● Protein molecules scattered through bilayer
● Some proteins have polysaccharide (carbohydrate) chain attached - called
glycoproteins
● Some lipids also have a polysaccharide chain attached - called glycolipids
Phospholipids:
● Form a barrier to dissolved substances
● Head (hydrophilic - attract water) and tail (hydrophobic - repels water)
● Molecules automatically arrange themselves into bilayer - head faces out towards
water on either side of membrane
● Centre of bilayer = hydrophobic so membrane doesn’t allow water-soluble
substances (ions) through it - acts as barrier to dissolved substances
● However, fat-soluble substances can dissolve + pass through the membrane
Cholesterol:
● Type of lipid with a hydrophilic and hydrophobic end
● Regulates fluidity of membranes
● Positioned between phospholipids in membrane bilayer, hydrophilic end interacting with
heads and hydrophobic end interacting with the tails, pulling them together
● Adds stability to membranes without making them too rigid
● Molecules prevent membranes becoming too solid by stopping the phospholipid molecules from
grouping too closely and crystallising
, Glycoproteins:
● Intrinsic proteins
● Embedded in the cell-surface membrane with attached carbohydrate (sugar) chains
of varying lengths and shapes
● Play a role in cell adhesion (when cells join together to form tight junctions in certain
tissues) and as receptors for chemical signals
● When chemical binds to receptor, it elicits a response from the cell - cause direct
response or set off a cascade of events inside cell - process known as cell
communication or cell signalling
○ Examples:
○ Receptors for neurotransmitters (such as acetylcholine at nerve cell
synapses). Binding of neurotransmitters tiggers/prevents an impulse in the
next neurone
○ Receptors for peptide hormones, including insulin and glucagon - affect
uptake and storage of glucose by cells
Glycolipids:
● Similar to glycoproteins
● Lipids with attached carbohydrate (sugar) chains
● These molecules are called cell markers or antigens and can be recognised by cells
of immune system as self (of the organism) or non-self (of cells belonging to another
organism)
Intrinsic proteins: transmembrane proteins that are embedded through both layers of a
membrane
Have amino acids with hydrophobic R-groups on their external surfaces - interact with
hydrophobic core of membrane, keeping them in place
Channel proteins: provide hydrophilic channels that allow passive movement of polar
molecules + ions down a concentration gradient through membranes.
Held in position by interactions between hydrophobic core of membrane and hydrophobic
R-groups on outside of proteins
Carrier proteins: important role in passive and active transport into cells. This often involves
the shape of protein changing
Proteins in cell membrane act as receptors for messenger molecules = membrane-bound
receptors
Receptor proteins have specific shapes - only messenger molecules with complementary
shape can bind to them
A cell that responds to a particular messenger molecule = target cell
Many drugs work by binding to receptors in cell membranes - trigger a response in cell, or
block receptor and prevent it from working
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