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Biological Sciences
Course Code
Tanya Da Sylva

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Plasma Membrane (II)
Protein Mobility: Some freely mobile
Others have restricted mobility
-tethered to the underlying cytoskeleton
-bound to the external ECM
Studied by FRAP: fluorescence recovery after photo-bleaching: Can not identify
individual protein units. You dont know if the individual proteins are moving or
they are attached to something else
Membrane Lipid Mobility
Phospholipids are small and make up the bilayer
Would expect them to move freely
They dont, instead seem to move freely within a compartment but movement
is restricted between compartments
Removing membrane skeletal components removes some barriers to movement
Membrane Domains & Cell Polarity
Membranes are rarely homogenous
Distinct variations in protein composition and mobility
Eg. Sperm
Distinct parts, continuous plasma membrane
Movement across PM: plasma membrane
The plasma membrane has two main functions

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Retain dissolved materials in a cell
Allow necessary materials in
Charged and polar molecules cant pass through hydrophobic membrane easily
Membranes must be selectively permeable.
Two means of movement across PM:
Passively by diffusion from high to low concentration [can involve protein
Actively by energy-coupled process : energy driven protein pump. Can
move against concentration gradeint
Solute movement
Diffusion =
Spontaneous process
Moves from high concentration to low
When solute is an ion (charged) movement depends on concentration gradient and
electrical gradient
If both forces = electrochemical gradient
- Voltage/potential difference
Electrochemical gradient can be very strong.
-intermembrane space is +ve
-matrix is ve. Therefore H+
wants to move into the
matrix because they are
A) Simple Diffusion through
bilayer: Passive
Substances must go down a gradient and membrane must be permeable to them

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Non-polar: dissolve in lipids
Small, uncharged (polar) molecules gases etc.
-Not large, polar molecules
Small polar molecule
Moves more rapidly than solutes
Process called = osmosis
Towards hypertonic, away from hypotonic
-Hypotonic solution: water flows inside, cell swells: net gain
-Hypertonic solution: water flows outside, cell shrinks: net loss
-Isotonic solution: no net loss or gain.
B) Simple Diffusion through protein channels: Passive
Move through ion channels
Movement is bidirectional
Channels are selective to particular ion down its [ ], voltage or electrochemical
-channels are gated: either an open or closed configuration
I. Ligand gated channels
A specific molecule (ligand) binds and opens channel
Ligand is usually NOT the solute that passes through
Eg. Ach (acetylcholine)-neurotransmitter
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