Lecture 1: Cell membrane and transport mechanisms A summary of the lecture about cell membranes, their properties, and descriptions of transport mechanisms in the cell membrane.

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16 Oct 2011

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Physiology Lecture 1
Cell membrane Made of a dilipid layer, with proteins and channels in it to allow the
movements of chemicals. Allows diffusion of gases and lipid-soluble molecules.
o Lipids have a phosphate head and two fatty acid tails. Straight tail for strength,
bent tail for flexibility. Head is hydrophilic, tails are hydrophobic.
o Membrane stabilized by cholesterol.
Membrane proteins are structural support for the cytoskeleton. Go through membrane
and allow for channels, and can also act as receptors to signal molecules. The proteins
criss-cross the membranes, like a needle and thread. On one end is NH2, on the other,
Glycocalyx is a matrix of carbohydrate filaments in the extracellular space, anchored to
the proteins (glycoproteins) or lipids (glycolipids). They slow diffusion and contain a net
negative charge, attracting positive ions, calcium in particular.
Channels are combinations of proteins to allow non-membrane permeable molecules in
and out of the cell. Parts of it dangle in the cell, forming “pore loops”. These act as a
filter, allowing only certain kinds of molecules through.
o An example of one is an “Aquaporin”, which allows water and small non-ionic
molecules in and out.
o Channels aren’t usually kept open and can switch between open and closed. This
is achieved through the binding of a chemical, or voltage across the membrane.
Active transport is moving molecules across the membrane AGAINST their concentration
gradient. This requires energy from ATP hydrolysis, with the substrate binding to the
protein to be moved.
Facilitated diffusion is carrier proteins moving polar molecules across the cell
membrane. No energy from ATP is needed, and the movement is down concentration.
Cotransport is several molecules moving across at once. Symporter is in the same
direction, antiporter has one or more moving in opposite directions.
o When these are powered by the kinetic energy of a substance moving DOWN it’s
concentration gradient, it’s called Secondary Active Transport.
Exo/endocytosis is the transference of a substance in a vesicle, either from the inside to
out (exo) or vice versa (endo). This can be quick, spread out, or in bulk. The full one uses
proteins to help this process. (Clathrin for scaffolding and dynamin for the “sewing” of
the top.)
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