APK 2105C Lecture Notes - Lecture 18: Basement Membrane, Epithelium, Paracellular Transport
23 May 2018
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Chapte 4, Lecture 4
Cell Membrane Transport
• Vesicular transport = transport of substances in membrane-bound compartments
o Endocytosis = molecules in the ECF are brought into the cell via the formation of
an endosome (bubble made of plasma membrane)
▪ Phagocytosis (relatively non-specific) = cellular eating
• Have a macromolecule that is too large to transport across the
membrane
• Plasma membrane has pseudopods that form pouch around
macromolecule
• Phagosome comes into the cell and fuses with a lysosome to
digest the components for use
▪ Pinocytosis (non-specific) = cellular drinking
• Too small of particles to bring in by phagocytosis
• Can grab multiple particles at once
• Form pit that fills up with ECF and dissolved molecules
• Pinches closed into endosome
▪ Receptor-mediated endocytosis (specific)
• Similar to pinocytosis but needs a messenger that binds to
receptor protein
• When messenger binds, in folding of plasma membrane initiates
• Brings in very specific molecules
o May be molecules that bind to receptors
o Bring in same way as pinocytosis
o Exocytosis = molecules are packaged into secretory vesicles inside the cell and
released into the ECF
▪ Molecules probably came from golgi apparatus
▪ Take secretory vesicle and fuse it with plasma membrane
▪ Once fused, plasma membrane opens up and releases contents into ECF
▪ Multiple used
• Used to secrete things from cells
• Used to put things into the plasma membrane
• Epithelial transport = movement of substances across 2 plasma membranes (2 layers)
o Absorption = transport of materials into the internal environment from the external
environment
o Secretion = transport of materials from the internal environment to the external
environment
o Epithelial structure
▪ Apical membrane = faces a lumen or surface
▪ Basolateral membrane = faces the interstitial fluid/basement membrane
• Basement membrane keeps cell in place
▪ The whole sell is covered in a plasma membrane
▪ Tight junctions between cells that keep things from passing between cells
▪ Absorption/secretion = must cross apical and basolateral membranes
o Different transport mechanisms on each side of the epithelial cell
▪ Na enters cell at apical membrane
▪ Na pumped out of cell at basolateral membrane by Na/K pump
▪ As a whole is an active process—ATP is spent
o Glucose transport
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