MEMBRANESThis text is divided into seven major sections:Overview of membranesThe chemical components of membranesMembrane structureMembranes and compartmentalizationMembrane receptorsSome receptors involve second messengersInsulin and growth factor receptors
Overview of membranes.The cell is not an amorphous sack of components, but a complex structure filled with organelles. Examples include:
Membranes are not passive barriers.
Membranes are dynamic.
Preview: In the following sections we will look at membranes from the perspectives of
The chemical components of membranesGeneral composition.
Distribution of lipids in membranes
The proteins of membranes.
Carbohydrates of membranes are present attached to protein or lipid as glycoprotein or glycolipid.
Membrane structureThe amphipathic properties of the phosphoglycerides and sphingolipids are due to their structures.
Membranes are currently pictured according to the fluid mosaic model.
The difficulty with which flip-flop movement of membrane components occurs relates to the sidedness of membranes. Membrane surfaces have asymmetry -- different characteristics on the two sides.
Membrane fluidity -- according to the fluid mosaic model, proteins and lipids diffuse in the membrane.
Membranes separate and maintain the chemical environments of the two sides of the membrane.Introduction: there are ion gradients across the mammalian plasma membrane. Here is a comparison of the mean concentration of selected ions outside and inside a typical mammalian cell, giving the ion, the concentration in the extracellular fluid, the intracellular fluid and the difference betwen the two. Na+ 140 mM 10 mM 14-fold
K+ 4 mM 140 mM 35-fold
Ca++ 2.5 mM 0.1 microM 25,000-fold
Cl- 100 mM 4 mM 25-fold
Cell membranes maintain these gradients by
Some substances can cross membranes by passive (simple) diffusion.
Protein channels transport specific ions.
Transport of molecules across membranes by carriers (mediated transport).
Passive mediated transport, or facilitated diffusion.
Active mediated transport involves transport against a concentration gradient, and requires energy.
Membrane receptorsCell-cell communication is by chemical messenger.
Messenger molecules which diffuse into the cell -- example: steroid hormones.
Plasma membrane receptors.
The acetylcholine receptor of nervous tissue exemplifies a direct response type of receptor.
Some receptors involve second messengers.Sometimes the binding of an effector to a receptor leads to the formation of an intracellular molecule which mediates the response of the effector.
Cyclic AMP (cAMP) is a second messenger that mediates many cellular responses.
Inositol triphosphate (IP3) and diacylglycerol (DG) are also second messengers.
Insulin and growth factor receptors.The insulin receptor exemplifies receptors for which no second messenger has yet been identified.
Structure: The insulin receptor is a tetramer with two kinds of subunits, alpha and beta. Disulfide bridges bind them together.
The mechanism of signal transmission is unclear.
Many of the cellular responses are well known, e.g.
Termination of the insulin and certain growth factor signals involves internalization and degradation of the hormone within the cell.
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Last modified 1/5/95