WEEK 1: MEMBRANE STRUCTURE Flashcards
What is a plasma membrane?
It is a lipid bilayer embedded with proteins.
State the roles of cellular membranes.
*They provide a barrier between cellular compartments or the cell and outside environment. This must be flexible and deformable, not rigid.
*They selectively transport of material between compartments.
*They detect and pass on signals.
*They provide mechanisms for cell-to-cell communication.
*They maintain cell – cell contacts.
*They carry out certain specific metabolic functions.
The lipids which make up membranes are always amphipathic molecule.
What does this mean?
Hydrophobic (water repelling) structure, and regions of hydrophilic (water associating) structure within the same molecule.
Describe the structure of a plasma membrane.
Is made up of phospholipids which have a hydrophilic, negatively charged phosphate group head and a two non-polar, electrically neutral fatty acid chains.
The polar head mix with water while the non-polar tails will not mix with water as they are hydrophobic.
The phospholipids assemble in a BILAYER, a double layer of lipid molecules when in water, the heads align on the outside while the tails bury themselves in the center of the bilayer.
How does the membrane maintain its fluidity?
How does it keep its stability?
The phospholipids are not held by strong chemical bonds, they are constantly moving.
By the presence of CHOLESTROL molecules which are tucked between the phospholipid molecules, where they prevent fatty acid chains from packing together and crystalizing.
Describe the behavior of phospholipid in water.
Phospholipids - their hydrophilic ‘heads’ are in contact with water molecules, but their hydrophobic ‘tails ‘are kept away from water.
How is this done ???:
1.Small amounts of phospholipid can form a monomolecular layer on the surface of water, with their ‘head’ groups in the water but ‘tails’ held out of the water.
2.They may form micelles – spherical aggregates with ‘tails’ pointing inwards and ‘heads’ outside, facing the water
They may form bilayers or multilayers. Again, the ‘tails’ face inwards and ‘heads’ face the water.
State the two types of membrane proteins.
State the two types of intrinsic proteins.
Intrinsic proteins:
-They are embedded in the membrane forming an integral part of its structure.
-Intrinsic proteins are the transmembrane proteins which are embedded in the lipid bilayer. They extend from one side to the other side.
Extrinsic proteins:
-They are attached to the membrane, but not part of its structure but were non-covalently bound to one side only.
-Extrinsic proteins are the membrane proteins which are located outside the membrane and bound weakly to the membrane.
2.Intrinsic type 1 and intrinsic type 2 proteins
State the function of plasma proteins.
1.They form channels through the bilayer.
2.They are carriers or transport molecules.
3.They act as docking marker acceptors.
What state is the plasma membrane?
Liquid crystal
State the need for the liquid structure of the plasma membrane.
*It allows deformability and flexibility - the cell is to change its shape is easily.
*Allows for conformational shapes change of proteins as they carry out their duties.
Outline the role of sterols in membrane.
*The sterol fills the space between phospholipids, making the membrane less permeable to small hydrophilic entities such as ions
*It also makes the membrane less fluid at higher temperatures by spacing out the phospholipids it gives the ends of the phospholipid ‘tails’ more room to move, so that the middle of the membrane is always fluid, even at low temperatures
*Thus, sterols ensure the fluidity of the membrane, and its supply of cholesterol is essential for the survival of animal cells.
