Plasma Membrane 1 Flashcards
What is a plasma membrane?
The structural component that organises cells into functionally distinct compartment, separating the cell from its environment and giving the organelles within the cell their own composition
Each membrane has unique function, protein and lipid components
Formed by non-covalent interaction of lipids and proteins
2 layers - hydrophilic heads face extracellularly and hydrophobic tails face intracellularly
What is the function of plasma membranes?
Control the intracellular composition of the cell and subsequent organelles, regulating when and what molecules move into or out of them
This regulation of movement allows for a unique composition within each compartment to facilitate their unique functions
Describe phospholipids in relation to the plasma membrane:
Membrane lipids have hydrophilic (polar) and hydrophobic (non-polar) regions
Phospholipids allow for the formation of plasma membrane due to philic + phobic
Amphipathic - one end hydrophilic and the other hydrophobic
The hydrophobic interactions with each other and their hydrophilic interactions with water direct the packing of lipids in the membrane
What is a leaflet?
Each side of the bilayer is called a leaflet when discussing the plasma membrane
Describe the complete structure of a phospholipid:
Hydrophilic head composed of - Phosphate head and
Glycerol backbone
Phosphodiester bond between phosphate head and glycerol backbone
Backbone - Glycerol-3-phosphate
3 carbons in glycerol
C1 and 2 are esterified by the addition of the fatty acid hydrocarbon tails.
C3 is linked to a phosphate which is attached by a phosphodiester covalent bond to ‘alcohol/OH’ or polar head group
Fatty acid tails - either saturated (no C=C) or unsaturated (have C=C) and are key regulators of the structure and mobility of the lipid bilayer
Describe the structure of a glycolipid:
Have a glycerol backbone
Instead of polar head groups there is a sugar residue covalently linked by a glyosidic bond/linkage to the C3 group
Sugar residue is typically glucose or galactose
No phosphate group
C1 and C2 of glycerol are esterified with fatty acids
Glycolipids are only found on the extracellular side
What are some functions of glycolipids?
Cell Recognition
Cell Adhesion
Membrane Stability
Describe the structure of sterols
Sterols are lipids that contain a hydroxyl group attached to a conserved 4 fused carbon ring structure A,B,C,D
Carbon A,B + C have 6 carbons each
Carbon D has 5 carbons
Fused ring is called a steroid nucleus
Cholesterol is the major sterol of animal membranes
What are the characteristics of the fluid mosaic model?
Structure - bilayer
Movement - fluidity, implies movement on the membrane
Function - asymmetric and selectively permeable
Mosaic implies that the membrane consists of many different molecules
Describe the bilayer as a characteristic of the fluid mosaic model:
Phospholipids are amphipathic
Polar head - charged, hydrophilic, form H bonds with H2O
Nonpolar tail - no charge, hydrophobic, no H bonds
Planar phospholipid bilayer with edges leaves hydrophobic tails exposed to water
So the bilayer rounds up and forms a sphere to protect the hydrophobic tails, forming an energetically favorable state
Describe fluidity as a characteristic of the fluid mosaic
Lateral diffusion of phospholipids within the plane of the leaflet
Individual phospholipid movement of rotation and flexion
Flip flopping - requires significant input of energy so flippase enzyme required
Ratio of unsaturated vs saturated hydrocarbon tails will influence the packing/tightness of the phospholipids and therefore the overall stiffness or room for movement of the lipids
Why is flipping important ?
Important in the building of new membranes as the lipids are synthesised on the intracellular side of endoplasmic reticulum
Ensures that there is symmetric growth of both leaflets of the bilayer and provide asymmetry to the leaflet components
Describe cholesterol’s influence on membrane fluidity:
Cholesterol stiffens the bilayer and decreases fluidity and permeability
Fits into spaces between phospholipids and prevent water-soluble molecules from diffusing across themembrane
Describe selectively permeable membrane:
Mainly lipids with hydrophobic interior, small hydrophobic molecules can readily diffuse across the membrane down their concentration gradient into the cell
In contrast, any large, uncharged molecules or ions are repelled from the membrane, requiring additional mechanisms to pass through the membrane.
Describe some sources of lipid asymmetry:
All lipids are synthesized on the cytosolic surface of the ER
Location of synthetic enzymes
Enzymes in ER lumen
