1.3 Membrane Structure Flashcards
How do phospholipids form bilayers?
The phosphate part is polar, as it is negative, and therefore hydrophilic, the lipid part is not polar and is therefore hydrophobic. They are amphipathic because they have both hydrophilic and hydrophobic parts. When phospholipids are placed in water, the phosphate heads are attracted to the water, but the lipids are attracted to each other, this means they form double layers with the hydrophobic hydrocarbon tails facing inwards towards each other and the hydrophobic hydrocarbon tails facing the water on either side.
What does amphipathic mean?
It means that a molecule, phospholipids, have both hydophilic and hydrophobic parts.
What were the problems with the models of membrane structure?
In 1920 Gorter and Grendel extracted phospholipids from the plasma membrane of red blood cells and calculated that the area that the phospholipids arranged in a monolayer was twice as large as the area of the plasma membrane. They deduced that the membrane contained a bilayer of phospholipids.
In 1930 Davson and Danielli tried to see where the proteins were in the membranes. They proposed layers of protein adjacent to the phospholipid bilayer. They though this sandwich model would explain how membranes, despite being very thin are a very efficient barrier to the movement of some substances.
What were the problems with the Davson and Danielli model?
They proposed that there was a layer of protein on either side of the membrane, however there were somethings that did not fit.
- Freeze etched micrographs - This technique involves rapid freezing of cells and then fracturing them. The fracture occurs along lines of weakness. When this was done with cell membranes they fractured down the middle of the membrane showing that there were globular structures in the centre of the membranes.
- Structure of membrane proteins - When proteins were extracted from membranes they were found to be varied in size and globular in shape so they would not have formed continuous layers. Also all of the proteins were hydrophobic on at least part of their surface so they would be attracted to the hydrocarbon tails of the phospholipids in the centre of the membrane.
- Florescent antibody tagging - Red or green florescent markers were attached to antibodies that bind to membrane proteins. The membrane proteins of some cells were tagged with red markers and other cells with green markers. The cells were fused together. Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell. This showed that membrane proteins are free to move within the membrane layer rather than being fixed in a peripheral layer.
So the Davson -Danielli model was replaced with the Singer Nicolson fluid mosaic model.
What do membrane proteins do?
They can be:
- Hormone binding sites
- Immobilised enzymes, that cannot move, with the active site on the outside, so attached to the membrane.
- Cell adhesion to form tight junctions between the groups of cells in tissues.
- Cell to cell communication, so neurotransmitters
- Channels for passive transport
- Pumps for active transport
What are integral proteins?
They are hydrophobic on at least part of their surface and so rather than just being stuck on the outside, they extend through the membrane.
What are peripheral proteins?
There are hydrophilic so are not embedded in the membrane. They are attached to the surface and this attachment is often reversible. Some have a single hydrocarbon in the membrane anchoring them.
How does protein content and activeness correlate?
The more active a membrane the higher its protein content because the proteins carry out activities.
What is cholesterol?
Cholesterol is a type of lipid. It belongs to a group called the steroids. Most of the molecule is hydrophobic so is attracted to the lipid part of the molecule. But the end of the cholesterol molecules has a OH- which is polar and therefore attracted to the water and will stick out from the membrane.
What does Cholesterol do?
Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
Membranes are odd - the hydrophobic tails usually behave like a solid but the phosphate tails are like a solid. Overall the membrane is fluid as components of the membrane are free to move. This fluidity needs to be controlled, if they were too fluid they would not be able to control what passed through, and if they were too fluid then they could not let anything through. Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules, so prevents them crystallising and behaving as a solid. However it restricts molecular motion and therefore the fluidity of the membrane. It also reduces the permeability to hydrophilic particles such a sodium ions and hydrogen ions.
Due to the shape of the molecule it can also help membranes to curve into a concave shape, which helps in the formation of vesicles during endocytosis.
