1.3 Membrane Structure Flashcards
Define hydrophilic
Substances which are attracted to water
Define hydrophobic
Substances which are not attracted to water
Define amphipathic
A molecule that is part hydrophilic and part hydrophobic
Why are phospholipid molecules unusual?
Because they are amphipathic - part of the molecule is hydrophilic, and part is hydrophobic
What is the hydrophilic part of a phospholipid molecule?
The phosphate group - or phosphate heads
What is the hydrophobic part of a phospholipid molecule?
2 hydrocarbon chains - or hydrocarbon tails
Define phospholipid bilayer
The 2 layers which are formed when phospholipid molecules are mixed with water - and the phosphate heads are attracted to the water, and the hydrocarbon tails are attracted to eachother - creating 2 layers
Stable structure
What forms the basis of all cell membranes?
Phospholipid bilayer
Who deduced that the cell membrane contained a bilayer?
Gorter and Grendel, 1920s
When was the model proposed by Davson and Danielli?
1930s
What was the model proposed by Davson and Danielli?
Sandwich model - layers of protein adjacent to the phospholipid bilayer, on both sides of the membrane.
Why did Davson and Danielli propose the sandwich model?
They thought it would explain how membranes, despite being very thin, are a very effective barrier to the movement of some substances.
What supported the Davson and Danielli model?
Electron micrographs from the 1950s showed a railroad track appearance - proteins appear dark in electron micrographs, and phospholipids appear light - fitted the model
When was the model proposed by Singer and Nicholson?
1966
What is the Singer and Nicholson model of membrane structure? (Fluid Mosaic Model)
Proteins occupy a variety of positions in the membrane
Peripheral proteins are attached to the inner and outer surface
Integral proteins are embedded in the phospholipid bilayer - some protruding out from one or both sides of the bilayer
Proteins are like tiles in a mosaic - phospholipid molecules are free to move in each of the two layers of the bilayer, proteins are also able to move
Who proposed the fluid mosaic model?
Singer and Nicholson
State what evidence falsified Davson and Danieli’s model
Freeze-etched electron micrographs
Structure of membrane proteins
Fluorescent antibody tagging
Describe the freeze-etched micro graphs technique - Davson and Danieli model falsification
Rapid freezing of cells, and then fracturing them
The fracture occurs along the line of weakness - including the centre of membranes
Globular structures scattered through freeze-etched images of the centre of membranes were interpreted as transmembrane proteins
Describe the structure of membrane proteins - Davson and Danieli model falsification
Improvements in biochemical techniques allowed proteins to be extracted from membranes.
They were found to be very varied in size and globular in shape- so were unlike the type of structural protein that would form continuous layers on the periphery of the membrane.
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.
Describe fluorescent antibody tagging - Davson and Danieli model falsification
Red or green fluorescent 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 rather than being fixed in a peripheral layer.
What is the currently accepted membrane model?
Singer and Nicholson - fluid mosaic model
What is the main function of cell membranes?
To form a barrier through which ions and hydrophilic molecules cannot easily pass
What are the 6 functions of membrane proteins?
- Hormone binding sites (hormone receptors) - insulin receptors
- Immobilised enzymes with the active site on the outside - in the small intestine
- Cell adhesion to form tight junctions between groups of cells in tissues and organs
- Cell-to-cell communication - receptors for neurotransmitters at synapses
- Channels for passive transport to allow hydrophilic particles across by facilitated diffusion
- Pumps for active transport which use ATP to move particles across the membrane
What are the 2 groups of membrane proteins?
Integral proteins and peripheral proteins
Describe integral proteins?
Integral proteins are hydrophobic on at least part of their surface
Embedded in the hydrocarbon chains in the centre of the membrane
Can be transmembrane - extend across the membrane, hydrophilic parts projecting through the regions of phosphate heads on either side
Describe peripheral proteins
Peripheral proteins are hydrophilic on their surface, so are not embedded in the membrane
Most are attached to the surface of integral proteins - this attachment is often reversible
Some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the membrane surface
What is the protein content of a very active membrane?
The more active a membrane, the higher the protein content
What features do you need to include when drawing the fluid mosaic model of membrane structure?
Phospholipids
Integral proteins
Peripheral proteins
Cholesterol
What are the two main components of cell membranes?
Phospholipids and proteins
What is a key component of cell membranes in animals?
Cholesterol
What group of substances does cholesterol belong to?
Cholesterol is a type of lipid - but is not a fat or oil
It belongs to a group of substances called steroids
Describe the cholesterol molecule?
Most of a cholesterol molecule is hydrophobic, so it is attracted to the hydrophobic hydrocarbon tails in the centre of the membrane
But one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic. This is attracted to the phosphate heads on the periphery of the membrane.
Cholesterol molecules are therefore positioned between phospholipids in the membrane
Describe the fluidity of cell membranes
The hydrophobic hydrocarbon tails usually behave as a liquid
The hydrophilic phosphate heads act more like a solid
Overall - the membrane is fluid as components of the membrane are free to move
Why does the fluidity animal cell membranes need to be carefully controlled?
Because if the animal cell membranes are too fluid, they would be less able to control what substances pass through
If they were not fluid enough, the movement of the cell and substances within it would be restricted
What is the function of cholesterol in animal cell membranes?
Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules - preventing them from crystallising and behaving as a solid
Restricts molecular motion, and therefore the fluidity of the membrane
Reduces the permeability to hydrophilic particles - such as sodium ions and hydrogen ions
Helps the formation of vesicles during endocytosis
What does the shape of cholesterol do?
The shape helps membranes to curve into a concave shape, which helps in the formation of vesicle during endocytosis
