Fluid Mosiac Model Flashcards
What does the cell membrane being partially permeable mean?
Small molecules can pass between the gaps in the phosolipds
What is a phospholipid made of?
A tail made up of 2 fatty acids with a phosphate group head attached to it
Is the phosphate head hydrophobic or hydrophillic and why?
Hydrophillic
The phosphate group heads are polar so they attract other polar molecules
Are the lipid tails hydrophobic or hydrophillic and why?
Hydrophobic
The lipid tails are non-polar so they repel polar molecules
How do the phospholipids molecules in cell membrane arrange themselves?
They automatically arrange themselves into a fluid bilayer
Why is the phospholipid bilayer fluid?
The phospholipid molecules are constantly moving
Describe the phospholipid bilayer structure
- The hydrophilic phosphate heads face out towards the water (either the aqueous tissue fluid/the aqueous cytoplasm) on either side of the membrane
- The hydrophobic lipid tails are on the inside, making the centre of the bilayer hydrophobic
What does the centre of the bilayer being hydrophobic mean?
Membrane doesn’t allow water-soluble, charged or polar substances e.g ions through it
Why do the phospholipids not form micelles in the membrane?
The 2 fatty acids in the tails are too bulky to fit in the micelle’s interior
Where are the proteins in the bilayer?
They are scattered throughout the bilayer (like tiles in a mosaic)
Why can some of the proteins move?
As the phospholipid bilayer is fluid
Define an “integral” protein
Protein that are fully embedded within the phospholipids, some even spanning both layers
Describe the structure of integral protein
- The regions at the end are polar hydrophillic amino acids
- The middle portion is made up of non-polar, hydrophobic amino acids
Define a “peripheral” protein
Proteins loosely attached on the outside surface of the cell membrane
What are the two types of protein?
- Integral
- Peripheral
What are the different kinds of proteins found in the cell membrane?
- Glycoproteins
- Channel proteins
- Carrier proteins
- Enzymes
What is a glycoprotein?
Proteins with a polysaccharide chain attached to them
What is the function of a glycoprotein?
To act as a receptor for hormones by producing specific shapes
What is the function of channel/carrier proteins?
Large molecules and ions can only pass through them
What are glycolipids?
Lipids with a polysaccaride chain attached
What is the function of a glycolipid?
To provide energy and be markers for cellular recognition
What is the function of cholesterol in the cell membrane?
To fit in between the phospholipids, making bonds with them and therefore make the membrane more rigid
How does the ratio of phosopholipds containing unsaturated fatty acids to those containing saturated fatty acids relate to the fluidity of the membrane?
The greater the ratio of phosopholipds containing unsaturated fatty acids to those containing saturated fatty acids, the more fluid the membrane
How do unsaturated phospholipds ensure membrane fluidity?
The kinks in the hydrocarbon tails of the unsaturated phospholipids prevent them packing too closely together
Describe the old model for the cell membrane
3 layers - a phospholipid layer between 2 continuous protein layers
What was the evidence for the old membrane model?
Based on the EM image of 3 layers where the dark outer layers were thought to be proteins and the lighter region within was thought to be the phospholipid layer
What 2 things did the old model not allow for?
Hydrophillic phosphate heads to be in contact with the water
Hydrophobic non-polar amino acids on the outside of the membrane proteins to be kept away from water
How was the EM image re-interpreted for the fluid mosaic model?
- The darker outer layers are the more electron dense phosphate heads
- The inner lighter layer are the lipid tails
Whats was the proof for peripheral and integral proteins?
Peripheral proteins could be disassociated from the membrane easily by increasing the ionic strength of a solution it was placed in whereas the integral proteins could only be removed by adding detergents
How did freeze/fracture electron microscopy prove the existence of integral proteins?
- Frozen membrane sections were fractured along a weak point between the lipid layers
- The inner fractured surface was covered in a heavy metal
- Scanning electron microscopy gave a 3D image revealing a smooth mosaic-like surface interspersed by much larger molecules
Describe the plant protein experiment and its results
Plant proteins (lectins) that bind to polysaccharides were labelled with feretin which is viewable under the EM
- These were mixed with membrane samples
- They only bound to the outer surface and never the inner of the membrane
What did the plant protein experiment prove?
- That membranes were asymmetric (the outer surface was different to the inner surface)
- Showed that polysaccharide chains were only on the outside of the membrane
- Proved the invalidity of the protein sandwich model as this would have had lectin binding on both sides
How was cell membrane fluidity proved?
- A mouse cell and a human cell were fused
- The mouse and human proteins completely mixed throughout the cell membrane
Why does a temperature below 0 degrees mean a high membrane permeability? 4
- Phospholipids don’t have much energy, don’t move much
- Tightly packed together, membrane is rigid
- But channel proteins and carrier proteins in membrane deform, increase permeability
- Ice crystals pierce membrane, make it highly permeable when it thaws
What happens to the membrane at temperatures between 0 and 45 degrees?
- Phospholipids move around, not as tightly packed together
- Partially permeable membrane
- Temperature increases, phospholipids move more, increases permeability of membrane
What happens to the membrane at temperatures above 45 degrees?
- Phospholipid bilayer starts to melt and membrane becomes more permeable
- Water inside cell expands
- Putting pressure on membrane
- Channel proteins and carrier proteins deform - can’t control what enters/leaves
