Chapter 5 Plasma Membranes Flashcards
Compartmentalisation
The formation of separate membrane-bound areas in a cell
It is vital because if you contain reactions in specific parts of cells then it allows the specific conditions for reactions to be achieved.
Required for cellular reactions and protects vital cell components
Membrane structure
The cell surface membrane is known as the plasma membrane.
Formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane.
Has the fatty acid tails of the phospholipids in between forming a hydrophobic core.
The outer surface of the hydrophilic phosphate heads can interact with water.
Cell membrane theory
Membranes were first seen in 1950s when electron microscopes were made. It showed that they were two black parrallel lines - supporting theorys of lipid bilayer.
Then the fluid-mosaic model was formed as the phospholipids are free to move within the layer, giving it flexibility.
A closer look at cell membrane components
Plasma membranes contain various proteins and lipids. The quanitity and type are dependant on the type of cell.
Glycoprotein - branchin carbohydrate portion of a protein which acts as a recognition site for chemicals. Used for cell signallinge.g. hormones
Glycolipid - acts as a recognition site, e.g. for cholera toxins. Used for immune system
Cholesterol - for stability/flexibility
extrinsic proteins - protein molecule partly embedded. or lying on the surface.
hydrophobic tails of phospholipid molecules.
intrinsic protein - protein molecul spanning the phospholipid bilayer
hydrophilic heads of phospholipds molecules point outwards.`
Intrinsic Proteins
are embedded through both layers of a membrane. Thye have amino acids with hydrophobic r-groups on their external surfaces, which interact withe hydrophobic core of the membrane keeping them in place.
Channel Proteins - provide a hydrophilic channel that allows passive movement of polar molecules and ions down a conc. gradient through membranes. They are held in place by interactions between the hydrophobic core of membrane and the hydrophic R - groups on the outside.
Carrier proteins - important role in passive transport and active transport
Glycoproteins
Are intrinsic proteins - embeedded in cell-surface membrane with attached chains of varying length
Play a role in cell adhesion and act as receptors for chemical signals.
When a chemical binds to the receptor it will cause cell signalling.
Some drugs act by binding to cell receptors
Glycolipids
similar to glycoproteins.
They are lipids with attached carbohydrate sugar chains.
These molecules are cell molecules or antigens and can be recognised by the cells of the immune system for themselves or others.
Extrinsic proteins
Present on only one side of the bilayer.
Hydrophilic R-groups on their outer surface and interact with the polar heads of the phospholipids or intrinsic proteins.
Some move between layers.
Cholesterol
is a lipid with a hydrophilic end and hydrophobic end, like a phospholipid.
regulates fluidity of membranes.
Are positioned between phospholipids in a membrane layer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails
Adds stability to the membranes without allowing them to become to solid by preventing them from grouping together and crystalising.
Factors affecting membrane structure: Temperature
When temperature is increased the phospholipid will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure. If the temperature continues to increase cells will eventually break down completely.
This loss in structure increases permeability allowing particles to cross it.
Carrier and channel proteins will denature in the membrane at higher temperatures. So membrane permeability will be affected.
Factors affecting membrane structure: Solvents
If a solvent is polar it makes the hydrophobic tails face away from the water and create a hydrophobic core with hydrophilic heads. However if there was an organic solvent which are less polar than water or a non-polar solvent then it will create disorientation in the membrane creating more gaps etc. and increasing the permeability for it. This is why alcohol is used to tried cuts since it breaks down the membrane of the bacteria and makes it more fluid and permeable allowing the bacteria to me destroyed.
Diffusion
is the net, or overall, movement of particles from an area of high concentration to an area of low concentration.
Passive transport since it doesn’t require energy.
Factors affecting rate of diffusion
Temperature - the higher the temp the higher rate of diffusion since particles have more kinetic energy.
Concentration difference - the greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.
With a barrier :
Surface area - the larger the area of exchange surface the higher the rate of diffusion.
thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion.
Diffusion accross membranes
involves particles crossing the phospholipid bilaye. It can only happen if the membrane is permeable to the particles - non-polar molecules such as oxygen (O2) diffuse through freely down conc. gradient.
The hydrophobic interior repels ions, polar molecule , water.
Small polar molecules move through at a very slow rate so they are partially permeable.
Facilitated diffusion
ions and polar molecules that are large can pass through channel proteins.
Protein channels are selectively permeable.
Can also involve carrier proteins which change shape when a specific molecule binds.
In facilitated diffusion the movement of the molecules is down the conc. gradient and doesn’t require energy.
Rate of facilitated diffusion is dependant on - temp, conc grad, SA, thickness, number of channel protein.
