Chapter 5- Biological Membrnes Flashcards
What is osmosis
This is the net movement of water molecules from a more dilute solution (which has more water molecules) to a more concentrated solution (which has fewer water molecules) across a partially permeable membrane.
What are the different types of solutions
Isotonic
Hypotonic
Hypertonic
What does isotonic mean
This is when the osmotic pressure is the same in both solutions on both sides of the partially permeable membrane.
What does hypotonic mean
When a solution on one side of the partially permeable membrane has a lower osmotic pressure than the solution on the other side of the membrane.
What does hypertonic mean
When a solution on one side of the partially permeable membrane, has a higher osmotic pressure than the other solution.
What does turgid mean
When a plant cell contains as much water as possible, it is turgid.
What is cell lysis (or haemolysis)
When too much water enters an animal cell, it will burst. This is called cell lysis.
In which solution, does cell lysis (or haemolysis) and turgid cells occur
Hypotonic solution. In a hypotonic solution, there is a higher water potential outside of the cell. This causes water to flow inside the cell making it swell up- an animal cell goes through cell lysis and makes the plant cell turgid.
What is plasmolysis
A plant cell can lose so much water that its membrane starts to pull away from the cell wall.
What is crenation
When an animal cell looses water, it shrivels. It is said to be crenated
In which solution does crenation and plasmolysis occur
These processes only occur in hypertonic solution.
In a hypertonic solution, there is a lower water potential outside of the cell. This causes water cells to flow inside the cell, making a plant cell plasmolysed and an animal cell crenated.
What happens in an isotonic solution
There is no change in the solute concentration and net water loss or gain inside the cell
There is equilibrium in and out of the cell
Functions of membranes
Controls the exchange between cell and environment Compartmentalises Involved in cell signalling Site for chemical reaction Cell communication
Fluid mosaic model
Is about 7mm thick
Mainly composed of lipid and protein
Appears a single straight line under a light microscope
This structure is fluid since the phospholipids can move around and so can the lipid layers and proteins
Mainly composed of a phospholipid bilayer (75%) and 25% of protein, polysaccharide and cholestrol
Glycolipids are scattered
Phospholipids have a hydrophilic polar head and a non-polar hydrophobic tail
Why is the structure of a membrane stable
The heads of the phospholipids face outwards since they are attracted to the water and the tails face inwards since they repel water.
The tails also attract each other further stabilising the structure.
These phospholipids are able to form miscelles, monolayers and bilayers
Channel and carrier proteins
Span the width of the membrane; are intrinsic
Known as transmembrane proteins
Transports water-soluble charged particles through the membrane because such substances cannot go through the lipid bilayer.
Polar and ionic molecules use protein channels to go through
The protein carriers are used to allow very large molecules (polar and non-polar molecules) to flow through
Glycoproteins and Glycolipids
Glycoproteins have a polysaccharide attached and are intrinsic. They carb chain protrude from the plasma membrane.
These will usually have two chains
These usually form hydrogen bonds with water and in doing so stabilise the structure.
important in cell signalling, acts as receptors for certain molecules (ex: hormones) and triggering specific changes in the cell when that molecule binds
Glycolipids also has three polysaccharide chains attached
Both form the surface antigens by which the immune system can identify the cell as belonging to the body (or identify as foreign if transplanted into the body of another individual)
Glycolipids are the site of cell recognition
Cholestrol
Maintains a suitable level of fluidity in the membrane.
Has a hydrophilic and hydrophobic portions.
This allows this to bind with the phospholipids and prevents it being too fluid.
Remember double or triple bonds in the hydrocarbon that form the fatty acids result in the fatty acid tail having a kink and thus making it more fluid.
The fluidity of the phospholipids also depend on the length of the tail- the shorter the tail, the more fluid the phospholipid.
At the same time, the fatty acid tails could come together and crystallise, making the phospholipids less fluid. Cholestrol prevents this.
Makes up 20% of lipid in animal cells; none in plants
Factors that affect the permeability of the membranes.
TEMPERATURE
Temperature- at a higher temperature, the phospholipid bilayer starts to break down and therefore become more permeable.
The channel proteins and carrier proteins in the membrane denature so they can’t control what enters the cell or leaks from the cell- this also increases the permeability.
At a very low temperature, the phospholipids don’t have much energy so they can’t move much. They are packed closely together and the membrane is rigid.
Ice crystals form and these pierce the membrane, making it highly permeable when it thaws.
The protein channels and carriers are inactive at a low temperature, so any substance can enter the cell and or leak from the cell.
What does a membrane permeability and temperature graph look like
Y axis- membrane permeability
X axis- temperature
The graph will start at the top and then go down with a very steep gradient.
Another line will start going up with a constant rate until it reaches a certain permeability.
Factors affect the membrane permeability
SOLVENT
Some solvent dissolve the lipids in a cell membrane so the membrane looses its structure.
This increases its permeability more than the others.
Increasing the concentration of the solvent will increase the membrane permeability
How does a solvent and membrane permeability graph look like
Y axis- membrane permeability
X axis- acid concentration
It starts off at the bottom and increase at a constant rate until it curves off at a high certain permeability value.
How is communication carried out in cells
Cells produce chemical messenger molecules which are transported to others. Some cells are unaffected by the chemical messengers because they have no means of detecting it.
Cells that do have the correct cell membrane receptors detect the messenger molecule and changes occur in the cell as a result.
Plasma membrane receptors
Each receptor is specific to a particular messenger
Cells have hundreds of different types of receptors.
A cell that responds to a particular messenger is a target cell for that messenger
This is because the receptors have the right complimentary shape for the messenger
Glycolipids could be responsible for this
Examples of target cells and messengers
Liver cells have receptors for the hormone, glucagon, which causes them to release glucose from stores of glycogen within the cells.
Skeletal muscle cells encounter glucagon as it circulates in the blood but do not respond to it, because they lack the right type of receptor that would have the complimentary shape
How do receptors change cells
Change in the receptor may:
- cause the release of a second messenger inside the cell which initiate a variety of effects
- cause the opening of a protein channel that was previously closed, or the closing of one that was open.
- activate an enzyme within the cell, or the membrane protein may be an enzyme itself which is activated
Membrane receptors and drugs
The messengers to which cell membrane receptors respond to could be initiated from within and out of the cell
Medicinal drugs affect only known target cells and create (or block) specific responses.
