Membrane Structure and Function Flashcards
exoplasmic face of the plasma membrane of a eukaryotic cell is …
… the part of the membrane that faces the exterior
The cytosolic face of the plama membrane is
… the part of the membrane which faces inside the cell towards the cytoplasm
What makes the organelles in eukaryotic cells different to prokaryotic cells
What is the benefit of this
They have membrane bound organelle
This allow areas of the cell to be comparmentalised in order for different acivities
What is the process of endocytosis and exocytosis
Endocytosis: the process of bring the contents from the outside the cell inside the cell body
Exocytosis: the process of releasing materials inside the cell, to outside the cell
What is the fluid mosaic model
created by Singer and Nicolson (1972) is describes the very basic form of any membrane
The lipid molecules are indicated in red, which form a liquid bilayers (hydrophobic tails face inwards and hydrophillic heads face outwards)
There is the outer leaflet - exoplasmic face and the inner leaflet - cytoplasmic face
There are many proteins embedded within this bilayers which can very in number, which can affect the width of the membrane + other things
What are the 3 main componets of membranes
Lipids
Proteins
Carbohydrates
What are the function of lipids in the membrane
- They form a barrier to prevent the traffic of molecules, ions etc from one side to the other/ molecules cannot permeate
- Compartmentalisation of different regions, so you can have specilised areas for different processes
- Cell-cell signalling and intercellular signalling
What is the function of proteins in the cell surface membrane
- Allows selective import and export through channels and pores
- (Receptors) receive and transmit information from one side of the membrane to the other
- Adhesion - highly proteinatious structures that would connect transmembrane proteins to the inside of the cell (influence the shape/mobility and cell behaviour using the cytoskeleton)
What is the use of carbohydrates in the cell-surface membrane
Sugars can be added to the head region of lipids, which are involved in cell identification and recognition
Membrane lipids are Amphipathic
This means
The lipids have a hydrophillic and a hydrophobic region
This allows lipids to self assemble is aqueous solutions
How do lipid micelles form
If you put lots of single tailed lipids in an aqueous solution they will from almost a cone shape through the tails coming together
So if single tailed fatty acids form a lipid micelle, what do two tailed fatty form in aqueous solution
They form a bilayers, which forms a cylindrical shape in aqeuous solution
How these fatty acid tails held together in lipid micelles and bilayers
Van der Waals forces
How do detergents work
They are also amphipathic, meaning their hydrophillic head and hydrophobic tails can disperse the lipid aggregates
Within a lipid bilayer, what can occur
There will be some degree of lateral diffusion that occurs
The individual lipid molecules are able to flex and rotate
Occasionally a lipid can flip inside the monolayer and a protein must be employed in order to facilitate that
What difference occur between lipid bilayers which have saturated vs unsaturated fatty acids
The lipid layers with unsaturated hydrocarbons are not as wide as those with saturated hydrocarbons
Note: the width of the bilayer can also be influence by cholestrol and proteins within the bilayer
What is the scientific FRAP
Fluorescence recovery after photobleaching
* Cell membrane proteins can bind to a fluorescent reagent and then bleach a particular area of interest with a laser (so it no longer produces a signal)
* Time-lapsed images of the region are taken and the recovery of fluorescence signalling in the beached region
* This recovery is only allowed through lateral diffusion as the beached proteins get redistributed
* Allows us to detect mobility within the bilayer
The longer the hydrocarbon chain of a lipid
The more fluid the membrane is
What are the 3 major classes of lipids which occur in the bilayer
Phospholids (Phosphoglyerides/phosphosphingolipids)
Glycolipids (Where the head group is a sugar residue)
III Cholesterol (straighten lipid tails to make more rigid structures)
The distribution of these liqids varies from membrane to membrane and where it is located
In animals the main type of phospholipid is phosphoglycerides
What are their structures like
The polar head is being based on a glycerol backbone to which the two hydrocarbon tails are attached
A choline group is attached to the phosphate group in the hydrophillic head
Carbon 1 and 2 on the glycerol form ester bonds to the hydrocarbon tail, and the carbon 3 bonds to the phosphate group
What is the effect of a double bond on the shape of the fatty acid hydrocarbon tail
it causes the tail to kink
It causes the membrane to be more fluid
Mammalian phospholipid can have variation within their head group and these group can interchange within the cell membrane
Name a few examples of this
- Phosphatidylethanolamine (PE) - smallest head size
- Phosphatidylserine (PS) - this group creates a charge as it is a neutral group and phosphate is negative - small head size
- Phosphatidylcholine (PC) - most commonly found and largest head size
- Sphingomyelin - this replaces the glycerol with sphingosine (has an additional NH group between C4-C5
What affect does head size have on the oritentation of phospholipids
Larger heads are more likely to form a bilayer, whereas a small head group is likely to form a cone shape
Mitocondria want a curved membrane so their phospholipids will have smaller head groups
What type of functional group do the hydrocarbon fatty acids tails have
Carboxylic acid
Fatty acid tail length is between 14-24 carbon long
What is the average length for a fatty acid tail
Commonly 16-18 Carbons long
The more unsaturated a hydrocarbon fatty acid tail is…
… the more fluid the membrane is
The lipid bilayer is asymmertrical, what does this mean?
that the types of phopholiqids on one side of the bilayer, is not the same on the other side
Note at the cytosolic surface, there is a high amount of phosphotidylserine, which the negative charged group sticks out of the membrane
Whereas phosphotidylcholine and glycolipids are found at the exoplasmic face
Does lateral diffusion occur actively or passively
Passively
Name an enzyme which is used to move a phospolipid from one bilayer to the other
Flippases
Allow the movement from one leaflet to another
How can the flipping of phospholipids be used in cell signalling
When normal platelets are activated at sites of vascular injury, phosphatidylserine (PS) is transported from the inner to the outer leaflets of the membrane
Phosphotidylserine is usually found on the plasma membranes cytoplasmic leaflet
Why would it occur on the exoplasmic leaflet
It is a sign of apopotosis - cell suicide
Glycolipids are only found on the exoplasmic leaflet, how does this relate to its function
Because glycolipids are used in cell recognition
Example: Blood cell recognition (A, B, AB etc)
What is the basic structure of a glycolipid
- Will havea sphingolipid base, rather than glycerol
- 1 or more sugar residues (glucose, galactose etc)
- No phosphate
- Neutral or acidic
- Found on exoplasmic leaflet
Where does lipid synthesis occur
In the Smooth endoplasmic reticulum
Where are lipids destroyed
In the lysosome
How does the insersion of new phospholipids into the plasma membrane occur
Newly synthesised phosphlipids are shipped from the SMR using the cytoskeleton and fuse the the existing membrane via exocytosis mechanism
Membranes can also come into contact, and movement of phospholipids can occur then (mitocondria)
Fatty acid binding proteins (FABP), which would encapture a particular fatty acid and traffic it to its destination
Cholesterol is found in both the inner and outer leaflet of the PM and can vary in quanity up to a 1:1 ratio of lipids to cholesterol
What is its main purpose
Is to stiffen the hydrocarbon tails on the phospholipids, making them more straight
This hence makes the membrane wider
The same applies for any other Sterols across other organisms
The phosphlipid bilayer created a barrier to prevent things entering and leaving a cell/organelle. Very little can move through this bilayer through passive diffusion
What moleulce can more through the bilayer
Hydrophobic molecules: O₂, N₂, CO₂, and steriod hormones (they can passively dissolve in the membrane)
H₂O is the only small uncharge polar molecule which can move through the bilayer
Name two instance where the ratio of lipids to proteins in the PM differs and why
Nerve cell are myelin coatrd: Protein:Lipid 1:4. This is due to the need for insulation
Mitocondria (Protein:Lipid 4:1) has a dense concentration of proteins in its membrane to allow maximum oxidative phsophorylation to occur in the membranes
A protein which is directly in contact/passes through the membrane is called
An intrinsic/integral protein
A protein which is indirectly associated with the membrane using another protein or an anchoring motif is called
Extrinsic/Peripheral proteins
What is a transmembrane protein
A protein which crosses the membrane completely
What do you call a protein which doesnt pass through the whole membrane/only associated with one side of the membrane
Cytosolic (cytosolic leflet)
Extracellular (exoplasmic leflet)
This can be done by adding some lipid motifs to the end of the protein, allowing it to anchor into the hydrophobic lipid molecules or by embedding in the membrane
How can a membrane be specilised for different microdomains
- Lots of phosphotidylethanolamine, allows the membrane to curve/bend
- Cholesterol as well as being a signalling molecules, also straightens the fatty acid tails, making the bilayer wider
What is the main purpose of protien in a lipid bilayer
As a means for transporting larger ion and polar molecules effectively across the hydrophobic lipid bilayer
This can be done through channels, transporters and through pumps
If you require energy to transport a molecule from one side of the membrane to the other the process is
active transport - using ATP
Passive transport …
Doesn’t require energy
However it can either be simple diffusion through the membrane or require a channel or transporter protein
How do transport mediated, transporter proteins operate
The bind of the substrate causes a conformational change in the protein and the subtrate is released on the other side of the membrane
This process is passive, and the binding alone is sufficient to cause a conformational change
When is active transport needed
When we are going against the concentration gradient
ATP must be used to facilitate a change in that particular molecules conformation
The proteins which do this are referred to as pumps
Describe channel proteins
Facilitated diffusion of molecules from one side of the bilayer to the other
It is a fast and passive process
Channels can be gated (voltage, ligand binding or mechanically gated) or non-gated
Give an example of a channel protein
Aquaporin: will traverse the membrane, will allows the movement of polar water through the hydrophobic membrane
Charged amino acids are found on the inside of the protein pore molecule, away from the hydrophobic core
Excludes water soluable ions
Describe the general features of a transporter/carrier protein
Slow
Require a conformational change
Solute unchaged
Specific (e.g. glucose transporter)
Can work in both directions
The ability of a transporter to deliver a molecule from one side to the other can be described in terms of
Michaelis-Menten kinetics
What would the line look like for simple diffusion and transporter-mediated diffusion when concentration of transported molecule is plotted against rate of transport
Simple diffusion: linear correlation
Transporter-mediated diffusion: indicates we get saturation of the transporter at Vmax (maximum rate of diffusion of a solute) because all binding sites become saturated
*note half Vmax shows Km which is the affinity of solute
What is the name of the transporters, where the transport of one molecule depends on the transport of another molecule either in the same or opposite direction (no energy requirement)
Smyport - same direction
Antiport - Opposite direction
The bind of both molecules is required to allow transport across the membrane (Obligatory)
Give an example of an ATP-driven pump
P-type pump: Is responsible for the transport of Calcium across the sarcoplasmic reticulum in muscle cells
They depend on the hydrolysis of ATP to cause a conformational change to deliver molecule from one side of the cell to the other (against a concentration gradient)
Give examples of 3 different types of active transport
- Ionic gradient (transport of ions against their gradient)
- ATP-driven pump
- Light-driven pump
What are receptors
Relaying information from outside of the cell through a receptor, across the receptors transmembrane, which sets off a signalling cascade which eventually leads to an alteration in intracellular processes (e.g. gene expression for HERs)
How do protein close to the plama membrane link with proteins which are embedded within the plasma membrane to allow an adherent cell to stick to a subtrate - Focal Adhesions
There is an extracellular matrix, which is a transmembrane protein, will bind to proteins that are on the inside of the cell
It is an extrinsically associated protein, that is now allow to be in close proximity to the membrane
