Membrane Structure and Function

Question 1

exoplasmic face of the plasma membrane of a eukaryotic cell is …

Answer 1

… the part of the membrane that faces the exterior

Question 2

The cytosolic face of the plama membrane is

Answer 2

… the part of the membrane which faces inside the cell towards the cytoplasm

Question 3

What makes the organelles in eukaryotic cells different to prokaryotic cells
What is the benefit of this

Answer 3

They have membrane bound organelle
This allow areas of the cell to be comparmentalised in order for different acivities

Question 4

What is the process of endocytosis and exocytosis

Answer 4

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

Question 5

What is the fluid mosaic model

Answer 5

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

Question 6

What are the 3 main componets of membranes

Answer 6

Lipids
Proteins
Carbohydrates

Question 7

What are the function of lipids in the membrane

Answer 7

• 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

Question 8

What is the function of proteins in the cell surface membrane

Answer 8

• 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)

Question 9

What is the use of carbohydrates in the cell-surface membrane

Answer 9

Sugars can be added to the head region of lipids, which are involved in cell identification and recognition

Question 10

Membrane lipids are Amphipathic
This means

Answer 10

The lipids have a hydrophillic and a hydrophobic region
This allows lipids to self assemble is aqueous solutions

Question 11

How do lipid micelles form

Answer 11

If you put lots of single tailed lipids in an aqueous solution they will from almost a cone shape through the tails coming together

Question 12

So if single tailed fatty acids form a lipid micelle, what do two tailed fatty form in aqueous solution

Answer 12

They form a bilayers, which forms a cylindrical shape in aqeuous solution

Question 13

How these fatty acid tails held together in lipid micelles and bilayers

Answer 13

Van der Waals forces

Question 14

How do detergents work

Answer 14

They are also amphipathic, meaning their hydrophillic head and hydrophobic tails can disperse the lipid aggregates

Question 15

Within a lipid bilayer, what can occur

Answer 15

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

Question 15

What difference occur between lipid bilayers which have saturated vs unsaturated fatty acids

Answer 15

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

Question 15

What is the scientific FRAP

Answer 15

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

Question 15

The longer the hydrocarbon chain of a lipid

Answer 15

The more fluid the membrane is

Question 16

What are the 3 major classes of lipids which occur in the bilayer

Answer 16

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

Question 17

In animals the main type of phospholipid is phosphoglycerides
What are their structures like

Answer 17

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

Question 18

What is the effect of a double bond on the shape of the fatty acid hydrocarbon tail

Answer 18

it causes the tail to kink
It causes the membrane to be more fluid

Question 19

Mammalian phospholipid can have variation within their head group and these group can interchange within the cell membrane
Name a few examples of this

Answer 19

• 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

Question 20

What affect does head size have on the oritentation of phospholipids

Answer 20

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

Question 21

What type of functional group do the hydrocarbon fatty acids tails have

Answer 21

Carboxylic acid

Question 22

Fatty acid tail length is between 14-24 carbon long What is the average length for a fatty acid tail

Answer 22

Commonly 16-18 Carbons long

Question 23

The more unsaturated a hydrocarbon fatty acid tail is...

Answer 23

... the more fluid the membrane is

Question 24

The lipid bilayer is asymmertrical, what does this mean?

Answer 24

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

Question 25

Does lateral diffusion occur actively or passively

Answer 25

Passively

Question 26

Name an enzyme which is used to move a phospolipid from one bilayer to the other

Answer 26

Flippases Allow the movement from one leaflet to another

Question 27

How can the flipping of phospholipids be used in cell signalling

Answer 27

When normal platelets are activated at sites of vascular injury, phosphatidylserine (PS) is transported from the inner to the outer leaflets of the membrane

Question 28

Phosphotidylserine is usually found on the plasma membranes cytoplasmic leaflet Why would it occur on the exoplasmic leaflet

Answer 28

It is a sign of apopotosis - cell suicide

Question 29

Glycolipids are only found on the exoplasmic leaflet, how does this relate to its function

Answer 29

Because glycolipids are used in cell recognition Example: Blood cell recognition (A, B, AB etc)

Question 30

What is the basic structure of a glycolipid

Answer 30

* Will havea sphingolipid base, rather than glycerol * 1 or more sugar residues (glucose, galactose etc) * No phosphate * Neutral or acidic * Found on exoplasmic leaflet

Question 31

Where does lipid synthesis occur

Answer 31

In the Smooth endoplasmic reticulum

Question 32

Where are lipids destroyed

Answer 32

In the lysosome

Question 33

How does the insersion of new phospholipids into the plasma membrane occur

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

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

Question 36

Name two instance where the ratio of lipids to proteins in the PM differs and why

Answer 36

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

Question 37

A protein which is directly in contact/passes through the membrane is called

Answer 37

An intrinsic/integral protein

Question 37

A protein which is indirectly associated with the membrane using another protein or an anchoring motif is called

Answer 37

Extrinsic/Peripheral proteins

Question 38

What is a transmembrane protein

Answer 38

A protein which crosses the membrane completely

Question 39

What do you call a protein which doesnt pass through the whole membrane/only associated with one side of the membrane

Answer 39

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

Question 40

How can a membrane be specilised for different microdomains

Answer 40

* 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

Question 41

What is the main purpose of protien in a lipid bilayer

Answer 41

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

Question 42

If you require energy to transport a molecule from one side of the membrane to the other the process is

Answer 42

active transport - using ATP

Question 43

Passive transport ...

Answer 43

Doesn't require energy However it can either be simple diffusion through the membrane or require a channel or transporter protein

Question 44

How do transport mediated, transporter proteins operate

Answer 44

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

Question 45

When is active transport needed

Answer 45

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

Question 46

Describe channel proteins

Answer 46

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

Question 47

Give an example of a channel protein

Answer 47

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

Question 48

Describe the general features of a transporter/carrier protein

Answer 48

Slow Require a conformational change Solute unchaged Specific (e.g. glucose transporter) Can work in both directions

Question 49

The ability of a transporter to deliver a molecule from one side to the other can be described in terms of

Answer 49

Michaelis-Menten kinetics

Question 50

What would the line look like for simple diffusion and transporter-mediated diffusion when concentration of transported molecule is plotted against rate of transport

Answer 50

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

Question 51

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)

Answer 51

Smyport - same direction Antiport - Opposite direction The bind of both molecules is required to allow transport across the membrane (Obligatory)

Question 52

Give an example of an ATP-driven pump

Answer 52

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)

Question 53

Give examples of 3 different types of active transport

Answer 53

1. Ionic gradient (transport of ions against their gradient) 2. ATP-driven pump 3. Light-driven pump

Question 54

What are receptors

Answer 54

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)

Question 55

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

Answer 55

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