Module 9

Question 1

Membrane functions

Answer 1

Seperate cells from the external medium to create an intracellular environment of unique and defined composition
Allows selective transport of substrates in and out of the cell

Question 2

Lipid Bilayer

Answer 2

Basic Structural element of membranes
Membrane formation is a consequence of the amphipathic nature of the membrane lipids
These molecules self assemble through the hydrophobic effect

Question 3

Fatty Acids Favour

Answer 3

Micelle formation

Question 4

Lipid Bilayers

Answer 4

Form membrane Vesicles
Exposure of hydrophobic tails at the edge of the bilayer to water is energetically unfavourable
Flat bilayer sheets are unstable and spontaneously form membrane vesicles with an internal volume
These Vesicles are the basis of cells and organelles

Question 5

Impermeable membrans

Answer 5

Lipid bilayer membranes have a vert low permeability to ions and Most polar molecules
Permeability of small molecules is correlated with their relative solubility in water
Some small non polar gases like O2 and CO2 can pass directly through the membrane

Question 6

Vesicles for Drug delivery

Answer 6

The cell membrane can represent a critical barrier per polar drug intended for intracellular targets
Encapsulation of a drug within a liposome can facilitate transport across the membrane
Liposomes can also be used to target specific cells and organ;lleles

Question 7

Membranes have

Answer 7

Higher ratio of protein than lipids

Question 8

Fluid Mosaic Model of Membrane structure

Answer 8

Membranes are dynamic structures due to the nature of the non covalent interactions
lipids and proteins freely diffuse in the plane of the membrane
Lateral movement of proteins and lipids within the membrane is very rapid
Movement across the membrane is restricted

Question 9

Transbilayer movement requires Catalysis

Answer 9

Transbilayer movement requires a polar head group to pass through hydrophobic environment
Uncatalyzed rate of lipid molecule crossing from one sheet to the other (Flip flop) is very slow

Question 10

Flippases

Answer 10

Translocation of lipids form one side of bilayer to other is catalyzed by enzymes called flippases

Question 11

membrane fluidity

Answer 11

Cells need to maintain an appropriate levels of membrane fluidity
Membranes undergo temperature - dependant phase transitions
Cells can adjust membrane composition to maintain liquid - ordered state

Question 12

Below the phase transition temp

Answer 12

Membrane is too solid

Question 13

Above the phase transition temp

Answer 13

Membrane is too fluid

Question 14

At the phase transition temp

Answer 14

just right

Question 15

Lipid Rafts

Answer 15

Arise from the spontaneous association of lipid molecules whose hydrocarbon tails are of similar length
Sphingolipids (With longer tails) for clusters that excel glycerphospholipids
Longer saturated hydrocarbons of sphingolipids form stable association making the rafts thicker and more orders than the rest of the membrane
Rafts are docking points in lipid anchored proteins that contain long chain saturated fatty acid anchors
Lipid linked proteins that associate with rafts often serve signalling functions

Question 16

Types of Membrane Proteins

Answer 16

Peripheral
Lipid Anchored
Intergral membrane proteins

Question 17

Peripheral Membrane Proteins

Answer 17

Associate with membrane through electrostatic or hydrogen bonding interactions
Can dock to either membrane lipids or integral membrane proteins
The bulk of the peripheral membrane proteins is in the cytosol or extracellular space
Changes in pH or ionic strength often releases these proteins from the membrane

Question 18

Lipid Anchored Proteins

Answer 18

Covalently attached lipids can anchor proteins to the membrane
These protein modifications are sometimes reversible, allowing for regulation of cellular location
GPI anchored protein are always at the outer face
Proteins with single chain hydrocarbons always to inner face

Question 19

Integral Membrane Proteins

Answer 19

Integral membrane proteins are immersed in and usually span the membrane
Protein positioning within a membrane is specific and directional
Integral membrane proteins tend to be of 3 varities

Question 20

3 types of integral membrane proteins

Answer 20

a- helical, a-helical bundles and B- barrels

Question 21

Hydrogen Bonding within membrane spanning regions

Answer 21

Side chains within the transmembrane region tend to be non polar however the carbonyl and amide groups of each peptide bond are polar
Polar unpaired carbonyl and amide groups in the bilayer core are energetically unfavourable
Carbonyl and amide group of the protein backbone within the bilayer have to be hydrogen bonded

Question 22

Simple Diffusion

Answer 22

Non polar gases (O2 and CO2) and hydrophobic molecules can directly cross the membrane
Their direction and rate of movement determined by their concentrations on either side of the membrane
Diffusion can only result in the net movement down a concentration gradient

Question 23

Facilitated Diffusion

Answer 23

Membrane transporters lower the activation energy barrier of crossing the bilayer
Activation energy for removing the hydration shell from a polar solute and transferring it into the non polar environment in the core of the bilayer is very high
Membrane transporters lower the activation energy for crossing the membrane by replacing the hydration shell with interactions with polar groups along the transfer path in the protein interior

Question 24

Channel Proteins

Answer 24

Membrane pore to transport molecules down conc gradient
hIgh conductance rates because they bind the substrate very weakly
Do not Saturate

Question 25

Carrier Proteins

Answer 25

Membrane proteins that undergo substrate induced conformational change, or membrane repositioning to release substrate to the other side of the membrane
Slower bevause they bind to substrate quite strongly
Can saturate

Question 26

Glucose Permease of Erythrocytes

Answer 26

Facilitated Diffusion through carrier
Specific to D- Glucose
Rate of uptake follows a pattern of resembling M-M kinetics
Kt about 1/3 concentration of blood glucose so the transporter is nearly saturated and operates near Vmax

Question 27

Coupled Transport

Answer 27

Transport of a single molecule is called uniport
Some transporters couple the movement of two molecules

Question 28

Antiport

Answer 28

Move molecules is different directions

Question 29

Symport

Answer 29

Move molecules in same directions

Question 30

Primary Active Transport: P Type ATPase

Answer 30

Cells maintain high gradients of Na+ outside the cell and K+ inside the cell
The gradient controls cell volume, electrical excitability, and enables uptake of nutrients through secondary active transport system
Maintaining activity the Na K sumo requires about a third of your energy
Na K ATPase pumps 3 Na+ out of the cell and two K+ Into the cell
Called P type as it undergoes a phosphorylated intermediate

Question 31

V type ATPases

Answer 31

Use the energy of ATP to move protons against a concentration gradient
Acidification of organelle
In chloroplasts and mitochondria, F type ATP synthases reverse this reaction to use proton gradients to generate ATP

Question 32

ABC Transporters

Answer 32

Contain ATP binding domains
Transport of a variety of biomolecules out of the cell against a concentration gradient
Multi drug resistance protein pumps drugs out of the cell rendering the drug ineffective

Question 33

Seconded Active Transport (Glucose Uptake into Intestinal Epithelial cells)

Answer 33

In intestinal epithelial cells glucose uptake from the gut in driven through symport with Na_+
The movement of glucose up its concentration gradient is enabled by the movement of Na+ions down their concentration

Question 34

Ion channels

Answer 34

Ion channels enable rapid movement of ions across the membrane\Actions of ion channels can cause changes in membrane potential

Question 35

K+Channel

Answer 35

k+ channels allow rapid movement of K+ ions out of cell
Although Na is small the channel os 100 fold more permeable to K