membranes transport and structure

Question 1

Importance of membranes

Answer 1

Needed to form a barrier between external medium and cytoplasm.
Act as a permeability barrier to some substances therefore maintain differences between external and internal environments
Needed to form internal organelles, which are devoted to specific tasks.
Enables a cell to respond rapidly (I) and selectively (II) to specific changes in its environment

Question 2

how do cell membranes enables response of only specific cells to stimulus

Answer 2

presence/ absence of cell surface recptor

Question 3

categories of membrane transport proteins

Answer 3

ATP pumps -ATPases
channels
transporters- uni/sym/anti

Question 4

what is the function of highly hydrophobic residues on membrane proteins

Answer 4

sit in trans membrane region

often alpha helical domains

Question 5

cell wall composition in
plants
fungi
prokaryotes

Answer 5

plants- cellulose
fingi- chitin
prokaryotes- variable peptidoglycan cell wall

Question 6

describe structure of gram positive and negative cell walls in prokaryotes

Answer 6

gram positive =plasma membrane> peptidoglycan cell wall

gram negative= plasma membrane> peptidoglycan wall>outermembrane

Question 7

why are human RBCs ideal for studying membranes

Answer 7

1. Large numbers of cells can be obtained
2. The cells are anucleate and are devoid of intracellular organelles i.e obtain just plasma membrane.
3. The cells are easy to manipulate, thereby enabling access to different parts of the membrane

Question 8

describe steps to obtain RBC membrane segments alone

Answer 8

1) hypotonic lysis

2) then either wash and reseal just wash or dysrupt and reseal

Question 9

4 Principal lipids are:

Answer 9

Principal lipids are:
PC= phosphatidylcholine (lecithin)
PS= phosphatidylserine - slight negative charge due to carboxyl group
PE= phosphatidylethanolamine
SM= Sphingomyelin

Question 10

Evidence for Bilayers

Answer 10

Electron microscopy: osmium tetraoxide die

Calculations

Question 11

membrane size is regulated by

Answer 11

vesicle formation and fusion with plasma membrane

Question 12

what is the differences in inner and outer membrane composition

Answer 12

SM and PC predominately on outer membrane ~4:1
PE predominantly on inner membrane ~4:1
PS exclusively on inner membrane

Question 13

what happens if PS phosphatidylserine is found on outer membrane

Answer 13

used as apoptotic signal to white blood cells

Question 14

describe function of phospholipid transfer proteins:
Scramblase
Flipase
Flopase

Answer 14

Scramblase (bidirectional)
Flipase (outer to inner)
Flopase (inner to outer)
Flipase and flopase require ATP

Question 15

Why have lipid asymmetry in the plasma membrane?

Answer 15

1.Different proteins function better when surrounded by certain phospholipids. e.g. calcium pump
prefers phosphatidylinositol4,5-bisphosphate;
Asymmetric protein distribution could therefore result in asymmetric lipid distribution.
2.Localizes proteins through protein - lipid binding.
3. There are lipid -derived signalling molecules which activate components of the cytoplasm. These
lipids are predominantly found in the inner leaflet of the plasma membrane.

Question 16

describe study demonstrating protein mobility within the membrane

Answer 16

demonstrated by fusion of human and mouse cell.

After incubation at body temperature fluorescently labeled human and mouse proteins will be mixed.

Question 17

how is protein moment restricted within the membrane?

clinical evidence

Answer 17

Lipid rafts - high density cholesterol and density of integral proteins (still a very contested model)
Ankyrin and other cytoskeletal proteins chaining channels together to prevent movement. E.g.
postsynaptic density.
In heart failure patients Ca2+ channels are disorganised in the membrane causing irregular
contractions

Question 18

define Autophagy

Answer 18

he delivery and breakdown of large worn out cell components.

Question 19

Lysosome features

Answer 19

specific to animal cells.
Contain very low pH as a consequence lysosome enzymes work best at low pH. Acidity also helps denature and breakdown proteins.
Secondary lysosomes form from the fusion of endosomes and primary lysosomes

Question 20

Endoplasmic reticulum features and functional difference between smooth and rough

Answer 20

Large network of interconnected membranes
Consists of large cisternae closed flattened membrane sacs
Primary function is synthesis of lipids membrane proteins and secreted proteins.

Smooth = fatty acid/ phospholipid synthesis
Rough =protein synthesis

Question 21

Golgi apparatus structure and function

Answer 21

Cisternae structures
Functions by controlling and directing membrane bound proteins.
The cis face of the golgi is bound to the RER

Proteins are sorted, processed and coded as they move from cis -medial-trans regions for release
Golgi body is also the site of the carbohydrate synthesis

Question 22

The nucleus as a membrane bound organelle

Answer 22

Largest animal cell organelle
Nuclear pores are bound to RER- membranes are normally continuous
Nucleolus is a subcompartment where RNAis produced.

Question 23

Nuclear pores are highly regulated spaces, function as gateway to the cell, how are they regulated

Answer 23

Protein and other structures create jelly like environments which regulate transport across.

Question 24

mitochondria membrane properties

Answer 24

outer membrane very permeable as has porins embedded in the membrane
Inner membrane is much less permeable and has a huge surface area.
Can take up to 25% of the cytoplasmic volume
ATP generation is analogous in mitochondria and chloroplasts

Question 25

what are the two families of vesicle formation proteins discovered by combination of biochemistry and yeast genetics

Answer 25

Cop-1 family notably includes clathrin, triskelions form hexagonal cotes. family also includes smaller vesicle formation proteins
COP II also a vesicle generator smaller structure smaller gaps gaps unlike clathrin

Question 26

key properties of vesicle formation proteins

Answer 26

Locally trap membrane cargo proteins through specific signals
Coating promote membrane curvature
Coats uncouple with vesicles before vesicle fusion.

Question 27

having multiple vesicle formation molecules enables specificity withing vesicle transport what are the sorting signals and pathways of the following molicules:
clathrin
Cop-1
cop-11

Answer 27

clathrin sorting signal tyrosine/Di-leucine and pathway=endocytosis
cop-1 sorting signal=Di-lysine. pathway= retrieval golgi to ER
cop-11 sorting signal= Di-acidic pathway= transport ER to golgi

Question 28

how are adapter proteins activated/ inactivated

Answer 28

Adapter proteins can be activated by a GDP-GTP exchange factor.
And inactivated by by GTPase proteins

Question 29

why is Spontaneous membrane fusion is very slow

Answer 29

polar phosphate groups repel each other such that their must be an energy input for efficient fusion.

Question 30

snare proteins drive membrane fusion what is their structure and how do they function to bring membranes into contact

Answer 30

Short alpha helical membrane proteins

Consist of alpha helical domains that twist together closing distance between membranes
Each membrane has unique snares enabling specificity

Question 31

how are snare proteins removed after vesicle fusion with membrane?

Answer 31

he ATPase NSF dissociates membrane snares after membrane fusion

Question 32

how where snare proteins discovered?

Answer 32

Snares were discovered by action of tetanus and botulinum toxin which digest snare proteins at neuromuscular junction

Question 33

snare proteins can only act on a microscopic scale thus another protein complex is needed to direct vesicle movement what is this system?

Answer 33

Tethering complexes promote long range transport to target organelle

Question 34

tethering protein structure

Answer 34

tethers use RABs activated by GTP/GDP exchange factors to bind to vesicle and use PIP molicules generated by PI-kinases to bind to target organelle

Question 35

describe how a negative resting membrane potential is created

Answer 35

K+/Na+ pump sets up gradient of K+ out of cell And Na+ gong into the cell
membrane is 40-50x more permiable to K+ than Na+
thus K+ diffuses out of cell untill electrical gradient in balances chemical gradient out and their is no net movement of K+

Question 36

state the equation which dictates the movement of K+ across the cell membrane

Answer 36

the nurnst equation
Vk=-RT/zFln*[K+]in/[K+]out
Vk=equilibrium potential difference
R=gas constant
T=temperature in kelvin
z=valience of ion 
F=faraday constant

Question 37

how are channels made selective only for k+ and not Na+as its smaller

Answer 37

Because Na+ ions are smaller than K+ ions any channel large enough for K+ Na+ will also be able to travel through. For this reason to create channel selectivity the chanel has OH groups that mimic the interactions of the water molecules however are at a distance that makes it un energetically viable for Na+ to move from aqueous solution but is analogous to being hydrated for K+

Question 38

define electrodiffusion

Answer 38

the direction of diffusion depends on the chemical gradient (determined by concentration differences) and in the case of ions the electrical gradient (determined by the potential difference)

Question 39

in what 3 ways can a membrane channel be gated

Answer 39

by voltage, extracellular or intracellular ligand gating or deformation of the membrane

Question 40

carriers bind the solute and undergo a conformational change what implications does this have for transport

Answer 40

transport is therefore slower and more sensitive to temperature.