membranes transport and structure Flashcards
Importance of membranes
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
how do cell membranes enables response of only specific cells to stimulus
presence/ absence of cell surface recptor
categories of membrane transport proteins
ATP pumps -ATPases
channels
transporters- uni/sym/anti
what is the function of highly hydrophobic residues on membrane proteins
sit in trans membrane region
often alpha helical domains
cell wall composition in
plants
fungi
prokaryotes
plants- cellulose
fingi- chitin
prokaryotes- variable peptidoglycan cell wall
describe structure of gram positive and negative cell walls in prokaryotes
gram positive =plasma membrane> peptidoglycan cell wall
gram negative= plasma membrane> peptidoglycan wall>outermembrane
why are human RBCs ideal for studying membranes
- Large numbers of cells can be obtained
- The cells are anucleate and are devoid of intracellular organelles i.e obtain just plasma membrane.
- The cells are easy to manipulate, thereby enabling access to different parts of the membrane
describe steps to obtain RBC membrane segments alone
1) hypotonic lysis
2) then either wash and reseal just wash or dysrupt and reseal
4 Principal lipids are:
Principal lipids are: PC= phosphatidylcholine (lecithin) PS= phosphatidylserine - slight negative charge due to carboxyl group PE= phosphatidylethanolamine SM= Sphingomyelin
Evidence for Bilayers
Electron microscopy: osmium tetraoxide die
Calculations
membrane size is regulated by
vesicle formation and fusion with plasma membrane
what is the differences in inner and outer membrane composition
SM and PC predominately on outer membrane ~4:1
PE predominantly on inner membrane ~4:1
PS exclusively on inner membrane
what happens if PS phosphatidylserine is found on outer membrane
used as apoptotic signal to white blood cells
describe function of phospholipid transfer proteins:
Scramblase
Flipase
Flopase
Scramblase (bidirectional)
Flipase (outer to inner)
Flopase (inner to outer)
Flipase and flopase require ATP
Why have lipid asymmetry in the plasma membrane?
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.
describe study demonstrating protein mobility within the membrane
demonstrated by fusion of human and mouse cell.
After incubation at body temperature fluorescently labeled human and mouse proteins will be mixed.
how is protein moment restricted within the membrane?
clinical evidence
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
define Autophagy
he delivery and breakdown of large worn out cell components.
Lysosome features
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
Endoplasmic reticulum features and functional difference between smooth and rough
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
Golgi apparatus structure and function
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
The nucleus as a membrane bound organelle
Largest animal cell organelle
Nuclear pores are bound to RER- membranes are normally continuous
Nucleolus is a subcompartment where RNAis produced.
Nuclear pores are highly regulated spaces, function as gateway to the cell, how are they regulated
Protein and other structures create jelly like environments which regulate transport across.
mitochondria membrane properties
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
what are the two families of vesicle formation proteins discovered by combination of biochemistry and yeast genetics
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
key properties of vesicle formation proteins
Locally trap membrane cargo proteins through specific signals
Coating promote membrane curvature
Coats uncouple with vesicles before vesicle fusion.
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
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
how are adapter proteins activated/ inactivated
Adapter proteins can be activated by a GDP-GTP exchange factor.
And inactivated by by GTPase proteins
why is Spontaneous membrane fusion is very slow
polar phosphate groups repel each other such that their must be an energy input for efficient fusion.
snare proteins drive membrane fusion what is their structure and how do they function to bring membranes into contact
Short alpha helical membrane proteins
Consist of alpha helical domains that twist together closing distance between membranes
Each membrane has unique snares enabling specificity
how are snare proteins removed after vesicle fusion with membrane?
he ATPase NSF dissociates membrane snares after membrane fusion
how where snare proteins discovered?
Snares were discovered by action of tetanus and botulinum toxin which digest snare proteins at neuromuscular junction
snare proteins can only act on a microscopic scale thus another protein complex is needed to direct vesicle movement what is this system?
Tethering complexes promote long range transport to target organelle
tethering protein structure
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
describe how a negative resting membrane potential is created
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+
state the equation which dictates the movement of K+ across the cell membrane
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
how are channels made selective only for k+ and not Na+as its smaller
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+
define electrodiffusion
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)
in what 3 ways can a membrane channel be gated
by voltage, extracellular or intracellular ligand gating or deformation of the membrane
carriers bind the solute and undergo a conformational change what implications does this have for transport
transport is therefore slower and more sensitive to temperature.
