Cell Membranes Flashcards
Name 6 organelles in a eukaryotic cell
Endoplasmic Reticulum Nucleus Lysosome Peroxisome Goglgi apparatus Plasma membrane Mitochrondrion
Can you see biological membranes under light microscopes?
Nope
What are cell membranes composed of?
Primarily proteins and lipids
Outer and inner monolayer.
Singer Nicholson model
What are the 3 major classes of lipid?
Phospholipid
Sphingolipid
Cholesterol
Describe the structure of a phospholipid
Glycerol backbone, CHOH x3
Phosphate on the sn-3 position
Two fatty acid chains on the sn-1 and sn-2 positions
Headgroup attached to phosphate.
Structally which 2 lipids are similar?
Phosphoglycerides and sphingolipids
In water, membrane lipids for what?
In water membrane lipids form vesicles
Lipids spontaneously form what in an aqueous environment?
And why?
Lipids spontaneously form bilayers in an aqueous environment
System tries to minimize the free energy (DG) of the system
Hydrophilic/charged sites try to make favorable interactions with the aqueous environment.
Hydrophobic regions try to interact with other hydrophobic sites, minimizing entropic cost of placing lipids in water (hydrophobic effect)
Describe the liquid crystalline phase
Membranes contain lipids with acyl chains with cis-double bonds.
These bonds prevent close packing of chains so have a bilayer with mobile acyl chains
At different temperatures the lipids move around in the plane of the bilayer
What are the 2 dymamics in lipid membranes and explain them.
Lateral diffusion- rapid movement of molecules
Flip-Flop- energetically unfavourable due to long time scale
preserves membrane asymmetry and can be enhanced by membrane fusion or flippases
At low temperature what happens to the liquid crystalline lipid?
At low temperatures the liquid crystalline lipid turns into a gel phase in which the acyl chains are effectively frozen
Why is the liquid crystalline phase is essential
The liquid crystalline phase is essential for the function of the membrane proteins, which require a fluid environment in which to operate
Molecules need to be able to move within the membrane to carry substrates between enzymes and mediate signalling events
Name the 4 major protein membrane proteins
Channels
Transporters
Receptors
Structural proteins
To cross the hydro-phobic (non-polar) core of the bilayer membrane proteins typically adopt what conformation?
To cross the hydro-phobic (non-polar) core of the bilayer membrane proteins typically adopt an alpha-helical conformation
What anchors the membrane proteins?
Describe the shape of the transmembrane domains
Transmembrane helices
Hydrophobic protein sequence with R groups projecting out to face the lipids
How many amphipathic helices are needed to generate a polar route through the membrane?
To generate a polar route through the membrane you need at least 4 amphipathic helices
What dictates what can pass through a pore?
Charges at entry to the pore dictate the charge of the ions that can pass
What can channels not do?
Move ions or any solute against their electrochemical gradient, i.e., they cannot pump
They are unsuitable for facilitating the diffusion of molecules such as glucose and amino acids across the plasma membrane.- as K and Na can travel through these as well
What can control passage of ions through a channel?
A gate
What dictates the direction of ion flow?
Is this active or passive?
Direction of ion flow is dictated by the electrochemical gradient of the ion (passive process)
What are the two types of transporters?
Passive ( down con. gradient)
Active (against con. gradient)
Where does energy for active transporter come from?
Electrochemical gradient
Energy rich substrates (e.g. ATP)
Why is it difficult to find out structure of membrane proteins?
Membrane proteins do not readily form 3-D crystals
too large for liquid state NMR
Little channels needed to perform so not large sample size
How was the early protein structures solved?
from naturally abundant proteins
(mitochondrial/chloroplast, bacterio-/rhodopsin,
bacterial proteins etc.)
What are the 3 main techniques used for structural analysis?
X-ray crystallography
Difficult to prepare crystals
Electron Microscopy (2D electron diffraction)
Low to medium resolution
Requires formation of 2D crystals
NMR Spectroscopy
Solution state NMR techniques – structure in micellar systems
Solid state NMR techniques – structure in the bilayer
Describe the hydropathy plot
Each amino acid residue is assigned a value corresponding to its hydrophobicity
The average length of a transmembrane helix is 20 amino acid residues
A computer algorithm calculates and plots the total hydrophobicity of residues 1-20, 2-21,3-22, etc to reveal potential transmembrane helices
What is the less common structure of transmembrane proteins?
Beta-barrel
Where are Beta-barrels usually found?
β-barrels form hydrophilic pores (porins) in the outer membranes of bacteria and mitochondria
Found in only a few specialised locations where it is important for the membranes to be relatively leaky to small polar molecules. So in mitochondria they allow substrates for ATP production to pass through the mitochondrial outer membrane. They are then transported using transporters across the inner membrane.
In bacterial outer membranes they allow nutrients like maltose or phosphate to cross the outer membrane Likewise they are then carried across the plasma membrane by transporters. Too leaky to be of use in general membranes.
Name an example of a Beta porin
This porin allows sucrose to diffuse across the outer membrane of the bacterium S. typhimurium
The pore is made up of 16 β-strands
