Membrane Structure Flashcards
What are some general functions/benefits of a plasma membrane?
- Holds the internal mixture separate from the environment
- via transporter proteins and channels it allows import and export of molecules
- may include receptors that enable reactivity to environment
- can grow and reshape occording to the cells needs
- it can enlarge, or fix small tears
What is the general structure of the membrane - type of lipids (their 2 parts)?
Membranes are composed of lipid bilayer - most commonly phospholipids
- amphipathic -> have a hydrophobic head and hydrophillic tail
- hydrophillic - dissolve in water due to charged or uncharged molecules that can make links with H20
- hydrophobic - insoluable in water due to their uncharged and nonpolar molecules -> even “repel” H2O which is then forced to form cagelike structure around them
How come lipids form a bilayers?
Hydrophobic parts cannor deal with water -> water will try to form a cagelike structure around it -> BUT that (as it creates order) requires energy (i.e. energetically unfavourable and harder to achieve) -> hydrophobic parts will try to bend on itself to avoid water
=> smart solution is putting the hydrophobic parts against each other shielded by the “external” hydrophillic components
Why else is the hydrophobic X hydrophilic conflict so essential?
It also forces the membrane to come together and form a closed bondage
- if there is a tear in the space it creates an unfavourable environment for the lipids -> will try to bend and connect to restore theit stable state
- In the case of greater tearing lipids could even bend on itself and create multiple small vesicles
What kind of movement do we see in lipids of membrane (4-5)?
1) Bending of the membrane itself
2) Lateral diffusion = exchanging positions with the neighboring lipid (one lipid may actually travel the entire length of a bacterial cell)
3) Rotation around its axis
4) Flip-Flop = swithiching inner with external position
- happens only rarely since hydrophillic head would need to pass through the hydrophobic space which is energetically unfavourable
5) Flexion = the movement of the tails
The fluidity/movement within bilayer can be impacted by size, temperature, molecular composition -> what is meant by the last one?
Phospholipid refers to hydrocarbon tails -> the closer and more regularly packed they are => less fluid, more solid
- How tightly they are packed depends on:
- Length -> the shorter -> less interactions between hydrocarbon chains => more fluidity
- Number of double bonds
- Double bonds do NOT contain the maximum number of hydrogens => unsaturated
- Single bonds do => saturated
-> double bonds create “kinks” in the tail -> harder to interact with (have more space around them) => more fluid
How is the previous question relevant for the fact that bacteria can survive at all kinds of temperatures?
This ability stems from the fact that the mambrane can ajust the lenght of the hydrocarbon chains and change the number of double bonds -> this way if temperature increases it can still adapt by making itself more solid
How can fluidity be adjusted in animal cells?
It is modulated by the inclusion of cholesterol
- short and rigit -> fill in spaces between phospolipids created due to kinks of double bonds -> stiffens the tail => less flexible, less permeable
NOTE: could work in other cell but they do not produce cholesterol
Why do we even need fluidity in cell membranes?
- proteins can diffuse through and interact with each other (essential for cell signaling)
- permits the lipids to diffuse from the site of insertion to a needed one
- allows membranes to fuse together and mix their molecules
How are new phospholipids added?
Free fatty acids bind as substrates to specific enzymes at the surface of endoplasmatic reticulum -> products are new phospholipids added to the cytosolic half of the bilayer.
But fif they are only added to the cytosolic layer -> how can they ever get to the other?
This process is energetically unfavourable (hydrophilic head through hydrophobic space) -> catalyzed by enzymes = Scramblases which removes randomly selected phospholipids from one layer and inserts them to the other => newly formed phospholipids can be redistributed equally
What happens with the newly assembled membrane?
- Some parts remain in the ER
- Rest will be used to supply the plasma membrane and other membranes of the cell
- Small bits will get pinched off as vesicles and send to fuse with other membranes e.g. Golgi apparatus
Most membranes are asymetrical i.e. each layer has a different composition -> but how is that possible? Does it stay that way?
Since membranes emerge from ER symetrically - where does their asymetry comes from?
-> enzymes of Golgi apparatus = Flippase, remove a specific phospholipid from the non-cytosolic (external) layer and place it into the internal one
-> asymetry is preserved even in the case of fusing => gives membranes certain orientation
- NOTE: directionality is also maintained for proteins within the membrane
Name a lipid group which shows this asymetry pattern.
Glycolipids - specific group that is located only on the non-cytosolic layer
- enzymes of Golgi apparatus add a sugar group to lipids facing external environment (sugar is sticks out) -> there are no flippase for glycolipids so they stay that way
Why do we have proteins in cell membranes? Is there more proteins or lipids?
Functions: transport of specific nutrients as transporters or other molecules (e.g.ions) as ion channels, detection of outside stimuli as a receptor, catalysis of chemical reactions as an enzyme
- NOTE: each plasma membrane could have a different composition of them -> reflected in its specialized properties
- There are more lipids than proteins as the latter is bigger.
What are the 4 ways by which proteins associate with the lipid bilayer?
1) Transmembrane proteins = spanning through the membrane
- amphipathic: hydrophobic segment nested against the hydrophobic segments of phospholipids while the hydrophilic head is exposed to aqueous environment
2) Monolayer-a helix = associated primarily with the cytosolic layer via an amphipathic alpha helix on the surface of the protein
3) Lipid-linked = located at either side of the membrane attached only by one or more lipid groups
4) Protein attached = on either side attached by an interaction with another protein embedded in the membrane
Depending on the way of attachment - how can we remove the types of proteins? How do we call them?
Integral membrane proteins - directly attached to the membrane
- removed only by disrupting the lipid bilayer with a detergent
- transmembrane, lipid-linked, monolayer a-helix
Peripheral membrane proteins - indirect
- can use more gentle approach without the need to destroy the membrane (e.g. using an enzyme)
How come the transmembrane proteins form a-helices?
Polypeptide chain that runs through the hydrophobic segment has to adjust to this environment
- side chains of the amino acids are hydrophobic => cannot deal with H2O, but wouldn’t mind neighboring the hydrophobic tails
- peptide bonds are hydrophilic (polar) -> since H2O is absent atoms are forced to form hydrogen bonds with one another => maximized when forming an a-helix
=> side chains on the outside, peptide bonds on the inside
What is the difference between transmembrane proteins functioning as receptors X channels?
Receptors = only one a-helix needed = Single pass transmembrane protein
Channel = multiple a-helices that form an aqueous pores to let molecules pass through = Multipass transmembrane proteins
- amphipathic side chans (some phobic some philic)
- concentrated in a circular shape with hydrophobic side chains sticking out into the lipid layer and hydrophilic forming the lining of the pore
What other shape can transmembrane proteins take on (you may also provide an example)?
B sheets that are rolled into a cylinder = B barrel
- inside lined by hydrophilic side chains
- outside formed by hydrophobic side chains
- e.g. porin - allow for passage of small nutrients and ions across the outer membrane of mitochondria
Why may it be an issue to study membrane proteins? How do we go about it?
Problem: we usually purify and study proteins in aqueous solutions -> not possible, as these proteins were build to function in partly aqueous and partly fatty environment
So how do we go about it?
Separate the protein of interest from other proteins
- we need to destroy the lipid bilayer by disrupting the hydrophobic associations -> most common are detergents = small amphipathic molecules that have ONLY a single hydrophobic tail -> in water shaped as cones and tend to gather into = Micelles
-> when put in contact with a membrane -> the detergent hydrophobic ends start to interact with hydrophobic ends of the protein and phospholipid molecules -> separates proteins from the membrane -> the detergent hydrophilic part interacts with the protein forming protein-detergent complex => can be separated in further analysis
If cell wall in case of plants and bacteria provides additional support to the innate fragility of cellular membranes -> what can be done in the animal cells? Can you give a specific cell example?
Animal plasma membrane tends to be stabilized by a meshwork of fibrous proteins = cell cortex
- e.g. aktin, myosin
Red blood cells form a spherical flattened shape thanks to the dimeric protein Spectrin -> meshwork connected to specific transmembrane proteins
NOTE: if there is a genetic defect in Spectrin -> red blood cells can end up as spherical and fragile (they have to be rigit since their pumped through the body)
Considering the fluidity/diffusion witthin the cell membrane -> do all proteins float freely?
No. Cells have found a way to confine specific proteins to localized areas -> which creates functionally specialized regions = Membrane domains
- these can be placed in all kinds of ways
