Chapter 11 Lipids
How can you prevent an integral protein from diffusing in the plane of the membrane?
By tethering it to the cell cortex or the extracellular matrix, or to proteins on the surface of another cell, or add diffusion barriers.
What are the 3 functions of cell membranes?
Cell communication, export/import of molecules, and cell growth and motility.
What do SDS and Triton do to membranes and why?
Common detergents They displace lipid molecules from proteins by interacting with the hydrophobic regions of proteins and with the hydrophobic regions of the lipids.
Where does membrane assembly occur and how?
In the ER phospholipids are made from fatty acids. They are made on the cytosolic monolayer, then are evenly transferred to the non-cytosolic side by an enzyme -scamblase.
Describe the origin of the asymmetry and describe why it persists?
In the Golgi an enzyme called "flipases" removes specific phospholipids from the non-cytosolic side and flip them on the monolayer that faces the cytosol. This creates asymmetry in the bilayer.
If the plasma membrane is continuously lost, they how does the cell maintain its shape?
The ER consistently produces phospholipids which for vesicles which then fuse to the plasma membrane of the cell.
What can we see in "freeze-fracture replicas that is not visible via any other forms of microscopy?
We can see the plasma membrane very detailed, so we can see the distribution of intermembrane proteins.
How are hydrophilic channels created?
a series of a-helixes cross the bilayer a number of times. the helices pack side by side in a ring because of the hydrophobic environment. The backbone forms the hydrophilic ore through the bilayer.
How is the lipid membrane asymmetrical?
phospholipids and glycolipids are distributed asymmetrically in the bilayer.
What explanation is there that cold-blooded organisms alter their lipid composition based on season.
when its cold they need more double bonds (unsaturated fats) to keep them from freezing, and when its hot they need less double bonds (saturated fat) in order to keep its shape.
peripheral membrane proteins
On top of bilayer -protein attached removal leaves bilayer intact
why do lipid bilayers form spontaneously?
Because their hydrophobic tails cluster together spontaneously limiting their contact with water, since they are non polar, therefore achieving the lowest free-energy rearrangement.
What lipids are part of the bilayer and which ones are not?
Amphipathic lipids are part of the bilayer (phospholipids, cholesterol, and glycolipids) Triglycerides are not part of the bilayer (hydrophobic)
How do we know that membrane proteins are free to move in the plane of the membrane?
An experiment that fuses a mouse cell with a human cell to see the distribution of the membrane proteins. Results showed that the 2 sets of proteins became evenly mixed over the entire cell surface, shows confirming that cells do freely move in the membrane.
Why are bilayer self sealing?
Because a tear in the bilayer is energetically unfavorable, the molecules in the bilayer will spontaneously rearrange to eliminate the tear made possible because phospholipids can readily move in the plane of the membrane
Why might it be important to restrict the freedom of proteins in the membrane? by tight junction
Because proteins are designed for specific functions they are restricted to a certain area where they can perform that function. For example in epithelial tissue in the gut the proteins that uptake nutrients must be in the apical surface of the cells, and the proteins that export solutes must be in the basal and lateral surfaces.
Integral membrane proteins
Integral are proteins that are embedded in the lipid bilayer or interact with interior of membrane -transmembrane -monolayer associated a-helix -lipid-linked removal only by detergents
Describe the freedom of motion characteristics of individual phospholipids in the bilayer.
Phospholipids readily diffuse laterally in the same monolayer. They rarely "flip-flop" across the membrane.
What does the fact the the poison ricin is a lectin suggest about how it might get into cells?
Ricin will bind to a particular oligosaccharide side chain, thus letting ricin into the cell.
What is a cell membrane? What does it do? How does it do it?
The membrane assists with cell communication, import and export of molecules, growth and motility of the cell. A cell membrane is made up of amphipathic phospholipids that spontaneously rearrange into a bilayer. The bilayer consists of glycolipids, cholesterol, and proteins. The bilayer's fluidity lets individual lipids diffuse within their monolayer, thus having the ability to reseal itself. Its amphipathic characteristic helps in forming ion channels, whom small polar molecules can diffuse through.
Why do integral membrane proteins have a-helix of B-sheets in portion of the protein which reside in the membrane?
The polar peptide bonds of the polypeptide backbone can be completely shielded from the hydrophobic environment of the lipid bilayer by the hydrophobic side chains. internal hydrogen bonds between the peptide bonds stabilize the a-helix and B-sheet
What causes the phospholipids to form a bilayer?
Their hydrophobic tails cluster next to each other because they cant interact with water so they cluster to themselves and their hydrophilic heads form the outer part of the lipid bilayer because they can make electrostatic attractions with water.
Membranes maintain what when they transfer between cell compartments?
Their orientation. Like the cytosolic side will always face the cytosolic side.
What does the cytoskeleton add to the properties of the plasma membrane?
cell's shape, mechanical properties, support, and to move (restrain diffusion)
How are plant cell wall and glycocalyx alike and how are they not?
glycocalyx: cell-to-cell recognition, prevent cells from sticking to one another, absorb water Both: protect, made of carbohydrates Plant cell wall: structure, prevent water loss
How do you increase fluidity in the lipid bilayer?
shorter tails and more double bonds increased fluidity of bilayer.