Membrane Structure (Lec. 4) Flashcards
Illustrate the hydrophobic and hydrophilic interactions that lead to the formation of lipid bilayers
Phospholipids have hydrophobic heads and hydrophilic tails. They spontaneously form bilayers in aqueous solution, with the tails on the interior and the heads exposed.
Explain the difference between integral and peripheral membrane proteins
Integral membrane proteins are embedded directly within the lipid bilayer. Peripheral membrane proteins are associated with the membrane indirectly, generally by interactions with integral membrane proteins
Summarize the lipid composition of plasma membranes
The are five kinds of phospholipids: 4 kinds of phosphoglycerides, and sphingomyelin. Plasma membranes also contain glycolipids and cholesterol.
Illustrate how proteins are associated with the plasma membrane
Fluid mosaic model, integral vs. peripheral membrane proteins, transmembrane proteins
Explain the significance of plasma membrane domains
Many cells are polarized, with different domains of their plasma membranes responsible for performing distinct activities. The plasma membranes of epithelial cells are divided into distinct apical and basolateral domains. Lipid rafts on the membrane have a variety of functions, including cell signaling, cell movement, and endocytosis.
What is the function of a biological membrane that is a selective barrier?
Prevents the mixing of cellular contents and the environment, serves as the primary zone of interaction for the cell, can maintain a chemical concentration gradient between compartments (and therefore can participate in the production of cellular energy), provide a ‘scaffold’ for biological activities
Why do membranes form spontaneously?
They’re made of amphipathic molecules in an aqueous environment. They can form micelles (a sphere) or a bilayer
Are the two faces of the membrane symmetrical?
No! Each has different structural and functional characteristics
What is the fluid mosaic model?
A two-dimensional mosaic of laterally mobile amphipathic lipid and protein molecules
What are the membrane lipids?
Sterols (cholesterol: 30-50%); 5 kinds of phospholipids (30-50%); glycolipids (~1-4%)
What is considered the primary structural backbone of the membrane?
Phosphoglycerides
What are sphingolipids?
Have a similar overall shape to phosphoglycerides, but have a serine head instead of a glycerol one. If the head group is phosphate + choline, it’s called sphingomyelin. Mostly found on the outside of cells.
What are glycolipids?
Functions in recognition and protection beyond the simple membrane structure role. Only on the outside of cells.
What do sterols do?
Fill space created by a kink in the chain of an unsaturated fatty acid (the phospholipid to which it is most attached displays reduced mobility); tends to increase the stiffness of the membrane at high temperatures (limits lipid movement) but makes it more fluid at low temperatures (by preventing dense packing)
What determines membrane fluidity?
- Temperature (more fluid at higher temperatures)
- Lipid composition: unsaturated fatty acid chains have double bonds that result in kinks (reduces packing and increases membrane fluidity); longer acyl-chains have stronger interactions (decrease fluidity); sterol content buffers fluidity (increases stiffness at high temps and decreases stiffness at low temps)
What are lipid rafts?
“microdomains” of membrane material; mostly cholesterol, sphingolipids, and glycolipids; move as a unit through the mosaic membrane; serve as a base for specific proteins
Describe integral membrane proteins
Embedded within the membrane bilayer; regions project out onto one or both sides; are receptors and transporters; extraction of them destroys the membrane
What is the relationship between integral and transmembrane proteins?
All transmembrane proteins are integral proteins, but not all integral proteins are transmembrane proteins.
Describe peripheral membrane proteins
Stuck to either side of the membrane, have ionic interactions with lipid heads; have non-covalent interactions with integral membrane proteins.
Describe lipid-linked proteins
‘Hang’ from the membrane by their lipid tail; are covalently linked to a modified fatty acid-like molecule
Describe single-pass transmembrane proteins
Have an N and C terminus on opposite sides. Are anchored to an alpha helix within the membrane, which consists of consecutive amino acids that are all hydrophobic
Describe multi-pass transmembrane proteins and their two relevant subclasses .
Can have an N and C terminus on same or opposite sides.
1) Seven-pass or ‘Serpentine Proteins’ are common G-protein-coupled receptors and are very common. They have no pore, but undergo conformational changes when they bind to a ligand on the extracellular side, which leads to changes inside the cell.
2) Amphipathic-Tunnel Transmembrane Proteins: multiple alpha-helices passing through the membrane, 4 or more of which are amphipathic (one side of the cylinder is lipophilic and the other side is hydrophilic)
What is the glycocalyx?
A sugar ‘coat’ composed of glycolipids and glycoproteins on the outer side of the membrane; protects cells from ionic stress, mechanical stress, and invading microorganisms; can participate in cell-cell interactions
Can all membrane proteins move?
No! Some are locked in place by the underlying cytoskeleton
