Lecture 8 - Membrane Structure Flashcards
Micelle vs. liposome?
Liposome has a lumen.
The ratio of the head to tail changes the shape of lipids. If it has a large head group and a small tail = cone shape, causing the membrane to curve. If it has a cylindrical shape = membrane forms a sheet.
Why are planar sheets unfavourable?
They are energetically unfavourable because they have an end where the hydrophobic tails are exposed to water.
What makes a membrane more fluid?
Higher temperatures, unsaturated fatty acids.
What makes a membrane less fluid?
Lower temperatures, cholesterol, saturated fatty acids.
Saturated vs unsaturated?
Saturated = single bonds only. Unsaturated = contains at least one double bond.
Who proposed the Fluid Mosaic Model? In what year?
Sanger and Nicholson, 1972.
How did ____ and _____ identify the fluid mosaic model?
They studied the rabbit erythrocyte membrane using ricin, which binds to carbohydrates on the surface of the plasma membrane.
In the electron micrograph the membrane is folded, and the ricin would only bind to the outside. Confirms that the membrane is asymmetrical.
Explain the technique of fluorescence recovery after photobleaching (FRAP).
A more modern technique used to confirm what Sanger and Nicolson identified - the clustering of membrane components.
FRAP of a membrane bound dye, e.g. GFP protein. Fluorescence of the dye will only recover if there is diffusion into the region where the dye was bleached. Shows free diffusion of the membrane.
Give examples of differently shaped biological membranes.
- ER consists of tubules and sheets. Composed of slightly different lipids and proteins. Sheets = studded w/ ribosomes, protein synthesis. Tubules = smooth ER = lipid synthesis and secretion.
- Golgi = many folded membranes called cisternae.
- Vesicles = tightly curved structures.
How do proteins curve membranes? (4)
- Amphiphatic helices - hydrophobic and hydrophilic AAs on opposite sites. Creates a wedge.
- Loop insertion - hydrophobic AAs. Creates a wedge.
- BAR domain proteins - bind to phopshate heads, curving the membrane.
- Curved lattices - bind to cargo proteins forming a cage like lattice that will ultimately form a vesicle.
Lipids are altered throughout the cell to fit the function of each membrane type. Explain this process.
- Lipids are synthesised in the ER.
- FA synthesis occurs in the cytoplasm.
- Membrane bound acyl transferases turn it into phosphatidic acid intermediate.
- Phosphatase then generates DAG.
- Other membrane bound enzymes (transferases) stick headgroups on (e.g. choline).
Where does extensive lipid remodelling occur?
In the secretory pathway.
Explain some of the adaptations that occur in the secretory pathway.
- Unsaturated FA chains are replaced with more saturated ones from ER to PM.
- More cholesterol added.
- The carbohydrate head-groups of glycolipids are synthesised into glycans in the Golgi.
- Membranes become more asymmetrical towards PM.
- Membrane thickness increases towards PM.
What does a phospholipid consist of?
Glycerol backbone, 2 FA chains, phosphate group with an alcohol head attached to C3.
Most lipid synthesis occurs in the ER. Must then be transferred to the target membrane. 3 different ways?
- Vesicle-lipid trafficking - vesicle transports largely one type of lipid. Fuses with target membrane.
- Lipid transfer proteins (LTPs).
- Free diffusion - unfavourable due to hydrophobic nature.
- Membrane contact sites (MCSs)- where one type of organelle gets in close proximity with another. At these sites there is intense lipid transfer. Less than 30nm apart.
MCSs have functions beyond lipid transfer. Name these.
- Coordinate Ca2+ release to facilitate signalling/cytoskeletal dynamics.
- Aid in organelle fission.
- Aid in protein sorting within/between organelles.
Two mechanisms by which LTPs work?
- Net phospholipid transfer - transfer phospholipid from one membrane to another.
- Phospholipid exchange between 2 membranes.
Describe the structure of a cholesterol molecule.
Largely hydrophobic. Has a hydrophilic OH group that sticks into the membrane.
Where do we get our cholesterol from?
Eating, or is synthesised in the liver.
Role of LDL and HDL in cholesterol?
Cholesterol binds to a LDL receptor; endocytosed; lysosome releases cholesterol via destruction of the receptor.
HDL transports cholesterol from the peripheral tissues back to the liver for destruction.
Where are lipid droplets found?
Adipose tissue.
Main role of lipid droplets?
Act as storage compartments for neutral lipids (lipids with a phosphate) as these can be toxic - they produce reactive oxygen species.
What encloses a lipid droplet?
They are surrounded by a lipid monolayer. Surface also contains proteins but they do not span the membrane because the inside is hydrophobic.
Function of membrane proteins?
Cell-to-cell communication, signalling, transport.
Structure of a glycolipid?
Glycerol backbone, 2 FAs, phosphate on the C3 with a sugar attached.
What is the lipid part of a glycolipid called?
Ceramide.
Example glycolipid?
Blood group antigens.
Another type of glycolipid = ganglioside. Function?
- Found in neuronal membranes.
- Important component of lipid rafts.
- Used by many pathogens for attachment and/or entry into the cell, e.g. cholera.
