Membrane structure Flashcards
1. Describe the molecular components of a membrane
Membranes are composed of lipids, carbohydrates, and proteins and function as physical barriers that define boundaries. Membranes spontaneously form sealed structures. Proteins that span the membrane control the movement of molecules between the inside and outside of the structure (cell or organelle). The plasma membrane defines the boundary of the cell, and membrane proteins sense the extracellular environment. Organelles are membrane-bound compartments that have specific structures and functions. Each membrane type has a unique complement of proteins and lipids. Lipids, which form the primary structure of the membrane, often have carbohydrates attached on the extracellular surface. Proteins embedded in the membrane also often have carbohydrates attached to the extracellular domains of the protein.
The internal membranes of the organelles and vesicles possess a ________ surface area than that of the plasma membrane but less area than that of the cytoskeleton, as schematically represented by the red, black, and green boxes. The enormous surface area of the cytoskeleton allows it to function as a scaffold on which cellular reactions can take place
greater
2. Describe the concept of membrane fluidity.
Membrane fluidity depends on composition and temperature.
Cholesterol has a polar hydroxyl group, a rigid steroid ring group, and a hydrocarbon tail. Cholesterol is intercalated among membrane phospholipids. The interaction of the steroid ring with the hydrophobic tail of other phospholipids tends to immobilize the lipid and decrease fluidity.
Lipids are forced to be straightened by
cholesterol. The thickness of a membrane
depends on the amount of cholesterol.
Intracellular membranes have l_ess_ cholesterol
than the plasma membranes and are thinner than the plasma membranes. The mole percentage of cholesterol roughly doubles from the ER (7%) to the Golgi (13%) and again from the Golgi to the plasma membrane (26%).
Carbohydrates are on the ________ side of the membrane.
extracellular side
3. Identify the parts of a phospholipid, sphingolipid, and cholesterol.
There are three classes of lipids in a membrane and all 3 classes are amphipathic (contain hydrophilic and hydrophobic domains). All are synthesized in the endoplasmic reticulum (ER).
The most common phospholipids are phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylinositol (PI). All the lipid molecules shown are derived from glycerol except for sphingomyelin, which is derived from sphingosine.
Acronym: PECI
Cholesterol has a polar hydroxyl group, a rigid steroid ring group, and a hydrocarbon tail. Cholesterol is intercalated among membrane phospholipids. The interaction of the steroid ring with the hydrophobic tail of other phospholipids tends to immobilize the lipid and decrease fluidity.
4. Describe the asymmetry of membrane bilayers and provide at least two reasons that this asymmetry is functionally important
The two leaflets of the plasma membrane bilayer are not equivalent in terms of the distribution of specific phospholipids.
Negatively charged phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylinositol (PI) are more abundant on the internal surface.
PC, sphingomyelin, and glycolipids are more abundant on the ***external surface. Cholesterol is thought to be distributed equally in the two leaflets/monolayers. Cholesterol is extremely important for membranes and its abundance is closely regulated in the body.
Elegant mechanisms regulate cholesterol synthesis. Synthesis depends on approximately 30 enzymes. The first and rate-limiting enzyme in this pathway is _________; statins, used to lower cholesterol, block this step.
HMGCoA reductase (3-hydroxy-3-methylglutaryl coenzyme A reductase)
Both uptake and synthesis are regulated by the __________, a protein containing a transcription factor that regulates both LDLR and all 30 of the synthesis proteins. If cholesterol is low, the transcription factor is released, moves to the nucleus and activates all these genes
Sterol regulatory element binding protein (SREBP)
The transcription factor is a basic helix loop helix (bHLH) DNA- binding protein and is held inactive because it is part of a larger transmembrane protein (SREBP). The transcription factor only becomes active when it is cleaved from SREBP and then translocates to the nucleus. Where are the proteases that cleave SREBP to release the bHLH?
These proteases (there are two of them) are located in the Golgi complex. SREBP must be held in the ER until cholesterol is low and then SREBP must move to the Golgi where it gets cleaved and the bHLH released.
two key proteins, one which binds both SREBP and sterols like cholesterol (it is called SCAP: SREBP cleavage activating protein) and the second protein binds SCAP when cholesterol is _______ (it is called Insig).
high
