What are the main properties of membranes?
Answer: The main properties of membranes are that they are 5-8 nm thick (50-80 Å), appear trilaminar, are fluid, and are impermeable to most polar or charged solutes but permeable to non polar compounds.
How do lipid molecules move within the membrane?
Answer: Lipid molecules are able to diffuse freely within lipid bilayers, and there is rapid lateral movement within one leaflet (monolayer). There is also rotation and flexion, but very little movement from one monolayer to the other (flip-flop) occurs, which happens less than once a month.
How do scientists study the properties of membranes?
Answer: Scientists study the properties of membranes using liposomes, which are closed self-sealing, solvent filled vesicles formed by phospholipids in water. They can also use fluorescent molecules or gold particles attached to individual lipid molecules and measure fluorescence recovery after photobleaching (FRAP).
What is the fluidity of membranes dependent on?
Answer: The fluidity of membranes is dependent on composition, and can be affected by the presence of cholesterol. The temperature of the phase transition, which is the change in physical condition from a liquid state to a 2-D rigid crystalline (gel) state, is lower for membranes with short tails or double bonds.
What is the semi-permeability of membranes?
Answer: Membranes are semi-permeable, meaning that they are able to allow the passage of certain molecules while blocking others. Polar and ionic molecules are unable to traverse the membrane.
What are lipid domains and what do they do?
Answer: Lipid domains are regions within a membrane that are enriched in certain lipids. They can form transient domains, such as lipid rafts, which are composed of certain lipid mixtures such as PC, sphingomyelin, and cholesterol. Lipid domains can play a role in various cellular processes, including intracellular signalling and the formation of blood clots.
- What is lateral diffusion? Lateral diffusion is the rapid movement of lipids and many membrane proteins in biological membranes.
- What is fluorescence recovery after photobleaching (FRAP)? FRAP is a technique used to visualize the lateral movement of membrane proteins using fluorescence microscopy. A small region of the cell surface is labeled with a fluorescent chromophore and viewed through a fluorescence microscope. The fluorescent molecules in this region are then destroyed by a very intense light pulse, and the fluorescence of the region is subsequently monitored as a function of time.
- How do proteins vary in their lateral mobility? Proteins vary significantly in their lateral mobility. Some proteins are nearly as mobile as lipids, while others are virtually immobile.
- What is the fluid mosaic model? The fluid mosaic model is a model proposed by S. Jonathan Singer and Garth Nicolson in 1972 to describe the overall organization of biological membranes. It suggests that membranes are two-dimensional solutions of oriented lipids and globular proteins, with the lipid bilayer serving as both a solvent for integral membrane proteins and a permeability barrier.
- What is transverse diffusion or flip-flop? Transverse diffusion, also known as flip-flop, is the transition of a molecule from one membrane surface to the other. It is a very slow process, occurring at a rate of about 10^-9 to 10^-10 s^-1 for phospholipids.
- What is the diffusion coefficient of lipids in a variety of membranes? The diffusion coefficient of lipids in a variety of membranes is about 1 μm^2/s.
- What is the diffusion coefficient of rhodopsin, a very mobile protein? The diffusion coefficient of rhodopsin, a very mobile protein, is 0.4 μm^2/s.
- What is the diffusion coefficient of fibronectin, a peripheral glycoprotein with low mobility? The diffusion coefficient of fibronectin, a peripheral glycoprotein with low mobility, is less than 10 μm^2/s.
- What is the function of the lipid bilayer in the fluid mosaic model? In the fluid mosaic model, the lipid bilayer has a dual role as both a solvent for integral membrane proteins and a permeability barrier.
- What is the rate of transverse diffusion or flip-flop for phospholipids? The rate of transverse diffusion or flip-flop for phospholipids is about 10^-9 to 10^-10 s^-1.