6th Summary- Experiments with Membranes, Osmosis& Selective Transport Dr. Matt Holden

March 29, 2014

Membranes are barriers that selectively allow the passage of certain components and retain other

components found in the liquid. Most membranes show this selectivity behavior due to their selective

transport, the transport of specific substances by means of their components found in their membrane

such as proteins, cholesterol, ions, etc. An example of this selective passage through a membrane is

osmosis, the net movement of solvent molecules through a partially permeable membrane from a

region of lower solute concentration to a region of higher solute concentration. Its goal is to equalize the

solute concentrations on the two sides. The purpose of the lab is making droplets to create a membrane

by investigating the relationship between lipid membrane composition and osmosis or selectively

transport rates of droplet interface bilayer (DIBs). These droplets are created by plunging two aqueous

droplets of lipid solution in hexadecane, which creates balance. Eventually, lipid monolayers will form

around the droplets when the vesicles inside the droplets supply the lipids. Since there is affinity

between the lipid monolayers, an entropic gain occurs, where a pair of droplets surrounded by lipid

monolayers will come in contact to form a lipid bilayer. Given that the B vesicles were more

concentrated than the A vesicles, water passed from the A vesicles to the B vesicles by osmosis. From

the eight lipid bilayers formed, seven coalesced. The second part of the experiment consisted of creating

more lipid bilayers between A vesicles and resorufin dyed vesicles, which must be amphipathic so that

the dye can pass from one molecule to another. After the DIBs are formed, a diffuser is placed on top of

the chip containing the DIBs to diffract the light for better image quality. Then, the DIBs are illuminated

using a laser pointer, which excites the electrons in the dye when absorbing a photon. The excess energy

is given off as heat or fluorescence light, and since the fluorescence concentration was high, residual

energy transfer between molecules occurred. As a result, the membranes were selectively permeable,

because polymers in the membrane did not let the dye molecules pass through. In this lab, transport

rates of water and other molecules depending on the composition of membranes were observed as

osmolytes and dyes were inserted into droplets of DIBs. The experiment can be applied to treating

diseases related to lipids by manipulating its composition so it can either absorb more water or inhibit

its water absorption. Examples of these applications are highly effective enzyme replacement therapy,

blood transfusions and enlarged spleen removal. It also has other applications such as creating bio

batteries by engineering toxins, and light receptors from Halobacterium salinarum.