Self-Assembly Of Real Cell Membranes Experiment
Experiment by Science101
Description:
When we see a cell membrane in a biology textbook it looks like each of the molecules had to be one by one purposely placed there then somehow bonded into place. But that is not the case.
In this experiment a small amount of egg yolk provides the cell membrane forming molecules so that with a shake we amazingly "self-assemble" real cell membranes around oil and water droplets to demonstrate how easily polar forces construct such an important part of a living thing. These same membrane forming compounds (amphiphiles) can also be found coming from deep sea hydrothermal vents and were found in a meteorite from outer space but an egg is much easier to get them from.
We will with our own eyes witness the fascinating life-giving properties of simple molecules being guided by the forces of nature which make even parts of cells miraculously all come together on their own. This is origin of life science at its best! :happy:
Materials:
125 ml flask with stopper, or suitable jar with lid. A larger volume container is OK, roughly adjust proportions for 2/3 full.
Cooking oil.
1 egg
Egg-dish or other small volume bowl shaped container.
Eyedropper or small spoon
Water
Procedure:
Add 100 ml of water to flask. To that add 25 ml of oil.
Cover then shake for a second or two. Mixture should at first appear milky but quickly start separating.
While waiting for the oil to return to a clear layer on top, crack open the egg then place in small bowl. Notice that the egg yolk is a single giant cell which gravity turns so that the white spot, the nucleus containing the DNA, points upwards. The yellow yolk that (if it were fertilized) feeds the developing chicken contains among other fats and oily compounds, phospholipid molecules that form strong flexible cell membranes.
By now the oil should have formed a clear layer on top. We first shook it to prove that it will do this. Notice that it is pure oil that can be seen through to the other side.
Now squeeze the eyedropper, plunge halfway into the egg yolk like a syringe, get a sample, then add one drop into the flask. If using a spoon then carefully dig out a small amount of yolk, then drip one small drop into flask. Adding too much can cause the oil to form such small droplets that it becomes a colloid, like milk.
The drop of yolk will fall through the oil but float on the water so you can see it stuck in the middle. Shake for a second or two like before.
Now watch what happens to the oil layer this time. It should soon look like a giant piece of tissue, like you're looking at cells under a microscope but in this case you see them with your own eyes! The less dense ones that contain only oil and have a phospholipid monolayer around them, go to the top. The smaller phospholipid bilayer vesicles are even more like living cells and contain water inside. They would be at the bottom of the layer, with some moving in the thermal convection currents of the water. If they picked up particles of something heavy such as from clay or iron then they will sink to the bottom.
How it Works:
Polar forces between water molecules pull them together as if they are attracted like magnets, providing a force which squeezes the nonpolar (hydrophobic, like they are afraid of water) oil out of solution. That is why oil and water do not mix. Oil is less dense so it heads towards the top. If oil were heavier than water then it would form a layer at the bottom.
Phospholipid molecules form membranes due to their having one end called a "head" which is attracted to the polar water, and on the other end are two hydrocarbon "tails" which are attracted to the nonpolar oil. Shown below is a phospholipid molecule. The carbon atoms of the tails are black while the hydrogen atoms are light purple. The head (shown as a single large sphere for clarity) contains phosphorus, nitrogen, oxygen, carbon and hydrogen.
Phospholipids will form a single "monolayer" membrane around an oil droplet due to the nonpolar "hydrophobic" tails being attracted to the oil while the water loving polar "hydrophilic" heads point outward to contact the water. This is what we saw forming in the oil layer of this experiment.
A phospholipid membrane that forms around a small droplet of water (instead of oil) is called a "Vesicle" or "Liposome". Vesicles have a phospholipid "bilayer" where instead of a single membrane with all the tails stuck into an oil droplet there is a second inner membrane where the phospholipid molecules point the other way, with their heads contact the water droplet on the inside. The tails of the inner membrane are strongly attracted to the tails of the outer membrane, squeezing out anything that tries to come between them.
Vesicles are about the size of a red blood cell. And believe it or not under the right natural (or lab) conditions vesicles even grow and divide without the aid of a genome! Vesicles are now in the spotlight of origin of life science due to being easily naturally produced in nature and their already possessing some of the remarkable properties of living cells including growth and division. They do after all start off with a real cell membrane like cells in our body have and will readily attract other molecules into the membrane such as cholesterol that will improve its performance. Since there is cholesterol in egg yolk the vesicles that were formed will contain it too.
Vesicles are also very good at trapping particles and large molecules including RNA and DNA. This might have ended up included inside some of the vesicles in your experiment, especially if you poked through the egg yolk's nucleus when you took the sample or the water you used already contained some.
Vesicles do not possess all of the requirements to be considered "life" but all by themselves they are still surprisingly "life-like". So if by chance you think you saw something very small slowly moving around inside your flask, then maybe you did!
Soap works much the same way. Soap has molecules very similar to phospholipids with only one tail. They do not form bilayer vesicles so they cannot self-assemble into a cell membrane, but the same kind of membrane shown surrounding the oil droplet also forms around sticky dirt so that it will easily go down the drain without resticking to something else. The tails are also attracted to each other. When in water they clump together with their tails inside the mass with their heads pointing outwards. These are called "Micelles". Soap is readily self-assemble into micelles while phospholipids in water readily self-assemble into bilayer vesicles.
More Things To Do:
Flask can be reshaken to attempt making a higher population of smaller membrane enclosed oil droplets and vesicles, in essence, replicate some of the ones that are there. More egg yolk will further decrease size but cloud water.
The contents can be poured into a clear tray-like vessel to spread out the oil layer. More water can be added to make the bottom easier to see. Iron filings or other heavy particles are then sprinkled onto it. As the particles pass through the oil-membrane-water interface (shake a little if they get stuck) they are first coated with oil, then pick up a membrane, and after passing into the water quickly sink to the bottom where they will stay.
Use two flasks or jars, equally shaken, with only one containing egg yolk so that the two results can at the same time be compared.
A 2000 ml flask or fishbowl with aquarium pump and airstone can be used to swirl them around like lipid-cell pets. With luck you will form strong ones and can watch them bump into each other without breaking apart, while new ones will form out of the churning oil.
Container can be left to see how long the membranes stay stable, but in time will likely begin to rot while building up pressure inside vessel so do not cover so tightly air under pressure cannot escape.