Friday, February 15, 2013

biology membranes minilab

Modeling Origins of Life: the phospholipid membrane
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 phospholipid membranes around oil and water droplets to demonstrate how easily polar forces construct such an important part of a living organism.
Materials: 125 ml flask with stopper, 50 ml graduated cylinder, Cooking oil, egg yolk , Eyedropper pipette, Water

Test tube brush & soapy water

1.   Add ~100 ml of water to flask. To that add 25 ml of oil.

  1. Cover then shake for a second or two. Mixture should at first appear milky but quickly start separating.
  2. While waiting for the oil to return to a clear layer on top.  Use the eyedropper to extract ~2 drops of egg yolk.
  3. 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.
  4. Then add one drop of the egg yolk into the flask.  Adding too much can cause the oil to form such small droplets it becomes a colloid, like milk.
  5. The drop of yolk will fall through the oil but float on the water so you can see it stuck in the middle.
  6. Shake for a second or two like before.
  7. 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 which contain only oil and have a phospholipid monolayer around them go to the top.

·       The smaller phospholipid bilayer vesicles contain water inside and will be at the bottom of the layer, with some moving in the thermal convection currents of the water.


Phospholipid and cholesterol form membranes due to their having one end called a "head" which is attracted to the polar water, and on the other end are "tails" made of oil chains which are attracted to the nonpolar oil. Phospholipids and similar compounds will form a single "monolayer" membrane around grease, oil, and dirt, by their nonpolar "hydrophobic" tails sticking to the dirt while the water loving "hydrophilic" heads point outward to contact the water.

A phospholipid membrane which forms around a small droplet of water (instead of oil) is called a "Vesicle" which has a phospholipid "bilayer" where instead of a single membrane where all the tails stuck into an oil droplet there is a second inner membrane that has the phospholipid molecules pointing the other way so their heads contact the water droplet on the inside, with the tails of the inner membrane strongly attracted to the tails of the outer membrane which squeezes out anything that tries to come between them.

Vesicles are also very good at trapping such things as RNA and DNA which might end up included in your experiment, especially if you poked through the egg yolk's nucleus when you took the sample.

Analysis Questions:

  • What are the bilayer membranes most like in cell biology?  Monolayer membranes?
  • Describe how this activity is representative of evidence for a possible theory in the origin of life. 
  • Did you notice any of the droplets growing or dividing?  Explain how this might be relevant to the origin of life.
  • What aspects of complete living single cell are missing in this demo?
  • What happened when you added Fe particles?  How might this relate to endosymbiont theory?

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