Download Formation of a membrane

ISCI 2001 – Life/Earth Science Activity
Self-Assembly Of Real Cell Membranes Experiment
Polar forces between water molecules pull them together as if they are attracted together like magnets,
providing a force which squeezes the nonpolar (hydrophobic) oil out of solution. That is why oil and
water do not mix.
Phospholipids and cholesterol form cell 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 hydrophobic.
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. This is how soap works.
Phospholipid tails are 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". A phospholipid “bilayer”
which forms around a small droplet of water (instead of oil) is called a "Vesicle". 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.
Cholesterol molecules have several functions in the membrane: They make the lipid bilayer less
deformable and decrease its permeability to small water-soluble molecules. Without cholesterol (such
as in a bacterium) a cell would need a cell wall. Cholesterol also prevents crystallization of hydrocarbons
in the membrane. The amount of cholesterol may vary with the type of membrane. Normally, plasma
membranes have nearly one cholesterol per phospholipid molecule. Other membranes (like those
around bacteria) have no cholesterol . The following figure shows the steroid structure of cholesterol.
The non-polar and polar regions are also illustrated (Figure modified from Alberts et al. Molecular
Biology of the Cell, Garland Publishing, N.Y., 1994, Third Edition, Figure 10-8.
Transmembrane proteins are also amphipathic, in that they have hydrophobic and hydrophilic regions
that are oriented in the same regions in the lipid bilayer. Another name for them is "integral proteins".
Other types of proteins may be linked only at the cytoplasmic surface (by attachment to a fatty acid
chain), or at the external cell surface, attached by an oligosaccharide. Or, these non-transmembrane
proteins may be bound to other membrane proteins. Collectively these are called "peripheral
membrane proteins".
The following figure shows transmembrane proteins passing through the lipid bilayer (from Wolfe S.L.,
Molecular and Cellular Biology, Wadsworth Publishing Company,
1993).
Cell membrane formation is highly dependent upon the biochemical properties of the phospholipids, as
well as those of other membrane lipids and proteins. These experiments demonstrate how easily polar
forces construct such an important part of a living organism.
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
Experiment:
PART 1: Add 100 ml of water to flask. To that add 25 ml of oil.
BEFORE MOVING TO THE NEXT STEP, READ THE INSTRUCTIONS.
CAN YOU MAKE A HYPOTHESIS ABOUT THE NEXT PART OF THE EXPERIMENT? WHY/WHY NOT?
Cover the flask, then shake for a second or two.
What happened to the oil and water in the flask?
Why?
BEFORE MOVING TO THE NEXT STEP, READ THE INSTRUCTIONS.
CAN YOU MAKE A HYPOTHESIS ABOUT THE NEXT PART OF THE EXPERIMENT? WHY/WHY NOT?
PART 2: 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 inside which feeds the developing
chicken (if it were fertilized) contains among other fats and oily compounds,
phospholipid molecules (can be used as soap), proteins, and cholesterol. Now
squeeze the eyedropper, plunge halfway into the egg yolk like a syringe, get a
sample, then add one drop into the flask.
What happens to the drop of yolk?
Why?
Is this what you expected?
Why/why not?
BEFORE MOVING TO THE NEXT STEP, READ THE INSTRUCTIONS.
CAN YOU MAKE A HYPOTHESIS ABOUT THE NEXT PART OF THE EXPERIMENT? WHY/WHY NOT?
PART 3: Shake for a second or two like before.
Describe what happened in your flask.
If you made a hypothesis, was it supported by your result(s)? Explain.
If you did NOT make a hypothesis, could you now? Do it!
Suggest a control for part 3 of this experiment.
Suggest a further experiment.
Modified from © 1996 - 2008 HotChalk, Inc. All Rights Reserved.