Draw students to consider the remarkable capabilities of membranes: allowing some materials to diffuse through, actively transporting others, endo- and exocytosis. What is the structural, chemical nature of membranes that give them these capacities? Using the electron microscope and techniques of chemical analysis, scientists have discovered much in the last few decades. Edge on images of membranes using the electron microscope shows them to have a “sandwich” structure as seen here:
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(The two parallel lines seen in these micrographs are edge-on views of the thickness of single membranes.)
Chemical analysis of membrane material shows that it is made largely of chemicals called phospholipids.
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The key features of phospholipids are:
1. a phosphate “head”, which is highly water soluble (attracted to water, hydrophilic)
lipid “tails”, which are water insoluble (essentially oils, repel water, hydrophobic) but readily combine with one another like the behavior of oil in water.
This structure causes phospholipids to spontaneously form the “sandwich” structure of membranes seen in photomicrographs: phosphate “heads” to the outside making the dense outside lines, and lipid “tails” to the inside, showing up as the less-dense space between.
This structure is diagrammatically illustrated in the following video. Certain proteins, which are also found in membranes, function in the various transport mechanisms as described. (Technical terms beyond what is used in the text, can be ignored.)