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Robert Lue: As we have all experienced, we live in a sea of chaos, noise, and entropy.

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The plasma membrane is a physical boundary

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between the highly organized inside of the cell

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and the chaotic external environment.

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So even though there is this important need

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to separate what is living from non-living,

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the plasma membrane must also allow for a dynamic interplay and exchange

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between the cell and its surroundings.

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A plasma membrane is a feature common to every cell.

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It surrounds every cell, and retains the contents of the cell.

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The cell's membrane is made up of mostly lipids and proteins.

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Lipids are fats like oil or butter, and the most common type

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of lipid in a plasma membrane is called a phospholipid.

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Phospholipids have both hydrophilic or water-loving, and hydrophobic or water

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avoiding, regions in a single molecule.

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Specifically their phosphate heads are water-loving,

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while their too long hydrocarbon tails are water-avoiding.

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Hydrophilic groups form favorable interactions

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with water via their polar or charge groups.

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They therefore dissolve readily in aqueous solutions.

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Hydrophobic groups avoid water due to their uncharged, non-polar

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hydrocarbons.

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The interaction between water and the hydrocarbon tails of a phospholipid

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is therefore energetically unfavorable.

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And they tend to cluster together in an aqueous solution

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to minimize their interaction with water.

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Phospholipids hydrophilic phosphate heads

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tend to partly dissolve in water, while their hydrocarbon tails pack amongst

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themselves to avoid exposure to water.

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The plasma membrane is a phospholipid bilayer,

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in which phospholipids arrange themselves

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so that the hydrophobic tails of the lipids face toward each other,

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and are isolated from the surrounding aqueous medium.

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While the hydrophilic head regions associate together

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and face the intracellular and extracellular faces of the cell.

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Hydrophobic interactions are the major driving force

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in the lipid bilayer formation.

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When dissolved in water, phospholipids self-assemble into a bilayer structure.

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It is remarkable that something as organized as the plasma membrane

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is primarily driven by spontaneous self-assembly.

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It is not made by somehow being put together on a factory floor

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before being delivered to the edge of the cell.

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This is one of the nicest ways to underscore

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the importance of self-assembly phenomena, the ability

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of a well-organized and quite complex structure

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to arise simply based on the chemistry of individual components.

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In a lipid bilayer, there is also dynamic diffusion of the lipids

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and other membrane components within the two-dimensional plane of the bilayer.

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This lateral diffusion gives the membrane its fluid structure.

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A fluorescence photobleaching experiment readily proves this concept,

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showing that the fluorescence from a particular region of a fluorescently

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labeled membrane can recover after the region is bleached by a laser beam.

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Fluorescent membrane lipids or other components diffuse into the region,

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and bleached membrane lipids or components

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diffuse out of the region over time.

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This is where the fluorescence recovery occurs.

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And this is why the method is called FRAP or FRAP,

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Fluorescence Recovery After Photobleaching.

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Biological membranes are composed of more than just phospholipids.

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They also contain many different types of membrane proteins

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that are associated in different ways with a lipid bilayer.

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These membrane proteins carry out many critical functions,

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facilitating the dynamic processes that occur at the membrane.

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In most mammalian cell's membrane proteins constitute 50% of the membrane

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by mass.

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There are some exceptions, such as the myelinated membranes of some neurons,

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in which case lipids constitute 80% of the membrane by mass.

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Membrane proteins can be classified into two groups in terms

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of their association with the bilayer, integral membrane proteins

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and peripheral membrane proteins.

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Integral membrane proteins are tightly bound

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to membrane lipids via hydrophobic forces,

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and can only be separated from the membrane under denaturing conditions.

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While some of the integral membrane proteins

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are embedded in only one leaflet of the bilayer,

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some others span both leaflets of the bilayer

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and are called transmembrane proteins.

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Because these proteins are designed to have large regions buried

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in the hydrophobic core of the phospholipid bilayer,

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they tend to be very insoluble in water, and tend

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to lose their structure upon purification

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from their membrane environments.

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Peripheral proteins, on the other hand, interact

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with the one surface of the membrane.

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They can be attached to the surface of a membrane

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via either electrostatic interactions or hydrogen bonds with lipid head groups.

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Alternatively, they can also associate with a different integral membrane

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protein.

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Peripheral proteins are typically water soluble,

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and can be separated from the membrane under mild conditions

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that denature neither the protein nor the membrane.

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One example of a peripheral membrane protein

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is cytochrome c, which we will cover in more detail in coming lessons.

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The wonderfully dynamic phospholipid and protein composed membrane

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provides all cells with an outer boundary,

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while also mediating the exchange of materials

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between the interior of the cell and its external environment.

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