Hey it's professor Dave, let's talk about the cell membrane. We probably know that living organisms
are made of cells. From unicellular organisms like bacteria, which are made
of just one cell, to large animals that are made of trillions of cells. One key
feature of a cell is the cell membrane, or plasma membrane, which separates
what's inside the cell from what's outside of the cell. This is a very
important function because without this separation there could be no such thing
as life, because an organism must be distinct from its surroundings. Inside a
cell is where metabolism takes place and genetic information is stored, under
regulated conditions, or homeostasis. The cell membrane is semipermeable, meaning
it lets some things through while preventing other things, and now that we
know about lipids we are ready to understand this outermost layer of the
cell in more detail. That's because the structure of the plasma membrane is
similar to the soap micelles we learned about, where the molecules arrange
themselves with the hydrophilic heads facing out and the hydrophobic tails
pointing in, except that instead of a micelle it is a phospholipid bilayer
that engulfs the contents of the cell. It's called a bilayer because it's two
layers of phospholipids, which each have a phosphate group similar to the
carboxylate group in the soap molecules with formal charges that can interact
with water molecules, and nonpolar fatty acid tails, one saturated, the other
unsaturated, which prefer to stay hidden from water molecules. One layer faces
outwards, to the aqueous environment outside of the cell, and the other layer
faces inwards, to the aqueous environment inside the cell. That means
there are two layers of nonpolar material in between. This is part of what
makes this membrane semipermeable, some compounds are able to traverse this
nonpolar section and others are not. We refer to this
bilayer as a fluid mosaic, because these phospholipids are not fixed in their
location with respect to one another. They are swimming around and constantly
changing places, like concert-goers navigating through the crowd. But it's
not just phospholipids in the membrane. There are molecules of cholesterol, which
give the cell membrane some rigidity, otherwise it would be too fluid. In
addition the cell membrane is filled with many different proteins that have a
wide variety of functions. Some of these are channel proteins, which float about
in the sea of phospholipids and facilitate the movement of some kind of
molecule across the membrane. Let's learn about these proteins and the different
ways that substances can move in or out of the cell. Some molecules don't need
help to get through the membrane. Small nonpolar molecules are able to go right
through the lipid bilayer because they are not repelled by this nonpolar
material. This is how oxygen and carbon dioxide get across, since they are tiny
and nonpolar. This process is an example of diffusion, where molecules move across
the membrane along with a concentration gradient, spontaneously moving from
higher concentration to lower concentration, just the same way gases
spontaneously fill up a room. Other molecules need help to get in and out of
the cell. Some molecules can get through via passive transport, which doesn't cost any energy. This includes polar molecules like water
and glucose, as well as various ions, which can move through special
transmembrane proteins that connect the intracellular space and extracellular
space, so that these polar or formally charged particles don't have to push
their way through the nonpolar tails of the lipid bilayer. This kind of transport
doesn't require energy expenditure when the particles are moving with the
concentration gradient, because it is an entropically favorable process, and we
can refer to this process as facilitated diffusion. So something
like a sodium or potassium ion, when moving with the gradient from high
concentration to low concentration, can pass through by passive transport, which
is a kind of diffusion, but when things need to move against the concentration
gradient, active transport is required. Since this is an entropically
unfavorable process there will have to be an energy expenditure involved, so
certain proteins, like a sodium-potassium pump use up ATP, the molecule that is the
currency of cellular energy, to move sodium ions out of the cell and
potassium ions into the cell against the concentration gradient, from low
concentration to high concentration. We will talk more about ATP later. Transport
proteins that assist in either passive or active transport are highly specific,
each allowing only a particular substance or small group of similar
substances to pass through. Aquaporins move water. Ion channels move ions, and so forth. Some of these are channel proteins that
remain open, and some are carrier proteins, which alternate between two
conformations, like the glucose transporter, where binding with glucose
causes it to change shape, and then once glucose is released it changes back to
its original conformation. Beyond transport proteins there are many other
types of proteins in the cell membrane. There are glycoproteins with
oligosaccharide chains jutting out that help cells recognize one another. There
are scaffold proteins that bind with structures inside or outside of the cell
to maintain cell shape and location, and lastly there are receptor proteins that
receive signals from outside of the cell. Let's learn about those next. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me:
