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.
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