The CELL MEMBRANE (PLASMA MEMBRANE) as a selectively permeable barrier
BIG IDEAS:
 Every cell is enclosed by a cell membrane (plasma membrane) which gives structure to the cell, separates the cell
contents from the outside environment, and regulates the passage of materials in and out of the cell.
 Note that both the outside and inside of a cell are mostly water. Thus, cells exist in as aqueous environment.
 The plasma membrane is often referred to as a “fluid mosaic” because a “mosaic” is a structure made up of
many different parts. Likewise, the plasma membrane is composed of different kinds of macromolecules,
including proteins, cholestorol, phospholipids, glycolipids, glycoproteins, and lipoproteins. Furthermore, the
plasma membrane is a flexible structure in which the various components float and move laterally (side-to-side).
 Cell membranes are composed primarily of lipids arranged in a characteristic two layered structure, known as the
phospholipid bliayer.
 Cell membranes also contain a variety of proteins embedded within the lipid bilayer and which provide various
functions for the cell, including transport, cell-to-cell communication, and enzymatic functions.
 The major classes of lipids that form the cell membrane are called “phospholipids”.
 Phospholipids contain two hydrophobic (“water-fearing”) non-polar fatty acid tails covalently bonded to a
glycerol. The third carbon of the glycerol is covalently bonded to a hydrophilic (“water-loving”) negatively
charged phosphate group.
 Thus, phospholipids are amphipathic
The basic phospholipid bilayer
molecules containing non-polar regions
that are NOT water soluble, linked to polar
regions that ARE water soluble.
 Due to their amphipathic nature,
phospholipids in an aqueous environment
will organize into a characteristic two
layered structure called a “bilayer”.
 The nonpolar fatty acid chains of the
molecules, termed the hydrophobic tails,
face the interior of the bilayer, where they
are shielded from water.
 The polar phosphate groups of the
molecules, termed the hydrophilic heads,
face outward where they interact with
water both inside and outside of the cell.
 But HOW do materials move in and out of a cell?
 Obviously, the cell membrane is the border that must be crossed…
To understand HOW, first we must consider DIFFUSION AND OSMOSIS…
DIFFUSION
DIFFUSION: The movement of ions or molecules from an area of higher concentration to an area of lower
concentration. The rate of diffusion is affected by temperature, the molecule’s size, the steepness of the
concentration gradient, etc.
How does diffusion ACROSS a MEMBRANE work?
Permeable membranes: Both water and the solute move freely across the membrane. In which case is it really a
“membrane” at all? Since everything gets through it’s not really a barrier….
Impermeable membranes: Neither water nor solute can move across the membrane. NOTHING gets in or out. This
membrane wouldn’t be very useful either, as the cell could neither get nutrients nor dispose of waste….
SEMI-PERMEABLE MEMBRANE: A membrane through which only certain molecules may pass.
 In other words, it lets certain molecules through, but prevents other molecules from crossing.
 The cell membrane (plasma membrane) of cells is such a selectively permeable membrane.
 Such functionality is critical to the homeostasis of cells – the ability to maintain and regulate internal
environment and to maintain the various processes of life.
 This selective permeability means that the cell membrane has some control over what can cross it, so that
only certain molecules either enter or leave the cell.
What determines whether or not a molecule can pass through the lipid bilayer of the cell membrane?
 SIZE and CHARGE!!!!!!
 Small molecules will pass more readily through the phospholipid bilayer than large molecules will.
 Non-polar molecules, since they are hydrophobic and NOT water soluble, ARE soluble in the non-polar fattyacids that form the hydrophobic interior of the cell membrane. Therefore, they can more easily pass through
the membrane.
 Polar molecules (such as water) or ions (positively or negatively charged atoms or molecules) are both
hydrophilic and water soluble. Therefore, they CANNOT pass through the non-polar fatty-acids that form the
hydrophobic interior of the cell membrane.
 For example: small non-polar molecules such as carbon dioxide gas (CO2) and oxygen gas (O2) can pass freely
through the phospholipid bilayer. And thank goodness for that, or cellular respiration would have problems…
 On the other hand, sodium ions (Na+) and potassium ions (K+) are small but are positively charged ions, and
therefore hydrophilic and water soluble. As a consequence, they cannot pass directly through the hydrophobic
interior of the cell membrane. And thank goodness for that, since the regulated movement of these ions
across the cell membrane of neurons constitutes the fundamental basis of what we call “thought”…
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 But wait, cells exist in and perform all of their functions in an aqueous environment, so understanding the
movement of water is crucial to our understanding of how cells, including neurons, function…
 Thus, we must also consider OSMOSIS
OSMOSIS
OSMOSIS:
The movement of WATER across a semi-permeable membrane from an area of low solute concentration (i.e. high
WATER concentration) to an area of high solute concentration (i.e., low WATER concentration)
Hypotonic solution: the concentration of solutes OUTSIDE the cell is LOWER than the inside of the cell; water moves
INSIDE the cell, in extreme cases an animal cell may burst in a process called “lysis”.
Isotonic solution: the concentration of solutes OUTSIDE the cell is EQUAL to that inside the cell; net water
movement = 0, as water is moving out of and into the cell equally.
Hypertonic solution: the concentration of solutes OUTSIDE the cell is HIGHER than the inside of the cell; water
LEAVES the cell, in extreme cases the cell shrivels up and dies
But HOW? Water is small but polar, so it can’t simply diffuse through the phospholipid bilayer…
HOW DO THINGS GET IN AND OUT OF A CELL?
Several different processes are involved in moving materials across the cell membrane….
PASSIVE TRANSPORT: The movement of a substance across a cell membrane WITHOUT the use of a cell’s ENERGY.
1. Simple diffusion:
 The passage of small, non-polar molecules (e.g. oxygen gas, O2,
carbon dioxide gas, CO2) directly across the cell membrane
 Moves molecules from HIGH concentration to LOW concentration
 [HIGH]  [LOW]
 Does NOT require any energy input (like a ball rolling down hill)
2. Facillitated diffusion:
 Materials pass through the cell membrane with the help of a
transport protein (such as a channel protein or carrier protein)
 These transport proteins basically form a doorway through the
hydrophobic interior of the phospholipid bilayer
 Moves molecules from high concentration to low concentration
 [HIGH]  [LOW]
 Does NOT require any energy input (like a ball rolling down hill…)
One of my favorite examples…
OSMOSIS!!! The movement of water molecules through the
“aquaporin” transport protein during osmosis
ACTIVE TRANSPORT:
 Uses cellular ENERGY (ATP!) to move materials across the cell
membrane
 Needed to move material from an area of LOWER concentration
to an area of HIGHER concentration
 [LOW]  [HIGH]
 It’s like “swimming upstream” or “rolling the ball uphill”
One of my favorite examples…
The sodium-potassium pump (Na+-K+ pump)!!!
Neurons constantly perform active transport to set up an ion
concentration gradient that is the basis for the electrochemical signal of
an active neuron!!!
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BULK MOVEMENT: Getting the really BIG stuff in and out…
Exocytosis:
 Movement of materials OUT of the cell (exiting the cell).
 Vesicles (e.g. from Golgi apparatus) fuse with the cell
membrane and release their contents OUTSIDE the cell.
 For example by gland cells to release their products into
the ducts within the gland.
 Or by neurons to release neurotransmitters into the
synaptic cleft (the space between the terminal end of
one neuron’s axon and the next neuron receiving the
signal…) and thereby regulate behavior, personality,
and mood!!!!
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Endocytosis:
 Movement of materials INTO the cell (entering the cell).
 Part of the cell membrane engulfs or surrounds the material, then pinches off to form a vesicle to bring the
material INSIDE the cell.
 THREE MAJOR TYPES of endocytosis:
1. Phagocytosis: “cellular eating,” for example of an invading bacterium by the white blood cells of your
immune system
2. Pinocystosis: “cellular drinking” to bring in a substantial volume of extracellular fluid
3. Receptor-mediated endocytosis: As used to control the extracellular concentrations of neurotransmitters,
removing them from the synaptic cleft (the space between the terminal end of one neuron’s axon and
the next neuron receiving the signal…) and thereby regulating behavior, personality, and mood!!!
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