Membranes, Permeability and Particle Movement
I.
Membrane Structure
a. Phospholipid bilayer; Fluid Mosaic Model
b. Proteins
i. Classified by their position:
1. Integral
2. Peripheral
ii. Functions:
1. Tunnels or carriers to let water-soluble
substances in or out of the cell
2. ID badges to identify cell as belonging to self
3. Hold adjacent cells together
4. Enzymes
5. Receptors: recognize important substances in
the ExtraCellular Fluid and send a message to
the interior of the cell (“signal transduction”).
For example, a cell in your body has receptors
that recognize the hormone insulin. When a
receptor on the cell recognizes insulin in the
ECF, it will tell the cell to take in glucose,
because there is plenty in the blood.
6. Anchors for the cytoskeleton
c. Carbohydrates; glycocalyx
-Glycoproteins- Cell Adhesion Molecules
-Functions- cell ID, communication, and “Velcro”
holding cell in place
II.
How Cells get stuff in and out (membrane permeability and
particle movement
a. Overview:
i. Hydrophobic substances can simply cross the cell
membrane. Examples: O2, CO2, alcohol, cholesterol,
fat-soluble vitamins
ii. Hydrophilic substances must be transported through
tunnels in the membrane. Examples: ions (Na+ etc),
glucose, amino acids, water-soluble vitamins
iii. When a cell wants to move large amounts of, and/or
large substances, in or out, it uses
endocytosis/exocytosis. Examples: secretory products
like mucin (exocytosis), food particles like a bacterium
(endocytosis)
b. Background: why stuff moves in the direction it does
(diffusion and osmosis)
i. Diffusion- describes the movement of solutes.
-because of random motion of particles, particles tend to
spread out and move from areas of high concentration
to areas of lower concentration, until they are, on
average, approximately equally spaced (equilibrium).
That is, they move down (or with) their concentration
gradient. When particles move down their concentration
gradient, energy is released (as if the particles slow
down). [A concentration gradient can be considered a
form of potential (stored) energy.]
-Each type of particle generally responds to its own
gradient (example, CO2 will respond to its own gradient
regardless of the gradient of O2)
-When a cell allows a solute to move in or out, following
its concentration gradient, the cell does not expend
energy. This is called “passive transport.”
-If a cell needs to move a solute from low concentration
to high concentration (that is, if it needs to move the
solute AGAINST its concentration gradient), the cell will
have to expend energy by using one of its ATP batteries.
ii. Osmosis- describes the movement of water molecules
-Water, like solutes, responds to its own concentration
gradient.
-Comparing 2 glasses of water: same volume. One
(glass A) has only H2O, one (glass B) has H2O plus
NaCl. There is more water in glass A; therefore, the
concentration of WATER is higher in glass A. Water, if
allowed, will spread out from A to B.
-Some terminology: hypertonic, hypotonic, isotonic.
These are relative terms, you have to be comparing 2
solutions in order to use them.
-In this class, the compartments we are interested in
comparing are the Extracellular Fluid (ECF) and the
Intracellular Fluid (ICF)
c. Using proteins to get hydrophilic substance across the
membrane (each protein only allows a specific solute to pass)
i. Facilitated Diffusion: passive transport using the help
of proteins. (no energy required!)
1. Channel proteins: act as a tunnel. These are
used for water and certain ions.
-The water tunnels are called aquaporins, and most
cells have lots of these, so water can move freely
back and forth by osmosis.
-Some ion channels are always open, and the ions
move in and out by diffusion. Many, however, are
gated, and only open under certain circumstances
(for example, if the electrical voltage changes).
2. Carrier proteins: act as a shuttle. When they
bind to the substance they are supposed to
transport, they change shape and move the
substance through.
ii. Active transport- similar to carrier proteins, but the
products are being moved AGAINST their gradients,
therefore ATP is required. (Binding of Pi causes shape
change that allows solute/s through)
-primary
-secondary
iii. Coupled transport- 2 different types of solutes are
moved at the same time. May be facilitated or active;
often used as part of a secondary active transport
system.
-Cotransport- both move in the same direction
-Countertransport- they are moved in opposite
directions
-Coupled transport sometimes moves both substances
against their gradients (requires ATP), sometimes moves
both substances with their gradients (does not require
ATP), and sometimes moves one substance with its
gradient and the other one against. In this case, the
energy released by the first is used to carry the other!
This is secondary active transport.
d. Endocytosis and Exocytosis: bulk transport. Both require
ATP regardless of gradients.
i. Exocytosis- moving stuff out. Ex, mucin
ii. Endocytosis- drawing stuff in
1. Phagocytosis- ex, bacteria
2. Pinocytosis- ex, ECF
3. Receptor-mediated endocytosis- ex, LDLs, folate
e. The membrane potential-Cells maintain a slightly
negative charge interior to the membrane relative to the
charge outside of the membrane (by putting more positive
ions outside the membrane). By keeping charged particles
separated, the cell maintains an electrical potential, which is
a form of potential (stored) energy. One of the primary
mechanisms by which animal cells do this is the sodiumpotassium exchange pump, which is a carrier protein that
pumps 3 sodiums out and 2 potassiums in at a time. The
concentration & electrical (electrochemical) gradient for
sodium is particularly high, and if it could, it would rush
into cells. The volt is a unit of measure of electrical potential.
Each cell type has a characteristic resting electrical potential
measured in mV