Some points about membranes----- See also class notes and related text material specified in syllabus
Membranes regulate the traffic of ions and molecules into and out of cells (i.e. across the plasma membrane) and from one
cell compartment to another within eukaryotic cells.
Prokaryote (adj. prokaryotic): cell than does not have extensive internal membrane-based organization; no nucleus or other
organelles. Bacteria and cyanobacteria are the prokaryotes; more primitive than eukaryotes.
Eukaryote (adj. eukaryotic): cell (or organism comprised of such cells) that has extensive internal organization based on
membranes that subdivide the interior into compartments. Animals, plants, fungi, protozoans, algae are the eukaryotes.
Organization of eukaryotic cells (one broad view): based on unit membranes
A. Cell surface: plasma membrane (all cells)
cell wall (plants, fungi, bacteria, but not animals)
cilia, flagella (some cell types)
B. Compartments: cytosol and organelles (nucleus, mitochondria, endoplasmic reticulum, etc.)
C. Other: e.g. cytoskeleton
e.g. spindle aparatus (cell division only)
Biological membranes are unit membranes: All have similar architecture.
a. phospholipid bilayer: forms a hydrophobic barrier between 2 aqueous solutions…e.g. within a cell and outside a cell.
Phospholipid molecules are mobile within each layer, not covalently linked to each other….the layers are "fluid"
b. proteins
1. integral proteins: those embedded in the bilayer partially or extending all the way through
2. peripheral proteins: those loosely bound to one bilayer surface or the other
The protein molecules form a "mosaic" as seen in surface view of the membrane. So, membranes are described as a
"fluid mosaic."
c. steroids. Cholesterol molecules (in animal cells) or related ones (in plants, fungi) fill some of the spaces between
phospholipid molecules of the bilayer.
Some points about permeability--- See also class notes and related text material specified in syllabus
Diffusion: physical process in which molecules or ions move from a region where they are more highly
concentrated to a region where they are less highly concentrated.
Case: Molecules of a gas such as CO2 will diffuse through the surrounding molecules of "air," which itself is a
mixture of gases, mostly N2 and O2.
Case: Solutes such as CO2 will diffuse through the water within a cell compartment.
Case: Solutes such as ions, amino acid molecules, and monosaccharide molecules will diffuse through the water
(solvent) within a cell compartment.
Note that molecules can move by diffusion from place to place without any involvement of a membrane: for
example, from one part of a cell's cytosol to another part of that same cell's cytosol.
a. Every one of the individual molecules of interest moves about randomly due to its internal energy. Given time, a
molecule will wander away from its starting point as a result of this random motion.
b. Every individual molecule in a solution will collide with other, surrounding molecules. Further, every solvent
molecule is in constant random motion also, colliding with each other and with solute molecules present.
c. When we speak of molecules moving from place to place by diffusion, it is large populations of them that move.
And although every individual moves randomly, the net effect of many of them moving that way is that they
disperse (spread out) from their initial, concentrated situation into the surrounding space.
Temperature (degrees F)
Concentration gradient. The concept of "gradient" is important for understanding movement of molecules from
place to place. First a familiar example of "gradient," a temperature gradient: If you stand close to a bonfire
outdoors on a cold night you notice that the temperature close to the fire is high. If you then slowly back away from
the fire, you notice that the temperature of the air around you gradually decreases, and that the drop in temperature
is a function of distance from the fire. This is a description of a temperature gradient, shown graphically here. A
graph of a concentration gradient similarly would show the differences in solute concentration between two regions:
higher concentration of molecules (of glucose, e.g.) in one region, lower concentration some distance away, and a
gradual slope of concentration between those two regions. Molecules moving by diffusion will move down their
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gradient, and they will continue to do so until equilibrium is reached. That is the condition in which the
concentration is the same everywhere. Every molecule still moves in its own random fashion, but at equilibrium
there is no longer any net movement, no change in concentration from region to region. For every molecule that
happens to move in one direction at one instant, another moves in the opposite direction. At equilibrium the
molecules have dispersed as much as possible within the available space. This is an application of the second law of
thermodynamics. It also tells us that the dispersed molecules will not spontaneously reconcentrate themselves into a
smaller space.
Passive movement is the movement of molecules (or ions) by diffusion through a selectively permeable membrane.
A given membrane will allow some chemical species, but not others, to pass through. If a membrane is permeable
to molecule "X," then X will move down (with) its concentration gradient, by diffusion, through the membrane.
The driving force for this net movement of X across the membrane is the concentration difference on opposite sides
of the membrane. A cell does not need to spend any energy to drive passive movement. Note that a molecule "X"
will not move against its own gradient. *Relatively few types of molecules will pass through a membrane by
passive movement; among the important ones that will do so are water, O 2, and CO2. Aside from the fact that most
molecules are too large to "squeeze through" the spaces between phospholipid molecules of the bilayer, the reason
for this impermeability of a membrane to most molecules and ions is the hydrophobic character of the membrane. In
moving from one side to the other, the molecule or ion would have to pass through the membrane's very
hydrophobic interior (the double layer of fatty acid tails of the phospholipids). Ions and most organic molecules
found in cells/tissues are hydrophilic.
Dialysis is an application of passive movement, as follows. As just described above, we could consider passive
movement of molecules of a single type through a membrane (e.g. only glucose molecules or only alanine
molecules). However, suppose that two types of molecules, "X" and "Y," are present on one side of a membrane at
higher concentrations than on the other side. And suppose further that the membrane is permeable to X but not to Y.
Then, X will move (by diffusion) through the membrane, but Y will not pass through. Thus, the selectivity of the
membrane allows a separation of one type of solute (X, in this case) from another (Y, in this case). That is dialysis.
Osmosis is the special case of passive movement in which the solvent moves, by diffusion, down its concentration
gradient. In biological systems, water is always the solvent of interest. Case: If we place pure water on one side of
a selectively permeable membrane and a solution of some molecule "X" on the other side and we stipulate that the
membrane is permeable to water but impermeable to X, then water will move down its gradient through the
membrane. Such movement of water through membranes in living organisms has very important consequences.
Osmotic pressure (O.P.) of a solution is the pressure that would have to be applied to a solution to prevent pure
water from passing through a selectively permeable membrane into that solution. Pure water O.P. = 0, by definition.
Dissolving a solute in the water increases the O.P. to a positive value. The greater the solute concentration, the
greater the O.P. of the solution. In comparing two solutions that are separated by a membrane, the solution having
the greater solute concentration is hypertonic (=hyperosmotic) relative to the other, and the solution having lower
solute concentration is hypotonic (= hypoosmotic) relative to the other. Isotonic (= isoosmotic) means that the
solutions have equal solute concentration.
Facilitated diffusion. Most types of ions and molecules in living systems are too hydrophilic to pass through
cellular membranes by passive movement. So carrier proteins (= permease) embedded in the phospholipid bilayer
of membranes serve to "assist" such chemical species in passing through the membranes. Each carrier protein is
specific (has the right shape) for binding to ("recognizing") a particular ion or molecule and moving it through the
membrane. The driving force for the movement is the concentration difference on opposite sides of the membrane;
that is, the movement is still by diffusion, though with the help of the carrier protein. The cell does not spend its
energy to enable this movement. Note that a molecule "X" will not move against its own gradient, even with the
"help" of the carrier protein.
Active transport. This is similar to facilitated diffusion except that the cell must spend chemical energy to move
the molecule "X" through the membrane. Like facilitated diffusion, active transport requires specific carrier
proteins, but they are not sufficient. Cellular energy (ATP) is needed. This means that molecules can be moved
across membranes against their concentration gradient; expenditure of chemical energy enables that.
Note that both active transport and facilitated diffusion are called carrier-mediated transport (movement) since both
require carrier proteins in the membrane.