ICF vs. ECF
The cell membrane is a highly selective
barrier MOLECULES
At any temperature above absolute zero, molecules are Molecules are constantly colliding with other molecules The number of collisions per unit time is dependent upon DIFFUSION
Although molecules move in all directions, everything else Physiologically important diffusion processes take place only [see Figures 4-1a,b; 4-3, 4-2]
[Fig. 4-4: The diffusion of substance X into a cell. Note that Diffusion time (t) increases with the square of diffusion distance
t is proportional to R squared
[see Table of diffusion times]
[see Figure of steady state diffusion across a membrane
FICK EQUATION: dQ = -D A (dc/dx) dt
where D = the diffusion coefficient, A = the area of diffusion In the Fick Equation, Q represents the quantity of We can simplify this relationship by using proportions
Q is proportional to A * t * delta c/x
This proportionality reveals that Q is directly proportional Diffusion does not require a membrane and can take place Example: (the diffusion of oxygen at 20 C) OSMOSIS
Osmosis (a special case of diffusion) represents the diffusion of There is an inverse relationship between the concentration of the There will be a net flux of water from a region of higher water With respect to osmosis, solute concentrations are typically For a nonelectrolyte (such as glucose) 1 Osmole = 1 Mole As you will recall, A one molal solution is slightly more dilute than a one molar Osmosis depends upon the number of osmotically active particles However, electrolytes dissociate in water (i.e. one molecule may The blood plasma of most mammals (including humans) is a Physiological saline (i.e. a salt solution having the same NOTE: 0.9% NaCl = 9 g NaCl/liter [see Fig. 4-19]
[see Animation]
[see Fig. 4-18]
[see Figure comparing penetrating and non penetrating solutes]
Although diffusion is classically defined as a net movement of Diffusion actually reflects the net movement of molecules from In the case of osmosis, we are dealing with the free energy Free energy increases as concentration increases.
It is important to note, however, that free energy is also affected For example: If we have an equal concentration of penetrating COLLIGATIVE PROPERTIES
These properties depend only on the number of particles in
solution, An increase in [solute] is associated with: Knowing any one colligative property, we can calculate any other.
In a thermodynamic sense, the colligative properties of a solution Dilute solutions behave like gases. We can therefore apply PV=nRT
where P = pressure, V = volume, n = the number of molecules, We can rearrange the equation (by dividing both sides by V) P = (n/V)RT
We can now substitute osmotic pressure (which we can symbolize) We then obtain the relationship:
pi = CRT
R is a constant. This equation thus reveals that the osmotic
pressure Most commercial instruments for monitoring osmotic concentration FREEZING POINT METHOD:
A one osmolar solution freezes at -1.86 C. Therefore, delta fp Rearranging: C = delta fp/1.86
[see figure depicting evolutionary transitions]
Marine invertebrates - most are isosmotic Some animals (especially those living in estuaries) are exposed to [see figure of osmoregulators and conformers]
As noted earlier, there is an inverse relationship between PV = nRT Therefore, P = (nRT)/V
P is proportional to 1/V
Therefore, osmotic pressure is proportional to 1/V
where V = cell or body volume
For osmoregulators, internal osmotic concentration is constant. Osmoconformers, in contrast, are often volume conformers.
Although osmoconformers are often volume conformers, a rigid Also, some osmoconformers can regulate volume by adding or [see Figure]
In a hyposmotic environment, animals may either osmoregulate The plasma membrane is SELECTIVELY PERMEABLE. Some For a given set of conditions, we can define a permeability constant F = Kp delta C
We can make some generalizations about the permeability of cell membranes (a) Small molecules enter more rapidly than do larger molecules.
[see Figure]
(b)Lipid-soluble (e.g. nonpolar) molecules penetrate more rapidly than do [see Figure] Note: The partition coefficient (solubililty in oil/solubility (c) Non-charged molecules penetrate more rapidly than do charged substances
However, membranes contain protein channels (some of which are [Fig. 4-7]
Some ion channels are always open. Other channels are sometimes open Gated channels may open in response to:
MEDIATED TRANSPORT
Some substances penetrate membranes faster than we would expect Glucose, for example, is a fairly large polar molecule with a low Specific carrier proteins (i.e. transporters) on the plasma [Fig. 4-8] This is a general model for mediated transport. Mediated transport systems have certain properties (a) Specificity
Mediated transport systems tend to be specific for a particular chemical, The system which transports glucose will not transport amino acids and vice versa.
(b) Saturation
There are a finite number of specific carrier proteins, and the system can (c) Competition
All substances diffuse independently; however, closely related molecules ACTIVE TRANSPORT
Cells can spend energy (in the form of ATP) to move molecules against their Some, but by no means all, active transport systems are "coupled" (i.e. they move If the molecules are moved in the same direction, the process is termed SYMPORT.
The sodium/potassium ATPase system is an important example of an antiport coupled [Fig. 4-11] This is a model of a generalized primary active transport system.
[see Animation]
[Fig. 4-12]
[Fig. 4-13] Secondary active transport systems are coupled systems which use [see Animation]
[Fig. 4-14] These are both examples of secondary active transport. The direction [Figure depicting transport of calcium]
[Fig. 4-15]
VESICULAR TRANSPORT
There are some special mechanisms whereby large molecules and other particles (a) Phagocytosis ("cell eating")
NOTE: Some consider phagocytosis to be a special case of endocytosis. However, [see Figure]
[Fig. 4-20]
[see Figure] Endocytosis can be non-selective (= pinocytosis or fluid endocytosis) BULK FLOW
As we have noted, diffusion is very rapid and effective over short distances, Bulk flow is proportional to delta P
TRANSEPITHELIAL TRANSPORT
The movement of materials across epithelia may involve a variety of EPITHELIA
Epithelial cells are very important in physiology. They form the integument These cells are connected together and organized into layers by various [Fig. 3-10a,b]
[Fig. 4-22]
[Fig. 4-23]
[see Figure]
[Fig. 4-24]
separating the ICF from the ECF.
constantly in
motion. Molecular motion increases with
temperature, and we experience this
motion as HEAT.
and thereby
changing direction. The net movement
is essentially random.
the velocity
(which in turn varies with molecular size
and temperature) and also, very
importantly, on the concentration
of the molecules (i.e. the number per unit
volume).
being equal,
there will be a net movement
(i.e. a NET FLUX) of molecules from a region of
higher
concentration to a region of lower concentration.
over very
short distances (normally a fraction of a mm).
the rate of
diffusion (i.e. the change in ICF concentration
per unit time) is dependent
upon the concentration gradient.
Diffusion is rapid initially, but declines
towards zero
as equilibrium is approached.
and t =
time
a substance diffused
to exposed
surface area (A), elapsed time (t), and the
concentration gradient (delta c),
and that Q is inversely
proportional to the diffusion distance (i.e.
membrane
thickness, x).
in gases,
liquids, and even solids. The rate of diffusion,
however, is dependent upon
the medium. Diffusion can be very
rapid in gases, is much slower in liquids,
and is slower yet in solids.
The rate of diffusion in water
is 2.4 times that in skeletal muscle
The rate of diffusion in air is 786,000
times that in skeletal muscle
WATER
across a selectively (or differentially) permeable membrane.
water (=
the solvent) and the concentration of substances (= solutes)
dissolved in the
water.
(lower
solute) concentration to a region of lower water (higher
solute)
concentration.
expressed in
terms of Osmoles (or milliosmoles)
= the molecular
weight in grams
a one molar solution = one mole plus enough water to
form
one liter of solution
a one molal solution = one mole plus 1000 g of
water
solution;
however, for physiological purposes, the distinction
is not very important.
in
solution. For a nonelectrolyte, one molecule is equivalent to
one particle.
Therefore, one mole = one osmole.
form two
or more particles). NaCl is a strong electrolyte and
dissociates almost (but
not quite) completely into sodium ions
and chloride ions. Therefore, a one
molar solution of NaCl
is almost 2 osmolar.
mixture of
solutes (mostly electrolytes) yielding an osmotic
concentration of about 0.30
Osm (or 300 mOsm).
osmotic
concentration as mammalian plasma) = 0.9 % NaCl
The molecular weight of NaCl =
58.5
Therefore, 0.9 % NaCl = 9/58.5 = 0.154 M
Assuming nearly complete
dissociation, this equates to
an osmotic concentration of about 0.30 Osm
molecules
from a region of higher concentration to a region of
lower concentration, the
molecules do NOT move because of
the concentration gradient per se.
a region of
higher towards a region of lower FREE ENERGY.
of the water
molecules in the solution (i.e. with the WATER ACTIVITY).
by
other factors, such as temperature and applied external pressure.
molecules,
but the solutions on the two sides of a membrane
are at different
temperatures, there will be a net flux of solute
molecules from the warmer
towards the cooler side.
irrespective of their chemical nature.
a. an increase in osmotic
pressure
b. a decrease in vapor pressure
c. an increase in the boiling
point
d. a decrease in the freezing point
depend
on the free energy of the water molecules (i.e. on the
"water activity").
the ideal gas
law:
R = the gas
constant, and T = temperature.
to obtain:
by pi) for
P, and osmotic concentration (C) for (n/V)
exerted by a solution is proportional to the product of the
osmotic
concentration times the absolute temperature.
measure
either vapor pressure or freezing point depression.
(the
change in freezing point) = 1.86 C
Freshwater animals - all are in
a hyposmotic environment
Marine bony fishes - all are in a hyperosmotic
environment
drastic
changes in their osmotic environment.
pressure and
volume:
They
therefore tend to also regulate body volume (i.e. they are
volume regulators.
carapace or
external covering can help to prevent or mimimize
body volume changes.
dumping solute
(e.g. amino acids).
(e.g. by
excreting water) or conform. If they conform, they may
either volume conform
or volume regulate. In any event, even if they
do not osmo- or volume
regulate, they will regulate ions.
substances penetrate
rapidly, others more slowly or not at all.
(Kp)
for a membrane such that the net flux (F) across that membrane
equals Kp
times the concentration gradient (delta C).
to various substances:
lipid insoluble substances.
in water) is a measure of lipid solubility
gated) which allow selected ions (e.g. Na+, K+, Cl-, Ca2+) to enter very rapidly
and sometimes closed (i.e. they are "gated")
(a) a mechanical change
(b) an electrical (i.e. voltage) change
(c) a chemical change
them to (based on their chemical nature) or even move in a direction
opposite to that expected for diffusion.
lipid solubility. We would expect it to diffuse into cells only very slowly,
but, in fact, it enters many cells quite rapidly.
membrane appear to be "helping" glucose across the membrane
barrier. This is a type of mediated transport referred to as
FACILITATED DIFFUSION. Note that the direction of movement is
that expected for simple diffusion.
It could, for example, represent facilitated diffusion from
the ECF to the ICF.
(i.e. specificity, saturation, and competition) which distinguish
them from simple diffusion.
or, at most, a group of related chemicals.
become saturated once all are functioning at maximum capacity.
[see Fig. 4-9]
may compete for a limited number of carrier proteins.
[see Figure]
concentration gradient (i.e. from a region of lower concentration to a region of
higher concentration). This is a constant, and very essential, activity of living cells.
As much as 25% of a cell's total energy expenditure may be used for active transport.
two or more different substances in the same or different directions.
If in the opposite direction, it is called ANTIPORT.
pump (trading 3 sodium ions for 2 potassium ions).
the energy associated with an ion gradient to move other molecules "uphill".
The actual expenditure of ATP is indirect (needed to maintain the ion gradient).
of movement may be either symport (i.e. cotransport) or antiport (i.e. countertransport).
can cross cell membranes.
(b) Endocytosis
(c) Exocytosis
most believe that phagocytosis is fundamentally different inasmuch as it involves
an evagination of the cell membrane, whereas endocytosis involves an invagination.
or selective (absorptive or receptor-mediated endocytosis). Some use the term
'pinocytosis' for both processes.
but slow and ineffective over large distances. Bulk flow is an important alternative
mechanism for molecular movement. Bulk flow depends on a pressure difference
between two regions of a liquid or a gas. Bulk flow and diffusion are NOT
mutually exclusive.
active and passive mechanisms.
(external lining of the body) and they line hollow structures such as blood vessels,
kidney tubules, and the intestine.
specialized membrane junctions.