The world of atoms and molecules is one mostly of empty space - particles
fly about in space, when they can, at relatively high rates of speed and
bump into each other and into the matrix formed when particles crowd
together into what we up in the macro world call a barrier. In some
barriers, the particles are so crowded (aided by their shared or
almost-shared electron zones) that very little can squeeze through - these
barriers would seem fairly solid to us, and would be largely impermeable,
a term meaning that nothing (at least nothing significant or practical)
could get through it. Some barriers are hardly barriers at all,
being for all intents and purposes permeable. These
two extremes are not absolutes - could anything be truly either of them? -
but are useful as concepts. Between these extremes, in barriers that
stop some particles - that are impermeable to that type of particle - but allow
others through - be permeable to those - we have the term
and many barriers in living systems can change permeability according to
needs, making them selectively permeable.
permeability can lead to potentials and voltages (video).
But what moves the particles in the first place? As
discussed before, particles just move, each with its own tiny kinetic
energy - speeding them up or slowing them down is associated with changes
in temperature, which is a measure of the average particle speeds
in the measured medium; warmer equals faster. And particles, when placed as a clump into a
medium through which they can move, will tend to spread out, randomly
distributing themselves from a crowded area to available less crowded
areas. This tendency to spread out, to appear to move from
areas of high concentration to areas of low concentration,
has a name: diffusion. We've all seen it at
work: a drop of ink or food coloring, or tea from a tea bag,
spreading through the water around it, spreading throughout a cup whether
we help it by stirring or not. When there is a permeable barrier
with an imbalance of particles on each side, there will be a net diffusion
movement through the barrier - a movement called flux - until
the concentrations are equal on both sides. At this point, the
material is in equilibrium, which means particles keep
moving through the barrier but for each one going one way, there's one
coming back. As a diffusion exchange moves toward equilibrium, flux slows
effects on molecules (animation video).
Introduction to flux.
Introduction to flux equilibrium.
The barrier that will be the main focus of this chapter
membrane, a lipid-based barrier with water-based (aqueous) solutions
on either side. Flux through a barrier can be increased in several
ways, some of which apply easily to cells and some of which don't.
For instance, the size of the particles affect the flux - smaller
particles move faster - but living things can't usually change particle
size in order to move something more quickly. It should be obvious
that speeding the particles up - raising the temperature -
will speed the flux, but that would have other effects on biological systems,
few cells or systems speed diffusion by warming themselves on
through a membrane (video).
factors are calculated.
But there are approaches that cells
can use to increase
One way to speed flux is to increase the concentration
gradient, which keeps the net movement of material speeding from a
crowded area into a much emptier one. Cells do this by either
processing particles just as they've crossed, or carrying them off, as blood systems
do with cellular wastes, or sequestering them in other less-accessible
parts of the cell that they can't come back from. Sometimes, by being permeable in just one
direction, the rate of entry slows as the amounts available to enter drop, but particles are
not lost in a return flux. Another feature of flux through a barrier
is the limitations presented by the surface to be penetrated - the more
surface area, the more particles can move through. Cells commonly increase
surface area (for example, with microvilli) to increase
particle movement in or out.
Many cells with microvilli.
A particular type of diffusion concerns a type of
particle that follows all of the diffusion rules we just discussed, but
because it is the solvent rather than the solute. Osmosis
is the diffusion of water,
moving from highly dilute areas to areas less
so. Typical cell membranes are permeable to water, and if the
dilution of cell contents and that of cell surroundings are not in
balance, there will be osmosis. It should be no surprise that many
organisms in the oceans have water concentrations inside that match the
water concentrations outside (although the solutes will be different), so
no net flux of water in or out has to be dealt with.
Osmosis in action (animation).
Solutions can be labeled according to their particle
concentrations - balanced solutions are isotonic. If
surroundings get too many solutes, as happens in some inland salt-water
lakes, they are hypertonic
to cytoplasm, and cells can lose so much water by osmosis that cell
chemistry is affected. If surroundings are too dilute, or hypotonic,
water can flood into a cell, interfering with chemical
interactions or even swelling it to the breaking
point. Organisms exposed to fresh water must somehow deal with
osmosis, or they will die. The flow of water inward can be so strong
that it exerts an osmotic pressure. In small plants,
the flow from groundwater into the roots - root pressure -
can be strong enough to push water up the plant against the pull of gravity
(usually not very far, maybe half a meter). Plants will also use
water pressure (turgor pressure) in a central vacuole
to provide a stiffness to their structure. This is done by pumping
particles, mostly potassium ions, into the vacuole, so that water
Basics of solutions.
Red blood cells
under the three conditions (video).
turgor pressure (video).
ADAPTATIONS TO DEAL WITH INCOMING OSMOSIS - In
small fresh-water organisms, a few mechanisms will stem the flux. One
is a structure like a cell wall - a rigid retainer that will not let the
cell expand beyond a certain size. Many plants and bacteria need no
other way to keep themselves from overfilling with water. But animal
cells, which lack cell walls, have a problem. Some animals do
produce casings to prevent swelling. For other animals, two basic
approaches work: first, water can be pumped out as it enters, such
as with a
vacuole; second, in
multicellular systems, the outer surfaces can be largely waterproofed.
In animals such as fish or crustaceans, only a few outer surfaces, such as
respiratory or digestive linings, are not waterproofed. They can't
be, as a waterproof surface is also impermeable to oxygen and
nutrients. Any extra
water that enters by osmosis across those surfaces is pumped out by
structures like kidneys.
vacuoles in action (video).
that can resist osmotic pressure.
Adaptations to fresh water or the changing dilutions of
tidal pools would have "set up" a capability to deal with no
surrounding water at all - a fish or crustacean waterproofed against osmosis in fresh
water would also be resistant to drying out in the air. For this
reason, animal life on land almost certainly moved there from tidal and / or