Biology 1009- Lab - Rebecca Teed
Diffusion and Osmosis - Study Sheet
The first phenomenon we are going to observe in this lab is Brownian motion.
The smaller the particle you observe, the more noticeable the motion is. The direction of
each particle is random, and they tend to bump into one another and go off in other, equally
random directions. There is no net overall movement. The particles move faster if you heat
the solution up, although they still don't go in any particular direction. Likewise, they
slow down if you cool them. Brownian motion stops at absolute zero. What we perceive as
heat is actually a non-directional form of kinetic energy. Brownian motion is simply
observable heat energy. Kinetic energy is proportional to the mass of the particle times its
velocity. For a given input of energy, a large particle moves more slowly than a small
particle. Cooling isn't a loss of energy, but rather a spreading out and dividing up of the
energy among multiple particles. When some of a particle's energy is passed on to a cooler
(slower moving particle), the first particle slows down.
I said above that there is no overall direction of motion since the particles are going
in all directions at once. The first part of that statement isn't always true, since if you put a
drop of dye in the middle of a larger drop of water, the dye particles can move outwards.
And they do, invariably. It's just a question of how long it takes. This spreading
outwards is diffusion. If I open a bottle of perfume, the particles float up into the air and
spread out across the room. If I leave it out long enough, the distribution becomes even,
so that the smell is as strong at the far end of the room as it is next to the bottle. Likewise
with the dye: the center starts out dark, and the edges of the puddle of water are light,
because the dye concentration is lower there. But the color will even out eventually, and
you won't be able to tell where you put the drop of dye to begin with. When this has
happened, the system has reached equilibrium. What we wanted to undo either of these
processes? It would be more difficult than it was to get them started. We would have to
add energy to the system (in the case of the perfume, we would suck all the air in the room
through a filter; don't be in the room when this happens) instead of making use of the
energy that's already dispersed throughout the system. Once we added the energy, it
would become heat energy and we couldn't pull it back out of the system. Diffusion is a
downhill sort of process.
The second Law of Thermodynamics says that there are a lot of "downhill"
processes, which make use of energy already present, and to stop or undo them, you must
add energy to the system. Downhill processes all contribute to the entropy of the system.
Most physics books describe entropy as chaos or disorder, which leaves a mental picture of
a very uneven situation. This is misleading. By disorder, physicists mean homogenaity.
"Order", in this context, means "sortedness". If all the blue ones are neatly sorted into one
pile and all the green ones are sorted into another, the system is highly ordered and has little
entropy. But it's so easy to inadvertently knock the piles over and mix them up, and it's so
much work to sort them out again.
Back to physics. Processes that make any compartment of the universe more mixed
and more homogenous add to that system's entropy. They tend to drive themselves once
they get going. Reversing these processes requires the expenditure of free energy,
which can do work and get directed motion and so forth. This energy isn't destroyed; it's
simply turned into more spread-out, less useful (from an entropy-resisting perspective)
energy, like heat.
The focus of this lab is on osmosis, diffusion through a membrane. If a
membrane is permeable to water, but not solutes (be they salts, sugar, protein, dye, etc.),
the flow of water into the membrane is unaffected by the concentration of solutes within the
membrane. If the solution inside the membrane has a greater goncentration of solutes than
the solution outside, the inside of that membrane is hypertonic relative to the outside. If
entropy is trying to force equal concentrations of water between the outside and inside of
the membrane, and the solutes within the membrane can't diffuse out to make room, there
will be water pressure against this membrane, possibly enough to burst it, thus dispersing
the solutes into the outer solution. Freshwater protozoans (including Amoeba, ciliates, and
Euglena) reduce the pressure by pumping out extra water with their contractile
vacuoles, expending hard-won food energy (obtained form outside their bodies). Plant
cell walls are simply strong enough to resist the pressure. Plant cells actually use the
pressure of water inside their central vacuoles (called turgor pressure) to keep their
stems (if they aren't woody) and leaves stiff and to stretch the cell walls as their cells grow.
If the outside solution has more solutes than the inside is relatively hypotonic. If
you dump a freshwater protist into saltwater, it shrivels up like a raisin as all the water
within its membrane is sucked out. The same thing will happen to a plant cell. Its cell wall
will be unaffected, but the membrane and the rest of the cell shrink. These cells would be
plasmolyzed. Organisms that live in salt water tend to be adapted to a specific salt
concentration and fare poorly in more or less salty water. All organisms need to maintain
an osmotic balance between themselves and their environment; no more water coming in
than leaving. This balance is called homeostasis.
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