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Unformatted text preview: is, from an area of low solute concentration to an area of high solute concentration. Thus
active transport allows cells to generate concentration gradients that support a diverse array of cellular
processes. You will be hearing much more about active transport in lecture. The other category of
solute movement does not involve a direct expenditure of energy and is called diffusion. Diffusion
will be the primary focus of today’s lab.
Your text defines diffusion as: “the spontaneous movement of a substance [solute] down its
concentration gradient from a region where it is more concentrated to a region where it is less
concentrated.” This movement continues until the solute is evenly distributed throughout the available
space. The driving force for diffusion comes from the activity of the water (solvent) molecules in the
solution. Water molecules are not static. In reality, they are in constant motion that is driven by the
heat energy of the system. Evidence for this can be seen at high magnification in the light microscope
when viewing suspensions of small particles rapidly vibrating in a random manner. This motion,
referred to as Brownian motion, is the result of randomly directed collisions between the water
molecules and the suspended particles. Similar interactions occur between solvent and solute
molecules. The random nature of these collisions and the probability of more frequent collisions
occurring in areas where solutes are more concentrated work together to move the solute molecules
into the available space, that is, down their concentration gradient. Thus, anything that influences the
number of collisions between solvent and solute can, in turn influence the rate of diffusion. Relevant
possibilities here include: the initial concentration of the solute and the amount of thermal energy
available (at higher temperatures the thermal agitation of the water molecules is more rapid and
forceful). Diffusion rate is also influenced by the mass of the solute; more massive molecules have a
greater inertia and thus require a greater force to move.
Thus far only the diffusion of solutes has been considered. Osmosis is an alternative version of
diffusion that involves the movement of the solvent across a differentially permeable membrane
separating two regions of different total solute concentration. A differentially permeable membrane is
a barrier that allows the free passage of water molecules but does not allow the free passage of solute
molecules. The region containing the lower total concentration of solutes is referred to as being
hypotonic (relative to the other side) and the region containing the higher total solute concentration is
referred to as being hypertonic (relative to the other side). Osmotic water movement occurs
spontaneously from the hypotonic to the hypertonic side. Here, the driving for osmotic water
movement is the thermal motion of the water molecules as described above. The reason that
movement is in the hypo- to hypertonic direction is that the total [chemical] potential energy of the Biology 05LA – Fall Quarter 2012 Lab 3 – page 2 water molecules on the hypertonic side is diminished by the greater number of energy consuming
interactions between solute and solvent on the hypertonic side. Thus, there is a potential energy
gradient between the two sides; higher on the hypotonic side and lower on the hypertonic side. In this
situation, water molecules will move down the energy gradient, that is, from the hypo- to the
FACTORS INFLUENCING DIFFUSION RATE
In these experiments we will study the influence of three factors upon diffusion rate. This will
be done by making comparisons between the distances moved by colored solutes through a gelatin
medium. The gelatin to be used is called agar and is composed of mostly water. As such, the agar
approximates an aqueous system, but is much easier to use for this kind of study. The basic plan is that
drops different solutions will be added to small test tubes containing agar. The tubes will then be
incubated under the specified conditions for at least one hour. Since the incubation time for each of
the tubes will be the same, the distances traveled by the solutes will be proportional to the relative
diffusion rates of the solutes.
The solutes that will be used are: KMnO4 (potassium permanganate, mass = 158 grams/mole)
and aniline blue (mass = 738 grams/mole). Table 1 lists the different solute concentrations and
incubation temperatures that will be used. With some careful thought, you should be able to deduce
which tube comparisons can be used to reveal the effects of varying molecular mass, solute
concentration, and temperature upon diffusion rate.
1. Each group of 4 students should obtain 6 agar-filled
test tubes and label them 1 – 6.
2. With Table 1 as your guide, and with the use of a
different Pasteur pipette for each solute, add 1 drop of
each of the specified solutions to the proper tube.
3. When all 6 tubes are complete, replac...
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- Fall '12