Investigation #4: Diffusion and Osmosis

Solution

A homogeneous mixture of two or more substances in which the molecules or atoms of the substances are completely dispersed. The constituents can be solids, liquids, or gasses.

Solute potential (Ψs = - i CRT)

A measure of the propensity of a given solution to gain water from surrounding solution of greater of lesser solute concentration. i = ionization constant (number of ions; Na+Cl- = 2, sucrose = 1) C = Molar Concentration (determined by the graph) R = pressure constant (0.0831 bars/mole, ° Kelvin) T = Temperature in Kelvin (273 + temp ° C) *room temperature = 20° C

Aqueous

A solution in which the solvent is water. It is usually shown in chemical equations b appending (aq) to the relevant formula.

Summary

A solution with a high water potential has more free water molecules, and a solution with lower water potential has fewer free water molecules. If these two solutions are separated by a selectively permeable membrane, then the water will tend to diffuse across the membrane from the solution with a higher water potential to the solution with a lower water potential (lower negative number)

Solvent

A substance in which another substance is dissolved, forming a solution.

How does cell size and shape influence diffusion into and out of the cell?

As cell size increases, its ability to facilitate diffusion across the cell membrane decreases. This is because its internal volume increases at a faster rate than its external surface area. Therefore, the cell will divide to maintain the proper ratio and rate of diffusion to survive. The more surface area that is exposed, the easier diffusion can occur. A thin flat cell will diffuse faster than a cube shaped cell because it has a higher SA to Volume ratio.

Which color mystery solution had the highest concentration of sucrose? How do you know this?

Clear because it was the most hypertonic to the cell and caused the potato core to lose the most mass.

How is dialysis tubing different from a cell membrane?

Dialysis tubing is not selectively permeable based on its needs, it only allows the movement of molecules based on its size, and cannot pump against the concentration gradient like a real cell could.

Hypertonic

Having a higher concentration of solutes when compared with the surrounding medium.

Hypotonic

Having a lower concentration of a solution, usually expressed as the number of moles of solute per liter of solution. When an animal cell is placed in a hypotonic solution the cell wall restricts expansion and turgor pressure builds to hold the plant upright (non-woody plants). Animal cells may burst or lyse in hypotonic solutions.

Volume

Length x Width x Height

Surface Area

Length x Width x Number of Sides

Review the graph of Rate of Diffusion vs. Surface Area-to-Volume Ratio. Do the results suggest that the surface area-to-volume ratio affects the rate of diffusion?

No, results do not suggest that the SA to Volume ratio affects the rate of diffusion because all three cubes had the same rate of diffusion.

Diffusion

Process by which molecules (liquid or gas) are passively transported from an area of higher concentration to an area of lower concentration.

Plasmolysis

Process where the cytoplasm pulls away from the cell wall as the cell loses water to osmosis in a hypertonic environment.

Why can't humans drink saltwater for hydration?

Seawater contains a higher concentration of salt than regular water and is hypertonic to human cells. Thus, drinking saltwater would actually dehydrate human, rather than replenish its water needs.

Isotonic

Solute concentration inside and outside the cell are at equilibrium; there is no net movement of water molecules across the selectively permeable membrane. Plant cells are flaccid in isotonic solutions.

Solute

Substance dissolved in a solvent.

Surface Area Volume Ratio

Surface Area --------------- Volume

What is the cell membrane described as semi-permeable? How is this important for cell survival?

The cell membrane will only allow certain substances to enter according to its metabolic needs. This prevents harmful substances from entering the cell and stops essential substances from leaving.

Concentration gradient

The change in the level of solute in a solution between a region of higher concentration and a region of lower concentration.

Equilibrium

The condition in which all acting influences are balanced or canceled by equal opposing forces, resulting in a stable system.

Water potential (Ψ = Ψs + Ψp)

The measure of the relative tendency of water to move from one area to another, and is commonly represented by the Greek letter Ψ (Psi). *if atmospheric pressure = 0, then Ψ = Ψs

Osmosis

The movement of water molecules separated by a selectively permeable membrane from an area of lower solute concentration to an area with higher solute concentration

Turgor

The normal distension or rigidity of plant cells, resulting from the pressure exerted by the cell contents on the cell walls.

Pressure Potential

The outward force exerted on a plant cell wall when a plant cell swells due to net movement of water into the cell.

Examine the initial and final masses of the model cells. What causes the mass of the dialysis bags to change? Was there more or less water in the dialysis bags at the conclusion of the experiment? Explain.

The process of osmosis causes water to passively diffuse from a high concentration to a lower concentration to reach an equilibrium. Because there was more water in the dialysis bags at the conclusion of the experiment because the solution was hypertonic to the cell and contained more water than what was in the dialysis tubing.

Why are celery sticks soaked in saltwater more flexible than those soaked in plain water?

The salt water is hypertonic to the celery stick and results in water diffusing out of the celery stick to reach equilibrium. There is less turgor pressure in the plant cell and results in a softer, more pliable shape.

A cell is observed to shrink after being immersed in an unknown solution. Would you predict that this solution was a highly concentrated salt solution or distilled water? Explain your reasoning.

The solution was most likely a highly concentrated salt solution that was hypertonic the cell and caused water to diffuse out of the cell through osmosis (high concentration to low). Thus, the cell would shrink.

Extent of Diffusion

Total Cube Volume of cube that Volume - has not changed color --------------------------------------------- x 100 Total Cube Volume

Review the graph of Rate of Diffusion vs. Surface Area-to-Volume Ratio. Do the results suggest that the surface area-to-volume ratio affects the extent of diffusion?

Yes because the larger the SA to Volume ratio, the greater the extent of diffusion and the higher the efficiency rate is. The smaller the ratio, the lower the extent of diffusion and the lower the efficiency rate.

If you paired two different 1 molar solutions (salt in the "cell" + glucose outside) did one solution appear more hypertonic than the other?

Yes, the solution would be hypertonic to the cell and cause water to diffuse out of the cell, and result in the cell shrinking and losing mass. In the lab we tested a cell of glucose in a salt solution and the cell had gained mass, therefore suggesting that this experiment would have the reverse effect and cause the cell to lose mass.

The size of human cells is 0.01 mm. a) calculate the surface area to volume ratio of such a cell (assume (0.01 cube) b) after one minute, would you expect the extent of diffusion in a human cell to be greater, lesser or the same as the smallest agar cube?

a) 600:1 (simplified from .0006:.000001) b) The extent of diffusion would be much greater than the smallest agar cube because it would has a greater SA to volume ratio and would allow for diffusion to occur more rapidly.