Osmosis and Surface Area/Volume Relationships

You should have a working definition for the terms osmosis, diffusion and an understanding of how the process works.

As we investigate unicellular and multicellular life we begin to realize that there are limits to practical cell size.

Factors Effecting Diffusion / Osmosis Rates

The speed at which solvents and solutes can enter and exit cells is of primary importance.

The following factors effect the rate of diffusion/osmosis

Depending on the cellular function, cells have adapted to be able to allow efficient diffusion. Examples Include:

• Alveoli are thin walled, tiny air sacs that have a huge surface area but very small volume to allow fast, efficient diffusion of oxygen into the blood
• Nerve cells are long and thin. The length allows the electrical signal to be transmitted long distances, the thin cel body allows for fast changes in concentration gradients.
• Epithelium of the gut have microvilli. Microvilli are tiny, finger like projections of the cell membrane that dramatically increase the surface area available for nutrient absorption across the gut.
• Mitochondrion has a double membrane system. The space between the membranes is called the inner membrane space. It is narrow with a low volume. In this space is where protons [H+] accumulate to establish the electrochemical gradient used to drive ATP synthesis by ATP synthase. This accumulation of protons is a potential energy source known as the proton motive force. Becasue the inner membrane space is narrow, with low volume, the concentration gradient is easier to establish and be maintained by the mitochondrion. The inner membrane is also heavily folded into what are called cristae. These increase the surface area and allow more surface for proton channels and enzymes required for respiration.

Surface Area to Volume Ratios:

This ratio is a primary selective factor in the evolution of cells. It has a huge effect on cell shape, cell size and and cell organization within tissues. The

By reducing cell size, cells are greatly increasing the surface area to volume ratio which makes diffusion/osmosis much more effective. Cellular concentrations of ions, nutrients, salts, etc can occur MUCH more quickly and efficiently. The ability to use the gradient, and then re-establish the gradient for the next ‘job’ is important. If the volume of a cell was large, the ability to quickly change the concentration would decrease.

Example: neurons send electrical signals along their axons by allowing Na+ and K+ ions to flow in and out of the cell. The axon is very thin. The movement of small amounts of ions can quickly alter the gradient and the charge distribution FAST, hence electrical signals can be sent quickly AND the cell can ‘reset’ for the next signal quickly.

Task: Lab Investigation

The investigation is meant to have you examine the relationship between surface area/volume ratio and the rate of diffusion.

Steps:

1. collect 5 beakers of same size, it needs to be large enough to hold 5 bags of dialysis tubing hanging from a glass rod or pencil as in diagram above.
2. cut 5 pieces of dialysis tubing to the same length.
3. repeat step 2 for another 4 lengths of tubing. Hence you will have 5 tubes of 5 different lengths for a total of 25 pieces of tubing.
4. soak tubing to be able to open.
5. tie off one end of each tube.
6. fill each tube until they are equivalently full…try to add the same amount of solution in each bag of same length.
7. tie off the second end of all bags.
8. mass and record every bag as accurately as possible
9. hang bags into beakers
10. Create your hypothesis NOW in the format: It is expected that <this> will cause <this> because…..Be sure to include your WHY you expect what you expect. An expected grapch outcome should be produced.
11. add distilled water until all the bags are submerged into the water. Staggering the addition of water may help.
12. remove all bags after an equal amount of time…..
13. blot bags dry and record new masses

Analysis:

The data you collect should allow you to be able to create a graph of the surface area (x-axis) against the amount of diffusion [% change in mass — y axis]

Questions: answer in the analysis

1. State the dependent and independent variables, and the controls we attempted to apply. Always explain WHY the controls were needed and what would happen if not controlled.
2. Explain the purpose in calculating the percent change in mass.
3. State and explain the relationship between surface area and amount of diffusion that occurred.
4. Describe an example, in an organism, of how a cell and/or tissue was selected by nature for an increase in surface area/volume ratio and explain its importance to the organism. Suggest an explanation of its evolution.

Final Report Format — what you need to show:

1. Introduction: you should inform the reader about diffusion, osmosis, permeability, importance of surface area to volume in cellular shape and limits to size.
2. Hypothesis: It is expected that <this> will cause <this> because…..
3. Procedure: omit — I told you what to do.
4. Data Collection — well organized data table, units, error of measurements, accuracy consistency,
5. Analysis — the graph with all parts and the answers to the questions. Use / refer to your graph. It is why we make graphs, to use them as evidence and explanation.
6. Conclusion — Restate your hypothesis and accept / decline and explain why by referring back to your evidence.
7. Evaluation — refer to the reliability — if you were to do again would you get similar data? Was the data between the bags in a beaker fairly consistent? Also refer to validity — did the experiment measure what it was suppossed to meaure? Discuss weaknesses and realistic improvements and why they are needed.