Week 4 Lab

Compare and contrast diffusion and osmosis.

Diffusion and osmosis are similar in the fact that osmosis is a type of diffusion involving water. Both instances involve molecules moving to achieve equilibrium. They differ in that diffusion does not have to involve water or a selectively permeable membrane.

Which substance(s) crossed the dialysis membrane? Support your response with data-based evidence.

Glucose crossed the membrane. this is known because the glucose test strips placed in the water after 1 hour turned green, indicative of glucose presence.

If the potato is allowed to dehydrate by sitting in open air, would the cells absorb more or less water? Explain.

If a potato is allowed to dehydrate, the potato cells would become more likely to absorb water. This is because the dehydration of the potato cells would cause the concentration of solute, and hence the osmolarity, of the cells' cytoplasm to increase. The potato would absorb more water (have greater water potential) because the concentration of solutes in the potato cells increases with dehydration. Thus, more water would move into the cell to establish an isotonic environment.

If you wanted water to flow into a tubing piece filled with a 50% solution, what would the minimum concentration of the beaker solution need to be?

If the bag contained a 50% solution, the beaker solution concentration would have to be less than 50% for water to flow into the bag.

If you want water to flow out of a dialysis bag filled with a 50% sucrose solution, what would be the minimum concentration of the beaker solution need to be?

If the bag contained a 50% solution, the beaker solution would have to be greater than 50% in order for the water to flow out of the bag.

Suppose you had a 1% sucrose solution in Beaker 2. How might this change your results?

In this case, the solution in all three bags would be hypertonic as compared to the beaker solution. As a result, you would see increases in the volumes of the red, yellow, and blue bag.

Suppose you had a 90% sucrose solution in Beaker 2. How might this change your results?

In this case, the yellow, red, and blue solutions would all be hypotonic as compared to the solution in the beaker. As a result, water would flow out of the tubing, decreasing the volume in the dialysis tubing.

Describe the similarities and differences between the dialysis bag and a cell membrane. Be specific.

In this experiment, the dialysis bag represents the cell membrane. Like the dialysis tubing, the cell membrane allows cells to loose and gain water to maintain isotonic conditions. Small solutes are also moving to maintain equilibrium. Homeostasis is maintained through this constant losing and gaining of water and solutes.

Did all of the molecules diffuse out of the bag into the beaker? Why or why not?

No, none of the starch molecules diffused outside of the dialysis tubing into the beaker, and only some of the glucose molecules diffused into the beaker. The starch molecules did not diffuse because they are too large to pass through the dialysis tubing. The glucose molecules did not all diffuse because the concentrations in the beaker and dialysis tube achieved equilibrium. At this point, molecules may continue to pass back and forth across the tubing, but equilibrium is maintained.

How do osmotic power plants work?

Osmotic power plants work by collecting energy from the act of osmosis between saltwater and freshwater. A semi-permeable membrane is put between the two water types and they begin the process of osmosis, releasing energy as the water moves.

Why is it necessary to have positive and negative controls in this experiment?

Positive and negative controls are necessary to accurately analyze the resulting colors at the end of the experiment. The positive control shows what colors should appear when glucose or starch is present. Negative controls indicate what color should appear when the substance is absent.

What do your results indicate about the concentration of the cytoplasm in the potato cells at the start of the experiment?

Positive displacement indicates that the solution of the cytoplasm was higher than the concentration of the test tube solution. Negative displacement indicates that the concentration was lower than the concentration of the test tube solution.

What does the net change in the potato sample indicate?

Positive net change indicates that water entered the potato sample. Negative net change indicates that water exited the potato sample.

Why don't red blood cells swell or shrink in blood?

Red blood cells don't swell or shrink because they are isotonic when in blood. In other words, the solute concentrations within the cell and outside the cell are the same.

How did the physical characteristics of the potatoes vary before and after the experiment? Did it vary by potato type?

Students should observe that the potatoes appear more dense if the strip is placed in the water solution (due to the inbound water displacement) and less dense if the potato strip is placed in the salt solution (due to the outbound water displacement). Water seeping out of the potatoes can cause the cells to lose their balance and become flaccid and less turgid than before the experiment began. Students may also notice discoloration due to oxidation or enzymatic contact after the potato was cut.

Suppose a green dye with a molecular weight of 1000 g/mol was also tested in this experiment. Rank the red, blue, and green dyes from fastest to slowest diffusion.

Students should recognize that the green dye has a higher molecular weight than both the red and blue dyes. As a result, they should rank the dyes red blue green.

Suppose a green dye with a molecular weight of 200 g/mol was also tested in this experiment. Rank the red, blue, and green dyes from fastest to slowest diffusion.

Students should recognize that the green dye has a lower molecular weight than both the red and blue dyes. As a result, they should rank the dyes green red blue.

Suppose a green dye with a molecular weight of 600 g/mol was also tested in this experiment. Rank the red, blue, and green dyes from fastest to slowest diffusion.

Students should recognize that the green dye would fall between the red and the blue. As a result, they should rank the dyes red green blue.

What do you hypothesize would happen if you began the experiment with glucose in the dialysis tubing and starch in the water in the beaker?

Students should recognize that the starch would not enter the dialysis tubing because it is to large. In contrast some, but not all, glucose would exit the dialysis tubing, resulting in glucose being observed in both the beaker water and the dialysis tubing solution.

What do you hypothesize would happen if you began the experiment with starch in the dialysis tubing and glucose in the beaker?

Students should recognize that the starch would not pass through the dialysis tubing. In contrast, some, but not all, of the glucose would enter the dialysis tubing, achieving chemical equilibrium. Thus, glucose would be observed in both solutions.

Research the structures that protect plant and animal cells from damage resulting from osmotic pressure. Write a few paragraphs explaining what they are, how they work, and where they are located.

The cell wall protects the plant from damage associated with osmotic pressure. The cell wall is a thick outer layer that protects the plant, as well as providing support, strength, and filtration. When performing osmosis by taking water up into the cell, the cell wall regulates the amount of water that can enter. Animal cells do not have cell walls, and therefore do not have the same protection from bursting and shrinking. Because of this, animal cells must always be in solution with the same osmotic pressure as in the cell.

Draw a diagram of the experimental setup. Use arrows to depict the movement of each substance in the dialysis bag and the beaker. Submit your diagram per your instructor's directions.

The diagram should include arrows that show glucose molecules traveling out of the dialysis tubing and into the beaker. This movement occurs until the solution in the tubing and the water in the beaker achieve chemical equilibrium. Additional arrows should also be included to show the starch molecules traveling within the dialysis bag. These arrows should not travel outside of the dialysis tubing.

Describe the factors that affect the direction of diffusion. Which factor has the greatest impact?

The direction of diffusion is affected by heat, pressure, and concentration gradients. Because temperature and pressure are generally constant in biological system, concentration gradients have the greatest impact on the direction of diffusion.

Which tubing decreased the most in volume? Why?

The green bag should decrease most in volume. This is because the solution in the green bag is hypotonic as compared to the beaker solution. The water will flow out of the bag to create an isotonic solution

From what you know of tonicity, what can you say about the plant cells and the solutions in the test tubes?

The potato was hypertonic compared to the solution in Tube A. The potato was hypotonic compared to the solution in Tube B.

Describe the factors that affect the rate of diffusion. What type of molecules exhibit a higher rate of diffusion in a biological system?

The rate of diffusion depends on membrane characteristics, the size of the solute, and the molecular polarity of the solute. In general, in a biological system, small, non-polar molecules will diffuse at a faster rate than large, charged molecules.

Which dye diffused faster?

The red dye diffused through the corn syrup faster than the blue dye.

Which molecules remained inside of the dialysis bag?

The starch molecules remained inside. This was indicated by the lack of color change when IKI was mixed with water in the beaker. If starch was present, the water in the beaker would have turned dark purple.

What is the water potential of an open beaker containing pure water?

The water potential is zero.

Which dialysis bag increased the most in volume? Why?

The yellow bag should increase the most in volume. This is because the difference in sucrose concentration between the yellow bag and the beaker is greater than any other combination. Water will flow into the bag to create an isotonic solution.

What does this tell you about the relative tonicity between the contents of the tubing and the solution in the beaker?

The yellow bag was more hypertonic as compared to the beaker than the red bag was (more water had to enter the yellow bag to achieve isotonicity).

What would happen if the tubing with the yellow rubber band was placed in a beaker of distilled water?

Water would flow from the beaker into the tubing, diluting the sucrose concentration.

For each dialysis bag, identify whether the solution inside was hypotonic, hypertonic, or isotonic in comparison to the beaker it was placed in.

Yellow - Hypertonic Red - Hypertonic Blue - Isotonic Green - Hypotonic

Does the rate of diffusion change over time? Why or why not?

Yes, it appears to change over time. Diffusion was fastest when the dye was first dropped into the corn syrup and then seemed to slow down over time.

Does the rate of diffusion correspond with the molecular weight of the dye?

Yes, the red dye has a lower molecular weight than the blue dye. As a result, it diffused faster.

Would this experiment work with other plant cells? What about animal cells? Why or why not?

Yes, this experiment would work on other plant and animal cells. This is because cell membranes are dependent on chemical balance to maintain integrity.

Different types of potatoes have varying natural sugar concentrations. Explain how this may influence the water potential of each potato type.

A higher sugar level would increase the amount of water that enters the potato cell. This is because the water molecules will naturally travel down the gradient from areas of high water concentration to areas of low water concentration.

What is a solution? What are the solvents and solutes in a cell?

A solution is composed of two or more substances (solute) dissolved in a solvent. In a cell, the intracellular and extracellular fluids are considered solvents. The dissolved materials (proteins, lipids, ions other molecules) are considered solutes.

Do you think your results would change if the corn syrup was replaced with water? Why or why not?

Although the water is less viscuous and the dyes will move faster than in the corn syrup, they will still see the red dye diffuse faster than the blue dye, due to its smaller molecular weight.

Did water flow in or out of plant (potato) cells? How do you know this?

Answers will vary based on potato type. However, students should be able to determine that water flows into the plant cells if the net displacement is positive and out of plant cells if the net displacement is negative.

Based on the data from this experiment, hypothesize which potato has the highest natural sugar concentration. Explain your reasoning.

Answers will vary based on the potatoes selected for the experiment. However, students should follow the logic that whichever sample exhibited the larger volume increase also had a higher sugar concentration.

What are aquaporins?

Aquaporins are a family of proteins that span the cell membrane. They facilitate the movement of water from the exterior of the cell to the interior of the cell.