Laboratory 3: Cell Membrane-Osmosis and Diffusion

Question 1

Objectives

Answer 1

This activity investigates osmosis and diffusion across a selectively permeable membrane to understand the properties and functions of the cell membrane, using model cells like dialysis tubing and plant cells such as potato or onion cells.

Question 2

Materials

Answer 2

• Microscope (light or phase-contrast)
• Dialysis tubing (to simulate a cell membrane)
• Beakers (100 mL, 250 mL)
• Slides and cover slips
• Pipettes
• Ruler or calipers (for measuring potato strips)
• Weighing scale (optional for measuring mass change)
• Elastic bands or string (for securing dialysis tubing)
• Sucrose solutions (0%, 5%, 10%, 15%, 20%)
• Distilled water
• Potato strips (or onion slices for microscope observations)
• Iodine solution (for staining)
• Glucose solution (or starch solution)
• Benedict’s reagent (for glucose test)
• Lugol’s iodine solution (for starch test)

Question 3

Steps for Part 1: Osmosis in Plant Cells (Using Potato Strips) (5)

Answer 3

1. Cut the potato into strips approximately 5cm long and 0.5cm wide. Measure the initial length of each strip using a ruler or calipers, and record the initial mass of each strip using a weighing scale (optional).
2. Prepare five sucrose solutions with concentrations of 0% (distilled water), 5%, 10%, 15%, and 20% to use as different environments for the potato strips.
3. Place one potato strip in each beaker filled with a different sucrose solution and let them sit for 30-60 minutes to allow osmosis to occur.
4. Then, remove each potato strip from the solution, gently pat it dry, and measure its final length. If mass was initially measured, record the final mass as well.
5. Compare the initial and final lengths (or masses) of the potato strips to assess the effects of osmosis across different sucrose concentrations.

Question 4

Steps for Part 2: Diffusion Across a Model Cell Membrane (Using Dialysis Tubing) (5)

Answer 4

1. Prepare the dialysis tubing: soak a piece of tubing in water until it becomes flexible, then the one and securely with the elastic band or string to create a sealed bottom.
2. Fill the dialysis tubing with glucose or starch solution, and tie the open end with another elastic band, forming a closed “cell.”
3. Place this model cell into a beaker filled with distilled water to observe diffusion.
4. After 30 to 60 minutes, test the water in the beaker for signs of diffusion. For glucose, add Benedict’s reagent and heat; a color change to orange or red will confirm glucose presence. If using starch, add Lugol’s iodine, which turns black if starch is present.
5. Record the results to determine if glucose or starch diffused out of the dialysis tubing. While glucose molecules can pass through the dialysis membrane due to their smaller size, starch molecules are typically too large to do so.

Question 5

Steps for Part 3: Osmosis in Onion Cells (Optional Microscopy Activity) (4)

Answer 5

1. Prepare the onion cells: peel a small piece of onion skin and place it on a microscope slide, then add a drop of distilled water and cover it with a cover slip.
2. Observe the onion cells under the microscope and focus to the cell membrane and vacuole structures.
3. Once you’ve observed the cells in water, add a few drops of a 10% salt solution to the slide. Notice how the cell membrane begins to pull away from the cell wall as water exits the cell, a process called plasmolysis.
4. Record your observations by drawing and labeling the onion cells to show the changes before and after adding the salt solution.

Question 6

The effects of plasmolysis:

Answer 6

When the onion cells were placed in a hypertonic solution, water moved out of the cells by osmosis, causing the cell membrane to pull away from the cell wall. This created a visible gap between the cell wall and the cell membrane, as the membrane shrank inward. Additionally, the plasmolyzed onion cells appeared with darker, more concentrated regions of the cytoplasm.

Question 7

Why does osmosis cause the potato strips to gain or lose mass depending on the concentration of the solution?

Answer 7

Osmosis is all about balance. When potato strips are placed in a solution, water moves across the potato cells’ membranes from areas of low solute concentration (high water concentration) to areas of high solute concentration (low water concentration). In a hypotonic solution (where the solution has a lower concentration of solutes than inside the potato cells), water enters the cells, causing them to swell and gain mass. In a hypertonic solution (where the solution has a higher concentration of solutes than inside the potato cells), water exits the cells, causing them to shrink and lose mass. This movement reflects the potato’s attempt to equalize solute concentrations on both sides of its cell membranes. This process aligns with the second law of thermodynamics, increasing entropy by dispersing water molecules more evenly across the membrane. This spontaneous movement of water drives the potato strips to gain or lose mass, aiming for equilibrium in solute concentration.

Question 8

Which molecules were able to diffuse through the dialysis tubing, and why?

Answer 8

In the experiment, the black coloration inside the tubing shows that Lugol’s iodine reacted with starch. The light brown color in the beaker (outside the tubing). The result indicates that starch is present only inside the tubing and no starch molecules exited.
According to Zweigart et al. (2010), the dialysis tubing’s average pore radius of 1.72 nm, allowing molecules with a size in the range of between 50-1000 Da to pass only slowly through the membrane and 1000 Da above cannot pass through. BeMiller (2019) notes that amylopectin molecules in commercial starch can be as large as 2 × 10⁹ Da (with a degree of polymerization around 10⁷), making them some of the largest molecules in nature. Thus, starch were too large to pass through the semi-permeable membrane.
Glucose, on the other hand, is only about 1 nanometer, about 100 Da, thus can pass through the semi-permeable membrane of dialysis tubing. Thus, glucose molecules can pass through the dialysis tubing.
Disclaimer: Only starch molecule was used in the experiment, the discussion about glucose was from experiments from online sources. Moreover, cellophane as an alternative for dialysis tubing was used.

Question 9

In the onion cell experiment, what happened to the cell membrane when the salt solution was added? Why?

Answer 9

When the salt solution was added to the onion cells, the protoplasm began to shrink away from the cell wall due to plasmolysis. This occurred because the salt solution created a hypertonic environment outside the cell, causing water to move out of the cell by osmosis. As water left the vacuole and cytoplasm shrank, it results in the cell membrane detaching from the cell wall and forming a visible gap. This movement of water out of the cell reduced turgor pressure, leaving the cell in a collapsed, plasmolyzed state.
In the experiment, the onion cells showed both concave-type and convex-type plasmolysis. Concave-type occurs when the cell membrane pulls inward unevenly, creating concave pockets along the cell wall. In contrast, convex-type happens when the cell membrane shrinks in a rounded, uniform shape, forming a smoother gap between the membrane and cell wall, due to more uniform water loss (Lang et al. 2014).

Question 10

How do the cell membrane and the selectively permeable nature of biological membranes regulate the movement of substances in and out of cells?

Answer 10

The cell membrane regulates the movement of substances in and out of cells through selective permeability, allowing only certain molecules to pass. The lipid bilayer restricts large or charged molecules, while embedded proteins help specific ions and larger molecules cross. Channel proteins facilitate passive movement, while carrier proteins can use ATP in active transport to move substances against their gradient. This selective control helps maintain cellular homeostasis by managing the cell’s internal environment (Cooper, 2000).