Investigation 4 Lab Report #YoungMoola #$$$

Devansh Taori
Biology AP, Period 2


Introduction
In all living things, cells are of paramount importance to ensure the effective functioning of micro-tasks. Transport is a major issue for cells because maintaining homeostasis is necessary, or else the cell will lose its effectiveness. Often, movement is regulated by cellular membranes (phospholipid bilayers with embedded proteins) or by simple diffusion. Osmosis refers to the diffusion process for water – this is common in cells. However, cells can also use aquaporins (specialized protein channels) for the transport of water.

Generally, in diffusion, solutes move from an area of higher concentration to an area of lower concentration, seeking equilibrium. Diffusion doesn't require energy. On the other hand, the opposite process (moving a solute from a lower concentration to a higher concentration) does require energy (in the form of ATP) and is usually mediated with pumps. There are three main terms that describe solutions separated by selectively permeable membranes – hypertonic, hypotonic, and isotonic. Hypertonic solutions have higher solute concentrations and lower water potential, causing water to move into them via osmosis. Too much water flowing into a cell can actually cause that cell to burst, which is why it's dangerous within a plant or animal cell. Hypotonic solutions have lower solute concentrations and higher water potential, causing water to leave them. This could result in a cell shrinking or shriveling, hampering cell activity. Isotonic solutions have equal water potentials.

An important term when defining water movement is water potential – this is the free energy per mole of water and it's calculated from two main components: 1) the solute potential (Ψs), dependent on solute concentration; and, 2) the pressure potential (Ψp), which results from the exertion of pressure on a solution. Water potential can be defined in the following equation: Ψ = Ψs + Ψp. Ψs can be described as -iCRT, where i is the ionization constant, C is the molar concentration, R is the pressure constant (0.0831), and T is the temperature in K. The water potential of an open beaker is zero because both Ψs and Ψp are zero. Increasing positive pressure raises the pressure potential (and thus the water potential). Increasing the solute concentration decreases the solute potential (and thus the water potential).


Experimentation
There are three main experiments we conducted. In Procedure 1, we used artificial cells to study the relationship between surface area, volume, and diffusion. In Procedure 2, we created models of living cells to explore osmosis and diffusion. In Procedure 3, we concluded by observing osmosis in real living cells.

Procedure One
Objective
In this procedure, the objective was testing the effect of surface area and volume of an artificial cell on the rate of diffusion.

Materials

• 2% agar containing NaOH and the pH-indicator dye phenolphthalein
• 1% phenolphthalein solution
• 0.1 M HCl
• 0.1 M NaOH
• Squares of thin, hard plastic and knives
• Petri dishes and test tubes
• 2% agar with phenolphthalein preparation

Procedure

1. Place some phenolphthalein in two test tubes. Add 0.1 M HCl (acid) to one test tube, swirl to mix the solutions, and observe the color. Using the same procedure, add 0.1 M NaOH (base) to the other test tube. Remember to record observations. 
2. Using a dull knife or a thin strip of hard plastic, cut three blocks of agar of different sizes. Put each block into the solution and observe which block through which the solution diffuses the fastest. 

Hypothesis

We hypothesized that the solution would diffuse faster through the block with the smaller surface area and volume. This is because there's less overall volume that the solution has to diffuse through, so the entire process is much faster than in a gigantic block. 

Data

The sizes of our blocks were as follows – 

Large: 2 cm x 2.5 cm x 1.5 cm

Medium: 1 cm x 1.5 cm x 1.75 cm

Small: 1 cm x 1.5 cm x 1.5 cm

Results
Our results line up perfectly with the hypothesis. The block with the smallest surface area and volume encountered the fastest diffusion. The medium block was next fastest. In last place was the large block. Our photos and a time-lapse validate this conclusion.

Before

After

Time-lapse

Procedure Two

Objective
In this procedure, the objective was testing the rate of diffusion with various solutions. We created models of living cells with dialysis tubing, a semi-permeable material that allows water and some solutes.

Materials

• Distilled water
• 1 M sucrose
• 1 M NaCl
• 5% egg white protein
• 20 cm-long dialysis tubing
• Cups and balances

Procedure

1. Choose up to two pairs of different solutions. One solution from each pair will be in the model cell of dialysis tubing, and the other will be outside the cell in the cup. Your third model cell will have water inside and outside; this is your control. Before starting, use your knowledge about solute gradients to predict whether the water will diffuse into or out of the cell. Make sure you label the cups to indicate what solution is inside the cell and inside the cup. 
2. Make dialysis tubing cells by tying a knot in one end of five pieces of dialysis tubing. Fill each "cell" with 10 mL of the solution you chose for the inside, and knot the other end, leaving enough space for water to diffuse into the cell. 
3. Weigh each cell, record the initial weight, and then place it into a cup filled with the second solution for that pair. Weigh the cell after 30 minutes and record the final weight. 
4. Calculate the percent change in weight using the following formula: (final - initial)/initial * 100. Record your results.

Hypothesis

We hypothesized that the water-in-water solution would experience no osmosis because both solutions are at equilibrium in terms of their concentrations of water. Additionally, we proposed that water would diffuse into the cell for the solutions containing solute. We predicted that the solution with NaCl would experience a greater rate of diffusion because NaCl as a salt ionizes more easily in water.

Data

We began with 12.4 g of water solution and ended up with 12.4 g of water solution. The percent change in weight was 0%.

We began with 11.2 g of NaCl solution and ended up with 12.5 g of NaCl solution. Per the formula, the percent change in weight was 11.60714286%.

We began with 12.1 g of sucrose solution and ended up with 13.1 g of sucrose solution. Per the formula, the percent change in weight was 8.26446281%.

Results

As can be clearly seen by the data above, the biggest change in grams of solution occurred with the salt solution (~12% change compared to ~8% change for sucrose). This means that water diffused into the NaCl solution at a higher rate than for any other solution. This validated our hypothesis and ultimately proves that with a solute in water, the rate of diffusion will be much higher as the water attempts to reach equilibrium in concentration again.

Procedure Three

Objective

In this procedure, the objective was to place a cell into either a hypotonic or hypertonic solution and observe how that would affect the cell's overall structure.

Materials

• Potatoes, sweet potatoes, or yams
• French fry cutters
• Balances, cups, and metric rulers
• Color-coded sucrose solutions of different, but unlabeled, concentrations

Procedure

1. Look at a single leaf blade under the light microscope. You should be able to see the cell wall and overall structure.
2. Expose the cell to the NaCl solution prepared in Procedure 2 and examine what happens to the cell.

Data

According to visual data, after exposing the plant cell to a 1 M NaCl solution, the plant cell actually shriveled up as water left the cell to equilibrate with the outside solution.

Before

After

Results

As we can see from the pictures above, the cell wall shrunk significantly after it was immersed in the salt solution. Because water left the cell, it shriveled up and lost its original shape. Therefore, we can conclude that this is how cells in general will react when exposed to hypertonic solutions.

Conclusion

At the end of the experiment, we ended up with some excellent observations of how diffusion operates within cells. In particular, we found in Procedure 1 that solutions can diffuse through cells with smaller volumes much easier (as our photos and time-lapse proves). In Procedure 2, we proved that water-in-water solutions don't experience osmosis (the percent change in weight was 0%). Additionally, we proved that solutions with higher concentrations of solute (the NaCl solution) would experience faster rates of diffusion (~12% change in weight). In Procedure 3, we examined how real living cells would react when placed in a hypertonic solution. The photos show how water left the cell, thus causing the cell to shrivel up.

If we were to conduct these experiments again, we would've tried using different solutions for Procedure 2 to figure out whether different solutions can affect diffusion rates. Additionally, we would've tried using different plant cells in Procedure 3 to evaluate how various cells would've shriveled up.

Therefore, this experiment was overall extremely useful and educational in learning about how cell sizes and solute concentration can affect diffusion. Special shout-out to Mr. Wong for letting us do the lab!

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