BIO 101 Lab 4

1. Does the concentration of water affect osmosis?

Quick version

Use a parer to shave peels from a carrot. Divide the peels into two groups. Place one group in a bowl with water and the other in a bowl of water with a tablespoon of salt. In a few minutes check the texture of the peels.

If the celery or lettuce in your refrigerator is a little limp, you should be able to make it more crisp by dunking it into water. If it is very limp because the cells have been damaged, the water won't help.

Extended version

Cut 3 equal sized cubes of potatoes that are about 1" (2.5 cm) on each side. Don't leave any skin on the surface. Weigh each of the cubes. Place one in a glass of water. Place a second in a glass containing a cup of water and a teaspoon of salt. Place a third in a glass containing a cup of water and two tablespoons of salt.

Near the end of the laboratory session, weigh each cube and determine the change in weight (if any).

The next day, weigh the cubes again. Make a bar graph to show what happened in each of the three cubes and explain what you saw.

2. Do dead cells do osmosis?

Take a potato and slice it in half. Cut a 1/2 " (1 cm) strip of the peel around the base of each one. With a knife or melon ball cutter, carve a small hollow in the top of both of the two potatoes. Place one half with the cut side down in a dish or pan of water. Place the other half in a small sauce pan and simmer for about 10 minutes so that it is cooked. Place a teaspoon of salt in each of the two hollows. Observe at the end of the laboratory session.

The next day, observe the potatoes again and explain any differences between the two.

3. Why do cells need to be very small?

Cut at least 5 equal sized cubes of potatoes that are about 1" (2.5 cm) on each side. Don't leave any skin on the surface. Place them in a jar of water that contains several drops of dark food coloring.

Each day or 2 afterward, remove a cube and slice it to see how far the dye has traveled (quit when the cubes start to get mushy). Estimate the rate of travel in mm/week.

Food needs to diffuse into the cell and waste material needs to diffuse out of a cell. Does the rate of diffusion you saw explain why it would not work to have cells that are a centimeter in diameter?

4. Is there diffusion through an egg membrane?

1. Place an egg in a pint jar and fill the jar with water. Observe every two or three days for two weeks. Make sure the egg is always submerged in water.

2. Tap the rounded end of a second egg with a spoon so that it has a small crack. Remove a few pieces of the shell so that there is a hole the size of a dime or smaller. There should be a membrane about a centimeter below the surface. If you break that membrane, start over. Try not to leave sharp jagged edges around the hole.

Place the egg in a pint jar filled with water. Observe every two or three days for a week. Make sure the egg is always submerged in water. If the membrane stays intact, make your final observation at the end of a week and discard. If the membrane breaks during the week, discard the egg or it will begin to smell.

3. Place an egg in a pint jar filled with white vinegar. Make sure the egg is always submerged in vinegar.

After a week, rinse the egg off under running water and remove any remaining bits of shell. Compare the size of the egg with egg number 1.

Put the egg back into a jar of fresh vinegar. Observe a week later, compare with eggs 1 and 4 and discard.

4. Place an egg in a pint jar filled with white vinegar. Make sure the egg is always submerged in vinegar.

After a week, rinse the egg off under running water and remove any remaining bits of shell. Compare the size of the egg with egg number 1.

Put the egg back into a jar containing light corn syrup. Observe a week later, compare with eggs 1 and 3 and discard.

Write-up - Describe what happened with each of the four eggs. Use your understanding of osmosis or diffusion to explain any changes you observed or didn't observe. What would you expect would happen if you placed an intact egg in a mixture of syrup and water?

5. How quickly does a dye diffuse in a liquid?

Diffusion in a liquid is often illustrated by dropping a dye into a liquid and watching it spread out over a few minutes. Real diffusion is actually much slower than that. Most of the movement observed in water is caused by convection currents, not diffusion.

To illustrate actual diffusion, place about a half a cup of light corn syrup in a glass. In a separate glass mix about a tablespoon of light corn syrup with a few drops of food coloring. With a dropper, carefully place a drop of the colored syrup on the top of the clear syrup. There may be some immediate mixing which is due to convection currents but if you are careful with adding the drop, it may be days or weeks before the color is completely spread throughout the glass. Cover the glass with plastic wrap and carefully place it on a shelf where it will be undisturbed.

Observe it every couple of days for at least a week. Describe what you saw.

6. How does water get around in a plant?

Obviously a plant can't wait for diffusion to move food throughout the various parts of the plant (Neither can you which is why we have a circulatory system). We'll use bok choy or celery cabbage to look at water travel in a plant.

Take a stalk of bok choy or celery cabbage that has leaves at the top. You can use the center stalks of regular celery that are light colored. The dark stalks don't show the movement of dye very well.

Cut off about an inch from the bottom of the stalk so there is a fresh end. Take two small glasses and add about a half a cup of water to each. In one put a couple of drops of red food coloring and in the other blue. If the stalk is very light you could use green. Yellow is usually not intense enough to show up in the plant. Slit the stem from the bottom so that you can have one end of the stalk in each of the two glasses.

Later in the day or the next day, look at the stalk to see where the color collected. Look at the leaves. Does it appear that the plumbing in the plant is connected at the top? Do the colors appear to mix or do the tubes appear to connect to specific parts of the leaves? Slice through the stem and see if you can identify the tubes (xylem) that carry liquid.

Variations that you could try sometime.

Instead of potatoes, use sweet potatoes, turnips, jicama, apples or carrots.

Measure changes in the volume of the potato cubes by dropping them in a graduated cylinder half full of water. Determine the amount of water displaced by reading the change in volume in the graduated cylinder.

Use sugar instead of salt.

Try other vegetable plants in colored water like you did the celery. Use white radishes, scallions or carrots. Be sure to use a color that will show up.

Try white carnations in different glasses of colored water or ink.

Supplies for 6 lab setups