Homeostasis Lab

Introduction

Maintaining aspects of your internal environment and/or plasma within a certain range is known as homeostasis. These aspects of your internal environment include pH, salinity, oxygen levels, carbon dioxide levels, temperature, etcetera. Many aspects of plasma are necessary to allow cells to maintain their integrity; cells also require transportation of molecules across the cell membrane. In this lab, we will demonstrate the importance of the surface area to volume ratio, some aspects of diffusion and osmosis, and the speed with which your body breaks down asparagus and transports asparagusic acid into your urine.

Pre-lab Questions

1. What is expected to happen to an animal cell when there is a higher saline concentration inside the cell rather than outside the cell?

2. Which is likely to cook faster: brownies with a high surface-area-to-volume ratio or brownies with a low surface-area-to-volume ratio? Why?

3. During osmosis, is the water transported through the semi-permeable membrane or is the sugar?

4. What type of blood vessel in your body has the highest surface-area-to-volume ratio? Why is the high surface-area-to-volume ratio of this blood vessel beneficial to you?

5. True/False: the salinity inside a freshwater fish is the same as the salinity of the water it is in because the two have reached an equilibrium.

Equipment & Materials

• Brownie mix for each student group, plus eggs and oil
• Pans of different sizes and shapes
• Solar ovens (can be homemade, many directions available online)
• Foil for solar ovens
• Drinking water, preferably in identical bottles, enough for each student
• Asparagus, at least 3 stems per student
• Dialysis tubing
• String to tie off tubing
• Molasses (can be diluted with water in a 50/50 mixture)
• 2-250 ml beakers for each student group
• Starch solution
• I-KI stain

Methods

Activity 1

Drink a bottle of the water provided during this first portion of the lab exercise to help ensure equal hydration of all lab participants, for best results with asparagusic acid.

You have been given a certain amount of brownie mix, add the appropriate amount of water and one egg, mix, and place into a pan. A wide variety of pans should be used among the groups, to ensure a variety of surface area to volume ratios. Cook outside in a solar oven until a fork poked gently in the center comes out with little or no raw dough attached. Record the baking time. Cool, and measure the surface area and volume and record below. Eat!!

Activity 2

Once food is consumed, your body transports the nutrients across membranes and into your blood plasma. In this experiment, we will be boiling asparagus for about 5-7 minutes until tender. Every willing participate will eat at least 3 pieces, noting the time eaten. Start a timer from the moment of consumption to the time of urination during which the smell of asparagus is detected. Asparagus contains asparagusic acid which is broken down into mercaptan, which can be quickly transported from the blood into urine, giving the distinct smell. (Some students will notice the smell within thirty minutes or less. Others may not detect the smell at all or not for several hours.)

Activity 3

We will make artificial cells out of dialysis tubing (a semipermeable membrane) and fill them with artificial cytoplasm. Use the supplied dialysis tubing, which must be soaked in water and then rubbed between the fingers to open. Tie off one end with string, then cut the other end about 8 inches further. Fill the open tube with the supplied “cytoplasm” and tie it off. Take a picture of the “cell” when finished, and then drop it into the “plasma.” Repeat, making another cell filled with starch mixture. Drop the second “cell” into the plasma mixture as well.

After thirty minutes, take another picture of the first “cell” you made, and describe the change under RESULTS.

Was any similar change noted in the second cell? Now add about ten milliliters of IKI to the “plasma” outside the second cell, observe, and report the change after thirty minutes.

What nutrient does Iodine-Potassium Iodide detect?

Activity 4

Do the supercool supercooling experiment: Try rapping a supercooled water bottle on a tabletop and observe the ice crystals that form immediately. Why didn’t they form before the rapping?

Results

Activity 1: Brownies

Cooked brownie SA/Vol ratio in centimeters:

Time to cook:

Fastest brownie to cook SA/Vol ratio in centimeters:

Time for fastest brownie to cook in minutes:

Activity 2: Asparagus

Time asparagus was eaten:

Time odor was detected:

Time elapsed between asparagus eaten and odor detected:

Fastest time:

Activity 3: Dialysis Tubing “Cells”

Notable difference between first picture and picture after thirty minutes of diffusion or osmosis:

Change that occurred to second cell after IKI was added:

Post-Lab Questions

1. Why is dialysis tubing a relatively accurate representation of an animal cell membrane? What is missing? Why did the starch not flow out of the tubing? What nutrient is stained blue by potassium iodide?

2. Which has a higher surface area to volume ratio: a hamster or an elephant? Therefore, which loses heat more easily? Which has much greater food needs FOR ITS SIZE?

3. The surface-area-to-volume ratio gets smaller as a cell gets…

4. Why do freshwater fish never have to drink yet they frequently urinate?

5. Through what process did the glucose from the brownies you ate go from your stomach lumen to your blood plasma? Draw the most relevant part of the process.

6. List every process that occurred between you eating asparagus and finally smelling it in your urine. Why don’t some people smell it at all?

7. Sketch the class graph (a spreadsheet may be used) of surface area to volume ratio vs cook time for brownies. What is an rsquared value? What does it mean?

8. Which brain structure often is most closely associated with maintaining homeostasis in vertebrates?

9. What does the supercooling experiment reveal about fish that swim in very cold water? How is it possible for some fish to swim in water that is below freezing?