Name:______Per:_____ Date:______
Why Does Cell Size Matter?
As you have observed, cells are small. Consider your little toe: it is made of about 2-3 billion cells! A newly-made cell will grow, but once it reaches a certain size it will divide to form two new cells rather than growing bigger. Why is this? Why aren’t you made of a few dozen, or a few hundred cells, instead of trillions? Why don’t single-celled organisms like amoebas and paramecia grow as big as a human? In this lab, we will investigate this question using various sized model cells.
Pre-lab Questions
1. Draw the basic structure of a cell?
2. What are some factors that could limit the ability of cells to survive?
3. How do cells obtain materials they need and get rid of waste products? 4. Fill in the blanks of the following sentence:
Solutes move from an area of ______concentration to ______concentration.
Now using this information, make an assumption about which way solutes will move when our model cell potato cubes are placed in iodine. This may be a verbal description or labeled drawing.
5. In this experiment, we will be using iodine to represent a nutrient needed by the cell. Iodine is known as an indicator, what does this mean?
6. In this lab, we will investigate the ability of three model cells of different sizes to obtain nutrients (iodine) from their environment. If the cell sizes are 2 cm3, 1 cm3 and .5 cm3, create a hypothesis stating which cell size you believe will be most successful in receiving nutrients.
If______
______than,______
______
Procedure
Check to make sure you have each the following at your lab station: Safety goggles (for each group member) Potato Metric ruler Knife Both a 50mL and 250mL beaker A single glove
Then, decide who in the group will be knife wielder and who will be the metric master. Put on the safety goggles. From the potato, the knife wielder will carefully measure and cut three cubes with the following dimensions: Be careful when handling sharp objects! Remember to cut away from yourself and others. o 1: 2cm³ (2 cm x 2 cm x 2cm) o 2: 1 cm³ (1 cm x 1cm x 1cm) o 3: 0.5 cm³ (0.5 cm x 0.5 cm x 0.5cm)
Metric master will verify accuracy and cubic dimensions, fill the 250 mL beaker about ¾ full with tap water, and place all three cubes into the 50 ml beaker. Finally, he/she will fill the 50 ml beaker about 1/3 full with iodine solution (high enough to cover cubes) obtained from the teacher station. If you get iodine on your skin, immediately wash the area with soap and water.
Allow cubes to soak in iodine solution for 15 minutes. Be sure to keep track of time! While waiting, answer questions seven and eight and complete Table 1.
7. What do the cubes represent?
8. What does the iodine represent?
Table 1 Cube Data Cube Size (cm) Surface area (cm2) Volume (cm3) Surface Area: Volume (length x width x number of (length x width x height) Ratio (reduced) sides) Surface Area/Volume
.5
1
2
Gathering Data
After 15 minutes, the knife wielder obtains three folded squares of paper toweling. Metric master puts on the glove and carefully removes the three cubes from the iodine solution, swirls them in the large beaker of water to rinse off excess iodine, and then places them on the paper toweling squares. Distribute the potato cubes to the knife wielder who created it. Metric master will return the beaker of iodine to the collection station at the teacher’s desk, and obtain a scalpel.
Knife wielder will slice each potato cube in half and then return the scalpel to the collection station; metric master will measure (in cm) and report how far the iodine penetrated into each cube.
Table 2 Diffusion of Iodine Rate of Diffusion Cube Size (cm) Depth of Diffusion (cm) Time (min) (cm/min)
.5
1
2
All team members help with the following calculations:
Percent Volume of Cube ((total cube volume) – (volume of cube that has not changed color)x 100% That Received Iodine = (total cube volume)
Table 3 Extent of Diffusion Volume of cube that has not Total Volume 3 Percent of total volume of cube of Cube (cm3) changed color (cm ) that received iodine (%) (length x width x height)
Wash and dry all utensils, including beakers, and return everything neatly where you found it. The iodine solution may be flushed down the drain with lots of water. Dispose of potato cubes, paper toweling and glove in the trash. Wipe your work surface clean!
If you have time, work with your team members on the Discussion Questions. Post-lab Questions
1. Why did the diffusion of iodine into the potato cube cause the color change from beige to black?
2. If each cube represented a living cell, and the iodine solution was a substance needed within the cell, what problem might a large cell have?
3. Examine your data in table 2. What pattern do you notice in the relationship between cube size and the rate of diffusion?
4. Examine your data in table 1. Describe what happens to the surface area and the volume as the cell grows larger.
5. Still considering table 1, what happens to the ratio between surface area and volume as the cell grows larger?
6. According to your data, which cell was most successful at receiving the needed nutrient (iodine solution) in the allowed time? 7. Evaluate your initial hypothesis (as stated in the Pre-Lab).
8. Graph the Percent Volume of Cube Changed by cell size (0.5cm, 1cm, and 2cm), then use your graph to predict the percent Volume of Cube Changed for a hypothetical cube of 0.1 cm, 0.25 cm, 3 cm and 4 cm. Be sure to label your graph properly.