Date ______Calculating Cell Size Name ______

First look at the photos of the cells. These are very young embryos from sea urchins, marine organisms. These photos were taken over a span of 105 minutes, beginning immediately after fertilization (Panel 1). The fertilized egg then divided once (Panel 3) to produce a two-cell embryo (Panel 4), and then again (Panel 5) to produce a 4-cell embryo (Panel 6). Eventually, hundreds of divisions would result in a tiny sea urchin larva that would settle to the sea bed and grow into a mature urchin.

Step 1: Measure the diameter of the cells

The scale bar on the photos tells you that a distance of 4 cm (the length of the line) equals 100 mm in the photos. In other words, 1 cm is equal to 25 mm.

Measure the diameter of the cells in Panels 1, 4, and 6 in mm, using the scale bar as a guide. The diameter is simply the distance from one end of the cell to the opposite end. In panels 4 and 6, just measure one cell; we’ll assume they are roughly the same size. If a cell is more oval-shaped than spherical, measure along the longer axis.

Record these diameters in column 1 of the table.

1 / 2 / 3 / 4 / 5
Panel / Diameter
(in mm) / Radius (r)
(in mm) / SA / V / SA/V
1
4
6

Step 2: Determining the radius of the cells

Divide the diameters in column 1 in half, and record these values in column 2.

Step 3: Determining surface area

Surface Area (SA) can be calculated using the formula: SA = 4 X 3.14 Xr2

Calculate the SA for each of your three cells and record it in column 3.

Step 4: Determining volume

Volume (V) can be calculated using the formula: V = 4/3 X 3.14X r3

Calculate the V for each of your three cells and record it in column 4.

Step 5: Determining SA/V ratios

Simply divide SA (column 3) by V (column 4). Record the ratios in column 5.

Use the data from the table and your knowledge of cell growth to answer the following questions:

1.  Which panel shows cells with the highest SA/V ratio? ______

2.  Which panel shows cells with the lowest SA/V ratio? ______

3.  What happens to the SA/V ratios of these embryonic cells over time (from 0 – 105 min)?

______

4.  In panel 7 (which is labeled) it shows the sperm cell penetrating the egg cell. Without doing any calculations, which cell (the sperm or egg) has a higher surface area to volume ratio and explain why?

______

5.  What must be copied before each cell division? ______

6.  How does the DNA compare in each of the 4 cells in panel 6?

______

7.  What is DNA overload?

______

8.  Which panel shows the cell(s) that are under the greatest stress of DNA overload? Why

______

9.  Why is it important for cells to have a high SA/V?

______

10.  What process is being shown in the 6 panels?

______