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salt concentration on potato cells.

For example, suppose there are two cubes. Cube 1 is 1 cm x 1 cm x 1 cm, and Cube 2 is 10 cm x 10 cm x 10 cm. To calculate the surface-to-volume ratio, the formula for de- termining the surface area (SA) of a cube (length x width x number of sides) and the formula for the volume (V) of a cube (length x width x height) must be known. Once the formulas for calculating surface area and volume of a cube are known, the surface area to volume ratios can be calculated, as seen below.


Surface Area: 1cm x 1cm x 6 sides = 6cm2 10cm x 10cm x 6 sides = 600cm2

Volume: 1cm x 1cm x 1cm = 1cm3 10cm x 10cm x 10cm = 1000cm3

SA/V: 6cm2/1cm3 = 6.0 cm2/cm3 600cm2/1000cm3 = .6cm2/cm3

As shown in the calculations above, the ratio for Cube 2 is significantly smaller than the ratio for Cube 1. The same trend holds true for cells. As a cell gets larger, the SA/V ratio decreases, meaning that it is not as efficient in moving material in and out of the cell. In other words, the size of the cell membrane relative to the contents of the cell decreases as the cell size increases.

An illustration of the importance in maintaining a high surface-to-volume ratio can be found in the human digestive system. Cells in the human digestive system contain villi, which are finger-like projections. Because of their shape, they have a large sur- face area for a small volume.


1. Cell Structure and Function a. Label the following idealized plant and animal cells.

b. Observing Cell Structures Under a Microscope i. Utilize the Virtual Microscope to view several cell structures. When

using the virtual microscope, complete the following steps in the order provided below. Failure to properly perform the steps in the correct order will result in failure to complete subsequent steps. Click to view

10/1/13 10:29 AMBIO156 – Lab 4

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optional detailed instructions. ii. Drag and drop the desired slide onto the microscope.

iii. Click on the stage clip knob on the left of the microscope stage. iv. Adjust the interpupillary distance. First click on the title interpupillary

distance. Next, place the pointer on the images and adjust them until the two images are observed as one image.

v. Adjust the slide position. Place the pointer on the positioner and ad- just the slide so there is a clear view of the specimen.

vi. Adjust the iris diaphragm until a comfortable light is obtained. vii. Adjust the diopter until a clear image is obtained. Use the line on the

slide and move it up or down. viii. Adjust the coarse focus. Use the line and move it up or down until a

clear image is obtained. ix. Adjust the fine focus. Use the line and move it up or down until a

clear image is obtained. x. Adjust the magnification by clicking on the objective numbers on the

microscope. xi. Using the virtual microscope, view Spirogyra. Identify and draw an

image of the chloroplasts. xii. Using the virtual microscope, view the slide of a paramecium. Identify

and draw an image showing the cilia. xiii. Using the virtual microscope, view the slide of the Euglena. Identify

and draw an image of the flagella. 2. Demonstration of Osmosis in a Potato

a. Learn to use the caliper. 1. Take the Vernier caliper out of the lab kit. Examine the scale on the

tool and try to measure the length of an object. Look closely at the scale. The metric scale will be used for measurements in this lab.

2. Read the scale by measuring exactly 2 cm (20 mm). Next, measure 4.5 cm (45 mm).

3. This caliper is accurate enough to measure to the nearest tenth of a millimeter (measured by the small, scored lines in the window). With the caliper in hand, go to this instructional Web site, which describes how to use a Vernier caliper.

Watch the scale move as in an actual measurement.