Beet the Bleach: Understanding Cell Size and Diffusion
Science Lab · Grade 8 · ngss
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science_lab
Pre Lab
Background: Every cell needs a steady supply of materials such as oxygen, water, and nutrients, and it must get rid of wastes. Many of these substances move into and out of the cell by **diffusion**, the movement of particles from an area of higher concentration to an area of lower concentration. Diffusion happens through the cell's outer boundary, the cell membrane. The amount of membrane a cell has is its **surface area**. The amount of space inside the cell that needs supplies is its **volume**.
When an object grows, its volume grows faster than its surface area. Look at a cube. Its surface area is 6 × side × side, and its volume is side × side × side. When the side doubles, the surface area becomes 4 times larger, but the volume becomes 8 times larger. Scientists compare these two numbers with the **surface area-to-volume ratio (SA:V)**, which is surface area divided by volume. Small objects have a large SA:V. Large objects have a small SA:V. That means a big cell has less membrane for each bit of its inside to depend on.
In this lab you will build a **model** of cells using beet cubes. Beets contain a red pigment. Dilute bleach diffuses into the beet and breaks down the pigment, turning the beet pale. The pale outer zone shows how far bleach diffused, and any red center shows the part it did not reach. By comparing cubes of different sizes, you will see how **cell size** and SA:V affect how well diffusion can reach the center of a cell. A model is useful because it lets us see something too small to watch directly, but every model also has limits.
Vocabulary
Term: Diffusion
Definition: The movement of particles from an area of higher concentration to an area of lower concentration until they are evenly spread.
Term: Surface area
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Post Lab
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