Name______

Block______

Osmosis Lab

Background – “Rub-a-dub-dub three men in a tub out on the deep blue sea.” – so the song goes. But you say these men have plenty of water around to drink so that will stay alive. Recall though that molecules of a certain substance tend to move from a place where they are in greater number (Greater concentration) to a place where there are fewer (lesser concentration). This is the process of diffusion and it goes on in the body day and night whether we are asleep in a nice warm bed or afloat in a “tub” in the ocean.

Can you determine why a shipwrecked sailor might die if he drank sea water? Sea water does not poison him but the concentration (amount per given volume) of salts in sea water is greater than the concentration of salts in cells. Along with this, concentration of water molecules in the cell is greater than the concentration of water molecules in the sea water (which he might be tempted to drink).

Using living cells of a red onion leaf we can investigate the movement of water and other substances out of the cell as well as into the cytoplasm of the cell.

PROCEDURE

Mount a red onion leaf in a drop of freshwater.

  1. Draw onecell under high power. Label CELL WALL, CELL MEMBRANE, and CYTOPLASM.

(Scientific drawing- use white paper and pencil!)

Draw off the freshwater and replaces it with the “mini-ocean” (salt water).

  1. Draw onecell under HP. Label CELL WALL, CELL MEMBRANE, and CYTOPLASM.

(Scientific drawing- use white paper and pencil!)

  1. What change is taking place?
  1. Why is this happening?

“Rescue” the plant by drawing off the “mini-ocean” and replacing it with freshwater.

  1. Draw one cell under HP. Label CELL WALL, CELL MEMBRANE, and CYTOPLASM.

(Scientific drawing- use white paper and pencil!)

Has the cell wall changed shape or position during this activity?

Has the cell membrane changed shape or position during this activity?

How?

What did the salt in the “mini ocean” do to the inside of the cells of the red onion?

How did the salt water get into cells?

When you eat a lot of potato chips why do your lips become sore and sometimes crack?

A good way to get a blood-sucker off your arm or leg is to put salt on it. How do you explain this statement?

How may salt be used to kill weeds?

Why is salt not used on lawns?

Bacteria cause food to spoil and meat to rot. Explain why salted pork, strawberry jam, and sweet pickles do not spoil even though they are exposed to bacteria.