Download L-osmosis in cells online

AP Biology – Lab Activity: Osmosis in Cells – C. Gray Mitchell
INTRODUCTION
Cells lose or gain water due to the difference in solute concentrations between the cytoplasm
(the intracellular fluid) and the solution surrounding the cell (the extracellular fluid). The
movement of water in and out of a cell is governed by the laws of diffusion: water flows from a
region of higher water concentration to a region of lower concentration.
When a cell is in a hypertonic solution, it will experience a net loss of water. A hypertonic
solution contains a higher concentration of solutes than the cell and therefore a lower
concentration of water. Consequently, water will flow out of the cell from the region of higher
water concentration to the region of lower concentration.
When a cell is in a hypotonic solution, it will experience a net gain of water. A hypotonic solution
contains a lower concentration of solutes than the cell and therefore a higher concentration of
water. Consequently, water will flow into the cell from the region of higher water concentration
to the region of lower concentration.
When a cell is in an isotonic solution, it will experience neither a net gain or loss of water. An
isotonic solution contains an equal concentration of solutes as the cell and therefore an equal
concentration of water. Consequently, water will flow equally into and out of the cell.
Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out
of the cell and into a hypertonic solution surrounding the cell as shown below in Figure 1. During
plasmolysis the cell membrane pulls away from the cell wall. In this lab exercise, you will
examine these processes by observing the effects of a highly concentrated salt solution, isotonic
solution, and distilled water on plant cells and animal cells.
Online Lab
1.
Go to https://www.youtube.com/watch?v=VK-_YHakvho and watch the Plasmolysis of Elodea video clip. Next,
skim over the following video clip and see if you can identify when the salt solution is added to the elodea cell
https://www.youtube.com/watch?v=SooSsKkJo1o. Sketch a single elodea cell before and after the salt solution is
placed on the slide. Describe the response of the cell to the salt water.
Sketch:
BEFORE
AFTER
Description:
2.
Go to https://www.youtube.com/watch?v=OYoaLzobQmk and watch the Osmostic Burst of Red Blood Cells
video clip.
a) Sketch a single red blood cell under isotonic conditions and describe the appearance of the cell.
b) Sketch a single red blood cell after addition of NaCl and describe the appearance of the cell.
c)
Sketch a single red blood cell after addition of distilled water and describe the appearance of the cell.
Summary Questions
1. In the winter, icy roads are often salted to remove the ice and make them less slippery. Grasses and other herbaceous
plants often die near the side of these roads. What causes this to happen?
2. What if the unthinkable happened at the hospital! A patient was given an I.V. bag with distilled water in it rather than
saline solution. Describe what would happen to their red blood cells and explain why this would occur.
3. Many freshwater one-celled organisms, like Paramecium , have contractile vacuoles. These
structures collect and pump out excess water that accumulates in the cell. Explain why
these organisms needs such a structure.
4. Popcorn sold at movie theaters is very salty, causing people to become thirsty and to buy
soft drinks. Explain why salty popcorn causes this thirst.
5. Explain why soft-bodied invertebrates, like slugs, die when you pour salt on them.
6. In the space below, draw a molecular diagram of the cell membrane. Illustrate the following processes and the cellular
structures and materials that are involved:
a. diffusion of water molecules across the cell membrane
b. active transport of a sodium ion across the cell membrane
c. diffusion of non-polar molecules (i.e., a lipid) across the cell membrane
Adapted from: www.ExploreBiology.com