Biology, Answering the Big Questions of Life/Osmosis lab

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Osmosis is defined as the diffusion of water (solvent) from areas of high solvent (more amount of water) concentration to areas of low solvent (less amount of water) concentration across a semi-permeable membrane.

We call the liquid with more dissolved solutes in it a hypertonic solution and the liquid with less dissolved solutes is called a hypotonic solution. If the solutions on both sides of the membrane have the same concentration, it is called isotonic. In osmosis, water moves from a hypotonic solution to a hypertonic solution.

All organisms must have strategies to deal with the movement of water into and out of their cells by osmosis. Plant cells usually have more dissolved solvents inside their cells than the water found in the soil. Because of this, water will rush into the cells. To prevent the cells from bursting (lysis), plant cells are surrounded by a hard cell wall.

If the plant finds itself in a hypertonic solution, water will rush out of the cells causing the cells to shrivel up. This process is called plasmolysis.

In this lab we will observe plasmolysis in red onion cells.


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  1. Make a wet mount of red onion cells.
  2. Observe the red cells under the microscope at 100X and 400X.
  3. Draw the cells and label this drawing BEFORE.
  4. While keeping the slide under the microscope.

Add a few drops of 20% salt solution to the edge of the slide. You may need to pull the liquid across by placing a piece of paper on the opposite side of the slide at the edge of the coverslip.

  1. Observe the cells. You may have to refocus the image as the increased liquid will change the location of the coverslip.
  2. Draw the image and label this cell AFTER.
  3. Explain what happened to the cells.

Alternative Instructions:

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Osmotic pressure on red blood cells immersed in various salt solutions

Alternative 1: You may do the same using a small slice of potato, if red onion skin is not available.

Alternative 2 (only if there is no access to a microscope): Look at this image. Which diagram represents cells in regular blood? Which represents cells in pure water? Which represents cells in a saturated salt solution? Answer these questions, then look at this article and check your answers. Then write a conjecture about why one's fingers seem "pruny" after a long bath, but not after being in the ocean for an extended time.


Always wear gloves when doing experiments using blood samples.