LAB. 8 -- Osmosis
Osmosis is a special case of diffusion -- the movement of molecules from high concentration to low
concentration. Osmosis is the diffusion of water across a semipermeable membrane. If there is less free water
(more solutes) outside the cell then inside the cell, the solution is referred to as hypertonic, and water will
leave the cell. If there is the same amount of free water inside and outside the cell, the cell is said to be in an
isotonic solution, and there will be no net diffusion of water. If there is more free water outside the cell then
inside, the cell is in a hypotonic solution and water will enter the cell.
OSMOSIS IN PLANT CELLS
Background: If a plant cell is placed into a hypertonic solution, the plasma (cell) membrane will
pull away from the cell wall as water leaves the cell. The cell is said to be plasmolyzed. A cell in
an isotonic solution is said to be flaccid. If a plant cell is placed into a hypotonic solution, water will
enter the cell, causing the cell membrane to press against the cell wall. This is turgid pressure.
Procedure:
1. Label three slides A, B, and C.
2. Place a single Elodea leaf on each slide.
3. Place several drops of hypotonic solution onto the leaf on slide A, add a coverslip, and observe
at high power.
4. Place several drops of isotonic solution onto the leaf on slide B, add a coverslip, and observe at
high power.
5. Place several drops of hypertonic solution onto the leaf on slide C, add a coverslip, and observe
at high power.
OSMOSIS IN ANIMAL CELLS
Background: If an animal cell is place into a hypertonic solution, the cell will shrivel as water
leaves. The cell is referred to as crenated. An animal cell will retain its natural shape in an
isotonic solution. If placed into a hypotonic solution, the cell lyse due to the influx of water (it does
not have a cell wall to protect it from osmotic pressure).
Procedure:
1. Label three slides A, B, and C.
2. Disinfect your middle finger's tip.
3. Using a lancet, cause your middle finger tip (fleshy part) to bleed. ** do not share lancets"*
4. Place a drop of blood on slide A, add a coverslip, and observe under high power. Add a drop of
hypotonic solution to the edge of the coverslip and continue observing.
5. Place a drop of blood onto each slide, then add a drop of isotonic solution to slide B, and a drop
of hypertonic solution to slide B. Add a coverslip and observe.