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BIO 101 Lab 05: Osmosis and Diffusion in Elodea and Red Blood Cells.
Notification: If you have a disability that makes it difficult to complete this lab, please contact your
instructor. Please provide your instructor a copy of the Memorandum of Accommodation (MOA) from
NVCC Disability Support Services.
Review cell membrane structure and the components that make it a semipermeable barrier.
Observe plant and animal cells placed in solutions, hypotonic, isontonic, and hypertonic to the
cells and observe the effect of each of the solutions on the cells.
Observe plazmolyzed and turgid and flaccid plant cells.
Observe healthy red blood cells, crenated and hemolyzed blood cells.
Interpret images of plant cells or red blood cells in different solutions.
Cells and Cell Membrane structure
Living cells are filled with an aqueous solution, the cytosol. Many small solutes or particles, are dissolved
in this watery cytosol, including ions, nutrients, waste and enzymes.
Larger organelles (such as
mitochondria and chloroplasts) and large complexes of molecules (e.g. ribosomes) are also located in
this cytosol; organelles and complexes combine with the cytosol to form the cytoplasm of cells.
The exterior barrier of all cells is a cell membrane (plasma membrane). Because the cell membrane is
comprised of both phospholipids and proteins it is semi-permeable:
select molecules can easily pass
through the phospholipid bilayer, while other molecules cannot. Phospholipids are comprised of
hydrophobic fatty acid tails and polar phospho- heads. In the bilayer, the fatty layers are sandwiched in
between the hydrophilic polar heads; one layer of polar heads lines the inside (cytosolic face) of the cell
membrane and a second polar head, lines the outside of the cell membrane or surface.
Transport through membranes
Molecules that can easily pass through a cell membrane are small and nonpolar, such as gases: oxygen,
carbon dioxide, and nitrogen. Hydrophobic lipids, (fats and steroids) can dissolve in the hydrophobic fatty
layer of the cell membrane and also will diffuse into or out of a cell. Some small molecules (e,g. sugars)
and ions (sodium or chloride) can diffuse across a membrane if there is a protein (transport protein or
ion channel) These small solutes will move from areas of high concentration to low concentration
through a select protein that is part of the membrane.
Osmosis occurs when water moves down its
gradient. Active transport occurs when a solute, such as sugar, moves against its gradient. Both energy
and protein transporters are required for active transport to occur.