Chapter 4: ActivitiesActivity 4.1 Diffusion and Osmosis |
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The cell membrane defines the basic
unit of life. In single-celled organisms, it creates a physical separation between life and
non-life, making an internal environment possible. This is important because the
chemical reactions necessary for life can occur much more efficiently in an enclosed
volume with the membrane serving as a selective barrier. That is, it could keep
compounds out of the cell that might be damaging but allow the entry of necessary
material. Cell membranes are composed of phospholipid bilayers with embedded
proteins. This structure allows the cell to control its permeability to substances. Small,
uncharged molecules and water pass easily through such membranes, but larger
molecules and ions cannot cross the membrane unassisted. Diffusion, osmosis, and
protein-mediated transfer are just some of the mechanisms used by the cell to control its
internal environment.
Cell
Membranes Chapter 4, Section 4.3, Why Molecules Move across Lipid Bilayers: Diffusion and Osmosis |