Diffusion of Molecules

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LAB EXERCISE: Diffusion and Osmosis

Laboratory Objectives
After completing this lab topic, you should be able to:
1. Describe the mechanism of diffusion at the molecular level. 2. List several factors that influence the rate of diffusion. 3. Explain why diffusion is important to cells.
4. Describe a selectively permeable membrane, and explain its role in osmosis. 5. Define hypotonic, hypertonic, and isotonic in terms of relative concentrations of osmotically active substances.

6. Discuss the influence of the cell wall on osmotic behavior in cells. 7. Explain how incubation plant tissues in a series of dilutions of sucrose can give an approximate measurement of osmolarity of tissue cells.

8. Explain why osmosis is important to cells.

Maintaining the steady state of a cell is achieved only through regulated movement of materials through cytoplasm, across organelle membranes, and across the plasma membrane. This regulated movement facilitates communication within the cell and between cytoplasm and the external environment.

The cytoplasm and extracellular environment of the cell are aqueous solutions. They are composed of water, which is the solvent, or dissolving agent, and numerous organic and inorganic molecules, which are the solutes, or dissolved substances. Organelle membranes and the plasma membrane are selectively permeable, allowing water to freely pass through but regulating the movement of solutes.

The cell actively moves some dissolved substances across membranes, expending adenosine triphosphate (ATP) (biological energy) to accomplish the movement. Other substances move passively, without expenditure of ATP from the cell, but only if the cell membrane is permeable to those substances. Water and selected solutes move passively through the cell and cell membranes by diffusion, a physical process in which molecules move from an area where they are in high concentration to one where their concentration is lower. The energy...
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