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BIO 120 Grossmont College Biology Cell Membrane Questions

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  1. What is a cell membrane?
  2. There are four different ways that cells move molecules across their membranes. List and define each of these four mechanisms.
  3. Read this brief excerpt to help you understand Brownian Motion:

[Introduction to Brownian Motion] (Links to an external site.)

Brownian Motion

Michael Fowler, U. Va.

Introduction: Jiggling Pollen Granules

In 1827 Robert Brown, a well-known botanist, was studying sexual relations of plants, and in particular was interested in the particles contained in grains of pollen. He began with a plant (Clarkia pulchella) in which he found the pollen grains were filled with oblong granules about 5 microns long. He noticed that these granules were in constant motion, and satisfied himself that this motion was not caused by currents in the fluid or evaporation. Smaller spherical grains, which at first he took to be oblongs end-on, but later realized weren’t, had even more vigorous motion. He thought at first that he was looking at the plant equivalent of spermthey were jiggling around because they were alive. To check this, he did the same experiment with dead plants. There was just as much jiggling. Perhaps all organic matter, everything that ever was alive, still contained some mysterious life force at this microscopic level? Sure enough, he found the movement in tiny fragments of fossilized wood! But then he went on to find it in matter that never was alivetiny particles of window glass, and even dust from a stone that had been part of the Sphinx. The movement evidently had nothing to do with the substance ever being alive or dead, much to Brown’s surprise. So what was causing it? Perhaps it was evaporation currents, or the incident light energy, or just tiny unnoticed vibrations. But none of these explanations was very satisfactory.

Half a century later, a new possible explanation emerged. The kinetic theory of heat developed by Maxwell, Boltzmann and others was gaining credence. If all the molecules in the fluid were indeed in vigorous motion, maybe these tiny granules were being moved around by this constant battering from all sides as the fluid molecules bounced off. But there was a problem with this explanation: didn’t it violate the second law of thermodynamics? It had been well established that energy always degrades, as friction slows movement kinetic energy goes to heat energy. This seemed to be the other way roundthe molecular battering was certainly disorganized heat energy, but when the granule moved it had evidently gained kinetic energy. Since many scientists regarded the second law as an absolute truth, they were very skeptical of this explanation.

In 1888, French experimentalist Léon Gouy investigated the movement in detail, finding it to be more lively in low viscosity liquids. He established that it was unaffected by intense illumination or by strong electromagnetic fields. Despite the second law, Guoy believedcorrectlythe random motion was indeed generated by thermal molecular collisions.

It’s easy to see the Brownian movement, or Brownian motion (it’s called both) by looking through a microscope at tobacco smoke in air.

[Introduction to Brownian Motion] (Links to an external site.)

4. Use this interactive online simulation to better understand what is happening as Brownian Motion is observed.

[Brownian Motion Simulation] (Links to an external site.)

5. Watch this four-minute YouTube video.

Brownian Motion – setting up a Smoke Cell (Links to an external site.)Brownian Motion - setting up a Smoke Cell

6. In your composition book, create a diagram or series of diagrams depicting Brownian motion. Include whatever labels and captions you see fit.

7. Draw and label a cell membrane with an embedded aquaporin protein. Your lab manual should be a sufficient reference for this objective.

8. Define the following:

      • Colloid
      • Selectively permeable membrane
      • Osmosis
      • Hypertonic
      • Hypotonic
      • Isotonic
      • Concentration Gradient

9. Carefully read and understand Osmosis and the rate (speed) of diffusion along a concentration gradient.

10. Read the procedure (pg 13) and reference Figure 2 (pg 11)

11. Write your answers to the following questions in your composition book:

      • What is this experiment evaluating?
      • How does the change in mass of the three model cells over time relate to osmosis?
      • How do you expect the mass to change for each of the model cells? (Increase? Decrease? No Change?) Explain why.


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