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4: Osmosis and Diffusion

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    • 4.1: Introduction
      The cell membrane is the barrier that separates the cytoplasm from the external world. The cell membrane consists primarily of phospholipids in a bilayer. Phospholipids are amphipathic with a polar head (phosphate group) and a hydrophobic tail (2 hydrocarbon chains). Due to the chemical properties of the heads being attracted to water and the tails having a desire to avoid water, phospholipids self-assemble into micelles.
    • 4.2: Why Are Cells Small? (Activity)
      In this lab, you will perform a simulation using agar cubes (representing cells) and figure out why cells are generally extremely small.
    • 4.3: Do Larger Things Diffuse Faster? (Activity)
      Agar is a gelatinous substance derived from a structural carbohydrate found in seaweed. It is often used in cooking as a vegetarian alternative to gelatin and can be used as a thickener. Altering the amount of fluid solution will change the pores between fibers. More fluid will create a looser gel that has larger spaces between molecules. Reducing the fluid solution volume will conversely create a stiffer gel with smaller spaces between fibers.
    • 4.4: Gummy Bears- Tonicity (Activity)
      We’re all familiar with gelatin (Jello!). Gummy candies are made of gelatin. Gelatin is a protein that exists as long fibers. When gelatin is dissolved in a liquid and cooled, the gelatin fibers tangle together in a mesh-like network. Gummy candies are considerably more firm than the gelatin molds we have as desserts because they contain a lot less fluid. Like a cell, a gummy candy placed in solution will be affected by the properties of osmosis when submerged in different solutions.
    • 4.5: Observing Osmosis (Activity)
      In this lab, you will hypothesize and observe what happens when bags filled with different concentrations of sucrose is placed in beakers with different concentrations of sucrose.
    • 4.6: Agar Cubes (Preparation)
      In this page, you will find instructions on how to prepare the Agar Cube used in the "Why Are Cells Small" activity.

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