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Experiment II

Biological Membranes

All cells are surrounded by a plasma or cell membrane made up of two basic components, lipids and proteins. Membrane lipids are amphipathetic molecules, that is, they have both hydrophilic and hydrophobic properties. As seen in the figure below, cell membranes are composed primarily of phospholipids; these lipids have a hydrophilic, charged, "head" to which are covalently attached long hydrophobic "tails" of fatty acid.


Cell membranes are "lipid bilayers" in which the hydrophobic tails orient themselves toward the center of the bilayer shielding them from water. The polar head groups are oriented outward where they can interact with water molecules.

The current understanding of cell membranes is described in the Fluid Mosaic Model. In this model, cells are surrounded by a continuous phospholipid bilayer in which are embedded membrane proteins (see picture below). Phospholipids in the bilayer are fluid, able to diffuse laterally within one half of the bilayer. Conversely, phospholipids rarely "flip" from one half of the bilayer to the other; this means that different halves of the bilayer can have different phospholipid composition. Some membrane proteins can also diffuse to new locations in the bilayer.

For more information on the structure/function of cell membranes, see the following sites:

 Cell membranes serve as a barrier to the entry (or exit) of most molecules. Most small nonpolar molecules such as O2 and CO2 or uncharged polar molecules can cross the cell membrane by diffusion. Most charged molecules; however, cannot cross the cell membrane and thus must be transported across the lipid bilayer. This impermeability causes a concentration difference between molecules inside and outside the cell.

A solution that has a lower concentration of dissolved substances (solutes) than inside the cell is said to be hypotonic; whereas a solution with a higher concentration of solute than inside the cell is called hypertonic. A solution in which the concentration of solute(s) is equal to that inside the cell is called isotonic.

When cells are placed in distilled water or some other hypotonic solution; the concentration of water is greater outside the cell than inside. Diffusion tends to equalize the concentration of molecules that can freely cross the membrane, thus water will enter the cell. Since most solute molecules cannot diffuse through the membrane, water is entering the cell faster than any solute is leaving, causing the cell to swell and the membrane bursts, or lyses. In contrast, when the concentration of solute is greater outside the cell (hypertonic), water tends to diffuse from the inside to the outside of the cell, causing the cell to shrivel; the cell is often described as crenate.

In today's experiment, we will estimate the tonicity, or the concentration of dissolved substances, inside a red blood cell (from cows). We will place red blood cells (pictured below) in a graded series of solutions of different NaCl concentrations and determine at what concentrations of NaCl lysis occurs.

Human red blood cells, T-lymphocyte and platelets (c) Dennis Kunkel


Soaps & Detergents-

Soaps and detergents, like lipids, are amphipathetic molecules with both hydrophilic and hydrophobic properties. The long hydrocarbon chains of soaps and detergents are lipophilic, that is, these chains are able to solubilize hydrocarbons (remember, like dissolves like), while the polar portion of the molecule provides solubility in water.

Soaps are generally made from fats by a process called saponification (sapon is Latin for soap). Detergents are generally synthetic compounds produced from petroleum and their ionic ends are salts of sulfonic acid, rather than carboxylic acids (soaps). Soaps are more basic than detergents and thus generally harsher.

Soaps and detergents work on oily materials by dissolving their hydrocarbon tails in the oils in a manner such that the polar groups are facing outward into the surrounding water forming micelles around the oil.

Soaps and detergents tend to solubilize the lipids of the cell membrane causing cells to lyse. Today, we will evaluate whether soap alone is as effective as soap with antibacterial compounds at killing bacteria.

For more info on Soaps & Detergents:

To view more great electron microscope images, click on the image below: