Chapter 2. Movement of Materials Across the Cell Membrane I

General Purpose

Lab 5 Pre-Lab—Membrane Permeability
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This pre-lab will present some of the concepts related to the movement of materials across cell membranes.

Learning Objectives

General Purpose

Conceptual

  • Define and use the following terms: solvent, solute, diffusion, and selectively permeable.
  • Gain an understanding of some of the factors that influence the movement of materials across cell membranes including the principle of diffusion.

Procedural

  • Determine the diffusion rate of molecules with varying size and partition coefficients (polarity) across a biological membrane.

The ability of a molecule to cross biological membranes is a function of many factors, including: size of the molecule, charge, hydrophobic/hydrophilic character, and concentration. Hydrophobic refers to substances that are lipid-soluble whereas the term hydrophilic refers to substances which dissolve easily in water. Very small uncharged molecules or molecules (small or large) which are hydrophobic can easily pass directly through the phospholipid bilayer. Charged atoms or molecules (ions) and larger hydrophilic molecules cannot cross membranes unless they are assisted by a membrane protein (facilitated transport or active transport). This assistance is usually very specific and results in only one type of material being moved by a given protein. Also, facilitated transport may often be regulated so that a substance may pass through at one time but not at another time.

Membrane Permeability

The mammalian erythrocyte (red blood cell or RBC) is one of the most well-studied membrane systems and is used as a model to describe many membrane–solvent–solute interactions. The mammalian red blood cell is a membrane-bound, circular, biconcave disk, which at maturity has no nucleus (Figure 5-3).

Figure 5-3. Diagram of erythrocytes.

The disk shape of the RBC permits rapid gas exchange to support the oxygen and carbon dioxide carrying capacity of the blood. RBCs placed in a hypotonic solution (e.g., water) will rapidly swell and rupture (hemolysis). Conversely, cells placed in a hypertonic solution (e.g., concentrated salt water) will rapidly shrink and crenate. Further, RBCs placed in an isotonic solution will neither swell nor shrink. The amount of time that it takes for hemolysis or crenation to occur is directly related to the rate of osmosis across the cell membrane. Therefore, hemolysis can be used as an index of the rate of osmosis.

Hemolysis time can also be used as an index as to the rate of diffusion of substances into the cell. Suppose that we add RBCs to isotonic solutions of different solutes. If the solute cannot move through the cell membrane and therefore influence the water to move with it, no hemolysis will occur. On the other hand, if the solute can move through the cell membrane and if it is less concentrated inside the cell compared to the extracellular solution, then the solute will move into the cell increasing the concentration of solute molecules inside the cell. This now makes the inside of the cell hypertonic compared to the outside environment. As a result water is pulled into the cell due to the newly established osmotic gradient and hemolysis occurs.

A standard method for studying the permeability of RBCs to a specific substance is to place blood into a solution of the substance and record the time required to reach a specified degree of hemolysis. As hemolysis occurs, the RBC membrane ruptures and releases hemoglobin, causing the solution to clear.

Pre-Lab Quiz

Proceed to the Pre-Lab Quiz