Cell Membrane Transport: Key Terms and Learning Objectives

Cell Membrane Transport

Key Terms

• Active Transport: The movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration, requiring energy (usually in the form of ATP).

• Concentration Gradient: A difference in the concentration of a substance across a space or a membrane.

• Crenation: The process in which cells lose water in a hypertonic solution, causing them to shrink.

• Diffusion: The passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Hemolysis: The rupture or destruction of red blood cells, often due to being placed in a hypotonic solution.

• Hypertonic Solution: A solution with a higher solute concentration compared to the cell, causing water to leave the cell.

• Hypotonic Solution: A solution with a lower solute concentration compared to the cell, causing water to enter the cell.

• Isotonic Solution: A solution with the same solute concentration as the cell, resulting in no net movement of water.

• Osmosis: The diffusion of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

• Passive Transport: The movement of substances across a cell membrane without the use of energy by the cell.

• Selective Permeability: The property of a cell membrane that allows some substances to pass through while blocking others.

• Semipermeable: A membrane that allows certain molecules or ions to pass through by diffusion.

• Simple Diffusion: The unassisted movement of small or nonpolar molecules across a membrane.

Learning Objectives

1. Relationship Between Diffusion Rate, Molecular Weight, and Selective Permeability:

   • The rate of diffusion is inversely related to molecular weight; smaller molecules diffuse faster than larger ones.

   • Selective permeability refers to the ability of the membrane to allow certain molecules to pass while restricting others, often based on size, charge, or solubility.

   • Example: Oxygen (O2) diffuses faster than glucose due to its smaller size.

2. Predicting Diffusion Outcomes Based on Molecular Weight:

   • Given two molecules, the one with the lower molecular weight will diffuse more rapidly across a permeable membrane.

   • Example: In an experiment, potassium permanganate (KMnO4) diffuses faster than methylene blue due to its lower molecular weight.

3. Predicting Results Using Dialysis and Solubility Information:

   • Dialysis tubing acts as a semipermeable membrane, allowing small molecules to pass while restricting larger ones.

   • Solubility and molecular size determine which substances can move through the membrane.

   • Example: Glucose can pass through dialysis tubing, but starch cannot due to its larger size.

4. Benedict's Test: Description and Purpose:

   • Benedict's test is a chemical assay used to detect the presence of reducing sugars (e.g., glucose) in a solution.

   • A positive result is indicated by a color change from blue to green, yellow, orange, or red, depending on the amount of sugar present.

   • Example: After dialysis, Benedict's test can be used to determine if glucose has passed through the membrane.

5. Silver Nitrate (AgNO3) Test: Description and Use:

   • Silver nitrate is used to test for the presence of chloride ions (Cl-) in a solution.

   • A white precipitate of silver chloride (AgCl) forms if chloride ions are present.

   • Example: Testing the contents of dialysis tubing for chloride ions after diffusion.

6. Predicting Cell Behavior in Solutions of Different Tonicity:

   • Cells placed in a hypertonic solution will lose water and shrink (crenation in red blood cells).

   • Cells in a hypotonic solution will gain water and may burst (hemolysis in red blood cells).

   • Cells in an isotonic solution will remain the same size, as there is no net movement of water.

   • Example: Red blood cells in distilled water (hypotonic) undergo hemolysis, while in 0.9% NaCl (isotonic), they remain unchanged.

Additional info:

• Formulas related to diffusion rate (Fick's Law): $J = -D \frac{dC}{dx}$ Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmosis is a specific type of diffusion involving water molecules across a semipermeable membrane.