Diffusion, osmosis and dialysis

Solution Making a saline water solution by dissolving salt (NaCl) in water. The salt is the solute and the water the solvent.

Concentration These glasses containing red dye demonstrate qualitative changes in concentration. The solutions on the left are more dilute, compared to the more concentrated solutions on the right.

Diffusion The tendency of molecules to spread out Molecules move from areas of their higher concentration to areas of lower concentration until equilibrium is achieved and the molecules and distributed equally. The speed of diffusion is dependent on such factors as the temperature, the size of the molecule, and the type of medium.

Solute Diffusion Across the Plasma Membrane Small, non-charged molecules can diffuse across a plasma membrane, but large molecules cannot diffuse across a membrane. Glucose— Benedict’s reagent (small) Starch— Iodine (large)

Osmosis Special case of diffusion Osmosis is the diffusion of water across the plasma membrane of a cell. Water molecules follows its concentration gradient and moves from the area of higher concentration to the area of lower concentration. a. less water (higher percentage of solute) more water (lower 10% 5% <10% >5% solute differentially permeable membrane water b. c. beaker thistle tube Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Tonicity The relative concentration of solute (particles), and therefore also of solvent (water), outside the cell compare with inside the cell. Isotonic Solution Solute and water concentrations equal on both sides of plasma membrane Hypotonic Solution Concentration of solute lower than the cell Hypertonic Solution Concentration of solute higher than the cell

Osmotic effects on cells In an hypertonic solution Animal cells Plant plasma membrane chloroplast nucleus cell wall Vacuoles fill with water turgor pressure develops chloroplasts - next to the cell wall Vacuoles lose wate the cytoplasm shrinks(plasmolysis) chloroplasts- in the center of the cell Water mainly enters the cell, which may burst (lysis) No net movement of water Water mainly leaves the cell, which shrivels (crenation) central vacuole In an isotonic solution In an hypotonic solution No net movement of water Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Experiments and questions 1. Solute Diffusion across Plasma Membrane (page 48) which molecules can diffuse across the dialysis tubing, starch or glucose? 2. Osmosis (page 50) why the level of the syrup solution rises in the thistle tube? 3. Demonstration of Tonicity in Red Blood Cells (page 51) 4. Effect of Tonicity on Elodea Cells (page 52)