Lab 3 – Cell Membranes and Water Balance 8
PLAN AHEAD! This lab requires working with a partner and writing a lab report.
Osmosis is the vitally important process by which water moves in and out of cells. Overall, the cells of living organisms are composed of about 75-85% water. The concentration of water is even higher in the fluid inside cells; i.e., the fluid containing dissolved substances that are not attached to membranes. Life depends on having the proper concentration of water inside the cell. If there is too little water, the chemical reactions that support life will not be able to proceed and the cell will die. If there is too much water inside, the cell may burst. In this minicourse, we will examine factors affecting the movement of water in and out of cells. The earliest cells are thought to have originated and evolved in the ocean. Is the internal environment (i.e., the intracellular fluid) of cells still a lot like sea water (97% water, 3% dissolved salts)? In this minicourse, you will also compare the concentration of water and “solutes” (dissolved substances or “salts”) found inside plant cells from different environments — fresh water, marine and terrestrial.
Lab learning objectives You will have mastered the content of this minicourse when you are able to: 1. Define diffusion and osmosis in terms of concentration gradients and the movements of molecules. Explain (in terms of concentration gradients and internal pressure) the effects of osmosis on cells placed in hypertonic, hypotonic and isotonic environments. 2. Use an osmometer to measure changes in internal pressure in model “cells” (dialysis bags) placed in different “environments” (solutions) and graph your results. Explain why internal pressure increases over time and then stabilizes. 3. Describe how the effects of osmosis in plant cells are modified by the presence of a cell wall. Contrast turgor and plasmolysis. 4. Use the plasmolysis threshold of cells to compare the isotonic points (internal solute concentrations) of plant cells from fresh water, marine, and terrestrial environments. PART I: OSMOSIS Molecules (including water, oxygen, carbon dioxide) are in continual motion, colliding and bouncing off each other in random directions. Because the movement of each molecule is random, it is impossible to predict the direction any one molecule will be moving at any given time. However, if you look at huge groups of randomly moving molecules (billions of molecules at a time), predictable patterns emerge. Namely, the “net” (or overall total) movement of molecules is from areas where they are in high concentration to areas where they are in low concentration.