6 Lab 5. Diffusion and Osmosis
LAB 5—DIFFUSION & OSMOSIS
OBJECTIVES
- Practice applying hypothesis testing, and further your understanding of the basic principles underlying the scientific method and experimental design.
- Identify independent and dependent variables in an experiment.
- Describe diffusion and osmosis.
- Practice graphing data obtained in the lab and designing useful data tables.
- Understand and be able to define the following terms: diffusion, osmosis, concentration gradient, tonicity, hypotonic, hypertonic, isotonic, turgor, selectively permeable, semipermeable, osmotic pressure, turgor pressure, plasmolysis, hypothesis, prediction, theory, control, independent variable, dependent variable, controlled variable, quantitative variable, qualitative variable.
PRELAB
- Draw a “flow chart” (in your lab notebook) to diagram what you will do (and in what order) in this Lab.
Exercise A – Plasmolysis in green leaf cells
- Predict what you think will happen and why, when the fresh water around the leaf is replaced by salt water. Predict what will happen with respect to osmosis and also with respect to what will happen to the cell itself.
Exercise B – Osmosis and dialysis tubing
- Predict what will happen inside the tubing comparing the three concentration treatments.
Exercise C – ‘Osmosis Egg as Model Cell’
- What hypothesis is being tested by this “experiment”/demonstration?
- What is the dependent variable?
- What is the independent variable?
- Are they quantitative or qualitative?
EXERCISE A – INTRODUCTION
In this lab you will explore the processes of diffusion and osmosis. Diffusion can occur across a semipermeable membrane, however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This movement of water is called osmosis.
PROCEDURES
- Please work in pairs for Exercises A and B and work in groups of 4 for Exercise C.
- BEFORE YOU BEGIN make a group plan about what you will do and who will be doing what. Your Flow Chart that you made for the Prelab Assignment will be useful for this. Note that one of the exercises requires using a microscope and each student should make their own microscopic observations and drawing.
- All data/observations must be written in your lab notebook.
Exercise A: Plasmolysis – Osmosis in a Living System (plant leaf)
Plant cells rely on pressure exerted outwardly by the fluid in the their vacuole to help them maintain their shape, much as a tire must be pumped up with air to provide the outward pressure that maintains the tire’s shape. The pressure exerted by the vacuole’s fluid is called turgor pressure. That pressure is maintained by the cell by adjusting the concentration of solutes in the vacuole’s fluid. In this exercise you will not be testing a hypothesis. Instead, you will observe what happens wh