COVER PAGE

TITLE:

Introductory Exercise in Osmosis, Transpiration of Plants, and Root Pressure

AUTHOR:

KEYWORD TOPICS:

ABSTRACT:

FAIR USAGE STATEMENT:

This exercise is copyrighted (J. E. Armstrong, 1992). This exercise may be freely copied and disseminated for all non- commercial educational activities provided that appropriate credit is given to the author, this source (BIOLAB), and NSF. Its use is explictly permitted in laboratory manuals and compiled class handouts sold at or below the cost of printing or duplication.

Fair usage allows minor modification to suit the user's specific situation. The BIOLAB BBS SYSOPs request that major modifications and improvements to this exercise be resubmitted so they may be shared.

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BACKGROUND INFORMATION

OBJECTIVES

Osmosis, diffusion across a semi-permiable membrane, is a very basis cell function commonly demonstrated in introductory biology courses. The objective of this exercise is simply to present observations and pose questions so that students may generate a reasonable concept of what osmosis is and how it works, and then refine, adapt, and employ the concept on related phenonmena. Descriptive terminology is introduced only after a general concept is developed.

CONCEPTS

The basic idea is that small molecules like water can move across semi-permiable membranes while large molecules cannot. The consequences for a cell, bounded by a semi-permiable membrane, depend upon the cell's environment. The actual size and structure of molecules is not necessary knowledge to develop the concept of osmosis.

With only a small amount of essential knowledge, as presented in the student protocol, students are directed to make observations and based upon them, develop a concept of osmosis.

Related concepts concern the development of cell turgor from osmotic pressure, wilting from loss of osmotic pressure, transpiration (the loss of water from plants), and root pressure (cell osmotic pressure forcing water up vascular tissue, xylem). Students can conclude that osmotic pressure contributes to plant and water movement within plants. Finally these concepts are related to real world situations like crisping vegetables and fertilizer application.

MATERIALS:

      
Part I-A:    beaker
             corn syrup (colored) or molasses 
             6" piece of dialysis tubing, soaked in water
             string
             Benedict's solution
             test tube & test tube holder
             hot plate & beaker of hot water
             pipettes
        
Part I-B:    microscope
             slide & cover slip
             water in dropper bottle
             10% salt solution in dropper bottle
             pieces of paper toweling
             razor blade (if using Zebrina or Gynura
             Elodea - 1 small leaf
             -or- Zebrina - epidermal peel
             -or- Gynura (velvet plant) - use a thin cross section 
             of the leaf so that purple trichomes can be observed.
        
Part II:     potatoes
             cork borer (or french fry cutter)
             knife 
             finger bowls
             distilled water
             Any other materials from earlier observations.
        
Part III-A:  Same as Part I-A plus:
             glass tubing
             rubber bands
             label tape & marker or grease pencil
             beaker
             ringstand and clamp
        
Part III-B:  A bell jar
             Pot of wheat or barley seedlings
        
Part III-C:  Demonstration of wilted plant (Coleus, Geranium)
        
Part III-D:  Potted geranium, coleus, tomato, etc.
             Clear plastic tubing to fit stem
             razor blade
             rubber bands or string
             pipette with long thin tip
             beaker
             balence

An Exercise in Introductory Biology

OSMOSIS

Introduction

Cells are the basic functional unit of life, and the cell membrane delimiting the cell is a universal structural feature of cells. This exercise will allow you to examine part of the cell membrane's function by investigating the process of osmosis ("oz- MOE-sis"), which is defined as diffusion across a semi-permeable membrane. In addition to a cell membrane, plant cells have a vacuolar membrane delimiting the vacuole, a water-filled sac that composes the majority of the cell's volume. In the observations and experiments that follow, there will be no need to distinguish between these two membranes.

Diffusion is a physical process where molecules move along a concentration gradient from areas of high concentration to areas of lower concentration. When there is no concentration gradient, there is an equilibrium and no diffusion.

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Part I: Observation of Simple Osmosis

A. Osmosis in a Pseudocell.

Dialysis tubing provides us with a material that can demonstrate osmosis by acting as the cell membrane of a simple pseudocell. Dialysis tubing is constructed so that it has molecular sized pores of limited size. Like a sieve, small molecules can pass through the tubing, but larger molecules cannot.

Pseudocell Procedure:

1. Take a piece of soaked dialysis tubing. Fold 2.5-3 cm of one end back on the rest of the tubing and tie off tightly to make an end. Rub your thumb and forefinger together across the other end of the tubing to open the tube. Pour in sugar solution a depth of 5-6 cm. Fold the open end over and use string to tightly tie off the end producing a limp sack of sugar.

2. Place the limp sack in a beaker or bowl of water and continue to observe. Every 5 mins. take 1 ml of water from the bowl. Place in a test tube, add an equal volume of Benedict's solution, and heat in a beaker of hot water. For an initial control, test 1 ml of the sugar solution at the same time. The formation of a yellow-orange precipitate is a positive test for sugar. Very small amounts of precipitate will shift the blue color of Benedict's toward green.

B. Osmosis of Plant Cells

While the pseudocell is taking its bath, set up the following brief observation of osmosis with living cells.

1. Take a leaf of Elodea (or a piece of Zebrina lower leaf epidermis, or a thin cross section of Gynura), place in a drop of water, and add a cover slip.

2. Observe microscopically to determine the distribution of cytoplasm (organelles and vacuole) within the living plant cells. The purple pigment in Zebrina and Gynura is in a large central vacuole, which is membrane bound sac within the cell membrane. Elodea also has a central vacuole, but without pigment.

3. Add one or two drops of salt solution to one side of the coverslip. Use a piece of paper toweling to absorb water from the other side so that the salt solution is pulled under the cover slip. Observe what happens to the cell contents within the semi-rigid cell walls.

C. Answer the following questions to help explain the results. Give evidence to support your answers.

1. Is the membrane permeable to water? Y N

Evidence:

2. Is the membrane permeable to sugar? Y N

Evidence:

3. Can osmosis generate a pressure in cells? Y N

Evidence:

4. Hypothesis Formation: Explain osmosis in terms of what happens to the dialysis tubing pseudocell.

5. What does this demonstration allow you to conclude about the size of the pores in dialysis tubing relative to the size of water molecules and sugar molecules?

6. Predict what would happen if a dialysis tubing sack full of water was placed in a beaker of sugar solution?

7. Use these concepts to explain what happens to the water filled vacuole/protoplast of a plant cell when placed in a solution of salt water.

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Part II: Experimental testing of the Osmosis concept

Experiments test logical statements, predictions, derived from explanations, hypotheses, that were constructed to explain observations and phenomena. It is useful to begin by thinking about what would happen under different circumstances. IF a plant cell operates something like an osmometer, THEN under these conditions the cell should ...,etc. In this case, the circumstance is a salt solution, salt dissolved in water. What prediction(s) can you make based upon your explanation?

Remember, experiments consist of treatments and controls. A treatment consists of the application of some variable to a subject or system, and a control is treated identically except for the treatment.

Prediction: If a piece of plant tissue composed of normal plant cells is placed in a salt solution, then ... (write out rest of the prediction in your notebook or report)

The Potato - An Experimental Organism

Individual cells are a little too small to deal with easily, but we can obtain uniform masses of cells, a tissue, to provide an easier specimen to measure. Potato tubers are composed of a relatively uniform mass of living cells forming parenchyma tissue, cells specialized for storage of water and starch grains, and whose cell walls are thin and flexible.

Experimental Tissue Procedure: Use a knife or cork borer (or better yet a french fry maker) to cut potato parenchyma tissue into columns or sticks (basically uncooked fries) of uniform length.

A. Designing an Experiment

1. What will be your experimental treatment(s) and what result is predicted for each?

Treatment:

Predicted Result:

Treatment:

Predicted Result:

2. What will be your control for each treatment and what result is predicted for each?

Control:

Predicted Result:

Control:

Predicted Result:

3. What data will you collect and when will you collect it? What exactly will you record? How will it be labelled?

4. How will you know if your predictions were true or not?

5. How can you increase your confidence in the results of the experiment?

6. Now conduct your experiment for at least 1/2 hr. Record the final data from your experiment in a table. Clearly label the results so that anyone could understand what the data are.

7. Do the results support your prediction(s) or not?

8. Did you get any unexpected results? If so, what?

9. Based upon your experiment, do you accept your hypothesis as true? Have you proven it is true? Or based upon your experiment, do you have to reject or modify your hypothesis? If so, why?

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Part III: Transpiration and Root Pressure

Plants lose water from aerial parts into their environment constantly. Osmosis provides a partial explanation of how plants manage to avoid dessication and dehydration.

A. Observation of Osmotic Pressure from a Pseudocell

1. Take the dialysis tubing out of the water. Fold 2.5-3 cm of one end back on the rest of the tubing and tie off tightly to make an end. Rub your thumb and forefinger together across the other end of the tubing to open the tube. Pour in sugar solution to a depth of 5-6 cm. Insert one end of the glass tube into the dialysis tubing, and use one or more rubber bands to secure a tight fit aroung the glass tube leaving no empty space inside the "balloon" of sugar solution. This should force some of the sugar solution up the tube a couple to several cm, which is OK.

2. Place the pseudocell full of sugar solution into a beaker of water. Use the stand and clamp to hold the glass tube erect. Mark the initial position of the sugar solution in the tube. Measure the height of the solution in the tube in mm every minute for the next 15-30 min. Construct a table for data collection on the Note pages at the end of this exercise.

B. Observation of Guttation

1. What do you observe upon the tips of the seedlings' leaves? When would you be most likely to observe this phenomenon in your yard or garden? Could this exudate be confused with dew?

2. Hypothesis: What is the source of the drops of water?

3. Where does the water come from? What forces the water from the ends of the leaves?

C. Observation of Plant Wilting

Examine two typical plants, one has been well watered, and the other has not. Consider the following questions and write out answers in your report or notebook.

1. What plant parts look and feel different?

2. Explain wilting in simple terms of plant water loss and water uptake. Remember to take into account the observation of transpiration.

D. Observation of Root Pressure

Observe a plant whose leafy shoot has been cut off and replaced by a glass tube drawn out into a long, fine end. Note that the pot is well-watered. The water exiting the end of the tube is collected in a graduated cylinder or in a beaker on a scale. On the adjacent chart, the volume or mass of water collected is being plotted against time.

Add a new data point to the chart at this time.

Write out answers to these questions in your report or notebook.

1. Use osmosis to explain what is happening here? Remember to take into account prior observations.

2. What happens to a plant when the soil drys out? How might this be explained at the cellular level?

3. How does osmosis contribute to plant stature?

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Part IV: Terminology, Concept Development, Synthesis, and Conclusions

Write out answers to the following questions and define the following terms in your report or notebooks.

1. The following terms relate to the observations or concepts that have been developed in the exercise. Look up a definition of each term and use them in your explanations or answers below.

Turgor & Turgor Pressure

Turgid

Flaccid

Guttation

Hypertonic solution

Hypotonic solution

Transpiration

2. Based upon these observations, the hypotheses you have generated and tested, and the conclusions you have drawn, construct a concept, an explanation, of how a simple plant cell functions by osmosis.

3. What roll does osmosis play in maintaining plant stature, and how does this relates to plant wilting.?

PRACTICAL CONSIDERATIONS

4. Why do cooks routinely place carrot and celery sticks in a bowl of cold water for 30-60 mins before serving them as snacks?

5. There are horticultural and agricultural situations where plants encounter salt solutions, for example, fertilizer application. Have you ever heard people say plants get "burned" by too much fertilizer? How would you explain this phenomenon to them? After applying fertilizer, why should plants be watered thoroughly?