Turgor Pressure and Capillary action Introduction: Turgor Pressure: also called turgidity, is the main pressure of the cell contents against the cell wall in plant cells. Turgid plant cells contain more water than flaccid cells and exert a greater osmotic pressure on its cell walls. Turgor is a force exerted outward on a plant cell wall by the water contained in the cell. This force gives the plant rigidity, and may help to keep it erect. Turgor can result in the bursting of a cell. This demonstration should be carried out after demonstration 8 on osmosis. This demonstration can be adapted (as shown in the notes section below) to demonstrate the effect of placing a plant in different solutions (hypertonic, hypotonic or isotonic) due to osmosis. Turgor Pressure Diagram showing relationship of Osmotic Pressure (OP), Turgor Pressure (TP) and Wall Pressure (WP) The actual pressure exerted by the protoplasm of turgid cell against cell wall is the turgor pressure. Also Known As: Turgidity Concepts explored in this demonstration: Turgor pressure; plant structure; plants; capillary action; physiology; water This science activity uses matches and a few drops of water to demonstrate the concept of capillary action and turgor pressure. Younger students will be amazed, and older students will enjoy the mystery of why this happens. This short activity can easily be done as a class starter or be expanded with questioning and research to create a full laboratory experiment. In the Classroom: Have students work with a partner to set up the experiment as a class starter. Draw a picture of what the matches should look like at the start on the board. Have students recreate their own and add water. Have them record observations before and after the addition of the water. Have them hypothesize why the experiment caused a star of matches to be formed. Allow students class or outside of class time to explore the question further. Discuss the concepts of turgor as they relate to plants and capillary action of water transport in trees to help cement the concepts for your students. Level: Senior cycle Topic area: Water transport in plants- Turgor pressure and Capillary action 1
HEALTH & SAFETY: No issues should arise. Make sure students don t misuse the matchsticks as they can have sharp edges after bending them. Waste disposal: All apparatus and items used can be disposed of in normal refuse bins. Apparatus: Matchsticks An Eye-dropper Water A plate Chemicals: None Procedure: 1. Bend five matchsticks in the middle. Be careful not to break them. 2. Arrange the matchsticks on the plate so they are all touching, with the bends in the centre. It should look like a five-pointed asterisk. 3. Use the dropper to place three or four drops of water in the centre of the matches. 4. Watch the matches for a couple of minutes. What happens? 5. The matchsticks should straighten up a little bit, turning the asterisk into a star. Alternative Method: After a couple of mins: Trace and cut out two flower patterns (see patterns below). 1. Fold each leaf of flower into centre. 2. Float one flower on top of water and rest the other on the table. 3. Get students to observe what happens to both flowers. 4. Invite student to try and explain what they observed. 2
Questions: 1. Why is it important to keep flowers in a vase of water? 2. If you wanted to make your plant very limp what type of solution would you place it in; hypertonic, hypotonic or isotonic? 3. In the alternative method, explain what happens to the two flowers. Notes/Tips: (including literature) There are actually two processes happening here. The first is capillary action. The matchsticks are made of dry wood. Most of the water has been dried out from the cells of the wood, leaving empty space behind. There are also gaps between the wood cells. The surface tension of the water pulls the water into these gaps, so it is sucked into the wood of the matchsticks. This capillary action leads to the second process. When you bent the matchstick, the cells and the gaps between them were squashed at the point where the wood bent. As the water filled the gaps inside and between the cells, the pressure of the water pushed out on the inside of the gaps, so they tried to expand back to their original shape. When the pressure of a fluid inside an object pushes it into a certain shape, it is called turgor pressure. The turgor pressure was enough to slightly straighten the matchsticks, so they pushed against each other to form the star shape. Applications Living things use turgor pressure inside their cells to hold the cells in shape - like a balloon blown up hard inside a sock or washing-up glove. If plants do not receive enough water, they will go limp because there is not enough pressure in the cells to maintain their shape. Alternative Method: What's Happening? Plant cells are filled with water. This makes the cells firm and causes the plant to be very rigid (plants wilt when they are deprived of water the cells collapse causing leaves and stems to droop). The pressure of the water inside the plant cells is called turgor pressure. Some flowers, like Morning Glories, open and close because of changes in the amount of water inside their cells. This water movement through the petals of flowers and the paper flowers you have made is due to capiallary action. Plants, like paper, have tubes and tube-like structures through which water can move. As the water enters, the turgor pressure produced causes the paper structure to spread open. 3
EFFECTS OF OSMOSIS ON CELLS WHEN PLACED IN DIFFERENT SOLUTIONS SOLUTION TYPE ISOTONIC HYPOTONIC HYPERTONIC Concentration of dissolved substances (solutes) Same as the living cell Lower than the living cell Higher than the living cell Concentration Gradient Results in Animal Cells Results in Plant Cells Zero No change No Change Net water movement into the cell Net water movement out of the cell Cell swells and bursts Cell shrinks * plasmolysis ** Pressure created by excess water stored in central vacuole Central vacuole collapses and plasmolysis occurs * Plasmolysis - The shrinkage of cytoplasm resulting from loss of water by osmosis, in a cell placed in a hypertonic solution. ** Turgor Pressure - In plant cells, the pressure on the cell wall that results because of the influx of water in osmosis. Extension: Demonstration of celery in hypotonic and hypertonic solutions Celery after standing in distilled water for 4 hours Celery after standing in salt water after 4 hours - absorbed the water and was very stiff and rigid - the bottom 1/4 has turned very soft and mushy 4
- hypotonic solution - hypertonic solution The crunch when you bite into a stalk of celery is partly due to the fact that its cells are full of water, which keeps the cell walls firm. If you place a stalk of celery in an empty glass and leave it for a day. The next day the celery will be dropping and limp because the cells lost water by evaporation but have no water to replenish its cells. If you now add some water to the glass, within a short period of time, the celery will absorb the water and straighten up again. By placing the celery in a glass of water with salt in (hypertonic to the cells in the celery), water will move from the cells of the celery into the surrounding solution due to osmosis. This causes the celery to appear limp or flaccid. Likewise if you now place the celery in a glass of distilled water, water will move into the cells of the celery again due to osmosis and the cells will become stiff or turgid. Web Links: http://www.csiro.au/portals/education/programs/do-it-yourself-science/physics-experiments/movingmatches-activity.aspx http://www.letstalkscience.ca/educators/handsactivities.html?sobi2task=sobi2details&catid=8&sobi2id=9 http://legacy.teachersfirst.com/tchr-subj-date.cfm?var1=30&var3=9&var2=12&offset=21 5