Plants survive because of two main support systems that come from the ground – water and nutrients. Both enter the plant through its roots by a process called osmosis, the diffusion of molecules (water for instance) across a cell’s semi-permeable membrane (wall). This process in plants is also called passive transportation because water passes through the cell’s membrane from an area of high concentration area (outside the cell) to an area of low concentration (inside the cell) without the need of a catalyst.
As water passes through the plant’s cell wall membrane, it attempts to equalize the amount of water on both sides of the membrane. This is called osmotic pressure (passive transport). Plant cell walls allow water to pass freely back and forth across cell walls until the pressure is equal on both sides – called dynamic equilibrium. Osmosis allows plants to pass water throughout the plant to keep cells alive in roots, stems, and leaves.
This is a three part investigation designed to prove the osmosis process to students. They will observe the effects water passing through roots, stems, and leaves. Students use problem solving and science process skills in this investigation.
Osmosis Investigation in Plant Roots
Materials (per group): one potato, skewer, tablespoon, sugar, water, clear drinking straw, two beakers
Procedures:
- Place one tablespoon of sugar and two tablespoons of water in a beaker one and stir until sugar is dissolved.
- Use skewer to bore a hole the size of the drinking straw in the potato, then place the straw in the hole.
- Fill the drinking straw one quarter full of sugar water (sucrose) solution, which acts as a plant’s sugary sap.
- Place the potato in beaker two and fill the beaker with water enough to cover the potato.
- Observe what happens over a period of several minutes
Student Questions:
- Ask students to describe their observations using their own words.
- Ask students to explain the osmosis process as it relates to this investigation.
(Students will observe a rise in the level of sugar solution in the straw.)
Investigating Osmosis in Plant Stems and Leaves
Materials (per group): three beakers, red food coloring, water, knife, one celery stalk, one fresh leaf, one fresh flower (carnation is best)
Procedures:
- Make a fresh cut on each cut end of the celery stalk, flower stem, and leaf stem
- Fill each beaker three quarters full with water
- Add enough food coloring to each beaker to make a strong colored solution
- Place the celery stalk in beaker one
- Place the flower stem in beaker two
- Place the leaf stem in beaker three
- Observe what happens over a period of several minutes
Student Questions
- Ask students to describe their observations using their own words.
- Ask students to explain the osmosis process as it relates to this investigation.
(Students will observe the colored water passing through the cell walls of the celery stalk, stem and petals of flower, and stem and leaf veins.)
Transpiration the Final Osmosis Process in Plants
Materials (one per class): one potted plant, one clear plastic bag to cover plant, tape or large rubber band
Procedures:
- Cover the plant with plastic bag, making sure no air can escape
- Place in a warm sunny place several hours before the osmosis investigations begin
Student Questions:
- Ask students to describe their observations of the plant in the clear bag.
- Ask students how the water droplets appeared on the inside of the plastic bag.
(Students will observe water droplets on the inside of the bag. Water droplets are a result of the water escaping from the cell walls of plant leaves into the air by osmosis, called transpiration. Transpiration allows more water to enter the plant through its roots to continue the cycle.)
Making Connections
Now it is time for students to make the connection between all three investigations. Have students explain the osmosis process from the time water enters the plant roots and escapes the leaves. Conduct a class discussion; use 20 Questions to Ask Students in Science Projects as a guide. Now have each student describe the osmosis process using their own words and drawings in a journal, using Writing in Science as a guide.
David R. Wetzel - Dr. David Wetzel's experience includes more than 25 years in continuing, adult, and teacher education.
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