On Country
How Water Moves in Plants
Background
• Plants need water to maintain water pressure inside cells (called turgor) in order to maintain structure and cell growth. Most water passes out from the leaves (transpiration) by way of holes (called stomata) in the leaf’s under-surface; only a very small amount is used in the synthesis of food.
• With the passage of dry air across the stomata, a water gradient is set up. Loss of water from stomata and their surrounding cells, causes a decrease in the pressure of the water (hydrostatic pressure) in the water channels in the leaf. This decrease in hydrostatic pressure acts like a stretched spring, creating a negative pressure at the leaf end, pulling columns of water up through xylem channels in the stem.
• The continuity of the water columns remains intact due to two forces. Water molecules are composed of 2 hydrogen atoms (which are slightly positive) and an oxygen atom (which is slightly negative). This means that each end of the water molecule is attracted to the opposite end of another water molecule. As a result, water molecules are cohesive. Water molecules also stick well to other substances, called adhesion. If this force is stronger than cohesive forces, then water spreads out, sticking to the other substance.
• In a very narrow channel, like a capillary tube, or a conducting tissue in a plant stem, the adhesive forces are stronger than the cohesive forces between the water molecules themselves, and water is drawn upwards, against the force of gravity.
• Once this pull (it has been called ‘transpiration pull’, or ‘capillary action’) reaches the roots, water is drawn in through the root cell membranes, either by diffusion (passive absorption) or by osmosis (see Glossary). Figure from: https://www.nature.com/scitable/knowledge/ library/water-uptake-and-transport-in-vascular plants-103016037/
Activity
Understand the Structure of Plant Tissues:
Show ‘Professor Dave Explains’ on the YouTube clip:
https://www.youtube.com/watch?v=M-qDzKG3RB0
This provides an excellent outline of the different cells and tissues that make up a plant. It also explains the forces that control the passage of water through the tissues, as demonstrated in osmosis, transpiration pull and capillary flow.
• Have students draw diagrams of the various tissues, leaf and leaf veins, stem, root, and outline their function for transporting water to the leaf cell.
Activity
Demonstrate Osmosis:
From: https://www.homeschool.com/blog/2023/01/ homeschool-science-gummy-bear-osmosis/
AIM
To demonstrate the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell’s partially permeable membrane.
MATERIALS
• Gummy bears (available from Supermarkets)
• A small glass for each liquid
• Solids Baking soda Sugar Salt
• Liquids Tap water, salt water, sugared water Soda Vinegar Milk
• Paper towels
• Kitchen scale
• Ruler
• Paper/Pen
• Timer
METHOD
1. Have 6 small glasses, each one labelled for a different kind of liquid (tap water, salt water, sugar water, milk, vinegar, soda).
2. Add a half cup of water to the tap water glass.
3. Add a half cup of water and 1 tablespoon of salt to the saltwater glass. Stir the salt until it is dissolved.
4. Add a half cup of water and 1 tablespoon of sugar to the sugar water glass. Stir until the sugar is dissolved.
5. Add a half cup of water and 1 tablespoon of baking soda to the baking soda water glass. Stir until the baking soda is dissolved.
6. Add a half cup of vinegar to the vinegar glass.
7. Add a half cup of milk to the milk glass.
Teacher note - Any of these liquids can be omitted or changed according to your preference.
1. Weigh each gummy bear and record results.
2. Add one gummy bear to each glass of liquid.
3. Set a timer for 12 hours.
4. After the timer goes off, remove each gummy bear from its solution, pat dry, weigh, measure, and compare to a new gummy bear.
5. Discuss the results and download the WORKSHEET from: https://www.homeschool.com/wp-content/ uploads/2023/01/gummy-bear-osmosis-printable lab-notes-1.pdf
Questions to ask
• Why did the gummy bears grow in the liquid?
• Which liquid grew the gummy bears the most?
• Which gummy bear changed the least?
• What caused these differences? (the different solutes in each liquid).
• What did you learn from this experiment? Can you explain the concepts of diffusion, equilibrium, osmosis and isotonicity?
Activity
Demonstrate Transpiration:
AIM
To demonstrate the passage of water from leaves.
MATERIALS
• Plastic bags
• Ties
• Schoolyard tree
• Large-leafed garden plant, for comparison
METHOD
1. Choose leaves at the end of a small branch.
2. Enclose as many as possible inside the plastic bag,
3. Close the plastic bag around the leaves by tying it tightly to the branch, making sure water vapour cannot escape.
4. Leave for seven days, or until measurable water has accumulated in the bottom of the bag.
5. If a large-leaved garden plant is available, it serves as a comparison.
6. Carefully pipette the water into a small measuring cylinder, and record the volume.
7. Count the number of leaves in the bag.
8. You may be able to calculate volume water transpired per leaf.
9. Write up the experiment, expressing results as the volume of water transpired by the plant (per leaf) over the time of collection. If a comparison is made, conclude possible reasons for differences in the volume of water transpired.
Activity
Demonstrate Capillary Action:
Revise how water moves in plants.
AIM
To demonstrate the passage of water upwards in leaf stems.
MATERIALS
• A head of celery (stems with leaves)
• A thin slice of celery (called a cross-section)
• 2 jars, or plastic water cups
• Water
• 2 different food dyes
• Microscope
METHOD
1. Examine the cross-section of celery under the microscope. Draw and describe what you see.
2. Half fill each water container (jar) with water. Add one food colour to one container and the other food colour to the second container.
3. Split the celery stalk in half, up to the leaves. Place 1/2 of the stem in one water container and the other half in the second
4. Label the containers with your name and put them in a safe place
5. Predict how you think water travels up the celery stem (the capillarity forces of adhesion and cohesion) and what you expect to see after the coloured dye has had time to move up the stem to the leaves
6. Examine the next day; the coloured water should have moved up the celery stems to the leaves.
7. Draw and describe what you see, explaining your observations.
8. Cut a thin slice of celery from each of the different coloured stems, observe each slice under the microscope, draw and describe what you see.
9. Explain why only some parts of the stem have changed colour, what do you think these parts are?
Demonstrating transpiration, Mt Pleasant PS, Photo Stephen Clee