Moving Water Through Plants
Students’ Sheet
Introduction
Capillaries are very thin tubes. In your body, for example, there are three types of blood vessels – arteries, veins and capillaries. All are tubes and capillaries are the smallest and thinnest of them. The largest artery, the aorta, is 2-3 cm in diameter. Capillaries are about 0.005-0.01 mm in diameter.
In plants, xylem tubes carry water from its roots to all other parts of the plant. The diameters of xylemtubes are about0.2-0.5 mm.But the question is, how does water ‘climb up’ plants, apparently defyinggravity?There are lessons to be learned from glass and plastic tubes.
Activities
Activity 1: Capillary action
You will be given three or four tubes (glass or plastic) of various internal diameters, for example,4 mm, 2 mm, 1.5 mm, 1 mm and 0.5 mm. Your teacher will tell you what the internal diameters are.
1. Pour some water into a transparent dish to a depth of about 1 cm.
2. Put the tubes into the water, holding them just above the bottom of the dish.
3. Measure and record the height of the water in each tube above the level of the water in the dish.
Diameter of capillary/ mmCapillary rise/ mm
4. Use the tube that gave the highest rise of water to investigate what happens if it is held at different angle in the water.
Questions
1. Describe how the height water rises depends in the internal diameter of the tube.
2. What happens if a tube is held at different angles?
Activity 2: Analysing data
It’s difficult to carry out experiments with tubes that have very small internal diameters, so here is a set of data that shows the capillary risefor 12 tubes of various internal diameters.
d / mm / 0.05 / 0.10 / 0.20 / 0.40 / 0.60 / 0.80 / 1.00 / 1.20 / 1.40 / 1.60 / 1.80 / 2.00h / mm / 600 / 300 / 150 / 75 / 50 / 38 / 30 / 35 / 21 / 19 / 17 / 15
To spot trends in data it is often helpful to use histograms, bar charts or graphs.
Draw a histogram to show how the height water rises (h) in millimetres varies with the internal diameter of the tube (d) in millimetres.
Questions
1. Examine the data and say what happens to h when d is halved (remember that the equation used gives approximate values for h).
2. Which result looks anomalous (odd one out)?
3. Describe in your own words what the column chart tells you.
4. In a small group discuss why you think water rises and why to only a certain height depending of the diameter of the tube. Here are some things to consider:
- Anything that has mass, including water, is pulled towards Earth’s centre by gravity.
- Water consists of particles. There are attractive forces between water particles (remember the particle model of matter).
- There are attractive forces between water particles and the surface of the tube.
Activity 3: The 10 metre tree problem
If the height water rises (h) is plotted against the internal diameter of a tube (d) the line of best fit is a curve. You could try it if you have time.
It can be worked out that the equation for the graph is h ≈ 3 x 10-5 metres
d
The sign ≈ rather than an equals sign (=) means the relationship is approximate, not exact.
Making d the subject of the equation gives
d ≈ 3 x 10-5 metres
h
Questions
1. Show that a tube with an internal diameter of 0.003 mm is necessary to give a capillary rise of 10 metres.
Remember: 1 m = 1000 mm (1 mm = 0.001 m)
2. The diameters of xylem tubes are 0.025 mm, about 10 times wider than if capillary action was the only factor that accounts for water rises. What does this tell you about the movement of water in plants?
Activity 4: Sucking water from the sky
You probably know that water from the soli is absorbed by a plant’s roots and moves up through its roots and stems of a plant to the leaves where it is lost to the atmosphere by evaporation.
Read the articleHow plants can suck water from the sky( Explain how the research described in the article has added to our understanding of how water gets into some plants.
Science & Plants for Schools:
Moving Water Through Plants: p. 1