Moving Water Through Plants

Technical & Teaching Notes

Related sheets

Plant roots; Plant cells and water; About water; Plant leaves and water; Water and plant structure

Introduction and context

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?Glass and plastic tubes are used to investigate.

Possible barriers to learning

The following are misconceptions that some students may have and which may be a hindrance to further learning:

  • Gravity = ‘downness’.
  • When an object is stationary, no forces are acting on it.
  • Something stops moving because the force has run out. An object will keep on moving in a straight line unless another force is applied to it.

Also, the ubiquitous,in physics especially, of the word ‘object’ might be a barrier since students may not think of water as an ‘object’.

Links to National curriculum for science in England at key stage 3

In biologypupils should be taught about cells and organisation, and nutrition and digestion which include:

  • plants making carbohydrates in their leaves by photosynthesis and gaining mineral nutrients and water from the soil via their roots

In physicspupils should be taught about forces. This includes:

forces as pushes or pulls, arising from the interaction between two objects

opposing forces and equilibrium: weight held by stretched spring or supported on a compressed surface

  • forces being needed to cause objects to stop or start moving, or to change their speed or direction of motion (qualitative only)
  • change depending on direction of force and its size

Safety Notes

No hazardous equipment or materials are used.

Apparatus

Activity 1: Capillary action

Each student or pair will need:

three or four tubes (glass or plastic) of various internal diameters, e.g. 4, 2, 1.5, 1and 0.5 mm – students should be told the diameters

(These may not be readily available and can be quite expensive – see Suppliers below. However, they are only used with water and should last a very long time assuming breakages are avoided)

a transparent dish such as an evaporating basin or a petri dish – it needs to contain water to a depth of about 1 cm

30 cm ruler

Suppliers

There are a number of specialist suppliers, but products are quite expensive.

Melting point tubes could be used. For example:

Teaching Notes

Activity 1: Capillary action

It may be difficult to get tubing with narrow internal diameters. However, capillary tubing with an internal diameter of 0.8 mm is readily available and could be compared with standard glass tubing available in the lab,

This is intended to be a semi-quantitative investigation, so measurements need not be accurate, just sufficient show a relationship between internal diameter of a tube and the height of water rise

Typical results:

Diameter of capillary / mm / 2.0 / 1.5 / 1.0 / 0.5
Capillary rise / mm / 15 / 20 / 30 / 60

Answers

1. The smaller the inner diameter of the tube, the greater is the water rise.

2. No matter what angle to the tube in held at water rises to the same height above the surface of the water in the container.

Activity 2: Analysing data

The histogram

Answers

1. When d is halved, h is also halved.

2. The tube with internal diameter 1.2 mm appears to give an anomalous result for water rise. It might be expected to be between the values for tubes with internal diameters 1.0 and 1.4, so about 25 mm.

3. Students must choose words that reflect an exponential relationship, though they are unlikely to know this term. It is sufficient to say that initially, increasing internal diameter reduces the water rise dramatically. However, the decrease in water rise becomes smaller and smaller as the internal diameter of the tube increases.

4. There are attractive forces between water particles and the surface of the tube. It pulls water particles towards it.

In turn, the forces between these water particles and other water particles. on another and towards the tube. Water rises up the tube as long as these forces are greater than force of Earth’s gravity.

The water stops rising when the forces between water particles and between water particles and the tube are equal to force of Earth’s gravity. The forces are equal but acting opposite directions.

(See diagram on right)

Activity 3: The 10 metre tree problem

This calculation is designed to show students that capillary action alone cannot explain how water rises up the trunk and branches of a tree (nor, indeed, any plant).

Answers

1. Using d ≈ 3 x 10-5 metres

h

h = 10 m

d ≈ 3 x 10-5= 3 x 10-6m = 0.003 mm

10

2. Capillary action plays a part, but there must be other factors involved. A second is transpiration. When water is drawn through the plant and reaches a leaf, it evaporates from the surface, changing state from liquid to gas and escaping into the atmosphere. A third is osmotic pressure in plant cells. Water is absorbed through a plant’s root cells. This increases the osmotic pressure inside them. Water then moves from those cells to neighbouring cells, reducing the osmotic pressure in the root cells, but increasing it in the neighbouring cells. More water is absorbed into the root cells and this in turn is moved by differences in osmotic pressure to the neighbouring cells. A chain reaction is set up throughout the trunk and branches.

So movement of water is a result of three things:

  • capillary action
  • respiration
  • osmotic pressure.

Activity 4: Sucking water from the sky

Students read an article on the SAPS website, at

The recent research showed that our ‘water gets in through the roots’ understanding of the movement of water through plants is incomplete. Under certain specialised conditions, some plants have evolved the ability to absorb water through their leaves, move it down the xylem, and them release it into the soil. The plants are actually watering their own roots - and their own seedlings. This off-beat mechanism for water uptake works well enough that these plants can continue to photosynthesise and grow, even when the soil they are growing in are dry.

Science & Plants for Schools:

Moving Water Through Plants: p. 1