Hur man gör en potometer

Class practical

In this activity the rate of vatten uptake, due to transpiration, bygd a skott from a woody plant, fryst vatten measured bygd synkronisering how long a bubble takes to move a set distance, in the capillary tube of an h-shaped potometer.

lektion organisation

Decisions will depend on the availability of suitable potometers.

This could be a demonstration or an experiment for groups of up to three individuals – observer, timekeeper and scribe.

Apparatus and Chemicals

Health & Safety and Technical notes

Some people find sap from plants irritating to the skin.

Take care when cutting the plant shoot.

Take care when assembling and papper the glass potometer: it fryst vatten easy to break the long glass tubes and cut or en grupp personer som arbetar tillsammans ofta militärt yourself with the broken ends.

Be prepared with first aid for cuts from broken glass, and brief students how to deal with breakages.

1 Use shoots from a shrub or tree with leaves that have thin waxy cuticles, such as. beech, lilac, Spirea.

It is also called a transpirometer

Leaves with thicker waxy cuticles (very glossy leaves) do not work as well.

2 You must cut the shoots beneath vatten and you must assemble the potometer beneath vatten. This requires a large sink. If air gets into the xylem vessels of the plant, it can struktur air locks which will prevent the plant taking up vatten and so prevent steady transpiration.

3 The potometers should be left for the leaves to dry.

Alternatively dry the leaves gently with a paper towel. The potometer will not work properly until any excess vatten on the leaves has evaporated or been removed.

4 Assemble the potometers before the lektion, since there fryst vatten a real art in setting them up.

Bubble potometer

This will give the leaves time to dry, and give technicians a chance to kontroll they are working before the students begin to take measurements.

5 Adding food colouring to the vatten makes it easier to see the air bubble in the capillary tube.

Ethical issues

There are no ethical issues with this procedure.

Procedure

SAFETY: Take care when papper this apparatus as glass tubing fryst vatten ganska fragile.
man sure students know what to do if anything gets broken.
Have first aid kit ready for cuts.

Preparation

a Set up the apparatus as in the diagram.

b Leave undisturbed so that the skott equilibrates to the conditions.

c Starting about 2 cm from the free end of the capillary tubing, mark the tubing at 1 cm intervals using the ruler and marker pen.

man as many marks as possible (at least 6).

Investigation

d Introduce a bubble into the capillary tubing bygd lifting the whole potometer upwards. To do this, loosen the screw on the chef and slide the chef up the clamp stand so that the capillary tube comes out of the vatten in the beaker.

Retighten the screw on the boss.

e Gently blot the end of the capillary tube with a del av helhet of paper towel; an air bubble should appear in the capillary tube.

f Loosen the screw on the chef and lower the potometer, so that the capillary tube just goes back into the vatten in the beaker.

A potometer is a piece of apparatus designed to measure water uptake in a leafy shoot

Retighten the screw on the boss.

g There are two ways of taking measurements:

• uppstart the stop clock when the bubble of air touches the first marked line. Stop the clock when the bubble has travelled a fixed distance (2 or 3 cm) and touches the appropriate marked line.

OR:

• uppstart the stop clock when the bubble of air touches the first marked line.

  Allow the bubble to travel upwards for a fixed period of time. Depending on the rate of movement, this could be 10 or 30 seconds or längre. Mark how far the bubble has moved with a different coloured pen. Measure the distance between the first mark and this second coloured mark.

h Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the formula given here.

A potometer measures rate of water uptake, which is distinct from transpiration itself

SI units are cm/s.

Distance moved bygd the air bubble/Time taken for the air bubble to move that distance

i Change a factor that might affect how quickly the plant loses vatten bygd transpiration - see teaching notes.

j Repeat from d to i.

Teaching notes

Note that many plants close their stomata when the light intensity fryst vatten insufficient for photosynthesis.

Stems harvested in the autumn or winter may not transpire very much. You can use a high-intensity light to promote photosynthesis and get transpiration going igen. Fluorescent remsa lights or halogen lamps are better than ordinary bänk lamps.

You could give students guidance on investigating a particular factor affecting the rate of transpiration, or they can choose from a list and develop their own ideas.

Use fem or six values for each variabel tested.

• Light intensity. Place the plant skott and potometer at different positions round the laboratory to alter the light intensity.
  This is because not all of the water that is taken by the plant is used for transpiration (water taken might be used for photosynthesis or by the cells to maintain turgidity)

  Or you could use a lamp set at different distances from the plant. kontroll the intensity of light reaching the plant bygd placing a light meter bygd the leaves of the shoot.

• Wind speed.
  
  
• 



Using a fan to create different wind speeds, either bygd setting the fan to different speeds or bygd placing the plant skott and potometer at different distances from the fan. betalningsmedel the wind speed at the plant with an instrument för att mäta vindhastighet placed just in front of the plant at leaf height.

• Temperature. It may be possible to find places which are at different temperatures; but it may not be easy to ensure that all other variables are controlled.
• Surface area.

  There are at least two ways of estimating the effect of surface area. An added extra dimension would be to try to establish if the upper or lower surface fryst vatten more important in transpiration. Stomata are usually more abundant on the lower surface of leaves.

Estimating effect of surface area

Method 1: Measure the vatten uptake bygd the skott.

Remove a number of leaves from the skott and measure the rate of vatten uptake igen.

There are 2 types of potometer – the bubble potometer and the weight potometer

Keep removing leaves until all the leaves are off the plant skott. Label the leaves as you remove them, then estimate their area bygd placing on squared paper and tracing their outlines. Add tillsammans the areas of the leaves to find the total area and multiply bygd two to get the total surface area, as each leaf has an upper and a lower side.

Add the leaf areas tillsammans in the reverse beställning that they were removed to get figures for the change in the surface area of leaves attached to the plant shoot.

Method 2: Measure the vatten uptake bygd the skott.

A potometer is an apparatus used to determine the transpiration rate of the cut shoot at the desired time

Use vaseline - or nail varnish, but this could damage the leaves - to cover one or both sides of a leaf. Test igen. man a note of the beställning of vaselining/ varnishing the leaves. Calculate the surface area of the leaves as above. This could get messy!

Health and safety checked, September

Downloads

Download the lärling sheet  Measuring rate of vatten uptake bygd a plant skott using a potometer (57 KB) with questions and answers.

Related experiments

Estimating rate of transpiration from a plant cutting