precautions for potometer experiment

Potometer Experiment

A Potometer experiment is performed to estimate the transpiration rate in different plants. A potometer is an apparatus used to determine the transpiration rate of the cut shoot at the desired time. It is also called a transpirometer .

Using the potometer, we can record the readings of water uptaken by the cut shoot. There are two causes of water uptake. Plants uptake water to perform two major activities. One is transpiration, and the other is photosynthesis.

Transpiration is the evaporation of water by the leaves through the stomatal activity. Plants absorb sunlight, CO2 and H2O to give oxygen and carbohydrates during photosynthesis .

Thus, we can only get a rough idea about the transpiration rate using a potometer. But, the actual transpiration rate may be lower than the value indicated by the potometer. Because water loss can be due to both transpiration and photosynthesis.

Bubble and mass potometers are common devices used to measure transpiration in plants. A bubble potometer measures water absorbed by the shoot. Mass potometer measures the water loss by the shoot.

The working of a potometer depends upon the amount of water absorbed by the plant. This post discusses the purpose, design and working of the photometer. Also, the aim, requirements and steps are explained to measure transpiration using Ganong’s photometer.

Content: Potometer Experiment

Requirements of the potometer experiment, potometer experiment steps, factors affecting transpiration.

• Calculation

Precautions

Limitations, what is potometer.

A potometer is a set-up that helps in estimating the transpiration rate and factors affecting transpiration. We can measure the transpiration rate in plants by knowing the amount of water absorbed by the plant. Here, the amount of water absorbed is equal to the water loss during the transpiration by plants. Also, we can study the effect of different external variables on transpiration rate.

What is the Purpose of the Potometer?

A potometer aims to measure the water absorbed by the plant, which is equal to the value of transpiration.

What is the Design of the Potometer?

• Capillary tube : Here, a bubble travels a distance due to water absorption by the plant. We can measure water uptake through regular gradations on the tube.
• Reservoir : It is like a funnel with a tap. We can reset the bubble by turning on the tap. Some potometers also use a syringe in place of a reservoir.
• End tube : It holds the shoot. The stem end must be in touch with the water. Additionally, we need to fix the cork Bauer on the end tube to avoid air bubbles.

How does a Potometer Work?

There are two main types of potometers.

• In a bubble photometer , we require a leafy shoot fixed within the end tube. Then, over a specific time, an air bubble travels a distance that is equal to the water uptaken by plants.
• In a mass photometer , we require a plant with its root submerged in a beaker. This beaker is then placed on a digital balance. Readings depend upon the amount of water lost by the plant.

Thus, we can measure transpiration by recording the change in the volume of water taken up or the change in mass.

• Potometer to conduct the experiment and to take out the readings.
• Water bucket to submerge the apparatus.
• Healthy shoot to study the rate of transpiration.
• Vaseline seals all the connection points.
• Cutter cuts the shoot.
• Adhesive tape secures the airtight seal.
• Stopwatch is used to set the time.

• Bench lamp to know the effect of light intensity.
• Table fan to know the effect of wind speed.
• Polythene to know the effect of humidity.
• Dryer to study the effect of temperature.
• First, submerge the whole apparatus in the sink or water tray to remove air bubbles.
• Then, cut the plant shoot underwater and insert it into the hole of the cork Bauer fixed to the end tube. This step should be quick to avoid entering any air into the plant’s xylem.
• After that, attach the whole unit to the capillary tube underwater.
• Then, use vaseline at all the connections to ensure an airtight environment. Also, we could use adhesive tape to secure the airtight seal.
• After that, we can take out the potometer apparatus and mount it on the wooden base. Note : If any bubbles appear, restart the whole procedure to get accurate results.
• Then, place the open end of the capillary tube into a beaker. The capillary tube has graduations in millimetres.
• Add colour to the water to see the distance travelled by the air bubble within the capillary tube.
• Introduce an air bubble by dipping the capillary tube out of and back into the beaker containing water.
• Note down the distance of an air bubble before the experiment. Then set the time in the stopwatch. Allow the air bubble to move at a given time.
• Finally, record the reading by looking at the distance an air bubble moved.
• We can also repeat the experiment by refilling the system by turning the glass stop cock. During this stage, we can reset or adjust the air bubble.

We can also investigate the transpiration rate under different environmental factors. Below, we will discuss the effect of different variables on transpiration.

Light Intensity

We can study the effect of light intensity by using a bench lamp at different distances. High light intensity causes turgidity of guard cells, allowing stomatal opening. It permits more water loss .

To know the effect of wind speed on transpiration, we can use a table fan . By using an inch tape, we could place the fan at different distances to study the effect of high and low wing speed. More wind speed removes water vapour from the air surrounding the leaf. Thus, it generates a steep concentration gradient between the leaf and water. It leads to an increase in water loss or transpiration.

We can wrap a polythene bag around the leaves to know how the humidity affects transpiration. When the humidity is high , the air concentration saturated with water vapour is high. As a result, the diffusion of water vapour from the leaves reduces. Here, the concentration gradient is smaller, resulting in a less net flow of water vapours .

Temperature

To know how the temperature affects transpiration, we can use a dryer . Higher temperature increases the water molecules’ kinetic energy, resulting in high transpiration rate . Water molecules evaporate from the mesophyll and diffuse away faster.

How does a potometer measure transpiration?

Using a potometer, we could estimate the transpiration rate in two ways:

• Indirect Method : Here, we need to measure the reduction in the volume of water in the capillary tube over some time.
• Direct Method : Here, we need to measure the loss in mass of the potometer over some time.
• Plant shoot must be fresh and healthy.
• The whole apparatus must be airtight.
• The cut stem must be fixed to the end tube underwater.
• Leaves of the desired shoot must be dry before cutting.
• There should not be any air bubbles in the apparatus.
• It measures the transpiration rate by estimating water uptake by the plant.
• It helps to study the effect of variables like light, humidity, temperature and wind speed on the transpiration rate.
• It isn’t easy to set up, as an air bubble may alter the results.
• The potometer does not give accurate results. Because not all the water that the plant takes is used for transpiration. Plants may take water for photosynthesis or to maintain cell turgidity as well.
• The cut shoot does not remain active for a long time.
• The air temperature outside may change the position of the air bubble within the capillary tube.

Thus, a potometer is used to calculate the amount of water that the cut shoot has uptaken. The water uptake by the plant gives the value of water loss or transpiration rate in a given plant.

Related Topics:

• Corolla in Plants
• Life Cycle of Bryophytes
• ATP Synthase in Photosynthesis
• Characteristics of Mushrooms

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Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Observing earthworm locomotion

Practical Work for Learning

Published experiments

Measuring rate of water uptake by a plant shoot using a potometer, class practical.

In this activity the rate of water uptake , due to transpiration , by a shoot from a woody plant, is measured by timing how long a bubble takes to move a set distance, in the capillary tube of an h-shaped potometer .

Lesson 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

For each working group – equipment to investigate factors affecting rate of transpiration:

Light meter

Vaseline or nail varnish

Graph paper

For each working group – basic equipment:

Plant material ( Note 1 )

Large sink – to assemble apparatus under water ( Note 2 )

Potometer – designs vary

Scalpel or scissors, 1

Clamp stand with boss and clamp, 1

Marker pen, 1

Stop clock, 1

Plastic ruler

Paper towels

Thermometer (-10 °C to 110 °C), 1

Food colouring, optional ( Note 2 )

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 handling the glass potometer: it is easy to break the long glass tubes and cut or stab yourself with the broken ends. Be prepared with first aid for cuts from broken glass, and brief students how to deal with breakages.

Read our standard health & safety guidance

1 Use shoots from a shrub or tree with leaves that have thin waxy cuticles, such as. beech, lilac, Spirea . Leaves with thicker waxy cuticles (very glossy leaves) do not work as well.

2 You must cut the shoots under water and you must assemble the potometer under water. This requires a large sink. If air gets into the xylem vessels of the plant, it can form air locks which will prevent the plant taking up water 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 water on the leaves has evaporated or been removed.

4 Assemble the potometers before the lesson, since there is a real art in setting them up. This will give the leaves time to dry, and give technicians a chance to check they are working before the students begin to take measurements.

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

Ethical issues

There are no ethical issues with this procedure.

SAFETY: Take care when handling this apparatus as glass tubing is quite fragile. Make 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 shoot 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. Make as many marks as possible (at least 6).

Investigation

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

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

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

g There are two ways of taking measurements:

• Start 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.
• Start 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 longer. 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. SI units are cm/s.

Distance moved by 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 water by transpiration - see teaching notes.

j Repeat from d to i .

Teaching notes

Note that many plants close their stomata when the light intensity is 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 again. Fluorescent strip lights or halogen lamps are better than ordinary bench 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 five or six values for each variable tested.

• Light intensity . Place the plant shoot and potometer at different positions round the laboratory to alter the light intensity. Or you could use a lamp set at different distances from the plant. Check the intensity of light reaching the plant by placing a light meter by the leaves of the shoot.
• Wind speed . Using a fan to create different wind speeds, either by setting the fan to different speeds or by placing the plant shoot and potometer at different distances from the fan. Check the wind speed at the plant with an anemometer 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 is more important in transpiration. Stomata are usually more abundant on the lower surface of leaves.

Estimating effect of surface area

Method 1 : Measure the water uptake by the shoot. Remove a number of leaves from the shoot and measure the rate of water uptake again. Keep removing leaves until all the leaves are off the plant shoot. Label the leaves as you remove them, then estimate their area by placing on squared paper and tracing their outlines. Add together the areas of the leaves to find the total area and multiply by two to get the total surface area, as each leaf has an upper and a lower side. Add the leaf areas together in the reverse order 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 water uptake by the shoot. Use vaseline - or nail varnish, but this could damage the leaves - to cover one or both sides of a leaf. Test again. Make a note of the order of vaselining/ varnishing the leaves. Calculate the surface area of the leaves as above. This could get messy!

Health and safety checked, September 2008

Related experiments

Estimating rate of transpiration from a plant cutting

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Transpiration

Instructor prep, student protocol.

• Comparing Transpiration Rates of Leaves
• To begin the experiment to measure the transpiration rates of leaves from different plant species, you will first create a tool called a potometer, using a piece of plastic tubing and 5 mL calibrated pipette.
• To assemble the device, simply attach the rubber tubing to the tip of the pipette.
• Then, submerge the device into a bucket of water and move the pipette around under the water to fully fill the tube, taking care not to create bubbles.
• Next, cap the open end of the tube with a pipette bulb to keep the water in.
• Now, taking care not to spill water from the potometer, use a clamp and stand to hold the capped end of the pipette around 1 – 3 inches below the pipette bulb.
• Then, use a second clamp attached lower on the same stand to secure the open end of the rubber tubing.
• Next, use a transfer pipette or dropper to remove water from the open end of the tubing until the water level in the pipette portion of the potometer rests at the zero line. HYPOTHESES: In this experiment, the experimental hypothesis might be that leaves from plant species more adapted to hotter or arid environments will have lower transpiration rates than those from humid or wet regions. The null hypothesis would be that leaves of the different plant species will not differ in their transpiration rates.
• To begin the experiment, carefully use scissors to cut the end of the petiole, or stalk, of the first leaf and then insert the leaf into the tubing.
• Then, place a layer of lubricant around the area where the plant stem and rubber tubing meet to make a watertight seal.
• Next, remove the pipette bulb and taking care not to disturb the setup, let the experiment run for the appropriate experimental period, typically 30 – 60 minutes.
• At the end of the allocated time, record the level of the water in the pipette, noting how much the level dropped in milliliters.
• Then, refill and reset the potometer water level to zero, and then repeat the procedure for each of the remaining test leaves until the transpiration rates of all four leaves have been measured.
• Next, to determine the surface area of the leaf samples, lay each leaf on a blank piece of paper and carefully trace the outlines.
• Then, cut the outlines out and record the mass of each tracing.
• Finally, cut out and weigh one four by four centimeter square of the same type of paper used to make the leaf tracings. This will act as a reference with known area and weight.
• Next, you will need to quantify the number of stomata per unit area on each leaf. To do this, first prepare a slide mount by painting at least one square centimeter of the underside of each leaf with clear nail polish. IMPORTANT: Ensure you paint the bottom of the leaf, since the majority of the stomata are located there.
• When the nail polish is completely dry, carefully press a small piece of cellophane tape directly onto each patch and then gently peel off the tape to remove the painted on polish. NOTE: Use a piece of tape that is smaller than the microscope slide.
• Then, place each impression and tape onto a separate, clean microscope slide. HYPOTHESES: In this exercise, the alternative hypothesis could be that the leaves of the plants adapted to drier environments will have fewer stomata per unit area than the leaves of plants adapted to wetter environments. The null hypothesis might be that all plants species will have an equal number of stomata per unit area.
• To observe each impression under the microscope, first use a low magnification to find areas of the leaf impression that contain stomata. These will appear as darker dots or impressions.
• Then, switch to a higher magnification, keeping the sample viewing window centered on the stomata-containing area.
• Draw and label your observations, making sure to label each sketch with the plant species.
• Then, count and record the number of stomata in the field of view. You should perform this count a total of four times in four different areas of the leaf impression, and then determine the average number of stomata per counted area on the leaf.
• To calculate the number of stomata per mm 2 , place a transparent plastic ruler on the microscope stage under the objective and measure the diameter of the field of view.
• You can then use this number, the diameter or D, to calculate the area of the field of view in mm 2 .
• First, calculate the surface area of the 4×4 cm paper square to obtain the number of square centimeters.
• Then, divide the weight of the paper square by the number of centimeters squared it covers to obtain the weight of 1 cm 2 of paper.
• Finally, to calculate the surface area of each leaf, divide the weight of the leaf tracing by the weight of 1 cm 2 of paper. This will give you leaf area in centimeters squared.
• You should now convert this number to meters squared, the standard unit used for calculating transpiration rate.
• To calculate the transpiration rate for each leaf, divide the total water loss you measured in the potometer in mL by the time the experiment was run for in minutes.
• Then, divide this by the surface area of the leaf in m 2 .
• Make a bar graph with the four different leaf species on the X axis, and their corresponding transpiration rates on the Y axis. Do these rates appear different?
• Next, add a second Y axis to your chart and plot the average number of stomata per millimeter squared for each leaf that was observed under the microscope. Do you see a correlation between transpiration rate and number of stomata? If so, does this seem to have any relationship to the native environment the tree would typically inhabit?

16. Investigation into transpiration using a simple potometer

• 00:33 Why must the end of the stalk be cut underwater before assembly?
• 00:49 Why is Vaseline applied to the joints of the potometer?
• 01:31 Which metabolic processes use water?

small beaker of water

freshly cut plant stems (cut end in water)

clamp stand

scissors / sharp knife

paper towel

bowl of water

Syllabus Edition

First teaching 2014

Last exams 2024

Skills: Experiments Investigating the Rate of Transpiration ( DP IB Biology: HL )

Revision note.

Biology & Environmental Systems and Societies

Practical 7: Potometers & the Rate of Transpiration

• Because the amount of water used in photosynthesis is so small in relation to the total amount of water that passes through a plant, the rate of water uptake can reasonably be used to represent the rate of transpiration
• The position of the air bubble is recorded at the start of an experiment , and then a researcher can either measure how far the bubble moves in a set amount of time , or time how long it takes for the bubble to move a certain distance
• Mass potometers measure the change in mass of a water-filled test tube connected to a plant shoot as it loses water over a set amount of time
• Light intensity
• Temperature

Investigating the effect of light intensity on the rate of transpiration

• Plant shoot
• Cutting board
• Scalpel/scissors
• Paper towels
• Volume scale
• Capillary tube
• This is done to prevent air from entering the xylem ; this could block the movement of water through the plant
• Set up the apparatus as shown in the diagram , ensuring that it is airtight , and using vaseline to seal any possible gaps
• Any water present on the leaves might affect the rate of transpiration as it could block the stomata
• Remove the capillary tube from the beaker of water to allow a single air bubble to form and then place the tube back into the water
• This could be achieved by varying the light bulbs used or by varying the distance between the light source and the plant shoot
• Allow the plant to adapt to the new environment for 5 minutes
• Record the starting location of the air bubble, leave for a set period of time , and then record the end location of the air bubble
• Change the light intensity by a measurable amount e.g. moving the lamp 10cm further away from the plant shoot
• Reset the bubble by opening the tap below the reservoir
• Repeat the experiment at the new light intensity, and again at a range of different intensities

The Effect of Temperature & Humidity on Transpiration Rates

• A potometer can be used to test hypotheses about the effect of various environmental factors , including temperature or humidity, on transpiration rates
• A fan on different settings could be used to vary the flow of air around a plant shoot
• Enclosing the plant shoot in a plastic bag can increase the humidity
• A humidifier or dehumidifier could be used to give a measurable variation in humidiy
• A lamp at different distances or with different types of light bulb can be used to vary light intensity
• A thermometer or temperature probe can be used to find surroundings with different air temperatures
• A heater or air conditioner can be used to give a measurable variation in temperature
• A researcher would need to be aware of the importance of controlling any variables other than the variable of interest to ensure that any results were valid e.g. placing a plant shoot in different rooms could be a way of varying temperature, but might bring the risk of also varying light levels and humidity; these variables would need to be controlled

Factors affecting the rate of transpiration

• The air outside a leaf usually contains a lower concentration of water vapour than the air spaces inside a leaf, causing water vapour to diffuse out of the leaf
• When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity
• Less water vapour will diffuse out into the air due to the reduced concentration gradient
• Air currents, or wind, can carry water molecules away from the leaf surface , increasing the concentration gradient and causing more water vapour to diffuse out
• An increase in temperature results in an increase in the kinetic energy of molecules
• This increases the rate of transpiration as water molecules evaporate out of the leaf at a faster rate
• If the temperature gets too high the stomata close to prevent excess water loss
• This dramatically reduces the rate of transpiration
• Stomata close in the dark and their closure greatly reduces the rate of transpiration
• Stomata open when it is light to enable gas exchange for photosynthesis; this increases the rate of transpiration
• Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration
• If the humidity is high that means the air surrounding the leaf surface is saturated with water vapour
• At a certain level of humidity, an equilibrium is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves

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Ganongs Potometer: Design Principle, Procedure, Precautions

Collegedunia Team

Content Curator

Ganongs Potometer is a device used to measure the water uptake in transpiration . The transpiration rate is calculated by taking the gap included by using the air bubble in a fixed time period. Several readings of the time taken are noted and repeated by adjusting the air bubble to zero reading. The average of those readings gives transpiration rate. Ganong’s potometer is made up of a glass cylinder with a wide mouth and a glass tube bent twice. 

Also read: Transportation in Plants

Key Terms : Ganong’s Potometer, Transpiration, Twig, Beaker, Bubble, Readings

Design Principle of Ganongs Potometer

[Click Here for Sample Questions]

The Design Principle of Ganong’s Potometer is as follows:

• Reservoir- A tap is attached to a funnel. The bubbles are adjusted by rotating the tap on the reservoir. (Syringes can also be used in some designs.)
• Capillary Tube’s Length- The capillary is injected with a bubble. The bubble starts moving when the plants absorb the water. The amount of water is calculated by spotting the regular markings on the tube.
• Tube for Handling Leafy Shoot- There should always be water with the leafy shoot, and no air should come in contact with the surface of the water, as evaporation can cause interference. 

Fig. Ganongs Potometer

Procedure of Measuring Water Uptake

The rate of transpiration with the help of Ganongs’s Potometer can be calculated in the following ways:

• Directly – If the weight of Ganong’s potometer is measured after some time. Any loss is assumed to be because of Transpiration.
• Indirectly– If the water level drops are measured in the graduated tube it is calculated after some time. 
• An apparatus is mounted on a flat surface under bright sunlight and is filled with coloured water. A few drops of Eosin oil is poured into the coloured water. 
• A freshly cut twig is placed to the mouth of the glass cylinder with the help of a single-holed rubber cork. 
• A single air bubble is kept at zero reading, before the experiment, in the horizontal glass rod’s capillary tube. 
• The horizontal rod’s bent side is shifted from the beaker. This will help a couple of air bubbles to stay in the capillary tube. Only a single air bubble should be kept at the zero reading. 
• For this variation, cork is removed from the reservoir to allow water to flow through the horizontal rod. 
• Water makes the air bubbles move out through the nozzle. As the moisture is transpired by the leafy shoot’s aerial regions, a pull by transpiration is done to compensate for the moisture’s loss. 
• Due to this force, the air bubble moves towards the freshly cut twig. 
• The amount of water absorption may be calculated by measuring the distance travelled via the air bubble in a positive time frame and numerous measurements are recorded through adjusting the air bubble to zero reading. 
• Average of these measurements will give transpiration’s rate. Water saved within the reservoir is necessary to adjust the air bubble to zero reading and restart the experiment.

Fig. Water Uptake Measurement Procedure

Limitations of Ganongs Potometer

• The newly cut twig might not have a long shelf life..
• The injection of the air bubble into the horizontal rod is not easy and takes time.
• It does not calculate the rate with accuracy as the entire water absorbed by the plants is not used in transpiration. 
• A little change in the temperature can inflict problems on the air bubble’s position.

Necessary Precautions

The precautions that must be taken before using Ganong’s potometer are as follows:

• When a freshly cut twig is immediately placed underwater, a tiny portion needs to be cut underwater. This will help to block the entrance of the air into the xylem vessels.
• The potometer's state must not be changed while the experiment goes on, doing so will let external factors like temperature or pressure affect the water absorption.
• Any water leakage should be avoided by tightening everything.

Things to Remember

• Ganong’s Potometer is a device used to measure the water uptake in transpiration. 
• The transpiration charge is calculated with the aid of taking the gap blanketed by using the usage of the air bubble in a certain term. 
• Ganong’s potometer is made up of a tumbler cylinder with a large mouth and a tumbler tube bent two times. 
• A horizontal glass bar consistent with a reservoir inserted with a capillary tube and a pitcher cylinder with an extensive beginning is constant to the gadget’s the front cease, on the equal time as a holed-rubber cork is fixed on the glass cylinder’s establishing.

Sample Questions

Ques. What is Ganong’s Potometer? ( 4 Marks )

Ans. A device used to measure the water uptake in transpiration is known as Ganong’s Potometer or a transpirometer . The transpiration rate can be measured in a laboratory with the help of this device. Ganong’s potometer is made up of a glass cylinder with a wide mouth and a glass tube bent twice.

A horizontal glass bar fixed with a reservoir gets inserted with a capillary tube and a glass cylinder with a wide opening is fixed to the apparatus’s front end, while a holed-rubber cork is fixed on the glass cylinder’s opening. 

The experiment begins by cutting a twig and placing it in the cork’s hole. As the horizontal bar has reading marks on it so we can take readings accordingly. Its ending is with a nozzle opening and is bent.

The bent end that was inserted in the horizontal bar gets a beaker with coloured water placed under it and a reservoir is connected to the to store water in the horizontal bar. A flat surface is used for the entire setup.

Ques. What is the principle of the Design of Ganong’s Potometer? ( 3 Marks )

• Reservoir– A tap is attached to a funnel. The bubbles are adjusted by rotating the tap on the reservoir. (Syringes can also be used in some designs.)
• The Tube for Handling the Leafy Shoot- There should always be water with the leafy shoot, and no air should come in contact with the surface of the water, as evaporation can cause interference. 

Ques. What are the two methods of measuring Water Intake? ( 3 Marks )

Ans. The rate of transpiration with the help of Ganongs’s Potometer can be calculated in the following ways:

• Directly: If the weight of Ganong’s potometer is measured after some time. Any loss is assumed to be because of Transpiration.
• Indirectly: If the water level drops are measured in the graduated tube it is calculated after some time. 

Ques What are the limitations of Ganong’s Potometer? ( 3 Marks )

Ans. Limitations of using Ganongs Potometer are listed below:

• The newly cut twig may not live for a long period.

Ques. What are the necessary precautions to be taken during the experiment? ( 3 Marks )

Ans. The important precautions that are considered before starting the experiment are:

• The potometer's state must not be changed while the experiment goes on, doing so will let external factors like temperature or pressure control the water absorption.

Ques. How is the amount of water absorption calculated? ( 3 Marks )

Ans. The amount of water absorption may be calculated with the aid of measuring the distance travelled via the air bubble in a positive time frame and numerous measurements are recorded with the aid of adjusting the air bubble to the zero reading once more. 

The average of these measurements will give us transpiration’s rate. The water saved within the reservoir is necessary to adjust the air bubble to zero reading and restart the experiment.

Ques. How many bubbles are kept in the experiment and how? ( 2 Marks )

Ans. A single air bubble is kept at zero reading, before the experiment, in the horizontal glass rod’s capillary tube. The horizontal rod’s bent side is shifted from the beaker. This will help a couple of air bubbles to stay in the capillary tube. Only a single air bubble should be kept at the zero reading.

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A-level set practicals – using a potometer

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‘Using a potometer’ resource supports the use of practicals across various new 2015 A-level biology specifications (OCR, AQA, Edexcel and Eduqas).

This simple potometer allows students to work individually or in pairs to measure the rate of transpiration of a plant specimen within a one hour lesson. Assembled using low cost parts the apparatus offers an affordable potometer for all your students, giving them a chance to record water loss at regular intervals and an opportunity to investigate the effect of different abiotic factors on the rate of transpiration.

The ‘ Making and using a straw potometer ‘ offers a quick alternative to demonstrate how to use a potometer to measure the rate of transpiration in a plant. This setup allows students to revise what a potometer does and gives an opportunity for them to investigate further if they want to.

These resources and this practical allow students to develop their understanding of water transport through plants and factors that affect the rate of diffusion. There is also the opportunity to develop graph drawing skills as well as the ability to calculate rates of change. It provides students with the opportunity to develop (and demonstrate) their skills of taking quantitative measurements as well as their ability to consider variables that need to be controlled.

Through the suggested questioning in the teacher support materials or the student worksheet students can be encouraged to explore the implications of their findings to real life situations.

Materials include:

• Tested materials to support the A-level practical endorsement (CPAC)
• Support materials for teachers and technicians
• Student worksheet to help provide evidence for meeting the practical skills requirements
• Student revision materials to help prepare for exams

Support materials for other key practicals in A-level specifications can be found on the A-level set practicals main page.

Your FAQs Answered

Q: what plant is best to use.

A: You need a stem that is strong enough to push through the rubber bung and about 4mm in diameter. Often you can find perfect examples in your local area. Laurel being one of them.

What's included?

• SAPS - Potometer and water transport in plants - Learning Outcomes
• SAPS - Potometer and water transport in plants - Student Sheet
• SAPS - Potometer and water transport in plants - Technical and Teaching Notes
• SAPS - Potometer and water transport in plants - Student Worksheet
• Respiration
• Transport in plants
• A-level set practicals

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Ganongs Potometer

What is a ganong's potometer.

A Potometer is an apparatus which is used to measure the amount of moisture absorption by the leafy shoot during transpiration. It is also referred to as a transpirometer. The reasons for moisture uptake are transpiration and photosynthesis. The Ganong's Potometer finds its purpose in the measurement of the transpiration rate in a cut twig in lab settings.

Ganong's Potometer is a glass tube that is bent twice and consists of a broad-mouthed glass cylinder, a horizontal glass rod with a capillary tube inserted within and a reservoir connected with the horizontal rod. 

The broad-mouthed glass cylinder is present towards the front end of the entire setup. The opening of the cylinder is attached with a single-holed cork (rubber). The newly cut twig for the experiment is placed via this hole in the rubber cork.

The horizontal rod of the setup is a long glass tube within which a capillary tube is placed and has graduated readings marked on it. This horizontal rod has a bent end containing a nozzle mouth. This bent side is placed in a beaker that consists of colored liquid (water). 

As we move towards the front end of the horizontal rod, a vertically placed reservoir is present that is attached with the horizontal rod in which water can be accumulated. A pinch cock or stop cock is placed at the bottom of the reservoir to enable or halt the flow of water from reservoir to the horizontal rod. The entire apparatus is mounted on a flat surface like the table with the help of a stand.

A Typical Ganong's Potometer follows the Below-Mentioned Design Principle:

Length of Capillary Tube - A bubble is forced into the capillary. As the water is absorbed by the plants, the bubble starts moving. By spotting regular markings on the tube, it is possible to calculate the amount of water absorbed.

Reservoir – It is generally a funnel with a tap attached to it. Rotating the tap on the reservoir adjusts the bubble. Some other designs make use of a syringe instead.

Tube for Handling the Leafy Shoot - The leafy shoot should be kept in contact with the water. In addition, the surface of the water is not supposed to be exposed to the air. Or else, evaporation will interfere with the calculations. A rubber bung lubricated with petroleum jelly is sufficient for the setup.

Transpiration

Transpiration is the mechanism where there is loss of moisture or water in the form of water vapor from the aerial organs of the plants, particularly via the leaves. In every green plant, the process of transpiration takes place via the stomata of the leaves, in most cases. The rate of transpiration relies on different climatic factors such as temperature, intensity of sunlight, humidity, velocity of wind, availability of water to the plant, atmospheric pressure, and many more.

There are various types of transpiration which are:

Lenticular transpiration

Cuticular transpiration

Stomatal transpiration

Water loss mechanism

The rate of transpiration can be calculated in the following ways:

Directly – By calculating the weight of the Ganong's Potometer after a definite duration of time. In this case, it is assumed that any loss that takes place is because of transpiration.

Indirectly – By calculating the path the water level drops in the graduated tube after a measured period of time. In this case, it is assumed that this phenomenon takes place because of the taking in water that, in turn, is important to substitute an equal amount of moisture or water lost due to transpiration.

The entire setup or apparatus is full of colored water. The colored water is made by pouring a couple of drops of Eosin oil into the water. A newly cut twig is placed with the help of the single-holed rubber cork to the opening of the glass cylinder. The entire setup or apparatus is supposed to be mounted on a flat surface like a table under bright sunlight. 

Prior to the beginning of the experiment, a lone air bubble is supposed to be kept at zero reading in the capillary tube of the horizontal glass rod. To achieve this, the bent side of the horizontal rod is supposed to be shifted from the container or the beaker. Doing this will enable a couple of air bubbles to get held in the capillary tube of the horizontal glass rod. However, we need to observe that there must be only a single air bubble to get held at the zero reading.

To do this modification, we need to take off the stopcock of the reservoir to enable water from the reservoir to flow through the horizontal rod. The water now compels the extra air bubbles to move out from the nozzle. We need to do numerous trials until a single air bubble gets modified at the zero reading.

After a certain time duration post beginning of the experiment, we observe that the air bubble from the zero reading begins to move. As the moisture or water is transpired from the aerial regions of the leafy shoot, a transpirational pull is generated by the leafy shoot to compensate for the loss of moisture. Because of this force, the air bubble begins to move from the zero reading of the horizontal rod towards the newly cut twig. 

The rate of water absorption can be calculated by measuring the distance traveled by the air bubble in a certain period of time and numerous measurements are recorded by adjusting the air bubble to the zero reading again. The average or mean of these measurements will provide us with the rate of transpiration. To adjust the air bubble to the zero reading and to perform the experiment again, the water stored in the reservoir is to be utilized.

Precautions

The following precautions must be taken before using Ganong's Potometer:

When a twig is freshly cut from a plant, make sure that it is immediately placed under water (only the cut region). After that, a tiny portion is cut while it is still under water. Doing this avoids the entrance of the air into the vessels of the xylem .

The state of the Potometer, except the modification that is being measured, must not be manipulated during a test, as external factors such as temperature, and pressure can affect the water absorption.

Each and everything should be absolutely water-tight in order to avoid any leakage.

Limitations of Ganong's Potometer

The Ganong's Potometer comes with the following limitations:

The newly cut twig positioned on the glass cylinder may not stay alive for a significant period of time.

The forcing of an air bubble into the horizontal rod is not an easy task and takes a lot of time.

The Ganong's Potometer does not calculate the rate of transpiration with the required accuracy as not the entire water absorbed by the plants is utilized for transpiration. The device generally estimates the rate of moisture uptake.

A little alteration in the atmospheric temperature can have some impact on the position of the air bubble.

FAQs on Ganongs Potometer

1. From where can students study about Ganong's Potometer?

From Vedantu, which is an online learning platform students can study and learn about the Ganongs Potometer. The well-researched content is available on this website in the form of Pdf file. With the help of this article, students will get to know about the meaning of the Ganongs Potometer, its structure, its mechanism, its limitations and precautions taken while using this instrument. All these key points regarding the instrument are written by skilled teachers. Therefore, students must read this article thoroughly to understand the Ganongs Potometer. They can also ask their queries from the experts through the comment section.  

2. Describe the appearance of the Ganong's Potometer.

Ganong's Potometer is an instrument or device used to measure the amount of water taken by shoot during the phenomena of transpiration. This device has a glass tube and a glass cylinder. The glass tube is bent twice and the glass cylinder has a wide mouth. A reservoir is attached to the capillary tube in the horizontal glass bar. The glass cylinder is present at the front end of the apparatus and its mouth is closed with a cork. A hole is present on the cork in which a twig is kept. The reservoir helps in storing the water. The whole apparatus is placed on a flat surface. 

3. Which phenomenon is associated with Ganong's Potometer?

Transpiration is the phenomenon associated with Ganong's Potometer. Transpiration is defined as the excretion process in plants through which excess water is removed from the aerial parts of the plants. The plants use only a small amount of water for their development and growth. The rest is evaporated through the process of transpiration. There are various types of transpiration. These are:

Cuticular Transpiration

Stomatal Transpiration

Lenticular Transpiration

The factors that affect transpiration in plants are listed below:

Cellular factors

Temperature

The surface area of leaves

4. What is the procedure of experimenting with Ganong's Potometer?

The method of experimenting with the Ganong's Potometer is discussed below:

Firstly, make the water coloured by adding a few drops of eosin oil to it.

Then, put a small and fresh twig on the mouth of the glass cylinder. 

For writing the readings, the single air bubble should be kept at the zero reading of the horizontal bar.

The air bubbles will get trapped in the horizontal bar by lifting the bent end.

The air bubbles will start moving from zero reading. This is the stage where readings can be taken.

5. How to prepare the topic of "Ganong's Potometer" for the practical examination?

Practical examinations are conducted to assess the practical knowledge of the students. These examinations are part of the internal assessment exams. Thus, students have to prepare well to score good marks. To prepare the topic of "Ganong's Potometer" for practicals, students must know the principle on which it works. After that, they should see and understand the functions of each part of the instrument. After learning about the device and its working, students should solve the viva questions given in the practical book. This will help them in comprehending the mechanism of the instrument more easily.

Biology • Class 11

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Transpiration.

In this lab, students investigate the rate of transpiration in plants under normal and humid conditions. Then, they will have the opportunity to design their own inquiry experiments to test additional variables.

In the Initial Investigation, students create a potometer with a pressure sensor and monitor microclimate conditions using a weather sensor. The potometer measures transpiration by detecting changes in pressure due to the evaporation of water from the leaves of a plant sample. For their own experiments, students can use the potometer method or whole-plant method to measure how variables affect the rate of transpiration.

Grade Level: Advanced Placement

Subject: Biology

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What precautions would you take to set up a potometer? [4 MARKS]

Each part: 1 mark to set up a potometer, we would take the following precautions: (a) the twig should be cut obliquely and dipped into water. (b) all the joints of the apparatus should be airtight. (c) the air bubbles should be set on zero point by opening the reservoir carefully. (d) the free end of the capillary tube should be properly dipped into the water..

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