Aim: To investigate the effect of different concentrations of a heavy metal chloride, namely lead chloride, on the growth of cress seeds.
Introduction:
Heavy metals compounds, such as lead chloride are able to dissolve in rain and enter the soils surrounding plants. Some sources of such compounds are exhaust fumes from vehicles, additives in gasoline and paints, fertilisers and mining. Lead chloride is able to accumulate in the soil at sufficient concentrations and is easily absorbed by plants. For plants, lead is a toxin and when present in significant amounts, can cause severe decreases in their growth as well as death. The toxicity of heavy metals is seen as the irregularities in the normal functioning of the plant rather than direct toxicity to plant cells. Symptoms include stunted growth and the yellowing of plants (called chlorosis). Heavy metals collect in different organs of a plant and produce variable effects. Lead disrupts the plant’s plasma membrane structure as well as permeability (proteins in the membrane), osmotic balance (the intake of water and ions) and indirectly, plant metabolism (the availability of nutrients for chemical reactions.) These factors are discussed below in further detail.
The root cells of a plant carry proteins called chelates in their cell membranes. These are the first set of proteins to encounter minerals and ions in the surrounding soil and are involved in the transport of micronutrients such as iron. However, lead has a high affinity for sulphur. Since sulphur is present in the molecules of chelate, lead irreversibly binds with the sulphur and causes the inhibition of iron transport. This means that iron deficiency occurs and there is discolouration of the plant which may eventually cause its death. For lead to be transported from the soil to the root cells, it must cross the cell membranes of the root cells. Lead is able to cross the cell membranes via voltage-gated calcium channels. These channels are for the transport of calcium. Lead blocks these channels and causes the inhibition of their activity, preventing calcium being transported.
Plants require water for photosynthesis. Photosynthesis provides plants glucose, an energy source, which is needed for the plants to grow. When lead is present in high concentrations in the soil, it decreases the water potential of the soil. It therefore, has a lower water potential than the root cells, causing water to move from a region of higher water potential (root cells) to a region of lower water potential (soil), via osmosis (Biology 1, OCR, pg 56). This upsets the osmotic balance of the plants and prevents sufficient amounts of water entering the cells.
In general, when lead is absorbed, it is present in higher concentrations in the roots rather than other organs of the plant. Therefore, at low concentrations of lead, there is a greater amount of lead retained in the roots instead of spreading to the shoots. Lead inhibits root growth by affecting mitosis – it prevents cell division in root tips by introducing mitotic abnormalities (such as destroying the microtubules making up the spindle in mitosis). Inhibition of growth also occurs in the meristems of plants when lead reaches the shoots and prevents mitosis taking place.
Enzymes are involved in almost all metabolic reactions and are very important to living organisms. In plants, the uptake of lead affects the functioning of enzymes. Enzymes are made from proteins and consist of amino acids which may contain cysteine. Components of cysteine include sulphur and as lead has a high affinity for this element, it is instantly attracted to the bonding between molecules of cysteine called disulphide bridges. More specifically, lead reacts with groups on the enzyme called -SH groups (containing sulphur and hydrogen). These are present in the active site of the enzymes (the region within which a substrate binds to the enzyme) and in the regions of the enzyme which are involved in the stabilisation of their tertiary structure.
Lead removes sulphur atoms from the disulphide bonding. This means that the shape of the protein and therefore, the enzyme is altered. The normal activity of the protein stops and the enzyme is said to be denatured (Biology 1, OCR, pg 47). The type of inhibition involved is called non-competitive inhibition. This means that the lead binds to an area of the enzyme other than the active site but eventually distorts it as well. Lead is also able to block -COOH groups on the enzymes which contributes significantly to the inhibition of enzymes as well.
The process of photosynthesis is also negatively affected by heavy metals. Lead inhibits photosynthetic enzymes (involved in the Calvin cycle such as ribulose bisphosphate carboxylase) and is highly effective at inhibiting ATPase – an enzyme required in the production of ATP in photosynthesis (and respiration). It also disrupts the fine detail of chloroplasts, reduces the production of chlorophyll and carotenoids, interrupts the electron transport chain and causes the closure of stomata which results in a lack of carbon dioxide. Many features of photosynthesis are therefore, affected. If the equation for photosynthesis is observed, the components that lead affects can be seen more clearly:
Water + Carbon dioxide Glucose + Oxygen
As discussed before, water uptake may be reduced due to the low water potential of the soil. The closure of stomata (which may have resulted because there is less uptake of water and so water moves out of guard cells by osmosis) may deprive the cells of carbon dioxide and the lack of chlorophyll means that enough light energy may not be obtained. The electron transport chains affected in photosystem I and photosystem II in the chloroplasts means that enough ATP and reduced NADP may not be produced. This would mean that macromolecules required by the plant to grow may not be made. If the structure of chloroplasts is also altered, then the processes involved photosynthesis may not take place at all as well. Overall, the rate of photosynthesis is reduced.
Water is involved in the germination of seeds. When the seed absorbs water, this results in the stimulation of a plant growth regulator called gibberellin. Gibberellin is present in the endosperm tissue (a food store) of seeds and in turn, stimulates the synthesis of amylase in the aleurone layer. This layer surrounds the endosperm and causes amylase to be produced. The amylase then hydrolyses starch molecules in the endosperm into maltose. The maltose is then converted to glucose, which is transported to the embryo and is ready to provide the energy source required for the embryo to grow. The existence of lead in plants results in the inhibition of seeds germinating and thus, reduced growth. This is also associated with the plant not being able to absorb enough water.
Nutrient uptake is affected in plants due to the presence of high concentrations of lead chloride in the soil. An imbalance of minerals and ions is established within the cells of plants under the influence of lead. It is able to block positively and negatively charged ions in the roots (an example using calcium has been discussed) from entering by altering membrane structure. Lead can also physically block the entry of ions in the roots and causes a general uneven distribution of ions in all organs of the plants.
It can be seen that lead has many adverse effects on the growth of plants. This investigation will observe exactly how varying concentrations of lead chloride affect the growth of cress seedlings.
Prediction: Based on my findings on the effects of lead chloride on plant growth, I predict that as the concentration of lead chloride increases, the growth of the cress seeds will decrease. The lead chloride will inhibit the growth of the cress seeds.
Preliminary work:
The aim of the preliminary work was to find out:
1. The medium in which cress seeds will grow most effectively.
2. The method in which to distribute the cress seeds on the growth medium for maximum growth.
3. The range of concentrations of lead chloride to use in the experiment, which will allow adequate growth of the cress seeds in order to produce measurable results. The test was also carried out to see if the heavy metal chloride really has an adverse effect on the growth of plants.
Test 1:
The first test was carried out to see which medium the cress seeds grew most effectively in. A stable environment was required and a number of different mediums were considered in which the cress seeds could be grown. These included a cotton pad, cotton wool, filter paper, sponge and soil. The sponge was rejected because of its excessive water holding capacities which would deprive the cress seeds of water and therefore allow insufficient growth. The sponge would also not fit properly into the petri dish and cutting it to size would be difficult. Soil was not used as a test medium because of its inconsistent composition. The mineral and nutrient content in the soil may not be uniform which would prevent a fair test being conducted, as some seeds would be in an environment with higher nutrient content than others in lower nutrient content environments. Since lead chloride is being investigated in this experiment, it is also best to have no other heavy metal ions involved which may be present in the soil.
As a result, the preliminary test was carried out using cotton wool, cotton pad and filter paper. The following points illustrate a simple method used to conduct the first test:
- Place a layer of each medium into separate petri dishes. Use three layers for the filter paper or else it will not soak up enough water. Label each petri dish with its corresponding medium.
- Measure 15ml of distilled water using a measuring cylinder and carefully pour into each petri dish.
- Using tweezers, place 40 cress seeds at almost equal distance apart on each medium.
- Place each petri dish into a separate polythene bag and fill with some air. Tie the bag and allow the cress seeds to grow for 7 days in an area with lots of sunlight.
- After 7 days, measure the length of the shoot (starting from the seed and not including the root) of each cress seed in all three petri dishes. This is done using a ruler.
The results of the experiment are shown below. The shoot length of all 40 cress seeds in each medium was measured. The length of the roots was not measured because the roots were found to be overlapping and twisted around each other in at least one petri dish. An average length of shoot of cress seeds was calculated for each medium by adding up all 40 lengths and dividing the result by 40 (which is the number of seeds planted) – see appendix
A table to show the average length of shoot of cress seeds grown in different mediums
Medium in which cress seeds are grown
Average length of shoot of cress seeds (mm)
Cotton pad
46.7
Filter paper
36.0
Cotton wool
39.9
Note: All figures in the table are correct to 3 significant figures as per the Institute of Biology guidelines.
The results convey that cress seeds grow most effectively in the cotton pads as a growth medium. The results show this because the greatest average length of shoot of cress seeds is found to be in the petri dish containing the cotton face pad as the medium. This may be because the cotton pads are of suitable and uniform thickness and can support the growth of the cress seeds well. They may also have moderate capacities of soaking water so that the cress seeds are not drowned underwater but are still in contact with damp surroundings. Although the length of the roots were not measured, observing the growth of the roots showed that the roots of the cress seeds in the cotton pad were not trapped in the pad and could be easily picked out.
However, this was not the case for cotton wool in which the roots were very entangled and sometimes caused the shoots to break when trying to remove them from the medium. The growth of the shoots in the cotton wool may also have been inhibited if the shoots could not grow out and beyond the cotton wool. The results for filter paper showed the lowest average length of shoot of cress seeds and this may be because of the filter paper could not soak up enough water and so the seeds were submerged under the water. The filter paper may have been too thin to provide enough anchorage for the growth of the cress seeds as well. For the final experiment, I will thus be using cotton pads as the growth medium for the cress seeds.
Test 2:
The second test was carried out to see which method of arranging the cress seeds in the petri dishes gave the maximum growth of the seeds. Various distributions were considered in which the cress seeds could be grown. These included a grid, a scatter and a cluster. The idea of distributing the seeds in a cluster was deserted because of the fact that a cluster would result in the seeds being far too close together and this may result in intraspecific competition for space. It would also not be practical to plant the seeds so close together given that there is a large amount of space in the petri dish. Therefore, the remaining methods of distribution were tested in the preliminary work. There were many ways in which to plant the cress seeds in a grid. Two methods were chosen, namely grid 1 and grid 2 (as discussed below). Since the first test portrayed that the best medium to use was cotton pads, the second test was carried out using these. The method below simply describes how the test was conducted:
- Place a cotton pad into three separate petri dishes.
- Measure 15ml of distilled water using a measuring cylinder and carefully pour into each petri dish.
- Take 40 seeds in the hand and scatter them across the cotton pad in the first petri dish.
- In a third petri dish, place 40 seeds using tweezers at each corner of the grid like shown:
- Distribute 40 seeds in the second petri dish by placing 2 seeds in each square of the grid (as shown).
- Label each petri dish with its corresponding seed distribution. Label the grid with the seeds at the corner and the grid with 2 seeds in each square, grid 1 and grid 2 respectively.
- Place each petri dish into a separate polythene bag and fill with some air. Tie the bag and allow the cress seeds to grow for 7 days in an area with lots of sunlight.
- After 7 days, measure the length of the shoot (starting from the seed and not including the root) of each cress seed in all three petri dishes. This is done using a ruler.
The results of the experiment are shown below. Once again the average length of shoot of cress seeds was calculated using the previous method – see appendix.
A table to show the average length of shoot of cress seeds grown in different arrangements.
Arrangement of cress seeds
Average length of shoot of cress seeds (mm)
Grid 1
47.3
Scatter
38.3
Grid 2
44.4
Note: All figures in the table are correct to 3 significant figures as per the Institute of Biology guidelines.
The results illustrate that the most effective distribution in planting the cress seeds is by using a grid 1 where one seed is placed at each corner of the squares making up the grid. The results show this because the greatest average length of shoot of cress seeds is found to be in the petri dish containing the seeds arranged in this way. This may be because the seeds in this arrangement are of equal distance apart so that each seed gets an equal share of the surrounding resources. They are not too close to result in intraspecific competition between them. Good use was also made of the space in the petri dish when the seeds were arranged in this manner. The distribution of seeds using the grid 2 method, showed the second highest average length of shoot of cress seeds. There may have been some competition for space between the cress seeds in this petri dish because 2 seeds were planted in each square of the grid. This means that each seed was quite close to at least one other seed. This can be seen in the diagram above of grid 2. Scattering showed the lowest average growth of cress seeds and this may be because scattering does not result in all the seeds being the same distance apart. Some seeds may have ended up very close together when scattered. This may have resulted in some intraspecific competition between the seeds for resources. Consequently, I will be using grid 1 as the method for distributing seeds which is to place one seed at each corner of the squares forming the grid.
Test 3:
The third test was carried out to see if lead chloride actually had a negative effect on the growth of cress seeds. The range of lead chloride concentrations that could be used in the final experiment, which would give sufficient growths of the cress seeds, also needed to be established. This was so that measurable results are produced which can be compared to see the effects of the different lead chloride concentrations on the growth of the seeds. 5 different concentrations of lead chloride were tested in the preliminary run. Each concentration was prepared from an initially concentrated solution of lead chloride with a molarity of 0.02moldm-3. The volumes of this lead chloride solution and distilled water required to produce the different concentrations were calculated using formula involving dilution factors (see appendix for calculations). Since the second test identified that the most effective method of arranging the seeds on the petri dish was to place one seed at the corner of each square, this arrangement was employed in the third test. The following steps outline the basic procedure that was used to carry out the test:
- Place a cotton pad into a petri dish.
- Prepare the first 0.000moldm-3 lead chloride concentration using the volumes of the concentrated lead chloride solution and distilled water shown in the table below. This is done using measuring cylinders
Concentration of lead chloride solution desired (mol dm-3)
Volume of 0.02moldm-3 lead chloride solution required (ml)
Volume of distilled water required
(ml)
0.000
0.00
15.00
0.005
3.75
11.25
0.010
7.50
7.50
0.015
11.25
3.75
0.020
15.00
0.00
- Pour this lead chloride solution into petri dish. Label this petri dish with its corresponding lead chloride concentration.
- Using tweezers, arrange 40 seeds in each petri dish using the method of distribution shown by grid 1 in the second test above.
- Repeat the above steps for the other concentrations of lead chloride. Use syringes for measuring the volumes of solutions below 5ml.
- Place each petri dish into a separate polythene bag and fill with some air. Tie the bag and allow the cress seeds to grow for 7 days in an area with lots of sunlight.
- After 7 days, measure the length of the shoot (starting from the seed and not including the root) of each cress seed in all five petri dishes. This is done using a ruler.
The results for this test are shown in the table below. The average length of shoot of cress seeds was calculated once more, using the previous method – see appendix.
A table to show the average length of shoot of cress seeds grown in different concentrations of lead chloride.
Concentration of lead chloride (moldm-3)
Average length of shoot of cress seeds (mm)
0.000
47.8
0.005
23.4
0.010
13.6
0.015
7.00
0.020
5.50
Note: All figures in the table are correct to 3 significant figures as per the Institute of Biology guidelines.
The results show that as the concentration of the lead chloride increases, the growth of the cress seeds decreases. This means that lead chloride, a heavy metal compound, does have a negative effect on the growth of cress seeds. The results also show that the growth of the cress seeds is measurable, with the lowest average length of shoot being no less than 5.50mm for 0.02moldm-3 lead chloride. There are also significant differences between the average lengths of the shoots of the seeds when grown in varying strengths of lead chloride. For example, the average length of the shoots decreases from 47.8mm to 23.4mm when the concentration of lead chloride increases from 0.000moldm-3 to 0.005moldm-3. This is a considerable difference of almost 15mm. Therefore, the good variation in the results of how lead chloride inhibits the growth of cress seeds can be clearly compared and explained well using scientific details. For the final experiment, I can consequently use these concentrations to demonstrate and analyse the effects of different concentrations of lead chloride on plant growth.
Using the preliminary work to inform the plan for the final experiment.
The results of the preliminary work have allowed a conclusion to be drawn regarding the method to be used in the final experiment. I have decided that cotton pads will be used as the growth medium for the cress seeds as they grew most effectively in this medium in the preliminary run. Cotton pads are of uniform thickness with moderate water holding capacities and are very suitable for the cress seeds. They also fit well in the petri dish. The arrangement of the seeds on the cotton pad will be in a grid form where one seed will be placed on each corner of the squares making up the grid. This will provide enough distance between each cress seed so that they have adequate space to grow and will not compete intraspecifically for room. The lead chloride concentrations used in the preliminary experiment will also be used in the final experiment because measurable results are obtained when using these concentrations and there are significant differences in the growth of the cress seeds when immersed in the different concentrations. This means that the five concentrations will be appropriate in allowing the results to be compared and contrasted well and will be used to explain exactly why lead chloride inhibits the growth of plants.
By conducting the preliminary experiment, there are other parts of the method that have been enlightened and which may need to be modified for the real experiment:
- The length of the shoots of the cress seeds was measured as an indication for the amount of growth – tall plants implied more growth. However, the length of shoots does not take in to consideration the lateral growth of shoots and the growth of roots. A better alternative would be to measure the dry mass of the cress seeds, as this would be proportional to the overall growth of the seeds. For instance, a dry mass of 1g indicates more growth of the seeds than a dry mass of 0.5g.
- To maintain a constant environment around the cress seeds whilst left to grow requires proficiency because this is an important factor in determining the growth of plants. For example, light, humidity and temperature are all variables which may affect plant growth and thus, need to be kept constant so that any differences in growth are due to the varying lead chloride concentration. In the preliminary experiment, polythene bags filled with air were used to provide a closed environment around the cress seeds. However, the amount of air in each bag may have varied, which may have varied the humidity as a result, the temperature of the room in which the bags were kept may have seen fluctuations and all the cress seeds may not have received equal amounts of light. In addition, some of the solution in the petri dishes may also have evaporated and this may have affected the concentration of the lead chloride solutions in Test 3. If this happened in the actual experiment, inaccurate results would be produced. It was also difficult to get hold of enough air into the bags without making the cress seeds in the petri dishes move position. Health and safety regulations when using lead chloride do not allow blowing into the bags as well. A substitute would be to use a Dewpoint propagator which controls light, temperature and humidity. It also prevents evaporation of solutions which would be perfect for the experiment in order to obtain accurate and reliable results.
Apparatus:
- 0.02moldm-3 lead chloride solution
- Distilled water
- 2 x beaker
- 5 x petri dish
- 5 x cotton pad
- 2 x 20ml, 10ml and 5ml syringe
- 200 x cress seed
- 5 x test tubes
- 1 x test tube rack
Diagram:
Method:
- Pour some 0.02moldm-3 lead chloride solution and some distilled water into two separate clean beakers and label them using the Chinagraph pencil.
- Label a petri dish 0.000moldm-3 and write the date that the cress seeds are planted.
- Place a cotton pad into the petri dish.
- Measure 15ml of the distilled water from the beaker using a 20ml syringe and pour it into a test tube, held in a test tube rack. Add the contents of the test tube to the petri dish, evenly pouring it onto the cotton pad so that all areas are damp.
- Count 40 cress seeds and use the acetate grid to distribute them evenly (with an equal space between them) onto the petri dish. This is done, using tweezers, by placing one seed at each corner of the squares making up the grid as shown in the diagram below. There will be 8 seeds in each of 5 rows. Do not place the lid onto the petri dish.
- Repeat the above steps for the remaining concentrations of lead chloride. These can be found in the table on the next page. For volumes of 5ml and under, use the 5ml syringe, for volumes between 5ml and 10ml, use the 10ml syringe and for volumes above 10ml, use the 20ml syringe. Do not mix up the syringes for lead chloride and distilled water. Put the solutions in the syringes into the test tubes as done previously.
Concentration of lead chloride solution desired (mol dm-3)
Volume of 0.02moldm-3 lead chloride solution required (ml)
Volume of distilled water required
(ml)
0.000
0.00
15.00
0.005
3.75
11.25
0.010
7.50
7.50
0.015
11.25
3.75
0.020
15.00
0.00
7. Place all the petri dishes into the Dewpoint propagator which controls the light, temperature and humidity of the surroundings of the cress seeds. Leave the cress seeds for 7 days.
8. After 7 days, take the petri dishes out of the Dewpoint propagator.
9. Carefully remove each seedling from the cotton pad of one petri dish, using tweezers. Place the seedlings together on an area of a clean baking tray.
10. Repeat step 9 for the other petri dishes. Do not mix up the seedlings (arrange them in small groups on the baking tray, separate from each other). Record the concentration of the lead chloride that each group of seedlings was grown in.
11. Put the baking tray into an oven set at 70oC and leave it to bake overnight.
12. Remove the baking tray from the oven. Take the seedlings grown in 0.000moldm-3 from the baking tray with tweezers and place them onto an electronic balance.
13. Weigh the seeds together and record the mass for this particular concentration.
14. Place the seeds back onto the baking tray in the same position.
15. Repeat steps 12 to 14 for the other seedlings grown in the various lead chloride concentrations.
16. Heat the seeds in the oven for a further 6 hours.
17. After 6 hours, repeat steps 12, 13 and 15 again. If the seeds grown in a particular lead chloride concentration are found to have the same mass again, then this is the actual dry mass (biomass) of the seeds grown in that particular lead chloride solution and is recorded.
18. If the mass reading is less when measured the second time, heat the seeds again for a further 6 hours. Continue doing this until two equal consecutive masses have been obtained. This will then be the mass of the cress seeds.
Justification of apparatus and method:
- Distilled water will be used to dilute the lead chloride because it has a greater purity than tap water. It will not be contaminated with any minerals or ions, which may be the case if using tap water. It is also best to have no other minerals or ions present, other than the lead chloride being investigated. This is so that any differences in the results are due to the lead chloride only.
- Beakers will be used to contain the lead chloride and distilled water because it will be easier to measure the required amounts of each solution from beakers rather than measuring them directly from the bottles. This will also reduce the chance of accidents occurring, as small amounts of the liquids will be dealt with at any one time. Clean beakers will be required to ensure there is no contamination by other substances which may affect the accuracy of the experiment.
- Petri dishes will be used to place the cotton pads in as they are of reasonable size and shape and the cotton pads fit well in them.
- The test tubes will be used to hold the different concentrations. For example, to make 0.005moldm-3 lead chloride, 3.75ml of 0.02moldm-3 lead chloride will be measured and added to the test tube which will also contain 11.25ml of distilled water. This allows the solutions to mix or else, pouring them separately onto the cotton pads would not give accurate results. This is because all parts of the cotton pads will not be surrounded by an equal concentration of the lead chloride. Separate test tubes will be used for each concentration so that they are not contaminated as this may lead to inaccurate results.
- The acetate grid will be used to distribute the seeds to make sure that the seeds are equally and accurately spaced. The squares of the grid are of reasonable size so that the seeds are a fair distance apart. There will thus, be reduced or no intraspecific competition giving accurate and reliable results.
- Tweezers will be used to handle the cress seeds, as this is the most convenient way of clasping and moving the small seeds around.
- A Dewpoint propagator will be used to provide a constant environment for the cress seeds whilst they are growing. The propagator is able to control light, temperature and humidity of the surroundings as well as prevent evaporation of the solutions in the petri dishes. It is therefore, an effective piece of apparatus to use and is better than using polythene bags which were used in the preliminary experiment (the reasons for this have been discussed).
- The petri dishes will be labelled to ensure that there is no confusion in remembering which concentration of lead chloride is present in each petri dish.
- When measuring the amounts of solutions, syringes will be used because these are more accurate than using measuring cylinders which were used in the preliminary experiment. The reasons for this are that syringes have a narrower lumen and consequently smaller increments. They are therefore, more accurate than using a measuring cylinder. Measuring cylinders are wider and so there is more chance of an error occurring when using these. The syringes used will be of varying sizes so that for different amounts of solution, different syringes can be used. For example, to measure below 5ml of a solution, it is more accurate to use a 5ml syringe than using a 10ml syringe, as once again, the increments will be smaller. This means there will be a smaller percentage error in measuring the volume. It would be impractical for instance, to use a 5ml syringe to measure 10ml because this means that the syringe would have to be filled twice, increasing the chance of error.
- Each petri dish will contain 15ml of the lead chloride concentration being tested. This is because during the preliminary work, 15ml was found to correspond well with the size of the petri dish and the cotton pads. The capacity of the cotton pads to soak up water was moderate – the seeds were neither flooded nor deprived of the solution. The same amount will be used in each petri dish to make it a fair test. Otherwise, inaccurate outcomes of the experiment will result.
- The cress seeds will be left to grow for 7 days because the tests done for the preliminary work resulted in measurable lengths of shoots. This means that 7 days is plenty of time for the cress seeds to grow. All the petri dishes will be left to grow for the same length of time to make it a fair test. If this did not happen, then the cress seeds in some of the petri dishes may grow more than those in others, leading to inaccurate and unreliable results.
- 40 cress seeds will be used in each petri dish. The preliminary work conveyed that this is an adequate amount of seeds to plant in the petri dish, as measurable results were produced. It would not be practical to plant only one seed in each petri dish, as the test would then have to be repeated to give reliable results. By planting 40 seeds, this is in effect, repeating the experiment and will give significant amounts of biomass to record and analyse. This in turn, increases the reliability of the results.
- The biomass of the cress seeds will be determined as an indication of the amount of growth in the different concentrations of lead chloride. This is has been discussed before; the reason being that measuring the length of the shoots does not take into account, lateral and root growth. Therefore, measuring the biomass gives a more accurate indication of the total amount of growth of the seeds that has taken place. The use of a ruler would also introduce errors in measurement, as it is difficult to tell exactly where the beginning of the shoot is, leading to inaccuracies in the results.
- The biomass will be found when the seeds are continuously heated and weighed several times until two consecutive masses are obtained. This strategy increases the reliability of the results greatly because if the mass of the cress seeds found the first time was taken as the actual biomass, this may have given inaccurate results. This is due to the fact that all the water may not have evaporated in the oven and so the mass obtained would not be the actual dry mass. By finding the biomass when two consecutive masses have been achieved, ensures that all the solution has evaporated from the seeds. Heating the seeds further would then have no effect on their dry mass. The seeds from all the lead chloride concentrations are put in the oven at the same time or else an unfair test would be produced if the seeds in some of the petri dishes are left to grow for a longer period. This would ultimately lead to inaccurate results.
Safety: Lead chloride is a potentially harmful substance for humans as well we plants. Safety goggles, protective gloves and lab coats should be worn at all times during the experiment, with long hair tied back as lead chloride is an irritant – it can cause painful skin irritations and serious damage when in contact with the eyes. Lead chloride, being toxic and a poison, can be very dangerous if it is inhaled or swallowed – it can damage internal systems such as the respiratory tract, the central nervous system, reproductive system and the blood. Lead poisoning can also develop causing muscle cramps and vomiting. It is therefore vital to keep the room well ventilated in order to avoid inhaling dust and fumes from the lead chloride. The lead chloride should be removed from the bottle and poured into a beaker, as it will be easier to handle smaller amounts of the lead chloride than using it straight from the bottle. However, at other times, it is necessary to keep the lead chloride in a tightly sealed bottle. In the event of broken glass or spillages, these should be attended immediately and cleaned safely and carefully. The lead chloride must not be disposed down the sink and the working area must be cleaned thoroughly with water. The cotton pads soaked with lead chloride should be placed in a bag to give to the teacher who can safely dispose it.
Calculating the volumes of 0.02 moldm-3 lead chloride and distilled water required to prepare the different concentrations of lead chloride used in the preliminary work.
The different concentrations of lead chloride were prepared from an initially given solution of 0.02 moldm-3 lead chloride. The lead chloride was diluted with known volumes of distilled water to give the various concentrations required. The volumes of both 0.02 moldm-3 lead chloride and distilled water used to prepare a solution with a certain concentration were calculated using the following formulae:
1) Dilution factor (DF) = molarity of concentrated lead chloride solution
molarity of desired lead chloride solution
2) 1 = volume of concentrated solution
DF volume of desired solution
3) Volume of distilled water required =
volume of desired solution – volume of concentrated lead chloride solution
The following example shows how to use the formulae to calculate the volumes of the 0.02moldm-3 lead chloride and distilled water, required to prepare a lead chloride solution with a concentration of 0.005 moldm-3.
Dilution factor = 0.02moldm-3 (this is the concentration of the lead chloride given)
0.005moldm-3 (this is the concentration of the lead chloride desired)
Dilution factor = 4
Therefore, the dilution factor in order to prepare 0.005moldm-3 lead chloride solution is 4. This is then substituted in the second formula:
1 = volume of concentrated solution
4 15ml
Thus, volume of concentrated solution = 15 x 1 = 3.75ml
4
So, to prepare a lead chloride solution with a concentration of 0.005moldm-3, 3.75 ml of 0.02 moldm-3 lead chloride solution is needed. To calculate the volume of the distilled water, the third formula is used. The volume of the concentrated solution (i.e. 3.75ml) is subtracted from the volume of the desired solution (15ml). This gives the following:
Volume of distilled water required = 15ml – 3.75ml = 11.25 ml.
Note: The solution in all the petri dishes has a volume of 15ml. The sum of the concentrated lead chloride solution and the distilled water must add up to 15ml.
The above method was used to determine the volumes of concentrated lead chloride solution and distilled water for all the desired concentrations of lead chloride (shown in a table in the method).
Calculating the average length of shoot of cress seeds in each petri dish.
To calculate the average length of shoot of the cress seeds in each petri dish, the sum of the length each cress seed is found and the total length is divided by the number of cress seeds in the dish (i.e. 40 seeds). An example for this is shown below for calculating the average length of shoot in the petri dish where the medium used was cotton wool (experiment 1).
Average length of shoot for cress seeds in cotton wool = sum of the length of each cress seed
Number of cress seeds
= 35 +55 + 46 + 33 + 52 + 51 + 31 + 41 + 30 + 39 + 41 + 41 + 24 + 36 + 31 + 34 + 43 + 44 + 37 + 48 + 43 + 49 + 41 + 35 + 23 + 48 + 47 + 50 + 34 + 42 + 46 + 52 + 23 + 45 + 33 + 35 + 33 + 46 + 29 + 51
40
= 1597 = 39.925 = 39.9 mm (3.s.f).
40
This method was used to calculate the average length of shoot for the cress seeds in all the petri dishes (shown in a table in the preliminary work).
