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An investigation into the effects of different concentrations of lead chloride on the growth of plants, (cress).

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A2 Biology coursework Skill P: Planning Title: An investigation into the effects of different concentrations of lead chloride on the growth of plants, (cress). Aim: To find out how changing different concentrations of a heavy metal chloride, namely lead chloride, affects the growth of cress seeds. Background information: Lead is a grey metal, derived from ore-bearing minerals. Heavy metals compounds, such as lead chloride are able to dissolve in rain and enter the soil surrounding the plants. Lead is largely emitted into the atmosphere in a gaseous state through emissions of vehicles using fuel that contains lead. Other sources include additives in gasoline and paints, fertilisers and mining. For plants, lead is a toxin and when present in large amounts it can cause severe decreases in their growth. 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.) I will go into detail about these factors further on. The first set of proteins to come in contact with the minerals and ions in the surrounding soil and are involved in the transport of micronutrients such as iron are called chelates. This is a protein carried by the root cell. Lead has a high affinity for sulphur, and as sulphur is present in the chelate molecule, lead binds with the sulphur and causes the inhibition of iron transport. This means that there is a deficiency of iron therefore slowing down all reactions, which mean there is a lack in chrophyll formation, which can lead to chlorosis, which is the decolourisation of the leaves. This therefore leads to a decrease in light absorption, less glucose made, and decreasing in photosynthesis meaning the plant may eventually die. ...read more.


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 * Once all the 5 concentrations are made up, place them over each cotton face pad in each petri dish, and label the dish with its corresponding label * Using tweezers, arrange 25 seeds in each petri dish using the method of distribution shown by the grid in the second test above. * 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 5 days in an area with lots of sunlight. * After 5 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. N:B whilst carrying out this experiment, make sure eye protection is worn at all times, with plastic gloves carrying the hands. Protective clothing should also be worn as well The table below shows the results of this experiment. The average length of shoot of cress seeds was calculated once more, using the previous method - (see appendix for calculations). 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 23.3 0.010 12.6 0.015 5.68 0.020 4.28 Note: All figures in the table are correct to 3 significant figures. The results show that as the concentration of the lead chloride increases, the growth of the cress seeds decreases. This means that lead chloride, 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 4.28mm for 0.02moldm-3 lead chloride. ...read more.


* The biomass of the cress seeds will be used as an indication of the amount of growth in the different concentrations of lead chloride, instead of the stem length. The reason being that measuring the length of the shoots does not take into account the width 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 biomass will be found when the seeds are heated and weighed several times until two consecutive masses are obtained. This increases the reliability of the results 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. Safety: Lead chloride is a harmful substance for humans as well as plants. Safety goggles, protective gloves and lab coats should be worn at all times during the experiment, with hair tied back. Lead chloride is an irritant and 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 essential 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. The cotton pads soaked with lead chloride should be placed in a bag to give to the teacher who can safely dispose it. ...read more.

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