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Effects of different concentrations of a heavy metal chloride on the growth of cress seedlings

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Effects of different concentrations of a heavy metal chloride on the growth of cress seedlings Introduction Heavy metals are everywhere, be it lead in car exhausts and industrial emissions, cadmium in paints, mercury in amalgams or zinc in batteries. These compounds invariably dissolve in rain, enter soil and are taken up by plant roots. If the metal is not present in sufficient quantities to kill the plant outright, it accumulates and is passed on to anything that eats it. Thus heavy metal accumulates down a food chain, causing the food we eat to be very contaminated. However I am more interested in those times when the metal is present in sufficient quantities to either kill the plant or inhibit growth. Contamination can affect plants in many ways: It can disrupt the normal plant/water relationship. It can indirectly affect plant metabolism, for example by disrupting nutrient availability. It may be directly toxic to plant cells. Heavy metals are systemic killers not contact killers. That is to say that the metal is not directly toxic but indirectly disrupts normal plant functioning. Heavy metals have a very high affinity for sulphur. Thus when they enter plants, they are immediately attracted to disulphide bridges between molecules of the amino acid cysteine (in proteins). ...read more.


5 x 0.00025M PbCl2 , 5 x 0.001M PbCl2 , 5 x 0.01M PbCl2 , 5 x 0.02M PbCl2 , 5 x 0.1M PbCl2 and 5 x 0.2M PbCl2 APPARATUS Dewpoint propagator 35 Petri dishes Lead Chloride Filter paper circles Acetate circles (marked with grid) 1500 cress seeds Distilled water Measuring cylinders, 1x500 cm 3 , 1x100 cm 3 Chinagraph pencil Syringes - 50 ml, 20 ml , 10 ml and 5 ml Spatula Plastic gloves Eyeglasses SAFETY PRECAUTIONS Plastic gloves and eyeglasses were worn when handling toxic lead chloride to avoid unnecessary contact. If any solid lead chloride is spilt, this must be removed into a bucket and the area well rinsed with water. Any solutions should be clearly labelled as TOXIC. All excess chemicals will be disposed of following the school safety policy. METHOD 1. A 0.2M solution was prepared; the appropriate amount (see appendix for calculations) of metal chloride was added to l000 cm 3 of water. This was diluted to give further concentrations. 2. An acetate circle was prepared for each Petri dish. This was ruled with a grid of 4 x 5 squares,(draw) 3. Filter paper was then placed over the top of this and 4 cms 3 of the 0.2M solution was added. This will make the grid visible. ...read more.


Dry mass measurements could have been taken. (graphs to show effects of lead chloride concentration on growth(height ) of cress seedlings) (Height(mm) on left, concentration on bottom(M)) APPENDIX Make up of solutions: PbCl2: RMM = 278 \0.2M ==>?55.6g PbCl2/1000 cm 3 distilled water. From the 0.2M solution prepared serial dilutions were carried out to give 0.1M, 0.02M, 0.01M, 0.001M and 0.00025M solutions. Lead Chloride concentration (M) 0.2M lead chloride (cm 3 ) Distilled water (cm 3 ) 0.20000 1000.00 0.00 0.10000 500.00 500.00 0.02000 100.00 900.00 0.01000 50.00 950.00 0.00100 5.00 995.00 0.00025 1.25 998.75 0.00000 0.00 1000.00 STATISTICAL ANALYSIS t test The t test is a test of significance. It is used to tell whether two results are statistically significant. In general when using more than 30 measurements per set of data, the value of should be compared with the critical value at ??degrees of freedom. This critical value is 1.96 for p = 0.05, and if the calculated value of t is greater than (or equal to) this critical value then the null hypothesis can be rejected. In all the following cases, the null hypothesis is that there is no difference between the data from the two samples being compared. t values calculated for control v various heavy metal concentrations Lead chloride concentration (M) 0.00025 0.00100 0.01000 0.02000 0.10000 0.20000 t value 1.61* 3.72 38.26 45.45 51.78 59.79 The results marked with a *, are not significant, all other results are significant. ...read more.

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