To investigate the affects of heavy metals such as Lead Chloride, on the length of both the stem and roots of cress seeds.

There are organic and inorganic lead compounds, for our experiments we will be using inorganic lead solutions. In general, inorganic lead compounds are of lower toxicity to plants than organic lead compounds such as trialkylead and tetralkylead compounds. Tetralkylead becomes toxic by decomposition into the ionic trialkylead. The toxicity of inorganic lead salts is strongly dependent on environmental conditions such as water hardness, pH, and salinity. When solid Lead Chloride is placed in water the following happens (in equilibrium):

PbCl2 (s) < = > Pb+2 (aq) + 2Cl- (aq)

The solubility of PbCl2 increases very rapidly as the temperature of the solution rises. However, PbCl2 precipitates very slowly, particularly when other ions that form insoluble chlorides are not present. The precipitation can be speeded up by vigorously rubbing the inside of the test tube with a stirring rod. Even then the precipitate may not form until 3 to 5 minutes after mixing the solutions. PbCl2 dissolves in excess chloride ion and results in the formation of a complex ion, tetrachloroplumbate (II) ion:

PbCl2 (s) + 2Cl- (aq) <==> [PbCl4]2- (aq)

The tendency of inorganic lead to form highly insoluble salts and complexes with various anions, together with its tight binding to soil, significantly reduces its availability to plants via the roots. Translocation of the ion in plants is limited and most bound lead stays at root or leaf surfaces. As a result high lead concentrations in the soil are needed to cause visible toxic effects on photosynthesis and growth. Thus, lead is only likely to affect plants high concentrations of lead.

There are degrees of tolerance to heavy metals in plants. Tolerance is inherited, i.e.tolerant parents transmit tolerance to their offspring. Within a plant species, there are tolerant and sensitive populations. Tolerance, therefore, develops by selection, rather than by adaptation of individuals. Two possible mechanisms for tolerance, to metals in general, are "external" mechanism prevents metal entering the plant, while an "internal" mechanisms allows entry but prevents the metal from coming into contact with sensitive processes within the organism.

Prediction:

We will perform the investigation seven times - each time using a different concentration of Lead Chloride solution. Hence we should be able to distinguish, clearly, the relationship between lead concentration and stem/root development.

Ali Mahdi