Biology coursework planning - the effect of lead chloride on the growth of cress seeds

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Biology coursework planning – the effect of lead chloride on the growth of cress seeds

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.
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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 ...

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This is a very detailed and well structured report. 1. The introduction is very well written and includes excellent research. 2, The sources of information need to be referenced. 3. The level of detail in all sections is good. 4. The report needs to have a centralized analysis section and an evaluation. *****