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Investigating the effects of varying pH levels on the germination of cress seeds

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Introduction

PLANNING & PREPARATORY WORK I will be investigating the effects of varying pH levels on the germination of cress seeds. A seed is a fertilised and ripened ovule, consisting of the plant embryo, varying amounts of stored food material (endosperm), and a protective outer seed coat (aleurone layer). The embryo secretes hormones such as cytokinin, and indole acetic acid, which promote cell division. The endosperm contains insoluble food stores including starch, proteins, and lipids, all of which are used in the initial germination stages. The aleurone layer contains proteins, and under the influence of gibberellic acid (manufactured and secreted by the embryo) synthesises amino acids into hydrolytic enzymes such as -amylase and a protease. These enzymes are then used to hydrolyse starch ( glucose, and proteins ( amino acids for further growth and development. The scale used to measure the acidity or alkalinity of a solution is the pH scale, which generally ranges in values from 0-14. Acidic solutions have pH values below pH 7, which is the value of a solution considered to be neither acid nor alkaline i.e. neutral. In pure water, the concentration of hydrogen ions is equal to 10� ; when an acid is added to pure water, the hydrogen ion concentration increases above this level. The pH level of the solution in which the seeds are germinating affects the availability of elements. Extreme acid/alkaline conditions may adversely affect plant growth by altering selected nutrient availability. Moreover, extreme acid/alkaline conditions can be very corrosive, and would more than likely denature vital enzymes due to their delicate nature as proteins. Slightly acidic conditions can be advantageous to germinating seeds, as the higher concentration of H� ions encourages the uptake of nutrients. I therefore predict that when grown in a variety of acidic pH values, the success of germination will decrease as the acidity increases. Null hypothesis: success of seed germination is not dependent on pH level. ...read more.

Middle

synthesises amino acids into hydrolytic enzymes such as -amylase and a protease. These enzymes are then used to hydrolyse starch ( glucose, and proteins ( amino acids for further growth and development (1). The scale used to measure the acidity or alkalinity of a solution is the pH scale, which generally ranges in values from 0-14. Acidic solutions have pH values below pH 7, which is the value of a solution considered to be neither acid nor alkaline i.e. neutral. In pure water, the concentration of hydrogen ions is equal to 10� ; when an acid is added to pure water, the hydrogen ion concentration increases above this level (3). The pH of a solution is a measure of the hydrogen ion concentration in that solution. The pH scale is logarithmic, so a small change in pH represents a large change in hydrogen ion concentration. For example, the hydrogen ion concentration of gastric juices (pH 1) is nearly 400 times greater than that of pure water (pH 7) (2). The pH level of the solution in which the seeds are germinating affects the availability of elements. Extreme acid/alkaline conditions may adversely affect plant growth by altering selected nutrient availability (3). Proteins have specifically charged groups such as carboxyl groups and amino groups. Since the charge of these groups depends on pH, a protein molecule can have different charges according to pH. In general, positive charges and negative charges on the surface of protein are well-balanced around neutral pH. Because of electrostatic attraction, the shape of the protein is compact and stable. However, for example, at extremely low pH levels, the carboxyl group is broken and negative charges are decreased. Thus, proteins will lose the stability which comes from electrostatic attraction, and gain more electrostatic repulsion between the increased positive charges. This is how proteins are denatured at extreme pH (4). Rain water is naturally slightly acidic due to the presence of carbon dioxide in the atmosphere which forms weak carbonic acid in water. ...read more.

Conclusion

Although the germination rates were different amongst the samples, there was a clear distinction between those that did germinate and those that didn't at all, and all those that did germinate reached 100% germination regardless. The highest concentrated samples (10% and 100%) did not germinate at all, most likely due to the enzymes required for germination being denatured. However, the seeds in the 10% concentrated sample always remained a lighter colour than those the 100% concentrated sample, indicating that although the concentrations were too strong for any germination, the minor difference between them still affected their outcome (although perhaps not significantly). Although limitations were fairly minor, I believe that the experiment could be improved as a whole if all the variables could be controlled absolutely. For example, if the seeds were grown in the same way, but placed on the cotton wool in a more controlled manner, one by one if necessary. This would help to prevent blocking of light or accessibility to the solutions by the cotton wool. Alternatively they could be grown in another medium similar to cotton wool in that it would not interfere with the germination process such as paper towels. Additionally, if the samples were kept in a room with a controlled temperature and the availability of natural light was controlled, this too would help to make the experiment more accurate as a whole. In terms of how acid rain affects the growth of plants in general, tests such as this one could be carried out with other acids found in acid rain such as Nitric Acid and Carbonic Acid varying the concentration of each; perhaps one particular acid in the solution of acid rain has more or less affect on plant germination than another. Furthermore, the same tests could be carried out using a variety of other seeds which may be more/less tolerant to the acidity, which could then be developed to research into what makes plants more/less tolerant to acidic conditions. ...read more.

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