Biology practical investigation

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Practical investigation.





Introduction





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Heavy metals like mercury and lead are toxic. They are non--competitive enzyme inhibitors, neurotoxins and reduce the partial permeability of cell membranes. Deposits of heavy metals accumulate in the body causing serious damage. People may ingest heavy metals � particularly lead ions � in various ways, chiefly; fumes from leaded petrol, lead based paints and drinking water from lead pipes (Biological Science).


Living cells have cell membranes, which regulate substances entering and leaving the cell. They are therefore semi-permeable. The membrane is made up of proteins like cholesterol and phospholipids. These form a phospholipids layer in which the hydrophobic tails point inwards and the hydrophilic heads point outwards. This is due to the way the react with water.


When the protoplast of a cell is in an area of high water potential, water enters through the semi-permeable membrane into the cytoplasm expanding it. This process is called Osmosis, the movement of a solvent (usually water) through a semi-permeable membrane. The solvent passes from a less concentrated solution (high water potential) to a more concentrated solution (low water potential) until the concentrations are the same. (The Hutchinson dictionary of science)


Extrinsic proteins line the surface of the membrane; they can even span the whole membrane and are known as Tran membrane proteins.


These proteins


sum Provide structure.


sum Act as recognition sites.


sum Are specific.


sum Are protein pumps, assisting in active transport.


Proteins are formed by a specific amino acid structure. If this is compromised i.e. denatured then the protein unravels. It is then deformed and ineffective.


Heavy metals ions bind to proteins causing them to denature, this reduces the partial permeability of the cell membranes. This means that the cells are unable to plasmolyse and deplasmolyse efficiently. This reflects the lead nitrate concentrations, and can therefore be used to measure the effect of different lead nitrate concentrations.





Variables


sum Independent Concentrations of lead nitrate.


sum Control Light, temperature, volume of substances, cells and time cells exposed to substances.


sum Dependant Plasmolysis.


To measure the amount of cells which will have plasmolysed I will look at them through a microscope and count out of ten how many have been effected, and how much. I will use cells from the same tissue and make sure they are all turgid at the start of the experiment, by placing them in distilled water and looking at them through a microscope. Therefore my only variable will be the lead nitrate concentration.





The cells can be plasmolysed in a concentrated sucrose solution; this will show me the typical level of plasmolysis for a normal cell. This will be my control, against which I can compare my results from the cells in nitrate.





Hypothesis


The heavy metal (lead nitrate) will compromise the cells membranes integrity. Therefore affecting its ability to plasmolyse and deplasmolyse efficiently. The cells that have been affected by the lead nitrate will remain turgid; the cells that have not will plasmolyse. I will try concentrations up to 1 mol of lead nitrate, as this should be more than enough to affect the cells membrane.


Fair Test





All the variables must be kept the same in order to ensure a fair test.


• The time the sample cells are bathed in the sucrose solution


• The time that the samples are exposed to lead nitrate


• The concentration of sucrose solution used


• The volume of the solutions (lead nitrate, sucrose and distilled water)


• The tissue sample must be left in distilled water long enough for all of the cells to become turgid.


• The number of cells looked at.


• The light and temperature levels.


• Repeat readings () to ensure a reliable average, minimizing anomalous results.


Safety


sum Wear goggles and lab coats when dealing with dangerous chemicals and toxins.


sum Do not ingest chemicals and toxins.


sum Clean up spillages.


Apparatus


sum Six Petri dishes


sum Chino graph pen


sum Graduated pipette


sum 10cm of lead nitrate


sum Knife


sum Cutting mat


sum Onion


sum Tweezers


sum Microscope


sum Slide


sum Cover slip


sum Distilled water


sum Sucrose solution


Method


sum Label six Petri dishes 1.00000M, 0.50000M, 0.5000M, 0.1500M and 0.0615M. Arrange them in order of descending concentration . Mix up concentrations, a put in corresponding Petri dishes.


Concentration (Mol)


Lead Nitrate


Sucrose solution


1.00000


10cm


0cm


0.50000


5cm


5cm


0.5000


.5cm


7.5cm


0.1500


1.5cm


8.75cm


0.0615


0.65cm


.75cm


sum Ensure that all of the solutions are thoroughly mixed.


sum Set up, for each lead nitrate dish, one containing 10cm distilled water and a dish containing 10cm 1Mol sucrose solution.


sum Remove a thin layer of onion.


sum Place cells in distilled water for minutes using tweezers, so that they become turgid.


sum Move the onion sample to the dish containing lead nitrate, submerge the sample for minutes.


sum Set up a microscope and desk lamp during this time.


sum Move the sample to the dish containing sucrose solution and submerge it for 5 minutes.


sum Examine cells under a microscope and see how many have been effected. NB count the same number of cells at random each time. This will give quantifiable results to plasmolysis.


sum Remove and place in Petri dish containing distilled water.


sum After three minutes, remove, place on slide and view under microscope again. Picking 0 cells at random look and see if they have deplasmolysed.


sum Clean out apparatus with distilled water.


sum Repeat for all samples.


sum Put away equipment when finished.


Table to show results of preliminary work


Lead Nitrate concentration (Mol/dm) Plasmolysed cells Deplasmolysed


1st nd rd Average 1st nd rd Average


1.00000 0 0 0 0 0 0 0 0


0.50000 0 0 0 0 1 0 1


0.5000 0 0 0 0 5 6 7 6


0.1500 0 0 0 0 1 1 11 1


0.0615 0 0 0 0 18 17 1 18


This method shows that the Lead nitrate has affected the cells membranes, and therefore their ability to plasmolyse and deplasmolyse. However I do not believe it is accurate enough, as it relies heavily on my interpretation on whether the cells have plasmolysed, deplasmolysed or are the same. My range seems to be fairly accurate, however I could take some more readings to improve the continuity of my results.





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