The ways that substances move across cell membrane are
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Diffusion
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Active transport
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Facilitated diffusion
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Osmosis
The one we are focusing on is diffusion. The rate of which diffusion occurs while temperature is being changed.
The polar heads lie at both surfaces, the kind of three-dimensional enclosed structure shown above can form. The real shape tends to be globular (minimizing surface to volume ratio), and there are so many more bipolar molecule pairs than shown here that the radius of curvature lacks the kind of strain and distortion shown at the corners of this drawing.
A membrane is a phospholipid bilayer studded with proteins, polysaccharides and other sorts of lipids. This patchwork of molecules is like a mosaic. Because the molecules move about within their respective layer, the membrane behaves like a fluid. This is why the model is called the Fluid-mosaic model.
There have been many theories as to the nature of the cell membrane. In 1935 Daniellie and Davson put forward a possible structure, which was widely accepted. Their model was composed of a double lipid layer coated on each side by protein. They Estimated that the membrane was so thick, with the introduction of the electron microscope, membranes could be clearly seen for the first time. Plasma membranes of both animal and plant cells showed a characteristic three-layered (trilarninar) appearance.
As we apply heat on to the beetroot, this denatures the proteins creating "holes" in the membrane allowing pigment to be leeched out. The denaturing of the proteins incorporated with the added kinetic energy added to the polysaccharides allows pigment to be released both through the holes where the protein where and through the mosaic as well.
Hypothesis –
- When temperature is cold little pigment would be given out
- As the temperature increases the more red pigment is going to be given out.
- At 40 degrees Celsius- 45 degrees Celsius the membrane gets denatured and thus more pigment would be released.
Appropriate observations have to be made to validly test a hypothesis or idea. The term “observations” is taken to include measurements. Validity is the measure of confidence that can be placed in a conclusion. After deciding upon a dependant variable to vary within the experiment, the method of measurement should also be known, the measurement taken should be valid e.g. the volume of oxygen collected in 30 seconds is a valid measurement of rate of catalase activity.
Valid results are derived through precise, repeatable measurements and observations, made with apparatus and experimental procedures that are suitable for the task.
Ideally a large number of replicates (repeat measurements) should be taken, and any results that vary considerably from the others should be repeated or discounted. A mean can be calculated to be representative of the set of results. The pressure of time usually puts a limit on the number of replicates that can be taken.
To be reliable, measurements should also be accurate. To be accurate a measurement should be close to the true value. Precision involves the choice of apparatus and the skill with which it is used.
I had to also be aware of the different types of error that could occur in my experiment, systematic (values differing from the true value by the same amount) or random (values lying equally above or below an average value). I will make sure I no how to use all the apparatus correctly, because it is this problem that leads to a systematic error in the results, making them invalid. I will make sure I know how to calibrate the colorimeter before use. I will also make sure that I give my full concentration and attention towards the experiment, as it is a lack or wane in students concentration, which causes a random error to occur. The occurrence off a random error can be reduced if appropriate sensors or data loggers are used, it can introduce objectivity.
Variables – The independent variable is the factor that is being varied. A rang of values for the independent variable can be chosen. The dependant variable depends on the value of the independent variable. The dependant variable is the one that is measured in some way. Any other variables that may affect the dependant variable should be controlled (kept constant) in order to produce results that are reliable.
In this experiment the independent variable is the temperature of the water, which the beetroot is being bathed in. The dependant variable is the percentage absorbency. The control variables would be size of beetroot slices.
Apparatus – To carry out this experiment, the following apparatus are needed:
Size 4 cork borer, white tile, knife, ruler, water baths, plastic beaker, 2 boiling tube racks, crushed ice, 8 boiling tubes, thermometer for each water baths, colorimeter, cuvettes, stop clock, distilled water, and pipettes.
Procedure – Firstly sections from a single beetroot have to be cut using a size 4 cork borer. Eight, 1cm length, slices have to be cut from this section. These slices have to be left in a beaker of distilled water over night, to wash away any excess dye.
The next day, eight boiling tubes have to be labelled, each containing 5cm3 of distilled water. These tubes then have to be left in water baths of temperatures at 0oC, 10oC, 20oC, 30oC, 40oC, 50oC, 60oC, and 70oC. The tubes are to be left in the baths for five minutes until the water has reached the required temperature. The next step is to place a beetroot slice in each tube and leave in the water baths for a further five minutes.
After five minutes, the slices are to be removed, and the solution is to be shaken to disperse the dye.
We then have to switch the colorimeter on and set it to read % absorbance. And set the filter dial to the blue/green filter.
The next step is to measure 2cm3 of distilled water and using a pipette, transfer it to a cuvette. The cuvette is then to be placed in the colorimeter, making sure that the light is shining through the smooth sides. We then have to adjust the colorimeter to read 0 absorbance for clear water. The setting is not to be altered during the experiment.
Once the settings are set, we shall take 2cm3 of the dye solution from each boiling tube and place it into the colorimeter and take readings for absorbancy.
Observation/Results – When the beetroot was left in the tube at low temperatures, very little or no dye leaked out. As we increased the temperature, we found that the amount of dye that leaked out increased rapidly, from 40-70oC percentage absorbance increases fast. As temperatures increase above 60oC, the amount of dye leaked out or % absorbance does not increase at the same rapid rate.
Below is a table of results which we recorded from the experiment.
The general trend has to be outlined; as the temperature increases the % transmission decreases. This backs up my hypothesis in my plan. This is because as temperature rises the movement of the phospholipids increase allowing more pigment to move across the membrane. As temperature rises to a point which the proteins in the membrane get de natured. The actually structure of the protein is destroyed due to the high temperature. Coincidently this leaves holes in the membrane. This is shown on the graph were there is a increase in gradient. . This occurs between 40 degrees Celsius to 60 degrees Celsius. After 60 degrees Celsius the structure of the proteins in the bilayer are non functional and thus the percentage transmission is at the least.
The denaturing occurs when high temperatures break the tertiary structure of a protein. The hydrogen bonds between the proteins break due to the high temperature and so do the di-sulphide bonds. Referring to my hypothesis I predicted that denaturing would occur at 45 degrees Celsius. Thus I would not of expect a lot of change in the percentage transmission between 20 and 30 degrees Celsius. Because at 30 degrees Celsius I would have presumed that the membrane would have been fairly stable. However my graph shows the contrary. This might be due to sources of error in my experiment.
Experiments - I have successfully gained my results that back my hypothesis. However the results have limitations and some sources of error. The size of the beetroot is essential. In my experiment I used a ruler and cork bore to measure out the same size beetroot. The cork borer was a a good aid as it ensured that the beetroot had the same diameter. However using a ruler could present possible error. Due to the surface area to volume ratio.
As the test tubes were taken out of the water bath and placed the in the colorimeter the temp reduced by a few degrees. This small error could of change the results by 0.5 percent inaccuracy. This is because of insufficient equipment.
I understand this experiment is only on a small scale but to gain better results each test tube going up in 10 degrees Celsius so we can tell the point of denaturing more accurately, however I feel that I my experiment showed breath of how a membrane functions under temperature change