An experiment to test the effect of different temperatures on the permeability of cell membrane.

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An experiment to test the effect of different temperatures on the permeability of cell membrane.

Aim

In this experiment, I am trying to find out how, why, and by how much does the difference in temperature affects the permeability of cell membrane of plant cells.

Prediction

Having done a pilot experiment and by using the scientific knowledge gathered, some predictions can be made. The permeability of cell membrane should increase as the temperature increases, and this is shown by the decrease in the percentage transmission of light on the colorimeter.  

As guidance, the results of the pilot experiment could be used:

Theory of prediction

There are five possible ways that the dye could leak out of the cell in normal condition without the aiding of heat, and these are simple diffusion through the lipid bi-layer, facilitated diffusion by protein carriers, and facilitated diffusion via ion channels, active transport, and exocytosis.

  • The dye would have no trouble passing through the cell wall by diffusion as it is fully permeable, but the tonoplast and the cell membrane are only partially permeable, and would only allow non-polar substances to pass through. As anthocyanin is a polar substance, it cannot leave the cell by diffusion. Active transport is the next possible way for the dye to leak out, but this process requires energy, and so it is not likely to occur. Exocytosis is possible through the cell membrane, but it is a process involving bulk transport, and this is not likely to take place through the tonoplast.

  • After looking at all the possible ways in which the dye could leak out of the cell, it can be certain that the dye would not leak out of the cell in normal conditions, and this means that the cell membrane is quite impermeable at room temperature. 
  • By increasing the temperature, it is predicted that the cell membrane would become more permeable, and this is due to several reasons:
  • The phospholipids bi-layer of the cell membrane is similar to a fluid layer as the phospholipids can move around and change places with each other, and when this happens, sometimes spaces appear between the phospholipids, and the anthocyanin particles could pass through. Although this is possible, but these spaces rarely appear as the movement of phospholipids are limited in normal conditions. But if the temperature increases, the lipids would have more kinetic energy, they would move more quickly and violently, resulting in more spaces to occur, and thus the cell membrane would become more permeable.

  • The cell membrane is often seen as a fluid mosaic model, and it is mosaic due to all the different proteins and cholesterol among the phospholipids. In normal condition the dye would not be able to pass through this complex membrane layer, but when temperature rises and gets above 40°C, the transport protein, receptor protein, glycoprotein and all other carrier proteins would start to denature. When the proteins denature, they would change shape or rupture, and no longer fit in the membrane, allowing spaces for leakage to appear and the membrane becomes more permeable. The anthocyanin particles can pass through these gaps created, and this shows that the membrane gets more permeable with increasing temperature.
  • Although the actual cell membrane is much more complex than the fluid mosaic model, the basic theory would still works, and the main reason for the membrane to get more permeable with increasing temperature is that the protein would get denatured. This means that temperature and permeability are not in direct proportion, but the ratio would change when it gets above 40°C when permeability increase more per amount of temperature increased.

Justification of apparatus.

  • The test tubes are used to contain the beetroot cells in the fixed amount of water so that the anthocyanin dye only go into the water in the test tube and not into the water bath. They can fit the beetroot cells in and have them totally emerged in the amount of water used.
  • Thermometers are used to check the temperature of the water baths to ensure that they are constant and correct. These thermometers are accurate to the nearest °C.
  • A digital colorimeter is used to record the % transmission of light through the water that contained the beetroot cells. This is used instead of an analogue one as digital would be more accurate than the human eye, and produce the required level of accuracy. This digital colorimeter is accurate to the nearest 1% while the analogue one is accurate to the nearest 5%.
  • The cuvettes are used to hold the water that was in the test tubes when the water needs to be read by the colorimeter. Cuvettes have unique and accurate shape, size and volume, and their sides are very transparent, reducing the amount of light being absorbed by the cuvettes to be a minimum and have a good accuracy.
  • Scalpel is used to cut the ends of the beetroot so that they are even and that each pieces are the same. The scalpel is very sharp, and so can cut the beetroot cells to be the same shape, size and have the same surface area every time, giving a high level of accuracy.
  • The timer is to make sure each test tube is placed in the hot water for the same amount of time and a fixed amount of time. The timer is definitely accurate enough as the accuracy is limited to the reaction speed of the human hand. It is also more accurate than having the time counted by a human.
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  • The white tile is where we cut the beetroot so that we do not contaminate them with the things on the bench, and also to keep the bench clean from dye.
  • The syringe is used to measure the exact same amount of water in each test tube. A syringe is used as it is much more accurate than the measuring cylinder in the level of measuring 5ml of water, and that they are measured to the nearest 0.2ml, while the measuring cylinder is accurate to the nearest 2ml, showing that the syringe is 10 times more accurate.
  • The cork ...

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