Their amphipathic characteristic means that they are composed of both hydrophilic and hydrophobic regions. They are formed of a hydrophilic head and a hydrophobic tail. The hydrophilic heads are polar molecules therefore can interact with water molecules, whereas the hydrophobic tails cannot because they are non-polar molecules, so instead of interacting with the water molecules, they stick together making it difficult for polar molecules and ions to pass through. Within the phospholipid bilayer, there are also lipids and proteins, these and the amphipathic property of the membrane is why it is selectively permeable.
This characteristic therefore allows the cell to control the passage of substances in and out of the cell; meaning that the necessary substances can be taken in, whilst the unnecessary and harmful substances are left out. Additionally, being selectively permeable is also important because it allows the maintenance of a concentration gradient between the cytoplasm and the external environment. The beetroot cell is composed of a pigment called betanin (a type of betalain pigment) which is contained within the vacuole of the cell, this pigment is what gives beetroot cells their purple-red colour. In a normal beetroot cell that has not be subjected to changes in temperature or exposed to solvents, it is difficult for it to diffuse out because of how enclosed it is within the cell. It needs to be able to pass through the tonoplast (the membrane layer of plant cell vacuoles), and then through the cell membrane of that beetroot cell.
However, if the pigment does manage to diffuse through the tonoplast and the cell membrane, it can readily diffuse into the external solution as it is water-soluble. Certain factors and conditions can damage a cell’s membrane therefore affecting the cell’s permeability. This means that large molecules and ions that aren’t normally able to pass through, such as the betanin pigment can now readily diffuse through the membrane into the surrounding solution. The disruption and destruction of the cell membrane means that the cell is no longer a contained system. Cell membrane damage can be caused by several factors including extremes in temperatures, incorporation of inorganic solvents and other physical and chemical factors.
All of these can disrupt the arrangement of the phospholipid bilayer and can denature the embedded proteins and lipids that regulate the passage of molecules. This report will therefore investigate the effect of temperature, solvents and freezing on cell membrane permeability of beetroot cells. A beetroot cylinder will be subjected to different conditions and the absorbance readings of these solution will be recorded. The absorbance reading will be able to tell us an estimation of the concentration of betanin that leaked out of the cell.