To investigate the effect of different concentrations of ethanol on the permeability of beetroot cell membranes - Biology Coursework

GCSE: Biology

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Prediction: By exposing a membrane to a solvent, ethanol, it will affect its permeability. Therefore higher the concentration of the solvent, the more permeable the membrane will be. But by increasing the concentrations of the ethanol beyond a certain point it may have no affect on the cell membrane because the ethanol would have broken down the cell membrane completely. This is because of the effect of ethanol on the lipids in the membrane. Also, more dye would be present as the lipids are essential to the structure of the cell membrane as they control the substances that enter and leave the cell. Ethanol could also destroy some the proteins or denature the structure because protein has a tertiary structure. The ethanol would destroy the hydrogen bonds that hold the protein structure. Therefore, with the lipids and proteins destroyed in the cell membrane, the pigment is allowed to escape from the cell due to there being no cell membrane holding substances in the cell.

Scientific Knowledge:
Transfer across cell membrane structure:
There are many ways that ions and molecules are transported across the cell membrane. By this, the cell must acquire the ions and molecules they need from their surrounding ‘extracellular fluid’. With the cell membrane being in contact with ethanol in the extracellular fluid, there is direct contact with the solvent. Therefore, by diffusion the ethanol is secreted into the cell and along the way destroys the permeability of the cell membrane.

The cell transports ions and small molecules across their membranes by the following means:
Osmosis:
By osmosis the diffusion of water through the plasma membrane is possible. Since the lipids bilayers are impermeable to essential molecules and also to a few small molecules like oxygen and carbon dioxide, these molecules and ions diffuse freely across the cell membrane.
Diffusion or Facilitated diffusion (Passive transport):
By diffusion the ions and molecules move spontaneously across their concentration gradient (from a region of higher to a region of lower concentration). Small, polar, hydrophilic molecules, like oxygen, and water, can pass through cell membranes by diffusion. Where as larger hydrophilic molecules, like glucose, can pass through the cell membrane by facilitated diffusion. In all cases of facilitated diffusion through channels, the channels are selective. Meaning that the structure of the protein admits only certain types of molecules through (hydrophilic pores). Where as non-polar, hydrophobic molecules are not impeded by the phospholipids in the cell membrane. This means that they are small enough to fit between the phospholipids layers. The factors that affect the rate of diffusion are: concentration gradient, temperature, pH (the charge), surface area, size of molecule and the width of the diffusing space.
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