Chapter 5-5.2- Factos affecting membrane structure Flashcards
What do membranes control?
The passage of different substances into and out of cells (and organelles).
If the membranes lose their structure they lose control of this and cell processes will be disrupted. A number of factors affect membrane structure including temperature and the presence of solvents.
How does temperature affect membrane structure?
Phospholipids in a cell membrane are constantly moving. When temperature is increased the phospholipids will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure.
If temperature continues to increase the cell will eventually break down completely.
This loss of structure increases the permeability of the membrane, making it easier for particles to cross it.
Carrier and channel proteins in the membrane will be denatured at higher temperatures.
These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.
How do solvents affect membrane structure?
Water, a polar solvent, is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipids are oriented away from the water, forming a bilayer with a hydrophobic core.
The charged phosphate heads interact with water, helping to keep the bilayer intact.
Many organic solvents are less polar than water for example alcohols, or they are non-polar like benzene.
Organic solvents will dissolve membranes, disrupting cells. This is why alcohols are used in antiseptic wipes. The alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection.
Pure or very strong alcohol solutions are toxic as they destroy cells in the body. Less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but still cause damage.
The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane.
When the membrane is disrupted it becomes more fluid and more permeable. Some cells need intact cell membranes for specific functions, for example, the transmission of nerve impulses by neurones (nerve cells). When neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal.
This also happens to neurones in the brain, explaining the changes seen in people’s behaviour after consuming alcoholic drinks.
How can membrane permeability be investigated?
Beetroot cells contain betalain, a red pigment that gives them their distinctive colour, because of this they are useful for investigating the effects of temperature and organic solvents on membrane permeability.
When beetroot cell membranes are disrupted the red pigment released into a solution is related to the disruption of the cell membranes.
To investigate the effect of temperature on the permeability of cell membranes a student carried out the following procedure.
Five small pieces of beetroot of equal size were cut using a cork borer. The beetroot pieces were thoroughly washed in running water, they were then placed in 100ml of distilled water in a water bath. The temperature of the water bath was increased in 10(*C) intervals.
Samples of the water containing the beetroot were taken five minutes after each temperature was reached.
The absorbance of each sample was measured using a colorimeter with a blue filter.
The experiment was done three times, each time with fresh beetroot pieces and a mean calculated for each temperature.
