2.5 Biological Membranes Flashcards
What are the roles of a cell membrane ?
•Serve as partially permeable barriers between cell and environment and cell and cytoplasm
•Sites of chemical reactions
•Sites of cell communication / cell signalling
What does the term ‘partially permeable’ mean in reference to cell membranes ?
A membrane that will allow certain molecules or ions to pass through it by diffusion and facilitated diffusion.
What are the roles of phospholipids in the cell membrane ?
•Phospholipids form the major structure of the bilayer and have hydrophobic and hydrophilic aspects.
•This means that the cell can control the movement of molecules in and out of the cell
•The phosphate of the molecule is hydrophilic, the fatty acid tails are hydrophobic.
What does cholesterol affect in the phospholipid bilayer ?
*Cholesterol increases permeability at low temperatures and decreases permeability at high temperatures.
What do transport proteins do ?
•Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
•This allows the control of the movement of molecules in and out of a cell
What do peripheral proteins do ?
*Peripheral proteins are involved in cell recognition / signalling and some transport.
What do glycoproteins and glycolipids do ?
•Glycoproteins / lipids have carbohydrate chains that act as receptors in cell signalling
•This allows for hormones to bind to cells as well as for recognition by the body
•The carbohydrate chain also allows cell-cell connection, increasing stability.
What is the point of membrane bound receptors ?
•Hormones and drugs can bind to these receptors to cause chemical reactions
•Allows inter and extra cellular communication
How does temperature affect membrane permeability ?
•Higher temperatures increase permeability and fluidity, lower temperatures decrease permeability and fluidity.
•This is because there is an increased kE of bilayer and therefore more movement, as well as the deformation of carrier proteins.
How would fluidity of the phospholipid bilayer change if ethanol is used as a solved instead of water ?
*It would increase as lipids are soluble in ethanol.
Suggest a method for a practical investigating how temperature affects permeability of a beetroot sample.
*Using a cork borer, cut 5 samples of beetroot to the same length in order to have roughly an equal volume, mass and surface area
*Rinse samples with water to take off pigment on surface and place into test tubes with water
*Place tubes into water baths of 10, 20, 30, 40 and 50°C for an equal length of time e.g. 30 mins.
*After this period, take out the tubes, take out the beetroot from them and place the liquids into individual cuvettes for the colorimeter.
After the practical investigating how temperature affects permeability of a beetroot sample, how would you use a colorimeter to get valid results ?
*Before inserting the various cuvettes with pigment, the colorimeter bust be zeroed using a cuvette with distilled water the blue filter.
*after this, place each cuvette in and measure %absorbance with the blue filter.
*The result should be that the highest temperature has the highest % absorbance as the most pigment has left the sample.
Why does the permeability increase with temperature?
*As temperature increases, the fluid mosaic model has more kE. This means that the model has gerater fluidity, is less tightly packed and can therefore allow more molecules to travel out of the cell.
*Also, at very high temperatures the structure of channel and carrier proteins is deformed, allowing uncontrollable movement of molecules in and out of cells.
What are the limitations of the experiment testing how temperature affects permeability of a beetroot sample?
*Cuvettes may differ in thickness or be damaged, causing invalid results from colorimeter. To solve this issue use the same cuvette for each reading.
*The beetroot samples may have varying amounts of tissue in each sample. To solve this issue cut each sample as accurately as possible and repeat results.
*Each part of the beetroot has varying amounts of pigment. To solve this, conduct lots of repeats and find a mean.
Suggest a method to Investigate the rate of diffusion using visking/dialysis tubing.
*Viksing tubing is a semi-permeable membrane which allows small molecules like glucose out but keeps larger molecules like sucrose or starch in.
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