Cell membranes and transport Flashcards
Fluid mosaic model
Fluid because both the phospholipids and proteins can move about by diffusion. Mosaic refers to the pattern of the protein molecules when the surface of the membrane is viewed from above, there are lots of different protein molecules
Features of the phospholipid bilayer
The membrane is a double layer (bilayer) of phospholipid molecules, they move about by diffusion within their monolayer. The phospholipid tails face inwards forming a non-polar hydrophobic interior. The heads face the aqueous medium.
How does the structure of the phospholipids affect the membrane
The more saturated they are the more fluid the membrane. The longer the tail the less fluid the membrane. As temperature increases the membrane becomes more fluid.
Phospholipids in membranes
Because the tails of the phospholipid are non-polar it is hard for polar molecules to pass though. Can be modified chemically to act as signalling molecules
Intrinsic proteins
Can be found in the inner or outer layer, or spanning the whole membrane as transmembrane proteins. In transmembrane proteins the hydrophobic regions are made up of alpha helical chains. Intrinsic proteins have hydrophobic and hydrophilic regions. They stay in the membrane because of the hydrophobic regions repelling the water.
Cholesterol
Have a hydrophilic head and a hydrophobic tail. At low temperatures it increases the membranes fluidity. It decreases the membranes fluidity at high temperatures.
Glycolipids and glycoproteins
Some of the lipid and all of the protein molecules have short carbohydrate chains attached to them. The carbohydrate chain forms hydrogen bonds with the water molecule and helps to stabilise the membrane structure.
Carbohydrate chain
Helps the glycolipids and glycoproteins act as receptor molecules which bind with different substances on the cell surface
What do the receptor molecules on the cell surface membrane act as
Signalling receptors - coordinates the activity of the cell
endocytosis
antigens - allowing cell to cell recognition
Transport protein
Provide hydrophilic channels for ions and polar molecules to pass through the membrane. The two types are channel and carrier proteins. Some proteins form part of the cytoskeleton
The process of cell signalling
The signal arrives at the protein receptor on the cell surface membrane. The receptor is a specific shape which recognises the signal. The signal changes the shape of the receptor, as this spans the membrane the message is passed to the inside of the cell (signal transduction). This activates a G protein which acts as a switch to release the “second messenger”, which diffuses through the cell relaying the message. Many second messengers can be made which amplifies the original message. The second messenger activates an enzyme which activates further enzymes increasing the amplification at each stage. Until an enzyme is produced which brings about the required change in cell metabolism
Signalling cascade
The sequence of events triggered by the G protein
Diffusion
The net movement of molecules or ions from a region of high concentration to a region of low concentration down a gradient, as a result of the random movement of particles.
Other ways a receptor can alter the activity of a cell
Opening an ion channel, resulting in a change of membrane potential.
Acting directly as a membrane bound enzyme.
Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane.
Demonstrating diffusion using Visking tubing
A partially permeable non-living membrane made from cellulose. Large molecules such as starch and sucrose will not pass through but glucose will. This can be demonstrated by filling visking tubing with starch and glucose and testing the water around it for it. You could investigate the amount of glucose over time by using a colorimeter.
