Membranes introduction

Importanat part of the cell as they provide structure. In eukaryotes they also provide compartments for different processes. They are also critical in the regukationa dn production of celluar energetics. 

Functions:

First evolved to be a barrier but soon needed to be able to regulate and allow nutrients in and watse out so developed channels. They then developed pumps which do this against a concentration gradient. (fundamental functions similar in all organisms) 

Later fuunctions devloped which are different in all organisms. These include;

• conversion of membrane potential/gradient to energy (most cells do this)
• cellular recognition (different in prokaryotes and eukaryotes)
• signalling (all cells but no universal system)
• membrane trafficking (eukaryotic cell) 
• compartmentalisation (eukaryotes only) 

Eukaryotic cells are roughly 1000x larger in volume than a prokaryotic cell. This means that for prokaryotic cells random transport/diffusion can transport most things around by eukaryotic cells need a transport system and compartments.

Compartments are vital due to this increase in size in prokaryotic cells to seperate different functions into different spaces. Almost all important functions in eukaryotes are conatined within compartments. Internal membranes make up more of the cell than the plasma membrane. 

Membranes are made of lipids (more detail in a few cards) which have a hydrophillic head and are hydrophobic elsewhere. This means they spontaneously aggregate in water. In lab conditions they can form liposomes, bilayers and vesicles etc but in cellular conditions only form bilayers. Hydrophobic proteins insert into this bilayer and move around laterally producing the fluid mosaic model. Which is basically correct but not quite. 

Lateral mobility of proteins can be seen using fluorescence recovery after photobleaching (FRAP). During this process fluorescent reagent is attached to the membrane problems and a section is bleached with a laser. This results in a dark patch on the cell surface and the fluorescence is supposed to recover as the proteins begin to move. However, only about half recover suggesting that not all lipids can move. Implies fluid mosaic model isnt whole story . 

Atomic force microscopy (AFM) shows the height is different components of the membrane. Images of membranes show that lipids form denser and thinner areas of membrane. Proteins embed in the denser areas and stick up above the lipid bilayer. Problem with fluid mosaic model is that it focuses on proteins and assumes all lipids are the same which they aren't.