Biological Membranes

• Controlls substances leaving and entering
• Partially mermeable (can move across plasma membrane by diffusion, osmosis or active transport
• Allow recognition of other cells eg: cells of immune system
• Allow cell communication

• Membrane around organelles divide cell into compartments- barrier for organelle and cytoplasm        

                - makes functions more efficiant (eg: substances for respiration are kept together in                              mitochondria)

• Can form vesicles to transport substances
• Controll what enteres and leaves organelle
• Can be reaction sites - eg: inner membrane of mitochobdria cintains enzymes needed for respiration

• Structure of proteins are usually same: composed of lipids (mainly phospholipids), proteins and carbohydrates
• Fluid mosaic model describes the arrangement of molecules in the membrane 
• Phospholipids form a continuous double bilayer 
• Cholesterol molecules present within bilayer
• Bilayer is fluid; constantly moving
• Proteins are scattered through bilayer (like mosaic)
• Some proteins have a polysaccaride chain attatched- glycoproteins 
• Some lipids have a polysaccaride chain- glycolipids
• About 7nm thick

• Form a barrier to dissolve substances 
• Have a hydrohpillic head and hydrophobic tail
• Molecules automatically arrange themselves to form bilayer (head faces out on either side of membrane)
• Middle of bilayer is hydrophobic so doesnt let ions through (no water solubles molecules, but fat-soluble substances can pass through)

• Gives membrane stability
  
• Cholestrol = type of lipid
• Present in all membranes (excluding bacterial cell membranes)
• Cholesterol molecules fit between phospholipids, makes them pack closer together  (makes it less fluid, more rigid)

• Proteins controll what enter and leave cell
• Some proteins form channels, these allow small or chaeged particles through
• Other proteins (carrier proteins) transport molecules and ions across membrane by active transport and facilitated diffusion
• Proteins also act as receptors for molecules in cell signalling (whena molecule sbinds to the protein, a chemical reaction is triggered inside cell)

• Glycolipids and glycoproteins act as receptors for messenger molecules
• They stabalise membrane bgy forming hydrogen bonds with water molecules 
• Also site where drugs, hormones and antibodies bind
• Act as receptors for cell signalling 
• Also anitgens

• Increasing temp. = increase membrane permeablity
• Temp below 0 'C:

> phospholipids have little engergy = cant move alot. Packed closely, membrane is rigid, channel and carrier proteins deform (increasing permeability). Ice crystals form and pierce membrane 

• Temp between 0-45 'C:

>phospholipds can move and arent packed as tight, partially permable. As temp increases, phospholipids move more

• Temp above 45 'C:

>phospholipd bilayer melts = membrane is permable, water inside cell expands putting pressure on membrane. Chanel and carrier proteins deform so cant control what eneters and leaves 

• Surrounding cells in solvents + permeability; solvents dissolve lipids in membrane so it loses its structure 
• Some solvents + permeability morethan others (ethanol more than methanol)
• Can investigate this by using the beetroot experiment
• + concentration of solvent will also +  permeablilty