MEMBRANE STRUCTURE

Phospholipids

* PHOSPHOLIPIDS FORM BILYAERS IN WATER DUE TO THE AMPHIATIC PROPERTIES OF PHOSPHOLIPID MOLECULES.

- Made of two fatty acid chains (reason why we need fat in our diet) joined to a polar, phosphate containing head.

- The head is hydrophillic (phosphate group).

- The tail is hydrophobic (2 hydrocarbon chains).

- Amphipathic is the property of being both hydrophillic and hydrophobic.

- Arranged in a (phospholipid) bilayer.

- An important component of all membranes found in cells.

* ANALYSIS OF EVIDENCE FROM ELECTRON MICROSCOPY THAT LED TO THE PROPOSAL OF THE DAVSON DANIELLI MODEL.

- In 1920, Gorter and Grendel extracted phospholipid molecules from the plasma membrane, they were twice as thick than expected, hence the bilayer.

- In 1930, Davson and Danielli proposed a model where proteins were located above and below the bilayer. They thought it explained how despite being thin, it was an effective barrier to the movement of certain substances.

- Theory excepted in 1950 because electron mocroscopy showed two dark lines with a lighter band in the middle.

* ANALYSIS OF THE FALSIFICATION OF THE DAVSON-DANIELLI MODEL LED TO THE SINGER-NICOLSON MODEL.

- When the phospholipid bilayer was freeze-etched, proteins were revealed to be found within the bilayer.

- Antibody tagging showed that proteins are free to move throughout the membrane.

- Structure: Proteins in the membrane did not form a continuous layer meaning that they each had specific functions in the membrane.

- Singer and Nicolson proposed that integral proteins were embedded in the membrane and peripheral proteins were attatched on the inner & outer sides. The proteins are free to move (fluid) and they looked like tiles in a "mosaic", hence the fluid mosaic model.

* DRAWING OF THE FLUID MOSAIC MODEL.

* CHOLESTEROL IN MAMMALIAN MEMBRANES REDUCES MEMBRANE FLUIDITY AND PERMEABILITY TO SOME SOLUTES.

* CHOLESTEROL IS A COMPONENT OF ANIMAL CELL MEMBRANES.

- A type of lipd but not fat or oil.

- Majority of cholesterol molecules are hydrophobic, so they are attracted to the phospholipid tails.

- The end of cholesterol is hrydrophillic (OH group), so it orientates itself nearer to the phosphate heads.

- Cholesterol disrupts regular packing of hydrocarbon tails to prevent them from crystalizing.

- They restrict molecular motion.

- They reduce permeability and fluidity of the membranes.

- If membranes are too fluid, control over what enters the cell would be lost. If membranes are too rigid, the movement of cells, the formation of vesicles, and the processes of endocytosis & exocytosis would not be able to occur.

* MEMBRANE PROTEINS ARE DIVERSE IN TERMS OF STRUCTURE, POSITION IN THE MEMBRANE AND FUNCTION.

- Proteins may be loosly fitting in capillaries. Only allows small digestive molecules into the gut.

Integral and peripheral proteins:

- Provide channels for passive transport of hydrophillic materials (channel and carrier proteins).

- Act as active transport pumps to move substances across the membrane (protein pumps).

- Act as hormone binding sites (hormone receptors do this and turns on the gene).

- Are immobilised enzymes with active site on the outside.

- Helps with cell adhesion forming junction between neighbouring cells in tissues and organs.

- Cell to cell communication (receptors for neurotransmitters at synapses.

- The molecules sticking out of the bilayer are all sugar molecules, hence the "glyco".

- If these sugar molecules are attached to proteins, they are called glycoproteins.

- If these sugar molecules are attatched to the phospholipids, they are called glycolipids.

- Glycoproteins and glycolipids: help to stabilise the membrane structure as they forms hydrogen bonds with water molecules in the fluid surrounding the cell. They also act as receptors and help in cell identification, hormonal responses and neurotransmission. Glycoproteins can act as carrier proteins.

- Cytoskeleton: a series of intercellular proteins that help a cell with shape, support, and movement.