Membranes: An Overview PDF

Membranes: An Overview The PLASMA MEMBRANE defines the limits the cell. Membranes also define the limits of sub-cellular organelles such as the NUCLEUS and MITOCHONDRIA. Membranes regulate the transport of solutes Membranes detect and transmit electrical and chemical signa...

Membranes: An Overview The PLASMA MEMBRANE defines the limits the cell. Membranes also define the limits of sub-cellular organelles such as the NUCLEUS and MITOCHONDRIA. Membranes regulate the transport of solutes Membranes detect and transmit electrical and chemical signals Membranes provide surfaces upon which reactions can be conducted, such as the ROUGH ENDOPLASMIC RETICULUM Biomembranes are composed of lipid bilayers A major component of bio-membranes are LIPIDS The lipids found in cell membranes are AMPHIPHATIC, because they contain a polar head unit as well as non-polar ‘tail’ The HYDROPHILIC heads and HYDROPHOBIC tails of lipids cause them spontaneously compose themselves into lipid bi-layers  Amphiphatic lipids can form bilayers Heads face into the aqueous solution (either the outside or inside of the cell) while the tails stack upon themselves Two layers of ‘stacked’ lipids create the hydrophobic core of the LIPID BILAYER  More about Lipids Examples of amphiphatic membrane lipids are are PHOSPHOGLYCERIDES, SPHINGOLIPIDS, (these are both types of PHOSPHOLIPIDS) and CHOLESTROL Lipids are not only important for membranes but are also important in energy storage and cell signaling  The classes of membrane lipids Phospholipids: Ø Phosphoglycerides Ø sphingolipids Glycolipids: Carbohydrate groups are attached to the lipid Cholestrol: Cholestrol is a steroid.  Different membranes have different lipid compositions Membranes fulfill many different roles in a cell therefore their properties must vary. Varying the lipid composition varies the physical properties of the membrane Fluidity of the membranethe membrane is dependent on Ø The ratio of saturated to unsaturated fatty acids Ø The length of the fatty acid tails The percentage of cholestrol can affect the fluidity of the membrane Ø Cholestrol can ‘buffer’ the fluidity Ø Stops in becoming too fluid Ø Stops it becoming too crystaline The head to tail size ratio can influence membrane curving  The lipid bilayer is fluid (the fluid mosaic model) Membrane lipids can move laterally in their lipid layer, but rarely flip into the opposing layer of the bilayer The long chain fatty acids of the phospholipids pack against each other stabilized by van der waals forces The longer the chain, and hence the greater the van der waals forces, the less freedom (fluidity) the lipid will have in the membrane  Flippases The two layers of the lipid bi-layer are sometimes referred to as LEAFLETS These leaflets often have different composition of lipids To move from one leaflet to another is a much slower process than lateral movement within the leaflet The process is facilitated by integral membrane proteins called FLIPPASES  Proteinsʻfloat in the lipid bilayer A variety of PROTEINS float in the lipid bi-layer. They represent the mosaic part of the FLUID MOSAIC model of membrane structure. The proteins associated with the lipid bi-layer can be broken down into 3 main groups 1) Lipid anchored membrane proteins 2) Peripheral membrane proteins 3) Integral membrane proteins  The layers of the lipid bi-layer are asymmetric The layers can be asymmetric both in the composition of the lipids and proteins. Some proteins are mobile in their layer, while others are locked in place by anchoring to either the CYTOSKELETON (inside) or the EXTRACELLULAR MATRIX (outside). The EXTRACELLULAR (ECM) matrix lies outside the plasma membrane of multicellular eukaryotes The CYTOSKELETON is an array of filaments beneath the plasma membrane that provides support to the cell. Blood grouping The three types of filament  The cell wall The CELL WALL is a rigid structure surrounding plant (and some fungi) cells It is composed of CELLULOSE MICROFIBRILSembedded in PECTIN and HEMICELLULOSE The plasma membrane is associated with the cell wall The cell wall provides support for the cell and helps prevent it bursting in a hypertonic solution  To recap Membranes define cells and organelles within cells The environment within a membrane is often different to the environment outside the membrane Membranes are composed of amphiphatic lipids Proteins ‘float’ within the fluid membrane The plasma membrane is sandwiched between an extracellular matrix and the cytoskeleton Plants cells have a cell wall