Janet Macaulay; Week 2 MED1011; Biochemistry
Fluid mosaic model is that of carbohydrates (attached to the outer surface of proteins forming glycoproteins or lipids), in animal cells some membrane proteins must associate with filaments in the extracellular matrix, peropheral membrane proteins do not permeate hte bilayer, cholesterol interspersed among bilayer controls permeability (regulates fluidity preventing rigidity at different temperatures), some membrane proteins interact with cytoskeleton, some integral proteins cross whole phospholipid bilayer but some only penetrate partially. All these molecules moving within the fluid bilayer form the fluid mosaic model. This is possible because the fatty hydrocarbon tail of the phospholipids in the bilayer are unsaturated and therefore the bilayer is not packed.
Glycolipids are only found on the outside of the cell. Receptor proteins are found within their binding site for extracellular hormones or ligands on the outside.
Transport proteins facilitate movement in one direction only. Passive tranport is down the membrane down a concentration gradient, facilitated transport is through a transport channel down a gradient, active transport is through a channel and against a gradient. Diffusion through channel is passive transport through a channel, ion channel for example, can be ligand gated, just like facilitated diffusion. Lipid membranes are permeable to small polar molecules or non-polar molecules (water, urea, carbon dioxide, ethanol).
Gated ion channels have polar compounds concentrated outside the cell, ligand binds and opens channel, ions move through. Facilitated diffusion is very specific, has binding site for one molecule. Active transport can be either uniporters, symporters or antiporters.
Different energy sources distinguish transport systems; primary active transport has direct hydrolysis of ATP, secondary active transport does not use ATP directly (driven by initial hydrolysis), energy supplied by ion concentration and electrical gradient established by primary active transport, uses the energy of ATP indirectly to create gradient
- For example, 3 Na bind, phosphorylation moves them through, shape change allows 2K to bind, they travel into cell, passage opens Na sites and releases phosphate
Secondary active transport is where primary active transport establishes gradient for another transporter, such as Na/K establishing Na/glucose to travel back into the cell. Glucose transport across intestinal epithelium has 2 mechanisms, one is across concentration gradient (entry through symport with Na), from cells to the blood is by facilitated diffusion (glucose uniporter).
Endocytosis uses protein called clathrin which coats the cytoplasmic side of the plasma membrane.
Fatty acid composition of membrane phospholiids is important in transport of nutrients through the membrane. Polyunsaturated fatty acids improve insulin stimulated glucose uptake in skeletal muscle. Saturated fatty acids are linked with insulin resistance.