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It has been known for decades that the addition of cholesterol to a fluid phase bilayer decreases its permeability to water. [6] [7] The mode of this interaction has more recently been shown to be due to cholesterol intercalating between lipid molecules, filling in free space and decreasing the flexibility of surrounding lipid chains. [8]
While the addition of one double bond raises the melting temperature, research conducted by Xiaoguang Yang et. al. supports that four or more double bonds has a direct correlation to membrane fluidity. Membrane fluidity is also affected by cholesterol. [2] Cholesterol can make the cell membrane fluid as well as rigid.
Because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails. In this way, cholesterol tends to stiffen the bilayer, making it more rigid and less permeable. [5] For all cells, membrane fluidity is important for many reasons.
Most polar molecules have low solubility in the hydrocarbon core of a lipid bilayer and, as a consequence, have low permeability coefficients across the bilayer. This effect is particularly pronounced for charged species, which have even lower permeability coefficients than neutral polar molecules. [ 61 ]
Cholesterol is the dynamic "glue" that holds the raft together. [3] Due to the rigid nature of the sterol group, cholesterol partitions preferentially into the lipid rafts where acyl chains of the lipids tend to be more rigid and in a less fluid state. [6] One important property of membrane lipids is their amphipathic character.
The more cholesterol a rat eats the lower the blood cholesterol. [16] During the first seven hours after ingestion of cholesterol, as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins (which transport all fats in the water outside cells), the concentrations increase.
Cholesterol molecules are also found throughout the plasma membrane and act as a buffer of membrane fluidity. [3] The phospholipid bilayer is most permeable to small, uncharged solutes . Protein channels are embedded in or through the phospholipids, [ 4 ] and, collectively, this model is known as the fluid mosaic model .
Thermodynamically the flow of substances from one compartment to another can occur in the direction of a concentration or electrochemical gradient or against it. If the exchange of substances occurs in the direction of the gradient, that is, in the direction of decreasing potential, there is no requirement for an input of energy from outside the system; if, however, the transport is against ...