Discuss how cholesterol is transported in the body, and describe these transport molecules.

Cholesterol, like other lipids, is insoluble in water. Therefore, it is transported in the blood in lipoproteins. These are spheres of lipid and protein, and are made in the liver. There are two types of lipoproteins in the blood, named according to their density; High density lipoproteins (HDL) and low density lipoproteins (LDL). Both HDLs and LDLs are taken up by cells that have the correct receptor sites. High Density Lipoproteins Contain unsaturated fats, cholesterol and protein Transport cholesterol from the body tissues to the liver Liver cell plasma membranes have receptor sites for HDLs Liver cells make bile from the cholesterol or break it down High levels of blood HDLs are beneficial since they bring about a reduction in blood cholesterol levels  Therefore, HDLs reduce the deposition of lipids in artery walls and reduce the formation of atheromas in the walls of these blood vessels Unsaturated fats are regarded as more beneficial than saturated fats because unsaturated fats are a component of HDLs Low Density Lipoproteins  Contain saturated fats, cholesterol and proteins Transport cholesterol from the liver to the tissues Therefore raise blood cholesterol Tissue cell plasma membranes have receptor sites for LDLs If the diet contains too much saturated fat and cholesterol, the concentration of LDLs increases in the blood A high concentration of LDLs in blood plasma increases the deposition of lipids and cholesterol in the artery walls and increases atheroma formation The activity of the LDL receptors on tissue cell plasma membranes is increased by unsaturated fats and decreased by saturated fats. If there is a high concentration of saturated fats, fewer LDLs are removed from the blood by the cells leading to more deposition in the artery walls Summary of Diet and Lipoproteins  Lipoproteins are not eaten in the diet. They are made in the body to transport non-polar lipids The ratio of HDLs to LDLs in blood is important. A higher proportion of HDLs to LDLs decreases the deposition of lipid in artery walls A low fat diet maintains low lipoproteins concentrations in blood Eating saturated fat will increase the LDLs Eating unsaturated fat will increase the HDLs Polyunsaturated and monounsaturated fats reduce the concentration of LDLs in the blood