Any of a class of organic compounds that contain both lipid ( fat ) and protein .
They may be soluble (those in egg yolk and blood plasma) or insoluble (those in cell membranes) in water and water solutions. Lipoproteins in blood plasma are the mode of transport for cholesterol , insoluble by itself. Low-density lipoproteins (LDLs) carry cholesterol from the liver, where it is made, to the cells, where it is used; high-density lipoproteins (HDLs) may carry excess cholesterol back to the liver for breakdown and excretion. LDL-bound cholesterol is primarily responsible for deposits in arteries (see coronary heart disease , angina pectoris , myocardial infarction , or {{link=stroke">stroke . HDL does not form such deposits and may actually retard or reduce their buildup.