Fill in the Blanks Biology IOnline version Fill in the missing words; by Kapolo Paulus 1 living phospholipids cholesterol bilayer membranes membranes permeable layers All cells and many of the tiny organelles internal to cells are bounded by thin membranes . These are composed primarily of and proteins and are typically described as phospholipid bi - . Cell membranes also contain in the phospholipid . In some there are only a few cholesterol molecules , but in others there are as many cholesterols as phospholipids according to Audesirk & Audesirk . Cholesterol makes the bilayer stronger , more flexible but less fluid , and less to water - soluble substances such as ions and monosaccharides .
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