Cholesterol is a waxy substance that is present in the blood plasma and in all animal tissues. Chemically, cholesterol is an organic compound belonging to the steroid family; its molecular formula is C₂₇H₄₆O. In its pure state it is a white, crystalline substance that is odorless and tasteless. Cholesterol is essential to life; it is a primary component of the membrane that surrounds each cell, and it is the starting material or an intermediate compound from which the body synthesizes bile acids, steroid hormones, and vitamin D. Cholesterol circulates in the bloodstream and is synthesized by the liver and several other organs. Human beings also (36)ingest considerable amounts of cholesterol in the course of a normal diet. A compensatory system regulates the amount of cholesterol synthesized by the liver, with the increased dietary intake of cholesterol resulting in the liver's decreased (37)synthesis of the compound.

High levels of cholesterol in the bloodstream are an extremely important cause of atherosclerosis. In this disorder, deposits of cholesterol and other fatty substances circulating in the blood accumulate in the interior walls of the blood vessels. These fatty deposits build up, thicken, and become calcified, eventually converting the vessel walls to scar tissue. The deposits narrow the channels of the blood vessels and thus can (38)constrict the blood flow, causing heart attacks and strokes. High levels of cholesterol in the blood (more than 240 mg of cholesterol per 100 cc of blood plasma) accelerate the buildup of cholesterol deposits in the vessel walls; people with high cholesterol levels thus eventually become more susceptible to coronary heart disease.

Cholesterol is insoluble in the blood; it must be attached to certain protein complexes called lipoproteins in order to be transported through the bloodstream. Low-density lipoproteins (LDLs) transport cholesterol from its site of synthesis in the liver to the various tissues and body cells, where it is separated from the lipoprotein and is used by the cell. High-density lipoproteins (HDLs) may possibly transport excess or unused cholesterol from the tissues back to the liver, where it is broken down to bile acids and is then excreted. Cholesterol attached to LDLs is primarily that which builds up in atherosclerotic deposits in the blood vessels. HDLs, on the other hand, may actually serve to retard or reduce atherosclerotic buildup.

• 36. The word “ingest” in line 9 is closest in meaning to
  • a. diet
  • b. become fat
  • c. synthesize
  • d. consume
• 37. The word “synthesis” in line 12 is closest in meaning to
  • a. production
  • b. reduction
  • c. charge
  • d. excretion
• 38. The word “constrict” in line 19 is closest in meaning to
  • a. condense
  • b. move
  • c. limit
  • d. expand
• 39. The phrase “susceptible to” in line 23 is closest in meaning to
  • a. subjected to
  • b. resistant to
  • c. unlikely to get
  • d. likely to get
• 40. The word “excreted” in line 31 is closest in meaning to
  • a. destroyed
  • b. traded
  • c. absorbed
  • d. expelled
• 41. According to the text, cholesterol is
  • a. necessary for liver function.
  • b. the main ingredient of cell membranes
  • c. synthesized from bile acids.
  • d. all of the above.
• 42. According to the text, the amount of cholesterol synthesized by the liver
  • a. goes down as the amount ingested goes up.
  • b. is unaffected by dietary intake.
  • c. is compensated for by the regulation of the circulation of the bloodstream.
  • d. is affected by the presence of atherosclerosis in the walls of the blood vessels.
• 43. According to the text, high levels of cholesterol in the blood increase the risk of coronary heart disease by
  • a. mating people more suspicious.
  • b. slowing the buildup of deposits in the vessel walls.
  • c. speeding up the accumulation of fatty deposits.
  • d. decreasing the susceptibility to heart attacks and strokes.
• 44. According to the text, cholesterol needs lipoproteins in order to
  • a. synthesize in the liver.
  • b. be carried through the bloodstream.
  • c. form protein complexes.
  • d. be absorbed into the blood.
• 45. According to the text, what do high-density lipoprotein complexes possibly do?
  • a. contribute to atherosclerosis
  • b. carry leftover cholesterol to the liver
  • c. break down bile acids and are then excreted
  • d. both b and c