A patient with familial hypercholesterolemia has defective LDL receptors on peripheral cells. What is the most direct consequence for blood LDL levels?
ALDL decreases because defective receptors signal the liver to reduce LDL production
BLDL increases because cells cannot internalize cholesterol from LDL particles, so LDL accumulates in the blood
CLDL stays normal because HDL compensates by delivering cholesterol directly to cells
DLDL decreases because the liver increases VLDL conversion to HDL in response to receptor defects
LDL delivers cholesterol to cells by binding to LDL receptors, which then endocytose the particle. If receptors are defective, cells cannot take up LDL, so LDL particles remain in circulation and accumulate. This leads to high LDL blood levels and increased risk of atherosclerosis. Option A confuses the uptake receptor with a production-feedback signal; options C and D invent compensatory mechanisms that do not exist.
Question 2 Multiple Choice
Which property of HDL most directly explains its protective role in cardiovascular disease?
AHDL contains more total cholesterol than other lipoprotein classes, enabling mass removal
BHDL is the densest lipoprotein because it is protein-rich, which accelerates its clearance by the kidney
CHDL performs reverse cholesterol transport — removing excess cholesterol from peripheral tissues (including arterial walls) and returning it to the liver for excretion
DHDL activates lipoprotein lipase to break down LDL particles in the bloodstream
HDL's protective function is directional: it moves cholesterol FROM peripheral tissues BACK TO the liver, where it can be excreted in bile. This reverse cholesterol transport counteracts LDL's delivery of cholesterol to peripheral cells and specifically removes cholesterol from arterial walls, reducing plaque formation. Options A and B describe structural properties of HDL but not its functional mechanism. Option D describes a role of VLDL and chylomicron-associated apolipoproteins, not HDL.
Question 3 True / False
Cholesterol acts as a fluidity buffer in cell membranes, both preventing excessive rigidity at cold temperatures and preventing excessive fluidity at warm temperatures.
TTrue
FFalse
Answer: True
Cholesterol's rigid steroid ring system inserts between phospholipid tails. At temperatures where the membrane would otherwise become too fluid (warm), cholesterol restricts tail movement and tightens packing. At temperatures where the membrane would become too rigid (cold), cholesterol disrupts tight crystalline packing by inserting between tails. This dual role maintains membrane fluidity within a functional range across temperature changes — a classic homeostatic mechanism at the molecular level.
Question 4 True / False
LDL is inherently harmful to cells and serves no normal physiological function; its primarily role is depositing cholesterol in arterial walls.
TTrue
FFalse
Answer: False
LDL's normal function is to deliver cholesterol to cells that need it for essential processes: plasma membrane synthesis, myelin formation, and steroid hormone production (cortisol, sex hormones). LDL becomes pathological only when it accumulates in excess — due to genetic LDL receptor defects, dietary overload, or other dysregulation — penetrates arterial walls, becomes oxidized, and triggers inflammation. LDL is a necessary delivery vehicle that causes disease only when the balance between delivery and removal (by HDL) is disrupted.
Question 5 Short Answer
Why can dietary lipids not simply dissolve in the bloodstream and travel directly to tissues, and how does the lipoprotein system solve this problem?
Think about your answer, then reveal below.
Model answer: Lipids (triglycerides, cholesterol, cholesterol esters) are hydrophobic and insoluble in the aqueous environment of blood plasma. Lipoproteins solve this by packaging lipids into spherical particles with a hydrophobic core (containing triglycerides and cholesterol esters) surrounded by a phospholipid monolayer and apolipoproteins — making the surface water-compatible. Different lipoprotein classes specialize: chylomicrons carry dietary triglycerides from the gut, VLDL carries liver-synthesized triglycerides, LDL delivers cholesterol to peripheral cells, and HDL returns excess cholesterol to the liver.
The key is amphipathic packaging: the phospholipid shell creates a water-soluble exterior while the lipid core remains hydrophobic. Apolipoproteins embedded in the shell serve as both structural components and functional signals — they identify the particle to specific receptors (ApoB-100 on LDL is recognized by LDL receptors) and activate key enzymes (ApoC-II activates lipoprotein lipase for triglyceride hydrolysis). The system is not just a solubility solution but a targeted delivery network.