Cholesterol is why animal cells can live without walls
Plants and most bacteria wrap their cells in rigid walls. Animal cells instead stiffen their flexible membranes from within, slotting cholesterol between the fatty molecules. The result is a skin that is sturdy yet able to change shape, and your body makes about a gram of it every day.
Cholesterol is the main sterol of all animals and is made by every animal cell, though the liver produces the most. Its name comes from Greek words for bile and solid, because François Poulletier de la Salle first found it as a solid in gallstones in 1769; Michel Eugène Chevreul named it cholesterine in 1815. Bacteria and archaea generally lack it, although a few, such as Mycoplasma, need it to grow.
In the membrane, each molecule's small water-loving hydroxyl end faces outward while its rigid, flat ring system and tail tuck among the fatty chains of neighbouring lipids. That packing tunes fluidity across normal body temperatures and makes the membrane less leaky to sodium and hydrogen ions. Cholesterol also helps form lipid rafts, patches that cluster receptor proteins with signalling molecules, and it is abundant in myelin, the layered insulation around nerve fibres that speeds electrical signals. Beyond structure, it is the raw material for steroid hormones, bile acids and vitamin D.
Making it is a marathon: a 37-step pathway, the first 18 steps forming the route targeted by statin drugs. A 68-kilogram man synthesises roughly 1,000 milligrams daily and carries about 35 grams, mostly in cell membranes. The brain runs a separate supply; astrocytes manufacture cholesterol for neurons, and cholesterol from the diet or liver does not normally cross the blood-brain barrier.
Diet plays a smaller role than many assume. Much dietary cholesterol is poorly absorbed, and the body cuts its own production to compensate, so food cholesterol has little effect on blood levels seven to ten hours after a meal. Plants make only tiny amounts but produce related phytosterols that compete for absorption. High blood cholesterol carried in low-density lipoprotein may raise cardiovascular risk.
Source: Cholesterol
