Which of the following is correct for cholesterol in humans ?
A. It maintains the fluidity of the cell membrane.
B. It is used to make all hormones.
C. It is a good source of energy because of the number of hydrogen bonds.
D. It forms hydrogen bonds with water which stabilises the cell membrane.
E. It transports fatty acids around the body.
The correct answer is A : it maintains the fluidity of the membrane.
Membrane fluidity
The membrane is made of a bilayer of lipids. The percentage of saturated and unsaturated fatty acids determine its fluidity. Indeed, double bonds in the saturated lipids increase it and, conversely, unsaturated fats decrease the fluidity.
Similarly, cholesterol affects fluidity : at high temperatures, it decreases it and increases it at low temperatures.
Hormone synthesis
There are 2 types of hormones : the ones that are made of chains of amino acids (polypeptide hormones) and the steroid ones, made from cholesterol.
Steroid hormones have very different roles : regulation of physiologic processes, such as homeostasis and reproduction.
Energy source
Cholesterol is considered to be a terminal metabolism : it does not provide any energy. Therefore, you can’t “exercice off” cholesterol.
When it is esterified however, it releases 1 fatty acid which produces energy.
Cholesterol and water
It does not form hydrogen bonds with water. In fact, it is mostly hydrophobic (apart from the polar hydroxyl group).
It stabilizes the cell membrane by decreasing the mobility of the fatty acid chains of the surrounding phospholipids when reacting with them.
Fatty acid transport
Lipids are transported as lipoproteins (= lipid + protein), which can either be chylomicrons, VLDL, LDL or HDL depending on their density and composition.