==Week 3

=Week 2
Cholesterol is a steroid lipid found in the body tissues and blood plasma of vertebrates. It's most concentrated in the brain, liver, and spinal cord. though cholesterol can cardiovascular disease when its exceeds a healthy level, it is part of several other biochemical processes.

Cholesterol is primarily synthesized from acetyl CoA through the HMG-CoA reductase pathway in the liver (~1 g/day), although other sites include the intestines, adrenal glands and reproductive organs.

Cholesterol is mostly insoluble in water and travels in the blood stream in the form of lipoproteins. It is carried from the intestinal mucosa to the liver in chylomicrons. In the liver it is converted into low-density lipoprotein (LDL) and takes cholesterol to the body cells, while high-density lipoprotein (HDL) carries it to the liver for excretion.
Note that the cholesterol in LDL cholesterol and the cholesterol in HDL cholesterol are identical. The only difference between the two is the carrier molecule (the lipoprote.)

The production of cholesterol is directly regulated by the body’s current cholesterol levels, and eating more leads to a decrease in production. Levels are generally monitored by Sterol Regulatory Element Binding Protein 1 and 2 (SREBP). SREBP is bound to two other proteins, SREBP-cleavage activating protein (SCAP) and Insig-1. When cholesterol levels fall, Insig-1 dissociates from the SREBP-SCAP complex, allowing the complex to migrate to the Golgi apparatus, where SREBP is cleaved by S1P and S2P (site 1/2 protease), two enzymes that are activated by SCAP when cholesterol levels are low. The cleaved SREBP then migrates to the nucleus and acts as a transcription factor to bind to the "Sterol Regulatory Element" of a number of genes to stimulate their transcription. Amongst the genes transcribed are the LDL receptor and HMG-CoA reductase. The former scavenges circulating LDL from the bloodstream, while HMG-CoA reductase leads to an increase of production of cholesterol.
On a side note, studies have shown that people have seasonal variations in cholesterol levels. There is no explanation for this phenomenon, but it may explain why more people are diagnosed with high cholesterol in winter.

Cholesterol is a component of cell membranes and provides them with stability. It has a role in the synthesis of vitamin D and various steroid hormones such as cortisol, cortisone, and aldosterone in the adrenal glands as well as the sex hormones progesterone, estrogen, and testosterone. Recent research implies cholesterol also has a role in the immune system and brain synapses.

Excretion Cholesterol is excreted from the liver in bile; under certain circumstances it can crystallise in the gallbladder to become a major constituent of gallstones, although lecitin and bilirubin gallstones can also occur.


== Weekly Molecule
Week 1 (Friday 10/31)==


This week’s molecule is ethanol (CH3CH2OH), also known as ethyl alcohol or grain alcohol. Though it should be obvious, ethanol is an alcohol, one in a group of chemical compounds containing a hydroxyl group (OH) bonded to a carbon atom.
Ethanol has a low freezing point, which makes it useful as thermometer fluid for temperatures below the freezing point of mercury (-40oC), as well antifreeze. It boils at 78.5oC, melts at -114.1oc, and has a density of .789 g/mL at room temperature.
Ethanol was first made in ancient times by the fermentation of sugars and all beverage ethanol and most industrial ethanol is made the same way. The yeast enzyme zymase changes simple sugars into ethanol and carbon dioxide in a fermentation reaction represented by a deceptively simple equation.
C6H12O6 xxv 2 CH3CH2OH + 2 CO2
The word alcohol is from the Arabic al-kuhul, which means a fine powder of antimony used as an eye makeup. Alcohol originally referred to any fine powder, but medieval alchemists later applied the term to the refined products of distillation, and this led to the current usage.
Ethanol is toxic, and the body gets rid of it as soon as possible, with 90% being processed by the liver. It is a depressant, suppresses certain brain functions, and can be mild tranquilizer in large amounts. Ethanol has also been suggested as a alternate fuel source.

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