Lipids

Fatty acid synthesis and oxidation 
The synthesis and breakdown of fatty acid chains are due to repetitive cycles that add and remove two carbons each time. Synthesis occurs in the cytosol while oxidation occurs in the mitochondrion. In the breakdown cycle, energy molecules NADH, FADH₂ and acetyl CoA are produced, which results in 17 ATPs for each two-carbon loss.

Hormonal controls
When blood sugar is low, epinephrine, norepinephrine and glucagon stimulate lipase in fat cells to break down triglycerides. The glycerol can then be used to produce more glucose.

In contrast, insulin promotes the transport of glucose into cells, where glucose is converted and stored as triglyceride. Insulin also inhibits lipid breakdown through several mechanisms and increases synthesis of glycogen, fatty acids, triglycerides and proteins.

Ketones
In states of starvation, fatty acid breakdown results in the four-carbon acetoacetyl CoA. Acetoacetyl CoA can break down further to acetyl CoA, be used for synthesis of cholesterol, or change to ketones. Ketones may be used for fuel by the brain as a last resort.

Lipids 
Fatty acids are stored on the three-carbon glycerol, forming glycerides. Triglycerides are the main storage molecules, which can revert to fatty acids and glycerol - to be used for cellular energy (glycerol to glycerol 3-P to glyceraldehyde 3-P). Acetyl CoA from the energy pathways with glycerol can form triglycerides. Therefore, excess sugar from the diet goes into glycerol to fatty acids (via acetyl CoA) to triglycerides, which are stored in fat cells.

Phosphoglycerides
Glycerol can also form phosphoglycerides when attached to phosphate attached to serine, ethanolamine, choline or inositol. Compared to triglycerides, phosphoglycerides are esters of only two fatty acids, and are important membrane components.

Prostaglandins
Prostaglandins are fatty acids synthesized throughout the body, having hormone-like effects.

Sphingolipids 
These molecules contain two fatty acids and a third group linked to the three-carbon serine. They are found in the nervous system.

Cholesterols
Cholesterol is ingested, but also synthesized from acetyl CoA, which transforms to isoprene. Isoprenes are the five-carbon building block forming isoprenoid molecules, or cholesterols. Cholesterol may then become bile acids and salts (mainly produced and exported by the liver), or form various steroid hormones.

Steroidogenesis by David Richfield and Mikael Häggström