Lipids serve three main functions in the human body: first, as an energetically cheap way to store metabolic fuel for long periods of time; second, as the main structural component of cell membranes; and third, as the precursor of steroid hormones and other molecules involved in cell signaling. Each of these roles will be discussed in turn.
Humans and other animals store some calories in the form of liver glycogen (starch) and proteins; however, the most important reservoir for long term energy storage is fat, specifically triacylglycerol (TAG). In the popular media they are more familiar as triglycerides. TAG molecules are made of three fatty acids linked by an ester bond to a three carbon unit called glycerol. Triacylglycerols form fat droplets in specialized fat storage cells called adipocytes. The average person stores enough calories in their adipocytes to survive approximately three months without food.
As a survival strategy, starch stored in the liver is consumed within the first 24 hours of starvation. If no food is ingested by then, elevated levels of cortisol and other stress hormones mobilize the body's fat reserves. In addition, the liver begins to produce high levels of substances called ketone bodies, which the brain can consume for energy as an alternative to glucose. When fat reserves are exhausted, the last fuel source broken down for energy is skeletal muscle protein.
Besides serving as an energy depot, fats also play a structural role in the body by forming the lipid bilayer component of cell and organelle membranes. In contrast to the triacylglycerol molecules found inside adipocytes, lipids found in the cell membrane are diacylglycerols, which contain a so called head region with hydrophillic phosphate groups and two fatty acid tails.
The most common lipid found in human cell membranes is phosphatidylcholine. Other structural membrane lipids include phosphatidylserine, phosphatidylethanolamine, cardiolipin, and phosphatidylinositol (more on that shortly). Another important lipid, cholesterol, is found in the membranes of animal cells, where it decreases the fluidity of the lipid bilayer. Cholesterol also serves as the precursor of bile acids, steroid hormones and vitamin D.
The third major role that fats play in the body is cell signaling. This can be broken down into three subcategories: a) intracellular signaling molecules derived from membrane lipids, chief among them inositol triphosphate(IP3); b) arachidonic acid derivatives including prostaglandins, prostacyclins, thromboxanes, and leukotrienes; and c) steroid hormones and vitamin D, both derived from cholesterol.
IP3 is produced in response to the actions of certain neurotransmitters (e.g. acetylcholine and norepinephrine) on their target cells. Briefly, in response to an extracellular signal, a cytosolic enzyme called phospholipase C cleaves IP3 from a special membrane lipid called phosphatidylinositol inositol. IP3 binds to a calcium channel on the endoplasmic reticulum (ER), resulting in the transient release of calcium ions from the ER lumen into the cytosol. Calcium ions, in turn, have multiple effects, like triggering muscle contraction, neurotransmitter release, and the activation of several types of enzymes.
Arachidonic acid, derived from a membrane lipid called diacylglycerol (DAG), is the precursor of a group of molecules called autacoids, including prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Autacoids are involved in pain, fever, inflammation, vascular tone, and platelet aggregation. NSAIDs (Non-Steroidal Anti-Inflammatory Drugs) like aspirin and ibuprofen treat pain and fever by blocking the production of certain inflammatory autocoids, prostaglandins and thromboxanes in particular.
Finally, all steroid hormonesare derivatives of cholesterol. These include cortisol, the main glucocorticoid; aldosterone, the main mineralocorticoid; and last, but not least, the so called sex hormones, chief among them testosterone, estrogen and progesterone. The adrenal glands produce all of these steroid hormones, however, the bulk of sex hormones are produced in the testes/ovaries. Vitamin D is also a cholesterol derivative, formed when a person's skin is exposed to UV light. After further activation in the liver, vitamin D acts in intestinal enterocytes to promote calcium uptake as well as in the kidneys to promote calcium reabsorption. A dietary deficiency of vitamin D combined with a lack of exposure to sunlight leads to a disease called rickets. Relatively few foods are rich in vitamin D with the exceptions of cod liver oil, and, in modern times, fortified milk and dairy products.
In addition to the links, see also:
Stryer (1988) Biochemistry, 3rd ed., Ch.12 Introduction to Biological Membranes, pp. 283-314; Ch. 20 Fatty Acid Metabolism, pp. 469-494; Ch. 23 Biosynthesis of Membrane Lipids and Steroid Hormones pp. 547-574, and Ch. 38 Hormone Action, pp. 975-1004.