EVOLUTION AND FATS

Carbohydrates versus fats

Both are utilized by the major mechanisms of storage and use of energy by animals.  For transport to and from locations of intake, storage, and use, glucose is the main blood sugar; fatty acids and triglycerides are the fats that are typically the most abundant forms in the blood.  The liver can convert glucose to fatty acids and fatty acids to glucose.

Storage of carbohydrates in the liver in the form of glycogen makes it easy for the liver to produce glucose needed by the liver or other organs.  Storage of glucose in fat cells involves its conversion to fatty acids before storage as fat.  Storage, release, and use have hormonal mechanisms regulating the processes.  Because fats are relatively insoluble in water but very soluble in other fats, oils, and waxes, they can be stored in large quantities in fat cells without adversely affecting the metabolism of those cells.

Energy content of fats versus carbohydrates

The efficiency of storage would seem to favor fats for energy storage even though liver glycogen can be more easily a source of glucose in the blood stream.  Fats store twice the energy per unit of weight than do carbohydrates.  Thus they have evolved as the energy store that enables many organisms to make long distance migrations or go for long periods without feeding.  Some birds lose most of their fat and over half of their body weight during annual migrations.  Fats need conversion to glucose in the liver or elsewhere to be used in the brain and energy production throughout the cells of the body.

Lipid is the generic term for most substances that dissolve or mix easily with other lipids and/or fat solvents.  Lipids include fats, oils, waxes, fatty acids, triglycerides and some other compounds such as steroids.  Whereas carbohydrates have the bulk of the carbon atoms of the molecule each associated with two hydrogen atoms and one oxygen atom, the chain of most lipid carbon atoms have no oxygen associated.  Most of the energy derived by the metabolic burning of the compounds comes from oxidation of the hydrogen atoms.  But half of the carbohydrate hydrogen atoms are already with oxygen atoms so the ratio of two hydrogen atoms to one oxygen atom is the same as in water and thus the origin of the name carbohydrate.

Uses of fats by organisms

The membranes within cells are readily formed from microscopic globules of certain lipids.  The myelin sheath insulating fibers within nerves of the central nervous system presumably functions to prevent short circuits between closely packed fibers as well as making the polarization of the fiber easier after depolarization during impulse transmission.  Fats in the diet are thought to be useful for efficient uptake of fat-soluble vitamins.  Fats secreted by sebaceous glands are useful lubricants for skin to keep it moist and pliable.  Fat can be stored at many locations in the body, but that stored under or in lower layer of the skin is especially valuable as insulation for animals living in cold habitats.

Fats having carbon chains having two adjacent carbons each lacking a hydrogen atom but sharing a second bond with each other are said to be unsaturated.  Unsaturated fats are liquid at lower temperatures than are saturated fats with the same number of carbon atoms.  We appear to be dependent on our food for necessary unsaturated fats.  Lipids with associated phosphate groups are called phospholipids and some seem to have an important role in brain function.

Steroid hormones are lipids with peculiar ring structures similar to the cholesterol molecule.  Small variations of atom clusters attached in one or more places around the rings can make big differences in their role in regulating body processes.  They are only one of the classes of hormones important in regulating life processes.

Evolution and lipids

The characteristic structure of hormones can lead to recognition of those that are the same or only have slight variation in various groups of animals.  The molting hormone of insects is recognizable as a steroid hormone.  Although we don’t have a hormone exactly like the insect’s ecdysone, it is part of an endocrine complex that seems unlikely to exist without having had a common origin from an earlier ancestor.  It is only one of many clues of chordate origin from ancient annelid ancestors (the most recent common ancestor of us and the insects).

The early evolution preceding living organisms may have been partially dependent on the insolubility of lipids in water, but lipid ability to form minute droplets (emulsify), as well as the potential of some long chain lipids to automatically form membranes in water.  Coating various mineral particles and being pulverized in countless tidal pools around the world provided much experimentation.  Along with ultraviolet light facilitating chance reactions of various concentrations of solutes produced by evaporation at low tide it was a suitable place to get some of the basics done for life to occur.  I think that might be how God did it.

Joseph G. Engemann      April 27, 2014