Fats and oils are the body's most concentrated form of energy, providing 9 kilocalories (kcal) of energy per gram of fat (oil). This is more than twice the energy per gram provided by carbohydrates (4 kcal/gram) and proteins (4 kcal/gram).

The molecular structure of fat is relatively simple. Fat molecules are constructed from one molecule of glycerol and three fatty acid molecules. This is why fats are also called triacylglycerols. Oils have the same chemical structure as fats, but are liquids rather than solids at room temperature.

Fatty acids have a long hydrocarbon chain, spanning from 4 carbon atoms to as many as 30 carbon atoms (although 12 to 24 carbons is more common) with a carboxyl (-COOH) group at one end. Fatty acids having one or more double bonds between carbon atoms in the hydrocarbon chain are called unsaturated fatty acids. Saturated fatty acids have no such double bonds between carbon atoms. Fats are insoluble in water because of the long hydrocarbon chains of the fatty acids.

Glycerol [CH₂(OH)CH(OH)CH₂OH] is an alcohol with three hydroxyl (-OH) groups. It is a thick, sweet tasting liquid that dissolves easily in water because the hydroxyl (-OH) groups in glycerol can form hydrogen bonds with water.

This type of bonding in fats is called esterification. Fat is an ester of glycerol and three fatty acids. The alcohol (-OH) part of glycerol reacts with the carboxylic acid (-COOH) part of the individual fatty acids, forming the fatty acid ester and releasing H₂O in the process. This is illustrated by the following reaction:

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___^{Esterification

Essential Fatty Acids

Different molecules of fat simply have different fatty acid molecules attached to glycerol.

Most of the fatty acids we need to build fats can be created or
"synthesized" by the body. There are three fatty acids that cannot be
made in this way and therefore they must be included in our diet. The three "essential" fatty acids are linoleic acid, linolenic acid and arachidonic acid. They are all unsaturated fatty acids.}

Fats are extremely versatile and can therefore be used be accomplish a variety of important tasks in the body. Metabolism of fats involves both the creation and the breakdown of fat molecules. Metabolism is usually divided into anabolism and catabolism. Anabolism is concerned with chemical reactions that synthesize or construct new (generally more complex) compounds from simpler compounds at the expense of an energy input. Catabolism, involves chemical reactions that breakdown more complex compounds (molecules) into simpler compounds often with the release of energy.

Before entering cells, fats (lipids) are broken down (catabolized) to fatty acids and glycerol by fat-specific enzymes called lipases (triglyceridases). Lipases attack ester bonds between glycerol and the respective fatty acids in the fat molecule in a process called hydrolysis, which adds H₂O across these bonds. This is the reverse of the esterification reaction described above. Glycerol can be converted to glyceraldehyde 3-phosphate and enter into glycolysis and then on to the Kreb's Cycle to produce more ATP. We examined this in the section called Converting Glucose to ATP. Glycerol can also be used to make glucose in an anabolic process called gluconeogenesis, which uses noncarbohydrates (i.e. proteins and fats). Our brain, for example, runs on glucose and must have a constant supply whether or not we are eating carbohyrates.

Within the inner mitochondrial membrane, the chemical energy contained in free fatty acids is released via a four-step process called the beta-oxidation cycle. The four steps include the following:

Fatty acids destined for beta-oxidation are activated for degradation (meaning "to reduce in size") by our old friend coenzyme-A (HS-CoA), which forms a chemical bond with the fatty acid (R-CH₂CH₂COOH). Activating fatty acids for degradation requires energy in the form of ATP, which is needed only once per fatty acid molecule degraded. The result is referred to as a fatty acyl-CoA thioester or fatty acyl-CoA for short. [Thio means, "containing sulfur"].

Beta-oxidation is so named, because in this process, the beta-carbon of the activated fatty acid is oxidized from CH₂ to a ketone C=O (where = signifies a double-bond between carbon C and oxygen O).

The first carbon in the figure above is called the carboxyl carbon (C=O) and it is where the fatty acid numbering system begins. The first carbon after the carboxyl carbon is called the alpha-carbon and it is carbon number 2 of the fatty acid chain. The beta-carbon is the second carbon after C=O and is located at carbon number 3 of the chain. The last carbon atom in the chain is designated the omega-carbon, which is reflective of the fact that omega is the last letter of the Greek alphabet. A table of the Greek alphabet is given at the end of our discussion about fats.

The ketone C=O is attacked by the enzyme beta-ketothiolase along with a molecule of acetyl-CoA, splitting the activated fatty acid into acetyl-CoA and a fatty acid that is now two carbon atoms shorter. In fact, each beta-oxidation cycle releases one acetyl-CoA molecule, thus reducing the starting fatty acid by two-carbon units. Beta-oxidation converts fatty acids having an even number of carbon atoms in their acyl chain completely to acetyl-CoA. For example, a fatty acid with 16 carbons in its acyl chain is converted to 8 acetyl-CoA molecules via beta-oxidation.

The acetyl-CoA made from beta-oxidation (like that derived from glycolysis) enters the Kreb's Cycle (Citric Acid Cycle), where it is further oxidized to CO₂, along with generating more of the electron carriers NADH and FADH2, which in turn, transfer their energy to the electron transport chain, thus driving the production of more ATP. This can be represented as a three-stage process as illustrated by the following diagram:

Fatty acids having an odd number of carbons in their acyl chain are also metabolized via beta-oxidation, however, the cycle ends with the three-carbon propionyl-CoA, which cannot enter another round of beta-oxidation. Propionyl-CoA from beta-oxidation is then converted to succinyl-CoA from which it enters the Kreb's cycle.

Types of Fats

Unsaturated Fats

Fats with only one carbon-carbon double bond are called mono-unsaturated fats (oils). Examples of mono-unsaturated fats (oils) include canola, olive and peanut oil.

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Polyunsaturated fats (oils) have two or more carbon-carbon double bonds in their fatty acid chains. Corn, sunflower and soybean oils are just a few of the many polyunsaturated oils available to consumers.

Saturated Fats

Fats with only carbon-carbon single bonds are called saturated fats. Butter and coconut oil are examples of saturated fats (oils).

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Stearic Acid, for example, is an 18-carbon saturated fatty acid.

The truth is that all fats and oils have a certain percentage of saturated, mono-unsaturated and polyunsaturated fatty acids. For example, butter (considered to be a saturated fat) does have a high percentage of saturated fatty acids (~ 62%), but it also has ~ 29% mono-unsaturated fatty acids and ~ 4% polyunsaturated fatty acids. Sunflower oil, which is high in polyunsaturated fatty acids (~ 66%) has ~ 20% mono-unsaturated fatty acids and ~ 10% saturated fatty acids.

Lipids found in animal cell membranes are mainly phospholipids and cholesterol. Phospholipids have a similar construction to triglycerides with one important difference. In phospholipids, one or more phosphate groups replace one of the three fatty acid chains normally found in triglycerides. The phosphate groups are contained in the polar end of the phopholipid molecule. The polar head group, as it is called, is attracted to polar liquids like water, hence the term hydrophilic or "water-loving" is used to describe this part of the phospholipid. The nonpolar end of the phospholipid molecule consists of two fatty acid chains, which by their nonpolar nature are repelled by water. This is referred to as the hydrophobic or "water-fearing" part of the phospholipid molecule.

__Phospholipid Molecule

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Cell Membrane

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This diagram illustrates how phospholipids align themselves in a typical animal cell membrane.

Greek Alphabet