Fatty Acid: Definition, Metabolism & Function

If you have taken a nutrition course or have even paid attention to the labels on food products, you are probably very familiar with three of the four main biomolecules of the human body. These biomolecules are carbohydrates, lipids, nucleic acids and proteins. Lipids include a broad range of molecules, including triglycerides, which are sometimes called fats.

Lipids perform many important functions in the human body. Some of the most crucial of these are storing energy and comprising cell membranes. Lipids also provide cushioning and insulation for vital organs.

Lipids are the most energy dense of all four basic biomolecules when it comes to energy storage and access. Lipids can supply 9 calories of energy per gram. This is more than both carbohydrates and proteins, which each supply only 4 calories of energy per gram.

Lipids also form cell membranes thanks to one very important characteristic of lipid molecules called hydrophobicity. This term comes from the Greek words hydor – meaning water – and phobos – meaning fear. Hydrophobic molecules, such as lipids, don't mix well with water because they repel water molecules.

As you will see, hydrophobic lipids can attach to hydrophilic molecules, meaning molecules that attract water molecules, for cell membrane formation.

Fat molecules, or triglycerides, have a backbone of glycerol and three fatty acid tails. These fatty acids are long chains containing a skeleton of carbon atoms with hydrogen molecules attached along the carbon skeleton and carboxylic acid attached at one end.

Because they contain so many carbons and hydrogens, scientists call these hydrocarbon chains.

There are two major types of fatty acids, saturated and unsaturated. Fatty acids receive their classification based on their chemical structure. Saturated fatty acids have single bonds between the carbon molecules of the hydrocarbon chains.

They are saturated with hydrogen, which means they contain as many hydrogen molecules as they possibly can.

Unsaturated fatty acids have double bonds or triple bonds between the carbon molecules of the hydrocarbon chains. They are not saturated with hydrogen, which means they have open sites available for other molecules to bind.

Because of the differences in the way single bonds and double (or triple) bonds affect molecular structure, saturated fatty acids with single bonds have straight, linear chains that can pack together very tightly. Unsaturated fatty acids, on the other hand, have kinks as a result of the double bonds and therefore can't stack together as well.

This structure affects the real-world functions of lipids.

One of these is the temperature at which the fatty acid melts. The melting point for unsaturated fatty acids is lower than the melting point for saturated fatty acids of the same length. For example, stearic acid melts at approximately 157 degrees Fahrenheit while oleic acid melts at approximately 56 degrees Fahrenheit.

This is why saturated lipids, such as the fat on a steak, tend to be solid at room temperature while unsaturated lipids, such as olive oil, are liquid at room temperature.

One of the most important roles of lipids and their constituent fatty acids is energy storage. This usually takes place in specialized tissues called adipose tissue. The cells that make up these tissues – called adipocytes – can contain fat droplets of triglycerides that take up 90 percent of the cell's volume!

All that fat has a crucial main purpose: to store the energy required to power the human body. This is an important way that evolution enables organisms to survive periods of low food availability by building up energy stores when food sources are readily available so they can tap into these stores during leaner times.

For example, animals that hibernate or migrate rely on fat stores to maintain necessary body functions and stay alive during times that they don't eat.

Some scientists drive home the idea that lipids are ideal for energy storage using the example of an average male human who weighs 154 pounds. If this model specimen stops eating, his carbohydrate stores (free glucose and glycogen stores in the liver and muscles) would keep him alive for about a day.

His protein stores (mostly muscle) would last for about a week, although some of the muscles he would eventually need to burn for energy are also crucial for his health, such as the cardiac muscles of the heart.

However, his lipid stores – which comprise about 24 pounds of his total body weight – could sustain him for 30 or 40 days. The type of metabolism his body would use to convert the energy stored in his adipose tissues into usable energy is lipolysis.

Fatty acids also make cell membranes possible. Biological membranes, such as plasma membranes, are selective barriers between the inside of the cell (or organelle) and outside of the cell. In this function, they allow some molecules to pass through and keep other molecules out.

The major component of these membranes are specialized lipids called _phospholipids. Phospholipids have two basic parts: a head and a tail. The head region is glycerol with an attached phosphate group. The tail region is made of fatty acid chains. These phospholipid molecules are amphipathic_; the fatty acid tail end repels water (hydrophobic), and the head end attracts water (hydrophilic).

Biological membranes usually form using lipid bilayers. This means that two rows of phospholipids line up tail to tail with the hydrophilic heads in contact with the interior and exterior of the cell, which comprise mostly water.

This makes the phospholipid membrane watertight while still allowing small molecules to pass through the semipermeable membrane without needing specialized transporters, such as protein pumps.

All that fat hanging out in the adipose tissues, storing energy for when it is needed, serves other helpful purposes, too. Adipose tissue is soft and therefore provides a cushion for vulnerable organs in the body, such as the heart, kidneys and liver.

This is why you can take a hard fall or even withstand a car accident without necessarily damaging your vital organs.

Adipose tissue also acts as insulation to help the body regulate its core temperature. This is especially important in circumstances that include extreme climates or temperature changes. This is why mammals that live in extremely cold environments, such as some whales that travel through freezing waters, maintain stores of fat called blubber.

Fat deposits just below the skin can even metabolize to make heat when the skin temperature gets too low.

Humans can synthesize many fatty acids using the carbon atoms found in biomolecules like carbohydrates and proteins. However, essential fatty acids are a type of fatty acid that the human body can't make on its own.

These are sometimes called dietary fatty acids since these molecules must instead come from the food in your diet.

Two well-known essential fatty acids are omega-3 fatty acids, also called alpha-linolenic acid, and omega-6 fatty acids, also called linoleic acid. Dietary omega-3 and omega-6 fatty acids form other essential fatty acids, such as arachidonic acid (AA), inside the body.

Foods that naturally contain these fatty acids include:

• Oily fish and shellfish.
• Leafy vegetables.
• Vegetable oils, especially canola oil, flaxseed
oil, olive oil and soy oil.
• Nuts and seeds, especially chia seeds, hemp
seeds, pumpkin seeds and walnuts.

These essential fatty acids are crucial for proper membrane function, especially in important nerve cell membranes and blood cell membranes. There, they contribute to membrane fluidity, which is critical for maintaining the concentration gradients that make life-sustaining processes like diffusion and osmosis possible.

Scientists believe that essential fatty acids play important roles in disease development and overall health. Conditions affected by fatty acid deficiencies may include:

• Cardiovascular disease, including coronary heart disease.
• Diabetes.
• Inflammatory diseases, such as asthma, inflammatory bowel
disease and rheumatoid arthritis.
• Neurodegenerative diseases, like Alzheimer's disease and dementia.
• Neuropsychiatric disorders, including bipolar disorder,
depression and schizophrenia.

Some fatty acids are essential only under specific conditions, such as disease or developmental states. For example, long-chain polyunsaturated fatty acids called docosahexaenoic acid (DHA) are crucial for brain structure and cognitive function as well as proper vision. Newborn humans, especially those born prematurely, require careful feeding of human milk rich in DHA and AA or infant formulas fortified with these essential fatty acids.

You have already become acquainted with the term lipolysis, which is the way fatty acids metabolize to release stored energy. When the cells in adipose tissues receive the signal that the body needs access to stored energy, lipase enzymes begin a multi-step process called hydrolysis, which breaks the triglycerides into their constituent parts, fatty acids and glycerol.

Each step of hydrolysis cleaves one fatty acid from the triglyceride molecule.

From that point, the citric acid cycle, also called the _Krebs cycle_, takes over. This series of chemical reactions further cleaves the fatty acid chains to release all the stored energy contained in the chains. All aerobic organisms, including humans, use this cycle to generate energy.

The opposite process from lipolysis enables the human body to store this energy in the first place. Lipogenesis, or esterification, converts simple sugars into fatty acids. Then these fatty acid chains are synthesized into triglycerides in order to store energy as fat in the body, especially in the adipose tissues.

You may have heard of another important lipid called cholesterol. This steroid molecule comes in two forms: high density (HDL) cholesterol and low density (LDL) cholesterol. Since cholesterol travels through the bloodstream, health care providers can check your cholesterol levels with a simple blood test.

While HDL cholesterol is beneficial for the human body, high levels of LDL cholesterol can harm the cardiovascular system.

Although most people equate the term cholesterol with LDL cholesterol and worry about having too much cholesterol in their blood, the cholesterol molecule plays very important roles in the human body. In addition to the protective effects of HDL cholesterol, the steroid molecule also acts as the precursor for many important hormones.

These include sex hormones important for your reproductive system, such as estrogen, progesterone and testosterone.

Cholesterol is also responsible for the production of stress hormones, including cortisol. These hormones help the body mount important stress responses in the face of danger, such as the flight-or-fight response.

Over the years, lipids have gotten a bad public image due to low-fat dieting trends. As you can see, this poor reputation is undeserved because the roles that lipids play in the human body – from energy storage to membrane formation to simple cushioning and insulation – aren't just important; they are crucial for life.