Lipid facts for kids

Structures of some common lipids. At the top are cholesterol and oleic acid. The middle structure is a triglyceride composed of three fatty acid chains attached to a glycerol backbone. At the bottom is the common phospholipid phosphatidylcholine.

Lipids are a large family of natural organic molecules found in all living things. They include familiar substances such as fats, oils, waxes, and cholesterol. They also include important nutrients like fat-soluble vitamins (vitamin A, vitamin D, vitamin E, and vitamin K).

Unlike many other biological molecules, lipids do not dissolve in water. Instead, they are hydrophobic (water-fearing) or amphiphilic (having one part that likes water and one part that avoids it). Because of these unique traits, lipids serve three major roles in living organisms:

Storing energy for long-term use.
Building the protective cell membranes that enclose all living cells.
Acting as chemical messengers to help cells talk to one another.

Lipids are also widely used by humans outside the body. They are essential ingredients in cooking, cosmetics, skincare products, and modern nanotechnology.

What Are Lipids and How Are They Classified?

Many people use the word "fat" to mean any lipid, but fats are actually just one type of lipid called triglycerides.

Chemists classify biological lipids into eight major categories based on their chemical building blocks. Most biological lipids are built from two main types of starter units: ketoacyl groups or isoprene groups.

Prostacyclin, an example of an eicosanoid fatty acid.

Leukotriene B4, another example of an active fatty acid messenger.

Eight Categories of Lipids

Scientific organizations, such as the LIPID MAPS consortium, divide lipids into eight groups:

Fatty Acyls (Fatty Acids)

Fatty acids are simple chains of carbon and hydrogen atoms with an acid group at one end. They are the primary building blocks of many complex lipids.

Saturated fatty acids have only single bonds between their carbon atoms. They form straight chains that pack tightly together, making them solid at room temperature (like butter).
Unsaturated fatty acids have one or more double bonds in their chain. These double bonds create kinks or bends in the chain. These bends keep the molecules from packing tightly, so they remain liquid at room temperature (like olive oil).
Special types of fatty acids include eicosanoids, which help control inflammation and body defenses.

Glycerolipids

Example of an unsaturated fat triglyceride. Left: glycerol; right: three fatty acids.

Glycerolipids are made by attaching one, two, or three fatty acids to a three-carbon molecule called glycerol.

The most famous glycerolipids are triglycerides (also called triacylglycerols).
Triglycerides make up the bulk of body fat in animals and vegetable oils in plants.
They are the body's main way of storing extra energy from food.

Glycerophospholipids

Phosphatidylethanolamine, an important membrane phospholipid.

Glycerophospholipids, usually called phospholipids, have two fatty acid tails and one phosphate-containing "head" group attached to glycerol.

The phosphate head loves water (hydrophilic), while the fatty acid tails hate water (hydrophobic).
This split personality allows them to form double-layered sheets that make up the walls of all living cells.
Common examples include phosphatidylcholine (found in egg yolk lecithin) and phosphatidylserine.
High concentrations of glycerophospholipids are found in the brain and nervous tissue.

Sphingolipids

Structure of sphingomyelin, which insulates nerve fibers.

Sphingolipids are built on a special chemical backbone called a sphingosine rather than glycerol.

They were named after the mythical Sphinx because their structure was once a great mystery to scientists.
They play critical roles in protecting cell surfaces and helping cells communicate.
A well-known sphingolipid is sphingomyelin, which forms the protective insulation around nerve fibers in the human body.

Sterol Lipids (Sterols)

Chemical structure of cholesterol, showing its four fused carbon rings.

Sterols are lipids made of four interconnected rings of carbon atoms.

The most famous sterol in animals is cholesterol. Cholesterol sits inside cell membranes to keep them flexible and stable.
Cholesterol is also used to make vital molecules like bile acids (which digest food) and hormones.
Plants produce related compounds called phytosterols, and fungi produce ergosterol.

Prenol Lipids

Prenol lipid structure (geraniol).

Prenols are synthesized from simple five-carbon units called isoprenes.

Carotenoids, which give carrots their bright orange color, are prenol lipids that act as antioxidants.
Fat-soluble vitamins, including vitamin E and vitamin K, belong to this lipid family.
They also include fragrant oils produced by flowers and trees.

Saccharolipids

Structure of Kdo2-lipid A, a saccharolipid found in bacteria.

In saccharolipids, fatty acids link directly to a sugar molecule instead of glycerol.

They are especially important in the outer protective walls of certain bacteria, such as Escherichia coli.
They help bacteria maintain their cell structure and interact with their surroundings.

Polyketides

Polyketides are complex, highly diverse natural molecules produced by microbes, fungi, and plants.

Many important medicines, including natural antibiotics (like erythromycin and tetracycline), are polyketides.
Organisms use them for defense against competing species.

History of Lipid Science

The scientific study of fats and oils began in the early 19th century:

In 1815, French chemist Henri Braconnot divided fats into two basic groups: solid fats (tallow) and liquid oils.
In 1823, Michel Eugène Chevreul performed detailed experiments showing that fats are made of fatty acids combined with glycerol.
In 1827, English physician William Prout recognized fat as one of the three primary nutrients necessary for animals, alongside proteins and carbohydrates.
In 1844, Théophile-Jules Pelouze created the first artificial triglyceride in a laboratory.
In 1847, Theodore Nicolas Gobley discovered phospholipids in egg yolks and brain tissue, naming them lecithin.
In 1923, French scientist Gabriel Bertrand introduced the official term lipide (later anglicized as lipid), derived from the Ancient Greek word lipos, meaning fat.

Vital Biological Functions of Lipids

Lipids are not just passive blobs of grease. They perform dynamic tasks that keep organisms alive.

Building Cell Membranes

How phospholipids arrange in water: a spherical liposome, a micelle, and a flat lipid bilayer.

Every living cell is enclosed by a thin barrier called a cell membrane (or plasma membrane). Organelles inside cells, like the nucleus and mitochondria, also have their own membranes.

Phospholipids are the primary building material for these membranes:

In water, phospholipid heads turn outward toward the liquid, while their water-hating tails hide inside away from water.
This natural reaction creates a double layer known as a lipid bilayer.
This bilayer forms a strong, flexible wall that keeps cell contents inside while blocking harmful substances.
Cholesterol molecules sit between the phospholipids to keep the membrane from becoming too stiff in cold temperatures or too runny in warm temperatures.

Long-Term Energy Storage

Triglycerides are the most concentrated energy source available to living things:

Burning one gram of fat releases about 38 kilojoules (9 calories) of energy.
In comparison, one gram of carbohydrate or protein provides only 17 kilojoules (4 calories).
Animals store triglycerides inside special cells called adipocytes (fat cells).
Migratory birds rely on stored fat to fly thousands of miles without stopping for food.
Fat stored under the skin also provides thermal insulation, helping polar bears and whales survive freezing arctic waters.

Chemical Signaling and Communication

Lipids act as essential messengers that send instructions across the body and between cells:

Steroid hormones, such as cortisol, travel through the bloodstream to regulate metabolism, blood pressure, and development.
Prostaglandins alert the immune system to injuries and trigger healing responses.
Phospholipids on the cell surface can flip positions to signal immune cells when an old or damaged cell needs to be recycled.

Nutrient Absorption and Protection

Fat-soluble vitamins (vitamin A, vitamin D, vitamin E, and vitamin K) cannot be absorbed or used by the body without dietary lipids. Lipids also form protective waxy coatings on plant leaves to prevent water loss and shield skin and feathers in animals from moisture.

How the Body Processes Lipids

Lipid metabolism involves breaking down fats from food and building new lipids needed by tissues.

Biosynthesis: Making Lipids

When an animal eats more carbohydrates or proteins than it burns for energy, the liver converts the extra energy into fatty acids and triglycerides through a process called lipogenesis.

Enzymes link two-carbon units together one by one to assemble fatty acid chains.
Desaturase enzymes can add double bonds to turn saturated chains into unsaturated ones.
However, human cells cannot place double bonds at every required position. Because of this, certain polyunsaturated fats, like alpha-linolenic acid (an omega-3) and linoleic acid (an omega-6), cannot be made by the body. These are called essential fatty acids and must come from food.

Degradation: Breaking Down Fats for Fuel

When the body needs energy, stored triglycerides are broken down by enzymes called lipases into free fatty acids and glycerol.

The fatty acids enter the mitochondria of cells.
Through a metabolic cycle called beta oxidation, the fatty acid chains are chopped into two-carbon pieces.
These two-carbon units enter the citric acid cycle and produce adenosine triphosphate (ATP), the universal energy currency of cells.
Breaking down a single 16-carbon fatty acid molecule can generate up to 106 ATP molecules.

Lipids in Nutrition and Health

Eating the right types of dietary fats is essential for good health, normal growth, and brain function.

Healthy Fats vs. Unhealthy Fats

Dietary fats differ based on their chemical structure:

Unsaturated Fats: Mostly found in plant oils (like olive, canola, and sunflower oils), nuts, seeds, and avocados. They support cardiovascular health.
Omega-3 and Omega-6 Fatty Acids: Essential fats found in flaxseed, walnuts, and oily fish (such as salmon and mackerel). They support brain health, eyesight, and normal growth.
Saturated Fats: Found in meats, dairy products, and palm oil. They should be eaten in moderate amounts as part of a balanced diet.
Trans Fats: Artificially altered fats created by industrial hydrogenation. Eating trans fats increases the risk of heart disease, leading many health agencies worldwide to ban artificial trans fats from processed food.