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What Is a Lipid?

You may be familiar with lipids because they are what create fats, oils, and the foods that are not nutritious but taste delicious. However, lipids are an important structural molecule in many parts of the body. For example, millions of lipids are used to construct the phospholipid bilayer that surrounds every cell, as well as the structure of organelles like the Golgi body or the mitochondria. Lipids can also be used to create important hormones and steroids that the body uses to communicate from cell to cell or from organ to organ.

What Are the Properties of a Lipid?

Lipids are nonpolar and insoluble in water. This is because their structure does not have a more electronegative or electropositive end. Water, however, is polar in comparison. When lipids mix with water, the lipids will closely associate and clump up instead of dissolving evenly into the water.

Some lipids have additional parts, such as phosphates, that make them amphipathic. This means that they will have a hydrophilic end, such as the phosphate, and a hydrophobic end, such as the lipid's carbon chain. This is how cell membranes form. The hydrophobic lipid carbon chains meet up in the center of the membrane, creating the double layer of phosphates. The hydrophilic phosphates are on the outside of the membrane, surrounded by the water that is both inside and outside of the cell.

What Types of Lipids Are There? What Are Lipids Made Of?

Fatty Acid

Fatty acids are the basic units of more complex lipids. Fatty acids are made of a hydrocarbon chain attached to a carboxylic acid group. Because of this, fatty acids are somewhat polar at the carboxylic acid end and nonpolar at the hydrocarbon chain end. Fatty acids always have an even number of carbons.

Waxes

Waxes are formed when a fatty acid combines with an alcohol in a dehydration reaction, turning the carboxylic acid into an ester. Waxes are usually used to waterproof plant leaves and bird feathers. Some fish produce bad-tasting waxes to avoid predation.

Glycerol Lipids

Glycerol lipids have a glycerol backbone that bonds to fatty acids. All glycerol lipids contain a fatty acid, a structure initially consisting of three carboxylic acids. When the fatty acid combines with the glycerol via a dehydration reaction, the hydroxyl groups on the fatty acid become esters.

Glycerols can be mono-, di-, or tri-subsituted, meaning they can have one, two, or three fatty acids attached. When a glycerol is trisubsituted, it is often called a triglyceride. Triglycerides are what the body uses to store energy in fat cells. Extra carbohydrates can be turned into triglycerides using a process known as lipogenesis.

Glycerol lipids can be saturated or unsaturated.

A saturated fat has no double bonds in its fatty acid section. This means that the carboxylic acid chain is "saturated" with hydrogens. Saturated fats, like butter, are usually solid at room temperature because the long, flexible, and nonpolar fatty acid chains all clump together.

An unsaturated fat has a few double bonds in the fatty acid chain. This means that some of the carbons have less hydrogens than the others, so the chain is not saturated with hydrogen. A monounsaturated fat only has one double bond. A polyunsaturated fat has many double bonds. Since the double bonds "bend" the structure of the chain and make it more rigid, it is harder for the fats to clump together. Because of this, unsaturated fats tend to be liquid at room temperature, such as olive oil.

Most natural unsaturated fats are cis unsaturated fats, meaning that the hydrogens on the double bond have the same orientation. Trans unsaturated fats are often formed when extra hydrogens are artificially added to natural unsaturated fats. This can result in the hydrogens on the double bonds having opposite configurations. Basically, the hydrogens in cis unsaturated fats are on the same side, while the hydrogens in trans unsaturated fats are not.

Glycerophospholipids

Glycerophospholipids, or phospholipids, are a di-substituted glycerol attached to a phosphate. The phosphate is polar while the glycerol fat is nonpolar. Phospholipids are the molecules that make up the lipid bilayer of cells.

Sphingolipids

Sphingolipids are made of sphingoids, or a molecule synthesized from the amino acid serine and a fatty acid. Sphingolipids are used to make cerebrosides and gangliosides, important components of nerve and muscle cells.

Sphingolipids were named after sphinxes because their structure and function was mysterious for a long time after their discovery.

Sterol Lipids

Sterols, or steroid alcohols, are made of interconnected carbon rings and an alcohol group. The carbon ring that the alcohol is connected to is referred to as the A ring. The alcohol group makes that end of the molecule polar, while the carbon ring sections are nonpolar. Sterols are another example of an amphipathic molecule.

An example of a sterol lipid is cholesterol. While high cholesterol can be bad for your health, cholesterol is necessary in cell membranes to keep it flexible and stop it from breaking apart. Cholesterol can also be used to synthesize steroids and hormones.

Prenol Lipids

Prenol lipids are produced using 5-carbon precusors and do not involve fatty acids. These types of lipids are common in vitamins A, E, and K. When prenols have cyclic parts, they are often referred to as isoprenoids or terpenes. Terpenes are the main component of essential oils and are basically what makes them smell good.

Saccharolipids

In saccharolipids, fatty acids are attached to a sugar backbone in a similar way that they are attached to glycerols in glycerol lipids. These types of lipids are usually placed in the cell membrane as a way to identify the cell and communicate with other cells. Saccharolipids are usually common in bacteria.

The image shows the structure of a specific saccharolipid known as Lipid A, an important part of a bacterial membrane. Multiple fatty acid chains are joined to a 2-sugar backbone that itself is connected to two phosphates. The important thing to remember is that these types of lipids use sugars instead of glycerols as backbones.

Polyketides

Polyketides are cyclic molecules that are highly modified through glycosylation, methylation, hydroxylation, oxidation, and other methods. Polyketides can be used for many purposes, from antibiotic activity to cancer prevention. The structure of polyketides can vary so much that it is very difficult to create a generic structure for them. The important thing to remember about polyketides is that they are made of alternating carbonyl and methylene groups.

Types of Lipids

Lipids are one of the four major groups of macromolecules. They make up things like fats and oils, and have many important functions. They store energy, form barriers, serve as precursors to other important biological molecules, help in absorbing fat soluble vitamins (A, D E, and K), and protect and insulate organs.

They can have diverse structures but generally consist of hydrocarbon chains. Other atoms, such as oxygen and phosphorus, can also be present in specific types of lipids.

Lipids typically share a nonpolar and hydrophobic characteristic rather than a specific structure. This means that they don't dissolve in or mix with water.

The most common building blocks of lipids are fatty acids. A fatty acid is a compound consisting of a hydrocarbon chain and a carboxyl group (−COOH) at one end. Typically, fatty acids are bound to a glycerol molecule which consists of three hydroxyl groups (-OH), each bound to a carbon atom.

There are several ways to further classify fatty acids. If a fatty acid contains only single bonds, it is called a saturated fatty acid. This name is given because the fatty acid has the maximum number of hydrogens in its structure. You can say that it is saturated with hydrogens. This type of fatty acid will be solid at room temperature. Butter is an example.

If a double bond exists in the fatty acid structure, it's called unsaturated. This type of fatty acid will be liquid at room temperature. Olive oil is an example. If a fatty acid has more than one double bond, it is called a polyunsaturated fatty acid.

We often hear bad things about trans fats. These are unsaturated fatty acids which are chemically treated to add hydrogen atoms and eliminate the double bonds. These types of fats have many health risks associated with them due to the way their structure is altered.

$$\color{green}{\text{Fats}}$$

A fat molecule forms when the hydroxyl (-OH) groups of a glycerol molecule react with the carboxyl (-COOH) groups of fatty acids. This dehydration reaction forms ester linkages.

The most common type of fats are triglycerides. A triglyceride consists of a glycerol molecule bound to three fatty acids. Triglycerides are the most common long-term energy storage molecules in the body.

$$\color{green}{\text{Waxes}}$$

Waxes are another type of lipid. These are commonly found on the surfaces of leaves of many plants. Waxes provide waterproof covering so that water doesn't soak through during heavy rain.

$$\color{green}{\text{Steroids}}$$

Steroids are an extremely important type of lipid. They are characterized by a distinct 4-ring structure. Steroids perform many vital functions in the body and make up things like hormones and some vitamins. Many steroids also have a hydroxyl (-OH) group and are then called sterols. Cholesterol is the most common steroid. It is a precursor of many hormones and is also found in cell membranes.

$$\color{green}{\text{Phospholipids}}$$

Phospholipids are a sort of outlier. They are amphipathic, meaning that they have a nonpolar region as well as a polar one. A phospholipid is made up of a polar, hydrophilic head and two nonpolar, hydrophobic tails.

This type of lipid makes up the cell's double membrane, which is why we also call it the phospholipid bilayer. The polar heads will face the outside and inside of the cell, while the nonpolar tails hide in the middle between them.

Want to know another really cool function of lipids? They are essential to our immune system. When you get hurt, certain compounds around the damaged area are broken down into special signaling lipids which then activate specific immune responses, like fever or inflammation, to help fix the problem. Many medicines, particularly pain killers, are made to target these lipids or their precursors to prevent some of these immune responses.

Structure and Function of Lipids

The structure and function of the six primary biological lipid forms is discussed here, including fatty acids, waxes, glycerides, phospholipids, prostogladins, and steroids.

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Fatty Acids

Fatty acids consist of a carboxylic acid (C-O-OH functional group) with a long, unbranched hydrocarbon chain. They serve as a precursor for other lipids and are used by the body for energy storage. Fatty acids always have an even number of carbons, which is most commonly 12 or 20. They can be divided into two categories: saturated or unsaturated. Saturated fatty acids have single bonds between each carbon. Dietary saturated fats are abundant in animal products such as cheese, butter, and meat. An example is lauric acid (found in coconut milk), whose structure is shown below.

Unsaturated fatty acids have one or more double bonds between carbon atoms. These can be divided into two categories: monounsaturated, which have one double-bond, and polyunsaturated, which have more than one double bond. Dietary sources rich in unsaturated fats include olive oil, salmon, and avocados. Two examples are shown below.

Saturated fats are typically solid at room temperature because their unbent structure allows tight packing of the molecules, forming lots of van der Waals interactions that stabilize the formation. On the other hand, unsaturated fats are typically liquid at room temperature because their bent structure prevents alignment of the molecules into a stable, packed formation.

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Waxes

Waxes are a class of simple lipids that are esters derived from a fatty acid and a long-chain alcohol. In general, their chemical structure consists of two hydrocarbon chains on either side of an ester group.

Biologically, waxes are used by both plants and animals for protection and water-repellence. For example, waxes coat the leaves of many plants to minimize water evaporation. They also coat the feathers of birds to minimize wetting. Beeswax produced by bees creates the structure of honeycomb cells. It includes wax called triacontanyl palmitate, whose structure is shown below as an example.

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Triglycerides

Triglycerides, also known as triacylglycerols, are fatty acid triesters of glycerol, consisting of a glycerol molecule attached to three fatty acid "tails". They are the most common form of lipids and can be labeled as a fat if solid at room temperature and an oil if liquid. The synthesis of triglycerides is shown below as an example of their structure.

Another triglyceride showing the glycerol connected to three individual fatty acids:

Triglycerides are used biologically for efficient energy storage because they are complex forms of fatty acids. In addition, the hydrolysis of tryglycerides produces soap.

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Phospholipids

Phospholipids are the second most common group of natural lipids. They are the main structural lipid group because they form the major component of biological membranes. Other cell membrane lipids include a related group, sphingolipids, and cholesterol. Phospholipids are formed from glycerol esterified with two fatty acids (a diglyceride) and one phosphate group. The general structure of phospholipids features a hydrophilic phosphate head attached to the hydrophobic fatty acid, as illustrated below.

This structure allows phospholipids to form the bilayer of cellular membranes.

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Prostaglandins

Prostaglandins are unsaturated carboxylic acids which consist of a 20-carbon skeleton that also contains a five-member ring, and are based upon the fatty acid arachidonic acid. The general skeleton of a prostaglandin molecule is shown below.

Since they are based on the same skeleton, all prostaglandins have very similar structures but different functions. Biologically, they serve as regulatory molecules and are involved in regulating the inflammatory response, blood clotting, and uterine contractions during labor.

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Steroids

Steroids are molecules that feature a 6-6-6-5 member ring system, as shown below.

All steroids are derivatives of this common structure made up of four fused carbon rings. However, each different steroid contains different chemical groups attached to these rings, as demonstrated in the following image.

Steroids are messenger molecules used by the body to control functions such as sex-related traits, metabolic traits, inflammation, and glucose metabolism. In addition, they aid in digestion by emulsifying fats. They can be divided into four classes: bile acids, which are responsible for emulsifying fats; sex hormones, which control sex traits; mineralocorticoids, which control electrolyte levels; and glucocorticoids, which control blood glucose levels.

Structures of Lipids

Lipids are the group of biomolecules that includes fats, oils, waxes, hormones, and steroids.

They are polymers of fatty acids. They contain a long, nonpolar hydrocarbon chain with a small, polar part containing oxygen. This means that they are mostly nonpolar so they are not soluble in water.

If you've ever tried to mix oil and water, you've seen how poorly they mix. This feature of lipids allows our bodies to use them as waterproof barriers and as biological membranes.

The fatty acids in lipids are either saturated or unsaturated.

Saturated fats occur when each carbon atom in the chain is linked by a single bond. The name saturated comes from the fact that each carbon atom has the maximum possible amount of hydrogen atoms attached to it; they are saturated in hydrogen atoms. Saturated fats have an unbent structure. They are typically solid at room temperature because their structure allows for tight packing of the molecules, forming lots of van der Waals interactions that stabilize the formation.

Monounsaturated fats have one carbon-to-carbon double bond. Polyunsaturated (poly- means many, so many unsaturated) fats have more than one carbon-to-carbon double bond. Unsaturated fats have a bent structure because of these double bonds.

Unsaturated fats are typically liquid at room temperature, like vegetable and olive oil. Their structure prevents the molecules from packing together in a stable formation, so they don't form a solid.

Lipids are used to form structural parts of cell membranes and form signaling molecules within the body. They are most commonly used for energy storage because they have a high energy content and are metabolized to release calories.

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Triglycerides

A triglyceride is a kind of lipid made of a glycerol backbone attached to three fatty acid chains.

Glycerol is a simple sugar alcohol, while fatty acids are carbon chains attached to a carboxylic acid group (-COOH), as seen below.

The bonds between the glycerol molecules and fatty acids are covalent bonds called ester bonds. They are formed from a condensation reaction which can be seen in the picture below. It's called condensation because water molecules are formed (they condense) during the reaction.

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Phospholipids

Phospholipids, like triglycerides, have a glycerol backbone and fatty acid tails. Instead of having three fatty acid tails like triglycerides, phospholipids have two fatty acids chains and a phosphate group attached to the glycerol backbone.

The phosphate group, sometimes called the "head", is polar, unlike the fatty acid chains. This uneven charge distribution make phospholipids polar molecules. This causes the phospholipid molecule to be partially soluble in water because "like dissolves like". That is, polar solvents can dissolve only polar solutes.

The polar portion of the phospholipid is called hydrophilic, which means water-loving. This is the portion of the molecule that is soluble in water. The nonpolar portion is called hydrophobic, or water-hating. It will not dissolve in water. These features make phospholipids ideal for cell membranes.

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Steroids and Waxes

Steroids have a carbon backbone made of four ring-like structures. Steroids are a category of lipid used for chemical signals, such as some hormones and vitamins. Some examples of these steroids are estrogen, testosterone, cholesterol, and cortisone.

Waxes are made of a fatty acid chain and an ester group (seen below). They are used by plants and animals for various reasons. Some animals even have a wax coating on their fur for insulation.

(Video) Structure Of Lipids/Fat

by Whats Up Dude

Watch this video to learn about properties of triglycerides, phospholipids, steroids, and prostaglandins.