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Lipids

Overview of lipids, covering fats and oils, saturated and unsaturated fats, triglycerides (triacylglycerols), phospholipids, and steroids.

Introduction

We sometimes talk about fat as if it were a malevolent substance bent on our dietary destruction. In reality, fats are elegant little molecules, each one made of three long hydrocarbon tails attached to a little coathanger-like molecule called glycerol. Like the other large biological molecules, they play essential roles in the biology of humans and other organisms. (Also, many recent dietary studies see sugar as causing a lot more health problems than fat!)
Fats are just one type of lipid, a category of molecules united by their inability to mix well with water. Lipids tend to be hydrophobic, nonpolar, and made up mostly of hydrocarbon chains, though there are some variations on this, which we'll explore below. The different varieties of lipids have different structures, and correspondingly diverse roles in organisms. For instance, lipids store energy, provide insulation, make up cell membranes, form water-repellent layers on leaves, and provide building blocks for hormones like testosterone.
Here, we’ll look in greater detail at some of the most important types of lipids, including fats and oils, waxes, phospholipids, and steroids.

Fats and oils

A fat molecule consists of two kinds of parts: a glycerol backbone and three fatty acid tails. Glycerol is a small organic molecule with three hydroxyl (OH) groups, while a fatty acid consists of a long hydrocarbon chain attached to a carboxyl group. A typical fatty acid contains 12–18 carbons, though some may have as few as 4 or as many as 36.
To make a fat molecule, the hydroxyl groups on the glycerol backbone react with the carboxyl groups of fatty acids in a dehydration synthesis reaction. This yields a fat molecule with three fatty acid tails bound to the glycerol backbone via ester linkages (linkages containing an oxygen atom next to a carbonyl, or C=O, group). Triglycerides may contain three identical fatty acid tails, or three different fatty acid tails (with different lengths or patterns of double bonds).
Image modified from OpenStax Biology.
Fat molecules are also called triacylglycerols, or, in bloodwork done by your doctor, triglycerides. In the human body, triglycerides are primarily stored in specialized fat cells, called adipocytes, which make up a tissue known as adipose tissue1. While many fatty acids are found in fat molecules, some are also free in the body, and they are considered a type of lipid in their own right.

Saturated and unsaturated fatty acids

As shown in the example above, the three fatty acid tails of a triglyceride need not be identical to each other. Fatty acid chains may differ in length, as well as in their degree of unsaturation.
  • If there are only single bonds between neighboring carbons in the hydrocarbon chain, a fatty acid is said to be saturated. (The thing that fatty acids are saturated with is hydrogen; in a saturated fat, as many hydrogen atoms as possible are attached to the carbon skeleton.)
  • When the hydrocarbon chain has a double bond, the fatty acid is said to be unsaturated, as it now has fewer hydrogens. If there is just one double bond in a fatty acid, it’s monounsaturated, while if there are multiple double bonds, it’s polyunsaturated.
The double bonds in unsaturated fatty acids, like other types of double bonds, can exist in either a cis or a trans configuration. In the cis configuration, the two hydrogens associated with the bond are on the same side, while in a trans configuration, they are on opposite sides (see below). A cis double bond generates a kink or bend in the fatty acid, a feature that has important consequences for the behavior of fats.
Image credit: OpenStax Biology.
Saturated fatty acids tails are straight, so fat molecules with fully saturated tails can pack tightly against one another. This tight packing results in fats that are solid at room temperature (have a relatively high melting point). For instance, most of the fat in butter is saturated fat2.
In contrast, cis-unsaturated fatty acid tails are bent due to the cis double bond. This makes it hard for fat molecules with one or more cis-unsaturated fatty acid tails to pack tightly. So, fats with unsaturated tails tend to be liquid at room temperature (have a relatively low melting point) – they are what we commonly call oils. For instance, olive oil is mostly made up of unsaturated fats2.

Trans fats

At this point, you may be noticing that I’ve left something out: I didn’t say anything about unsaturated fats with trans double bonds in their fatty acid tails, or trans fats. Trans fats are rare in nature, but are readily produced in an industrial procedure called partial hydrogenation.
In this process, hydrogen gas is passed through oils (made mostly of cis-unsaturated fats), converting some – but not all – of the double bonds to single bonds. The goal of partial hydrogenation is to give the oils some of the desirable properties of saturated fats, such as solidity at room temperature, but an unintended consequence is that some of the cis double bonds change configuration and become trans double bonds3. Trans-unsaturated fatty acids can pack more tightly and are more likely to be solid at room temperature. Some types of shortening, for example, contain a high fraction of trans fats3.
Partial hydrogenation and trans fats might seem like a good way to get a butter-like substance at oil-like prices. Unfortunately, trans fats have turned out to have very negative effects on human health. Because of a strong link between trans fats and coronary heart disease, the U.S. Food and Drug Administration (FDA) recently issued a ban on trans fats in foods, with a three-year deadline for companies to remove trans fats from their products4.

Omega fatty acids

Another class of fatty acids that deserves mention includes the omega-3 and omega-6 fatty acids. There are different types of omega-3 and omega-6 fatty acids, but all of them are made from two basic precursor forms: alpha-linolenic acid (ALA) for omega-3s and linoleic acid (LA) for omega-6s.
The human body needs these molecules (and their derivatives), but can't synthesize either ALA or LA itself5. Accordingly, ALA and LA are classified as essential fatty acids and must be obtained from a person’s diet. Some fish, such as salmon, and some seeds, such as chia and flax, are good sources of omega-3 fatty acids.
Omega-3 and omega-6 fatty acids have at least two cis-unsaturated bonds, which gives them a curved shape. ALA, shown below, is quite bent but isn’t the most extreme example – DHA, an omega-3 fatty acid made from ALA by the formation of additional double bonds, has six cis-unsaturated bonds and is curled up almost in a circle!
Image credit: OpenStax Biology.
Omega-3 and omega-6 fatty acids play a number of different roles in the body. They are precursors (starting material) for the synthesis of a number of important signaling molecules, including ones that regulate inflammation and mood. Omega-3 fatty acids in particular may reduce the risk of sudden death from heart attacks, decrease triglycerides in the blood, lower blood pressure, and prevent the formation of blood clots.

Role of fats

Fats have received a lot of bad publicity, and it’s true that eating large amounts of fried foods and other “fatty” foods can lead to weight gain and cause health problems. However, fats are essential to the body and have a number of important functions.
For instance, many vitamins are fat-soluble, meaning that they must be associated with fat molecules in order to be effectively absorbed by the body. Fats also provide an efficient way to store energy over long time periods, since they contain over twice as much energy per gram as carbohydrates, and they additionally provide insulation for the body.
Like all the other large biological molecules, fats in the right amounts are necessary to keep your body (and the bodies of other organisms) functioning correctly.

Waxes

Waxes are another biologically important category of lipids. Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants, where its hydrophobic (water-repelling) properties prevent water from sticking to, or soaking into, the surface. This is why water beads up on the leaves of many plants, and why birds don’t get soaked through when it rains.
Image credit: OpenStax Biology.
Structurally speaking, waxes typically contain long fatty acid chains connected to alcohols by ester linkages, although waxes produced by plants often have plain hydrocarbons mixed in as well6.

Phospholipids

What keeps the watery goo (cytosol) inside of your cells from spilling out? Cells are surrounded by a structure called the plasma membrane, which serves as a barrier between the inside of the cell and its surroundings.
Specialized lipids called phospholipids are major components of the plasma membrane. Like fats, they are typically composed of fatty acid chains attached to a backbone of glycerol. Instead having three fatty acid tails, however, phospholipids generally have just two, and the third carbon of the glycerol backbone is occupied by a modified phosphate group. Different phospholipids have different modifiers on the phosphate group, with choline (a nitrogen-containing compound) and serine (an amino acid) being common examples. Different modifiers give phospholipids different properties and roles in a cell.
Image modified from OpenStax Biology.
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic part and a hydrophilic part. The fatty acid chains are hydrophobic and do not interact with water, whereas the phosphate-containing group is hydrophilic (because of its charge) and interacts readily with water. In a membrane, phospholipids are arranged into a structure called a bilayer, with their phosphate heads facing the water and their tails pointing towards the inside (above). This organization prevents the hydrophobic tails from coming into contact with the water, making it a low-energy, stable arrangement.
If a drop of phospholipids is placed in water, it may spontaneously form a sphere-shaped structure known as a micelle, in which the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure. Formation of micelle is an energetically favored because it sequesters the hydrophobic fatty acid tails, allowing the hydrophilic phosphate head group to instead interact with the surrounding water7,8.

Steroids

Steroids are another class of lipid molecules, identifiable by their structure of four fused rings. Although they do not resemble the other lipids structurally, steroids are included in lipid category because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, also have a short tail. Many steroids also have an –OH functional group attached at a particular site, as shown for cholesterol below; such steroids are also classified as alcohols, and are thus called sterols.
Image credit: OpenStax Biology.
Cholesterol, the most common steroid, is mainly synthesized in the liver and is the precursor to many steroid hormones. These include the sex hormones testosterone and estradiol, which are secreted by the gonads (testes and ovaries). Cholesterol also serves as the starting material for other important molecules in the body, including vitamin D and bile acids, which aid in the digestion and absorption of fats from dietary sources. It’s also a key component of cell membranes, altering their fluidity and dynamics.
Of course, cholesterol is also found in the bloodstream, and blood levels of cholesterol are what we often hear about at the doctor’s office or in news reports. Cholesterol in the blood can have both protective effects (in its high-density, or HDL, form) and negative effects (in its low-density, or LDL, form) on cardiovascular health.

Want to join the conversation?

  • starky ultimate style avatar for user alina
    When a micelle is formed, are the hydrophobic tails packed together just because they don't want to touch the water or is there also bonding happening between the tails?
    (11 votes)
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    • old spice man green style avatar for user Matt B
      There is bonding too (van der Waals forces) although these are very weak. The packaging happens primarily because of the hydrophilic parts being attracted to each other and reducing the entropy of the system (=reducing the surface area, and forming a sphere with the hydrophobic parts in the center)
      (26 votes)
  • piceratops tree style avatar for user Michael
    How do saturated and unsaturated fats affect the fluidity of cell membranes? How do the length of fatty acid tails and the presence of cholesterol in cell membranes affect fluidity?
    (11 votes)
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  • leafers sapling style avatar for user Sai Saketh
    Do the number of carbons in a fatty acid affect the properties of the fat?
    (5 votes)
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    • blobby green style avatar for user Najam Zubair
      Yes, it does. You can use penguins as an analogy to this concept. Penguins contain warmth (energy, thermal energy) in a cold environment by clumping together. The more penguins, the more energy conserved and not lost to the environment. Let's say one penguin equates one carbon. In the context of a fatty acid, the more carbons you have, the more "stabilized" the fatty acid is. The net energy needed to break the carbon bonds would be higher, and the molecule will therefore have higher melting point (and less water solubility).
      (14 votes)
  • piceratops seed style avatar for user Todd Doering
    Where does lipolysis fit into this?
    (6 votes)
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  • starky ultimate style avatar for user alina
    "Sequestering the fatty acid tails on the inside of a micelle frees up the water molecules, allowing the system to take on a greater number of microstates (that is, increasing its entropy)." But I would think that the water molecules aren't "freed up" because they'll just form a bond with the phosphate group...(the head group)

    Or am i wrong? Thanks!
    (4 votes)
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    • old spice man green style avatar for user Matt B
      The bonds between H2O and phosphate are not permanent and not strong either. They are simply polar interactions. If micelle were not formed then more space is taken up by the hydrophobic parts which actually reduce entropy because water cannot do anything (bond) with unipolar things. With the micelle formation, there are more water molecules together which can rotate, move, and hydrogen bond to nearby water molecules, and this increases the entropy.
      (8 votes)
  • blobby green style avatar for user Julia Wilson
    Are saturated fats in plants and unsaturated fats in animals, or the other way around?
    (4 votes)
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  • blobby green style avatar for user Prince Baadu
    what is the biochemical functions of all the soluble fats, that is vitamin A,D,E and K
    (3 votes)
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    • blobby green style avatar for user Jarrah Anderson
      Vitamin A
      -you can get it from carrots, it's incredibly important for the photoreceptors in your eyes, without it you can't see.

      Vitamin D
      - necessary for proper bone and tooth mineralisation, your body can produce it if you have adequate UV/ sunlight intake otherwise you need to supplement it or you will get bone resorption which is not a good time.

      Vitamin E
      - shown to improve the reproductive system in rats, in humans however it is a very important antioxidant, takes radicals that would otherwise be harmful to the cells in the body out of circulation.

      Vitamin K
      - a very important clotting factor, helps you mitigate bleeding etc.
      (7 votes)
  • blobby green style avatar for user Vishal
    This sentence in the last paragraph doesn't sound right.

    "Of course, cholesterol is also found in the bloodstream, and blood levels of cholesterol are what we often hear about at the doctor’s office"

    Should it be "Of course, cholesterol is also found in the bloodstream, and cholesterol levels of the blood are what we often hear about at the doctor’s office" ?
    (5 votes)
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  • aqualine ultimate style avatar for user Zander Hall
    Out of curiosity are the "oils" secreted by your skin and hair also made up of fats?
    (4 votes)
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  • starky seed style avatar for user Emma
    Why do fatty acid tails provide us with so much energy when we eat them?
    (3 votes)
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    • female robot grace style avatar for user tyersome
      This is a good question, but one that I think you have enough information to answer on your own. Therefore, I'm going to ask you some questions in response to help you figure out (some of) the answers yourself.

      Answer #1
      What is the oxidation state of the carbons in the fatty acid tail?

      How does this compare with the oxidation state of the carbon in carbohydrates (the other group of macromolecules that are often used to store energy)?

      How would you expect this to affect a oxidative process like cellular respiration?


      Answer #2
      How many moles of carbon are present in a gram of tetradecane — a 14 carbon alkane (a reasonable comparison for the tails of the fatty acids found in food)?

      How many moles of carbon are present in a gram of glucose (a "typical" carbohydrate)?


      Do the answers to those two sets of questions help you answer your question?


      If you are not familiar with reduction and oxidation states, then I encourage you to start working through the Chemistry material on KhanAcademy:
      https://www.khanacademy.org/science/chemistry

      That may look like a lot of work, but you've probably watched many of the videos already under "Chemistry of life". A deep understanding of chemistry is essential to anyone interested in modern biological sciences or medicine, so I really encourage you to take the time to work though all of the chemistry material.
      (4 votes)