2.5: Lipids - Biology

2.5: Lipids - Biology

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Learning Objectives

  • Describe the chemical composition of lipids
  • Describe the unique characteristics and diverse structures of lipids
  • Compare and contrast triacylglycerides (triglycerides) and phospholipids.
  • Describe how phospholipids are used to construct biological membranes.

Although they are composed primarily of carbon and hydrogen, lipid molecules may also contain oxygen, nitrogen, sulfur, and phosphorous. Lipids serve numerous and diverse purposes in the structure and functions of organisms. They can be a source of nutrients, a storage form for carbon, energy-storage molecules, or structural components of membranes and hormones. Lipids comprise a broad class of many chemically distinct compounds, the most common of which are discussed in this section.

Fatty Acids and Triglycerides

The fatty acids are lipids that contain long-chain hydrocarbons terminated with a carboxylic acid functional group. Because the long hydrocarbon chain, fatty acids are hydrophobic (“water fearing”) or nonpolar. A triglyceride is formed when three fatty acids are chemically linked to a glycerol molecule (Figure (PageIndex{1})). Triglycerides are the primary components of adipose tissue (body fat), and are major constituents of sebum (skin oils). They play an important metabolic role, serving as efficient energy-storage molecules that can provide more than double the caloric content of both carbohydrates and proteins.

Saturated vs Unsaturated

Fatty acids with hydrocarbon chains that contain only single bonds are called saturated fatty acids because they have the greatest number of hydrogen atoms possible and are, therefore, “saturated” with hydrogen. Fatty acids with hydrocarbon chains containing at least one double bond are called unsaturated fatty acids because they have fewer hydrogen atoms. Saturated fatty acids have a straight, flexible carbon backbone, whereas unsaturated fatty acids have “kinks” in their carbon skeleton because each double bond causes a rigid bend of the carbon skeleton. These differences in saturated versus unsaturated fatty acid structure result in different properties for the corresponding lipids in which the fatty acids are incorporated.

In unsaturated fatty acids the double bonds can be in either the cis or trans configuration, illustrated in Figure (PageIndex{2}). When some of these bonds are in the cis configuration, the resulting bend in the carbon backbone of the chain means that triglyceride molecules cannot pack tightly, so they remain liquid (oil) at room temperature. On the other hand, triglycerides with trans double bonds (popularly called trans fats), have relatively linear fatty acids that are able to pack tightly together at room temperature and form solid fats. In the human diet, trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have reduced or eliminated their use in recent years. In contrast to unsaturated fats, triglycerides without double bonds between carbon atoms are called saturated fats, meaning that they contain all the hydrogen atoms available. Saturated fats are a solid at room temperature and usually of animal origin.

Exercise (PageIndex{1})

Explain why fatty acids with hydrocarbon chains that contain only single bonds are called saturated fatty acids.

Phospholipids and Biological Membranes

Triglycerides are classified as simple lipids because they are formed from just two types of compounds: glycerol and fatty acids. In contrast, complex lipids contain at least one additional component, for example, a phosphate group (phospholipids) or a carbohydrate moiety (glycolipids). Figure (PageIndex{3}) depicts a typical phospholipid composed of two fatty acids linked to glycerol (a diglyceride). The two fatty acid carbon chains may be both saturated, both unsaturated, or one of each. Instead of another fatty acid molecule (as for triglycerides), the third binding position on the glycerol molecule is occupied by a modified phosphate group.

The molecular structure of lipids results in unique behavior in aqueous environments. Figure (PageIndex{1}) depicts the structure of a triglyceride. Because all three substituents on the glycerol backbone are long hydrocarbon chains, these compounds are nonpolar and not significantly attracted to polar water molecules—they are hydrophobic. Conversely, phospholipids such as the one shown in Figure (PageIndex{3}) have a negatively charged phosphate group. Because the phosphate is charged, it is capable of strong attraction to water molecules and thus is hydrophilic, or “water loving.” The hydrophilic portion of the phospholipid is often referred to as a polar “head,” and the long hydrocarbon chains as nonpolar “tails.” A molecule presenting a hydrophobic portion and a hydrophilic moiety is said to be amphipathic. Notice the “R” designation within the hydrophilic head depicted in Figure (PageIndex{3}), indicating that a polar head group can be more complex than a simple phosphate moiety. Glycolipids are examples in which carbohydrates are bonded to the lipids’ head groups.

The amphipathic nature of phospholipids enables them to form uniquely functional structures in aqueous environments. As mentioned, the polar heads of these molecules are strongly attracted to water molecules, and the nonpolar tails are not. Because of their considerable lengths, these tails are, in fact, strongly attracted to one another. As a result, energetically stable, large-scale assemblies of phospholipid molecules are formed in which the hydrophobic tails congregate within enclosed regions, shielded from contact with water by the polar heads (Figure (PageIndex{4})). The simplest of these structures are micelles, spherical assemblies containing a hydrophobic interior of phospholipid tails and an outer surface of polar head groups. Larger and more complex structures are created from lipid-bilayer sheets, or unit membranes, which are large, two-dimensional assemblies of phospholipids congregated tail to tail. The cell membranes of nearly all organisms are made from lipid-bilayer sheets, as are the membranes of many intracellular components. These sheets may also form lipid-bilayer spheres that are the structural basis of vesicles and liposomes, subcellular components that play a role in numerous physiological functions.

Exercise (PageIndex{2})

How is the amphipathic nature of phospholipids significant?


The isoprenoids are branched lipids, also referred to as terpenoids, that are formed by chemical modifications of the isoprene molecule (Figure (PageIndex{5})). These lipids play a wide variety of physiological roles in plants and animals, with many technological uses as pharmaceuticals (capsaicin), pigments (e.g., orange beta carotene, xanthophylls), and fragrances (e.g., menthol, camphor, limonene [lemon fragrance], and pinene [pine fragrance]). Long-chain isoprenoids are also found in hydrophobic oils and waxes. Waxes are typically water resistant and hard at room temperature, but they soften when heated and liquefy if warmed adequately. In humans, the main wax production occurs within the sebaceous glands of hair follicles in the skin, resulting in a secreted material called sebum, which consists mainly of triglycerol, wax esters, and the hydrocarbon squalene. There are many bacteria in the microbiota on the skin that feed on these lipids. One of the most prominent bacteria that feed on lipids is Propionibacterium acnes, which uses the skin’s lipids to generate short-chain fatty acids and is involved in the production of acne.


Another type of lipids are steroids, complex, ringed structures that are found in cell membranes; some function as hormones. The most common types of steroids are sterols, which are steroids containing an -OH group. These are mainly hydrophobic molecules, but also have hydrophilic hydroxyl groups. The most common sterol found in animal tissues is cholesterol. Its structure consists of four rings with a double bond in one of the rings, and a hydroxyl group at the sterol-defining position. The function of cholesterol is to strengthen cell membranes in eukaryotes and in bacteria without cell walls, such as Mycoplasma. Prokaryotes generally do not produce cholesterol, although bacteria produce similar compounds called hopanoids, which are also multiringed structures that strengthen bacterial membranes (Figure (PageIndex{6})). Fungi and some protozoa produce a similar compound called ergosterol, which strengthens the cell membranes of these organisms.

Exercise (PageIndex{3})

How are isoprenoids used in technology?

Clinical Focus: part 2

The moisturizing cream prescribed by Penny’s doctor was a topical corticosteroid cream containing hydrocortisone. Hydrocortisone is a synthetic form of cortisol, a corticosteroid hormone produced in the adrenal glands, from cholesterol. When applied directly to the skin, it can reduce inflammation and temporarily relieve minor skin irritations, itching, and rashes by reducing the secretion of histamine, a compound produced by cells of the immune system in response to the presence of pathogens or other foreign substances. Because histamine triggers the body’s inflammatory response, the ability of hydrocortisone to reduce the local production of histamine in the skin effectively suppresses the immune system and helps limit inflammation and accompanying symptoms such as pruritus (itching) and rashes.

Exercise (PageIndex{4})

Does the corticosteroid cream treat the cause of Penny’s rash, or just the symptoms?

Key Concepts and Summary

  • Lipids are composed mainly of carbon and hydrogen, but they can also contain oxygen, nitrogen, sulfur, and phosphorous. They provide nutrients for organisms, store carbon and energy, play structural roles in membranes, and function as hormones, pharmaceuticals, fragrances, and pigments.
  • Fatty acids are long-chain hydrocarbons with a carboxylic acid functional group. Their relatively long nonpolar hydrocarbon chains make them hydrophobic. Fatty acids with no double bonds are saturated; those with double bonds are unsaturated.
  • Fatty acids chemically bond to glycerol to form structurally essential lipids such as triglycerides and phospholipids. Triglycerides comprise three fatty acids bonded to glycerol, yielding a hydrophobic molecule. Phospholipids contain both hydrophobic hydrocarbon chains and polar head groups, making them amphipathicand capable of forming uniquely functional large scale structures.
  • Biological membranes are large-scale structures based on phospholipid bilayers that provide hydrophilic exterior and interior surfaces suitable for aqueous environments, separated by an intervening hydrophobic layer. These bilayers are the structural basis for cell membranes in most organisms, as well as subcellular components such as vesicles.
  • Isoprenoids are lipids derived from isoprene molecules that have many physiological roles and a variety of commercial applications.
  • A wax is a long-chain isoprenoid that is typically water resistant; an example of a wax-containing substance is sebum, produced by sebaceous glands in the skin. Steroids are lipids with complex, ringed structures that function as structural components of cell membranes and as hormones. Sterols are a subclass of steroids containing a hydroxyl group at a specific location on one of the molecule’s rings; one example is cholesterol.
  • Bacteria produce hopanoids, structurally similar to cholesterol, to strengthen bacterial membranes. Fungi and protozoa produce a strengthening agent called ergosterol.

AP Biology 2.5 - Membrane Permeability

Section 2.5 of the AP Biology curriculum covers Membrane Permeability. In this section, we take a look at how lipid bilayers are semi-permeable and how adding proteins to the lipid bilayer makes it selectively permeable. This section covers the types of molecules that can permeate the lipid bilayer, the importance of membrane permeability for different organisms, and how cell walls can influence membrane permeability by adding an additional layer of filtration. Check it out!

Lipids and Fatty Acids

Fats are actually a type of lipid. Lipids are a major class of biochemical compounds that includes oils, as well as fats. Among other things, organisms use lipids to store energy.

Lipid molecules consist mainly of repeating units called fatty acids . There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids. Both types consist mainly of simple chains of carbon atoms bonded to one another and to hydrogen atoms. The two types of fatty acids differ in how many hydrogen atoms they contain and the number of bonds between carbon atoms.

Lipids are water-insoluble molecules that have a wide variety of functions within cells, including: 1) maintenance of electrochemical gradients 2) subcellular partitioning 3) first- and second-messenger cell signaling 4) energy storage and 5) protein trafficking and membrane anchoring. The physiological importance of lipids is illustrated by the numerous diseases to which lipid abnormalities contribute, including atherosclerosis, diabetes, obesity, and Alzheimer's disease. Lipidomics, a branch of metabolomics, is a systems-based study of all lipids, the molecules with which they interact, and their function within the cell. Recent advances in soft-ionization mass spectrometry, combined with established separation techniques, have allowed the rapid and sensitive detection of a variety of lipid species with minimal sample preparation. A “lipid profile” from a crude lipid extract is a mass spectrum of the composition and abundance of the lipids it contains, which can be used to monitor changes over time and in response to particular stimuli. Lipidomics, integrated with genomics, proteomics, and metabolomics, will contribute toward understanding how lipids function in a biological system and will provide a powerful tool for elucidating the mechanism of lipid-based disease, for biomarker screening, and for monitoring pharmacologic therapy.

Published, JLR Papers in Press, August 10, 2006.


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In summary, this is the first-ever animal study to demonstrate sex differences in insulin sensitivity and hepatic lipid accumulation after “real world” exposure to PM2.5. The present study suggests significant changes in HPA axis and changes in lipid metabolites. Our novel findings provide insights into the potential mechanisms of the adverse metabolic health effects in sex difference. More research regarding sex-dimorphic pathophysiological mechanisms of PM’s adverse effects could contribute to more personalized care in the future and would thus promote awareness in terms of sex-specific risk factors.

The 2.5 Å structure of CD1c in complex with a mycobacterial lipid reveals an open groove ideally suited for diverse antigen presentation

CD1 molecules function to present lipid-based antigens to T cells. Here we present the crystal structure of CD1c at 2.5 Å resolution, in complex with the pathogenic Mycobacterium tuberculosis antigen mannosyl-β1-phosphomycoketide (MPM). CD1c accommodated MPM's methylated alkyl chain exclusively in the A' pocket, aided by a unique exit portal underneath the α1 helix. Most striking was an open F' pocket architecture lacking the closed cavity structure of other CD1 molecules, reminiscent of peptide binding grooves of classical major histocompatibility complex molecules. This feature, combined with tryptophan-fluorescence quenching during loading of a dodecameric lipopeptide antigen, provides a compelling model by which both the lipid and peptide moieties of the lipopeptide are involved in CD1c presentation of lipopeptides.


Figure 1. The α1 and α2 platform…

Figure 1. The α1 and α2 platform domains of CD1c and optimized CD1c (CD1copt) are…

Figure 2. Structure of CD1c in complex…

Figure 2. Structure of CD1c in complex with MPM and a C 12 spacer lipid

Figure 3. Comparison of antigen-binding cavities of…

Figure 3. Comparison of antigen-binding cavities of human CD1 isoforms in complex with ligands

Figure 4. Unique features in CD1c: D′…

Figure 4. Unique features in CD1c: D′ and E′ portals

Figure 5. Residues and features forming the…

Figure 5. Residues and features forming the F′ groove in CD1c, and comparison to CD1b

Figure 6. Hydrogen bonding network between CD1c…

Figure 6. Hydrogen bonding network between CD1c and MPM

CD1c is shown as a ribbon…

Figure 7. Analysis of lipopeptide binding to…

Figure 7. Analysis of lipopeptide binding to CD1c with fluorescence spectroscopy

Lipids in Chemistry, a Definition

A lipid is a fat-soluble molecule. To put it another way, lipids are insoluble in water but soluble in at least one organic solvent. The other major classes of organic compounds (nucleic acids, proteins, and carbohydrates) are much more soluble in water than in an organic solvent. Lipids are hydrocarbons (molecules consisting of hydrogen and oxygen), but they do not share a common molecule structure.

Lipids that contain an ester functional group may be hydrolyzed in water. Waxes, glycolipids, phospholipids, and neutral waxes are hydrolyzable lipids. Lipids that lack this functional group are considered nonhydrolyzable. The nonhydrolyzable lipids include steroids and the fat soluble vitamins A, D, E, and K.


Lipids are the class of macromolecules that mostly serve as long-term energy storage. Additionally, they serve as signaling molecules, water sealant, structure and insulation. Lipids are insoluble in polar solvents such as water, and are soluble in nonpolar solvents such as ether and acetone.

Fats or triglycerides are made of glycerol and three fatty acid chains. They form through 3 dehydration synthesis reactions between a hydroxyl of the glycerol and the carboxyl group of the fatty acid.

Saturated versus Unsaturated fats

A saturated fatty acid. The molecule takes up little space in three dimensions. Many molecules can stack upon each other. Saturated fats are solid at room temperature.

A polyunsaturated fatty acid. A kink from the double bond increases the amount of three dimensional space that the molecule fills. Unsaturated fats tend to be liquid at room temperature.

A trans fatty acid. Despite an unsaturated bond, the molecule fills as much space as a saturated fatty acid and is solid at room temperature. Trans fats usually arise from artificial saturation techniques.

Butterfat is almost completely saturated. Notice how molecules can stack very closely.

Because butterfat can stack together very closely, it is dense and found as a solid at room temperature.Credit: Steve Karg (CC BY 2.5)

Feature: Human Biology in the News

Imagine squeezing through a 7-inch slit in rock to enter a completely dark cave full of lots and lots of old bones. It might sound like a nightmare to most people, but it was a necessary part of a recent exploration of human origins in South Africa as reported in the New York Times in September 2015. The cave and its bones were actually first discovered by spelunkers in 2013, who reported it to paleontologists. An international research project was soon launched to explore the cave. The researchers would eventually conclude that the cave was a hiding place for the dead of a previously unknown early species of Homo, whom they called Homo naledi. Members of this species lived in South Africa around 2.5 to 2.8 million years ago.

Figure 2.5.9 Comparison of skull features of Homo naledi and other early human species.

Homo naledi individuals were about 5 feet (about 1.5 metres) tall and weighed around 100 pounds (about 45 kilograms), so they probably had no trouble squeezing into the cave. Modern humans are considerably larger on average. In order to retrieve the fossilized bones from the cave, six slender researchers had to be found on social media. They were the only ones who could fit through the crack to access the cave. The work was difficult and dangerous, but also incredibly exciting. The site constitutes one of the largest samples of any extinct early Homo species anywhere in the world, and the fossils represent a completely new species of that genus. The site also suggests that early members of our genus were intentionally depositing their dead in a remote place. This behavior was previously thought to be limited to later humans.

Like other early Homo species, Homo naledi exhibits a mosaic of old and modern traits. From the neck down, these early hominins were well-adapted for upright walking. Their feet were virtually indistinguishable from modern human feet and their legs were also long like ours. Homo naledi had relatively small front teeth, but also a small brain, no larger than an average orange. Clearly, the spurt in brain growth in Homo did not occur in this species. The image to the left shows the different morphology of early human skulls.

Watch the news for more exciting updates about this early species of our genus. Paleontologists researching the cave site estimate that there are hundreds — if not thousands — of fossilized bones still remaining in the cave. There are sure to be many more discoveries reported in the news media about this extinct Homo species.



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