The four basic categories of molecules for building life are carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates serve many purposes, from energy to structure to chemical communication, as monomers or polymers.
Lipids, which are hydrophobic, also have different purposes, including energy storage, structure, and signaling.
Proteins, made of amino acids in up to four structural levels, are involved in just about every process of life.
The nucleic acids DNA and RNA consist of four nucleotide building blocks, and each has different purposes.
The longer version
Life is so diverse and unwieldy, it may surprise you to learn that we can break it down into four basic categories of molecules. Possibly even more implausible is the fact that two of these categories of large molecules themselves break down into a surprisingly small number of building blocks. The proteins that make up all of the living things on this planet and ensure their appropriate structure and smooth function consist of only 20 different kinds of building blocks. Nucleic acids, specifically DNA, are even more basic: only four different kinds of molecules provide the materials to build the countless different genetic codes that translate into all the different walking, swimming, crawling, oozing, and/or photosynthesizing organisms that populate the third rock from the Sun.
Big Molecules with Small Building Blocks
The functional groups, assembled into building blocks on backbones of carbon atoms, can be bonded together to yield large molecules that we classify into four basic categories. These molecules, in many different permutations, are the basis for the diversity that we see among living things. They can consist of thousands of atoms, but only a handful of different kinds of atoms form them. It’s like building apartment buildings using a small selection of different materials: bricks, mortar, iron, glass, and wood. Arranged in different ways, these few materials can yield a huge variety of structures.
We encountered functional groups and the SPHONC in Chapter 3. These components form the four categories of molecules of life. These Big Four biological molecules are carbohydrates, lipids, proteins, and nucleic acids. They can have many roles, from giving an organism structure to being involved in one of the millions of processes of living. Let’s meet each category individually and discover the basic roles of each in the structure and function of life.
You have met carbohydrates before, whether you know it or not. We refer to them casually as “sugars,” molecules made of carbon, hydrogen, and oxygen. A sugar molecule has a carbon backbone, usually five or six carbons in the ones we’ll discuss here, but it can be as few as three. Sugar molecules can link together in pairs or in chains or branching “trees,” either for structure or energy storage.
When you look on a nutrition label, you’ll see reference to “sugars.” That term includes carbohydrates that provide energy, which we get from breaking the chemical bonds in a sugar called glucose. The “sugars” on a nutrition label also include those that give structure to a plant, which we call fiber. Both are important nutrients for people.
Sugars serve many purposes. They give crunch to the cell walls of a plant or the exoskeleton of a beetle and chemical energy to the marathon runner. When attached to other molecules, like proteins or fats, they aid in communication between cells. But before we get any further into their uses, let’s talk structure.
The sugars we encounter most in basic biology have their five or six carbons linked together in a ring. There’s no need to dive deep into organic chemistry, but there are a couple of essential things to know to interpret the standard representations of these molecules.
Check out the sugars depicted in the figure. The top-left molecule, glucose, has six carbons, which have been numbered. The sugar to its right is the same glucose, with all but one “C” removed. The other five carbons are still there but are inferred using the conventions of organic chemistry: Anywhere there is a corner, there’s a carbon unless otherwise indicated. It might be a good exercise for you to add in a “C” over each corner so that you gain a good understanding of this convention. You should end up adding in five carbon symbols; the sixth is already given because that is conventionally included when it occurs outside of the ring.
On the left is a glucose with all of its carbons indicated. They’re also numbered, which is important to understand now for information that comes later. On the right is the same molecule, glucose, without the carbons indicated (except for the sixth one). Wherever there is a corner, there is a carbon, unless otherwise indicated (as with the oxygen). On the bottom left is ribose, the sugar found in RNA. The sugar on the bottom right is deoxyribose. Note that at carbon 2 (*), the ribose and deoxyribose differ by a single oxygen.
The lower left sugar in the figure is a ribose. In this depiction, the carbons, except the one outside of the ring, have not been drawn in, and they are not numbered. This is the standard way sugars are presented in texts. Can you tell how many carbons there are in this sugar? Count the corners and don’t forget the one that’s already indicated!
If you said “five,” you are right. Ribose is a pentose (pent = five) and happens to be the sugar present in ribonucleic acid, or RNA. Think to yourself what the sugar might be in deoxyribonucleic acid, or DNA. If you thought, deoxyribose, you’d be right.
The fourth sugar given in the figure is a deoxyribose. In organic chemistry, it’s not enough to know that corners indicate carbons. Each carbon also has a specific number, which becomes important in discussions of nucleic acids. Luckily, we get to keep our carbon counting pretty simple in basic biology. To count carbons, you start with the carbon to the right of the non-carbon corner of the molecule. The deoxyribose or ribose always looks to me like a little cupcake with a cherry on top. The “cherry” is an oxygen. To the right of that oxygen, we start counting carbons, so that corner to the right of the “cherry” is the first carbon. Now, keep counting. Here’s a little test: What is hanging down from carbon 2 of the deoxyribose?
If you said a hydrogen (H), you are right! Now, compare the deoxyribose to the ribose. Do you see the difference in what hangs off of the carbon 2 of each sugar? You’ll see that the carbon 2 of ribose has an –OH, rather than an H. The reason the deoxyribose is called that is because the O on the second carbon of the ribose has been removed, leaving a “deoxyed” ribose. This tiny distinction between the sugars used in DNA and RNA is significant enough in biology that we use it to distinguish the two nucleic acids.
In fact, these subtle differences in sugars mean big differences for many biological molecules. Below, you’ll find a couple of ways that apparently small changes in a sugar molecule can mean big changes in what it does. These little changes make the difference between a delicious sugar cookie and the crunchy exoskeleton of a dung beetle.
Sugar and Fuel
A marathon runner keeps fuel on hand in the form of “carbs,” or sugars. These fuels provide the marathoner’s straining body with the energy it needs to keep the muscles pumping. When we take in sugar like this, it often comes in the form of glucose molecules attached together in a polymer called starch. We are especially equipped to start breaking off individual glucose molecules the minute we start chewing on a starch.
Double X Extra: A monomer is a building block (mono = one) and a polymer is a chain of monomers. With a few dozen monomers or building blocks, we get millions of different polymers. That may sound nutty until you think of the infinity of values that can be built using only the numbers 0 through 9 as building blocks or the intricate programming that is done using only a binary code of zeros and ones in different combinations.
Our bodies then can rapidly take the single molecules, or monomers, into cells and crack open the chemical bonds to transform the energy for use. The bonds of a sugar are packed with chemical energy that we capture to build a different kind of energy-containing molecule that our muscles access easily. Most species rely on this process of capturing energy from sugars and transforming it for specific purposes.
Polysaccharides: Fuel and Form
Plants use the Sun’s energy to make their own glucose, and starch is actually a plant’s way of storing up that sugar. Potatoes, for example, are quite good at packing away tons of glucose molecules and are known to dieticians as a “starchy” vegetable. The glucose molecules in starch are packed fairly closely together. A string of sugar molecules bonded together through dehydration synthesis, as they are in starch, is a polymer called a polysaccharide (poly = many; saccharide = sugar). When the monomers of the polysaccharide are released, as when our bodies break them up, the reaction that releases them is called hydrolysis.
Double X Extra: The specific reaction that hooks one monomer to another in a covalent bond is called dehydration synthesis because in making the bond–synthesizing the larger molecule–a molecule of water is removed (dehydration). The reverse is hydrolysis (hydro = water; lysis = breaking), which breaks the covalent bond by the addition of a molecule of water.
Although plants make their own glucose and animals acquire it by eating the plants, animals can also package away the glucose they eat for later use. Animals, including humans, store glucose in a polysaccharide called glycogen, which is more branched than starch. In us, we build this energy reserve primarily in the liver and access it when our glucose levels drop.
Whether starch or glycogen, the glucose molecules that are stored are bonded together so that all of the molecules are oriented the same way. If you view the sixth carbon of the glucose to be a “carbon flag,” you’ll see in the figure that all of the glucose molecules in starch are oriented with their carbon flags on the upper left.
The orientation of monomers of glucose in polysaccharides can make a big difference in the use of the polymer. The glucoses in the molecule on the top are all oriented “up” and form starch. The glucoses in the molecule on the bottom alternate orientation to form cellulose, which is quite different in its function from starch.
Storing up sugars for fuel and using them as fuel isn’t the end of the uses of sugar. In fact, sugars serve as structural molecules in a huge variety of organisms, including fungi, bacteria, plants, and insects.
The primary structural role of a sugar is as a component of the cell wall, giving the organism support against gravity. In plants, the familiar old glucose molecule serves as one building block of the plant cell wall, but with a catch: The molecules are oriented in an alternating up-down fashion. The resulting structural sugar is called cellulose.
That simple difference in orientation means the difference between a polysaccharide as fuel for us and a polysaccharide as structure. Insects take it step further with the polysaccharide that makes up their exoskeleton, or outer shell. Once again, the building block is glucose, arranged as it is in cellulose, in an alternating conformation. But in insects, each glucose has a little extra added on, a chemical group called an N-acetyl group. This addition of a single functional group alters the use of cellulose and turns it into a structural molecule that gives bugs that special crunchy sound when you accidentally…ahem…step on them.
These variations on the simple theme of a basic carbon-ring-as-building-block occur again and again in biological systems. In addition to serving roles in structure and as fuel, sugars also play a role in function. The attachment of subtly different sugar molecules to a protein or a lipid is one way cells communicate chemically with one another in refined, regulated interactions. It’s as though the cells talk with each other using a specialized, sugar-based vocabulary. Typically, cells display these sugary messages to the outside world, making them available to other cells that can recognize the molecular language.
Lipids: The Fatty Trifecta
Starch makes for good, accessible fuel, something that we immediately attack chemically and break up for quick energy. But fats are energy that we are supposed to bank away for a good long time and break out in times of deprivation. Like sugars, fats serve several purposes, including as a dense source of energy and as a universal structural component of cell membranes everywhere.
Fats: the Good, the Bad, the Neutral
Turn again to a nutrition label, and you’ll see a few references to fats, also known as lipids. (Fats are slightly less confusing that sugars in that they have only two names.) The label may break down fats into categories, including trans fats, saturated fats, unsaturated fats, and cholesterol. You may have learned that trans fats are “bad” and that there is good cholesterol and bad cholesterol, but what does it all mean?
Let’s start with what we mean when we say saturated fat. The question is, saturated with what? There is a specific kind of dietary fat call the triglyceride. As its name implies, it has a structural motif in which something is repeated three times. That something is a chain of carbons and hydrogens, hanging off in triplicate from a head made of glycerol, as the figure shows. Those three carbon-hydrogen chains, or fatty acids, are the “tri” in a triglyceride. Chains like this can be many carbons long.
Double X Extra: We call a fatty acid a fatty acid because it’s got a carboxylic acid attached to a fatty tail. A triglyceride consists of three of these fatty acids attached to a molecule called glycerol. Our dietary fat primarily consists of these triglycerides.
Triglycerides come in several forms. You may recall that carbon can form several different kinds of bonds, including single bonds, as with hydrogen, and double bonds, as with itself. A chain of carbon and hydrogens can have every single available carbon bond taken by a hydrogen in single covalent bond. This scenario of hydrogen saturation yields a saturated fat. The fat is saturated to its fullest with every covalent bond taken by hydrogens single bonded to the carbons.
Saturated fats have predictable characteristics. They lie flat easily and stick to each other, meaning that at room temperature, they form a dense solid. You will realize this if you find a little bit of fat on you to pinch. Does it feel pretty solid? That’s because animal fat is saturated fat. The fat on a steak is also solid at room temperature, and in fact, it takes a pretty high heat to loosen it up enough to become liquid. Animals are not the only organisms that produce saturated fat–avocados and coconuts also are known for their saturated fat content.
The top graphic above depicts a triglyceride with the glycerol, acid, and three hydrocarbon tails. The tails of this saturated fat, with every possible hydrogen space occupied, lie comparatively flat on one another, and this kind of fat is solid at room temperature. The fat on the bottom, however, is unsaturated, with bends or kinks wherever two carbons have double bonded, booting a couple of hydrogens and making this fat unsaturated, or lacking some hydrogens. Because of the space between the bumps, this fat is probably not solid at room temperature, but liquid.
You can probably now guess what an unsaturated fat is–one that has one or more hydrogens missing. Instead of single bonding with hydrogens at every available space, two or more carbons in an unsaturated fat chain will form a double bond with carbon, leaving no space for a hydrogen. Because some carbons in the chain share two pairs of electrons, they physically draw closer to one another than they do in a single bond. This tighter bonding result in a “kink” in the fatty acid chain.
In a fat with these kinks, the three fatty acids don’t lie as densely packed with each other as they do in a saturated fat. The kinks leave spaces between them. Thus, unsaturated fats are less dense than saturated fats and often will be liquid at room temperature. A good example of a liquid unsaturated fat at room temperature is canola oil.
A few decades ago, food scientists discovered that unsaturated fats could be resaturated or hydrogenated to behave more like saturated fats and have a longer shelf life. The process of hydrogenation–adding in hydrogens–yields trans fat. This kind of processed fat is now frowned upon and is being removed from many foods because of its associations with adverse health effects. If you check a food label and it lists among the ingredients “partially hydrogenated” oils, that can mean that the food contains trans fat.
Double X Extra: A triglyceride can have up to three different fatty acids attached to it. Canola oil, for example, consists primarily of oleic acid, linoleic acid, and linolenic acid, all of which are unsaturated fatty acids with 18 carbons in their chains.
Why do we take in fat anyway? Fat is a necessary nutrient for everything from our nervous systems to our circulatory health. It also, under appropriate conditions, is an excellent way to store up densely packaged energy for the times when stores are running low. We really can’t live very well without it.
Phospholipids: An Abundant Fat
You may have heard that oil and water don’t mix, and indeed, it is something you can observe for yourself. Drop a pat of butter–pure saturated fat–into a bowl of water and watch it just sit there. Even if you try mixing it with a spoon, it will just sit there. Now, drop a spoon of salt into the water and stir it a bit. The salt seems to vanish. You’ve just illustrated the difference between a water-fearing (hydrophobic) and a water-loving (hydrophilic) substance.
Generally speaking, compounds that have an unequal sharing of electrons (like ions or anything with a covalent bond between oxygen and hydrogen or nitrogen and hydrogen) will be hydrophilic. The reason is that a charge or an unequal electron sharing gives the molecule polarity that allows it to interact with water through hydrogen bonds. A fat, however, consists largely of hydrogen and carbon in those long chains. Carbon and hydrogen have roughly equivalent electronegativities, and their electron-sharing relationship is relatively nonpolar. Fat, lacking in polarity, doesn’t interact with water. As the butter demonstrated, it just sits there.
There is one exception to that little maxim about fat and water, and that exception is the phospholipid. This lipid has a special structure that makes it just right for the job it does: forming the membranes of cells. A phospholipid consists of a polar phosphate head–P and O don’t share equally–and a couple of nonpolar hydrocarbon tails, as the figure shows. If you look at the figure, you’ll see that one of the two tails has a little kick in it, thanks to a double bond between the two carbons there.
Phospholipids form a double layer and are the major structural components of cell membranes. Their bend, or kick, in one of the hydrocarbon tails helps ensure fluidity of the cell membrane. The molecules are bipolar, with hydrophilic heads for interacting with the internal and external watery environments of the cell and hydrophobic tails that help cell membranes behave as general security guards.
The kick and the bipolar (hydrophobic and hydrophilic) nature of the phospholipid make it the perfect molecule for building a cell membrane. A cell needs a watery outside to survive. It also needs a watery inside to survive. Thus, it must face the inside and outside worlds with something that interacts well with water. But it also must protect itself against unwanted intruders, providing a barrier that keeps unwanted things out and keeps necessary molecules in.
Phospholipids achieve it all. They assemble into a double layer around a cell but orient to allow interaction with the watery external and internal environments. On the layer facing the inside of the cell, the phospholipids orient their polar, hydrophilic heads to the watery inner environment and their tails away from it. On the layer to the outside of the cell, they do the same.
As the figure shows, the result is a double layer of phospholipids with each layer facing a polar, hydrophilic head to the watery environments. The tails of each layer face one another. They form a hydrophobic, fatty moat around a cell that serves as a general gatekeeper, much in the way that your skin does for you. Charged particles cannot simply slip across this fatty moat because they can’t interact with it. And to keep the fat fluid, one tail of each phospholipid has that little kick, giving the cell membrane a fluid, liquidy flow and keeping it from being solid and unforgiving at temperatures in which cells thrive.
Steroids: Here to Pump You Up?
Our final molecule in the lipid fatty trifecta is cholesterol. As you may have heard, there are a few different kinds of cholesterol, some of which we consider to be “good” and some of which is “bad.” The good cholesterol, high-density lipoprotein, or HDL, in part helps us out because it removes the bad cholesterol, low-density lipoprotein or LDL, from our blood. The presence of LDL is associated with inflammation of the lining of the blood vessels, which can lead to a variety of health problems.
But cholesterol has some other reasons for existing. One of its roles is in the maintenance of cell membrane fluidity. Cholesterol is inserted throughout the lipid bilayer and serves as a block to the fatty tails that might otherwise stick together and become a bit too solid.
Cholesterol’s other starring role as a lipid is as the starting molecule for a class of hormones we called steroids or steroid hormones. With a few snips here and additions there, cholesterol can be changed into the steroid hormones progesterone, testosterone, or estrogen. These molecules look quite similar, but they play very different roles in organisms. Testosterone, for example, generally masculinizes vertebrates (animals with backbones), while progesterone and estrogen play a role in regulating the ovulatory cycle.
Double X Extra: A hormone is a blood-borne signaling molecule. It can be lipid based, like testosterone, or short protein, like insulin.
As you progress through learning biology, one thing will become more and more clear: Most cells function primarily as protein factories. It may surprise you to learn that proteins, which we often talk about in terms of food intake, are the fundamental molecule of many of life’s processes. Enzymes, for example, form a single broad category of proteins, but there are millions of them, each one governing a small step in the molecular pathways that are required for living.
Levels of Structure
Amino acids are the building blocks of proteins. A few amino acids strung together is called a peptide, while many many peptides linked together form a polypeptide. When many amino acids strung together interact with each other to form a properly folded molecule, we call that molecule a protein.
For a string of amino acids to ultimately fold up into an active protein, they must first be assembled in the correct order. The code for their assembly lies in the DNA, but once that code has been read and the amino acid chain built, we call that simple, unfolded chain the primary structure of the protein.
This chain can consist of hundreds of amino acids that interact all along the sequence. Some amino acids are hydrophobic and some are hydrophilic. In this context, like interacts best with like, so the hydrophobic amino acids will interact with one another, and the hydrophilic amino acids will interact together. As these contacts occur along the string of molecules, different conformations will arise in different parts of the chain. We call these different conformations along the amino acid chain the protein’s secondary structure.
Once those interactions have occurred, the protein can fold into its final, or tertiary structure and be ready to serve as an active participant in cellular processes. To achieve the tertiary structure, the amino acid chain’s secondary interactions must usually be ongoing, and the pH, temperature, and salt balance must be just right to facilitate the folding. This tertiary folding takes place through interactions of the secondary structures along the different parts of the amino acid chain.
The final product is a properly folded protein. If we could see it with the naked eye, it might look a lot like a wadded up string of pearls, but that “wadded up” look is misleading. Protein folding is a carefully regulated process that is determined at its core by the amino acids in the chain: their hydrophobicity and hydrophilicity and how they interact together.
In many instances, however, a complete protein consists of more than one amino acid chain, and the complete protein has two or more interacting strings of amino acids. A good example is hemoglobin in red blood cells. Its job is to grab oxygen and deliver it to the body’s tissues. A complete hemoglobin protein consists of four separate amino acid chains all properly folded into their tertiary structures and interacting as a single unit. In cases like this involving two or more interacting amino acid chains, we say that the final protein has a quaternary structure. Some proteins can consist of as many as a dozen interacting chains, behaving as a single protein unit.
A Plethora of Purposes
What does a protein do? Let us count the ways. Really, that’s almost impossible because proteins do just about everything. Some of them tag things. Some of them destroy things. Some of them protect. Some mark cells as “self.” Some serve as structural materials, while others are highways or motors. They aid in communication, they operate as signaling molecules, they transfer molecules and cut them up, they interact with each other in complex, interrelated pathways to build things up and break things down. They regulate genes and package DNA, and they regulate and package each other.
As described above, proteins are the final folded arrangement of a string of amino acids. One way we obtain these building blocks for the millions of proteins our bodies make is through our diet. You may hear about foods that are high in protein or people eating high-protein diets to build muscle. When we take in those proteins, we can break them apart and use the amino acids that make them up to build proteins of our own.
How does a cell know which proteins to make? It has a code for building them, one that is especially guarded in a cellular vault in our cells called the nucleus. This code is deoxyribonucleic acid, or DNA. The cell makes a copy of this code and send it out to specialized structures that read it and build proteins based on what they read. As with any code, a typo–a mutation–can result in a message that doesn’t make as much sense. When the code gets changed, sometimes, the protein that the cell builds using that code will be changed, too.
Biohazard!The names associated with nucleic acids can be confusing because they all start with nucle-. It may seem obvious or easy now, but a brain freeze on a test could mix you up. You need to fix in your mind that the shorter term (10 letters, four syllables), nucleotide, refers to the smaller molecule, the three-part building block. The longer term (12 characters, including the space, and five syllables), nucleic acid, which is inherent in the names DNA and RNA, designates the big, long molecule.
DNA vs. RNA: A Matter of Structure
DNA and its nucleic acid cousin, ribonucleic acid, or RNA, are both made of the same kinds of building blocks. These building blocks are called nucleotides. Each nucleotide consists of three parts: a sugar (ribose for RNA and deoxyribose for DNA), a phosphate, and a nitrogenous base. In DNA, every nucleotide has identical sugars and phosphates, and in RNA, the sugar and phosphate are also the same for every nucleotide.
So what’s different? The nitrogenous bases. DNA has a set of four to use as its coding alphabet. These are the purines, adenine and guanine, and the pyrimidines, thymine and cytosine. The nucleotides are abbreviated by their initial letters as A, G, T, and C. From variations in the arrangement and number of these four molecules, all of the diversity of life arises. Just four different types of the nucleotide building blocks, and we have you, bacteria, wombats, and blue whales.
RNA is also basic at its core, consisting of only four different nucleotides. In fact, it uses three of the same nitrogenous bases as DNA–A, G, and C–but it substitutes a base called uracil (U) where DNA uses thymine. Uracil is a pyrimidine.
DNA vs. RNA: Function Wars
An interesting thing about the nitrogenous bases of the nucleotides is that they pair with each other, using hydrogen bonds, in a predictable way. An adenine will almost always bond with a thymine in DNA or a uracil in RNA, and cytosine and guanine will almost always bond with each other. This pairing capacity allows the cell to use a sequence of DNA and build either a new DNA sequence, using the old one as a template, or build an RNA sequence to make a copy of the DNA.
These two different uses of A-T/U and C-G base pairing serve two different purposes. DNA is copied into DNA usually when a cell is preparing to divide and needs two complete sets of DNA for the new cells. DNA is copied into RNA when the cell needs to send the code out of the vault so proteins can be built. The DNA stays safely where it belongs.
RNA is really a nucleic acid jack-of-all-trades. It not only serves as the copy of the DNA but also is the main component of the two types of cellular workers that read that copy and build proteins from it. At one point in this process, the three types of RNA come together in protein assembly to make sure the job is done right.
Today’s guest post (originally posted here) is from Katie Hinde, an Assistant Professor in Human Evolutionary Biology at Harvard University. Katie studies how variation in mother’s milk influences infant development in rhesus monkeys. You can learn more about Katie and mammalian lactation by visiting her blog, Mammals Suck… Milk!. Follow Katie on Twitter @Mammals_Suck.
Milk is everywhere. From the dairy aisle at the grocery store to the explosive cover of the Mother’s Day issue of Time magazine, the ubiquity of milk makes it easy to take for granted. But surprisingly, milk synthesis is evolutionarily older than mammals. Milk is even older than dinosaurs. Moreover, milk contains constituents that infants don’t digest, namely oligosaccharides, which are the preferred diet of the neonate’s intestinal bacteria (nom nom nom!) And milk doesn’t just feed the infant, and the infant’s microbiome; the symbiotic bacteria are IN mother’s milk.
Evolutionary Origins of Lactation
The fossil record, unfortunately, leaves little direct evidence of the soft-tissue structures that first secreted milk. Despite this, paleontologists can scrutinize morphological features of fossils, such as the presence or absence of milk teeth (diphyodonty), to infer clues about the emergence of “milk.” Genome-wide surveys of the expression and function of mammary genes across divergent taxa, and experimental evo-devo manipulations of particular genes also yield critical insights. As scientists begin to integrate information from complementary approaches, a clearer understanding of the evolution of lactation emerges.
In his recent paper, leading lactation theorist Dr. Olav Oftedal discusses the ancient origins of milk secretion (2012). He contends the first milk secretions originated ~310 million years ago (MYA) in synapsids, a lineage ancestral to mammals and contemporaries with sauropsids, the ancestors of reptiles, birds, and dinosaurs. Synapsids and sauropsids produced eggs with multiple membrane layers, known as amniote eggs. Such eggs could be laid on land. However, synapsid eggs had permeable, parchment-like shells and were vulnerable to water loss. Burying these eggs in damp soil or sand near water resources- like sea turtles do- wasn’t an option, posits Oftedal. The buried temperatures would have likely been too cold for the higher metabolism of synapsids. But incubating eggs in a nest would have evaporated water from the egg. The synapsid egg was proverbially between a rock and a hard place: too warm to bury, too permeable to incubate.
Ophiacodon by Dmitri Bogdanov
Luckily for us, a mutation gave rise to secretions from glandular skin on the belly of the synapsid parent. This mechanism replenished water lost during incubation, allowing synapsids to lay eggs in a variety of terrestrial environments. As other mutations randomly arose and were favored by selection, milk composition became increasingly complex, incorporating nutritive, protective, and hormonal factors (Oftedal 2012). Some of these milk constituents are shunted into milk from maternal blood, some- although also present in the maternal blood stream- are regulated locally in the mammary gland, and some very special constituents are unique to milk. Lactose and oligosaccharides (a sugar with lactose at the reducing end) are two constituents unique to mammalian milk, but are interestingly divergent among mammals living today.
Illustration by Carl Buell
Mammalian and Primate Divergences: Milk Composition
Among all mammals studied to date, lactose and oligosaccharides are the primary sugars in milk. Lactose is synthesized in mammary glands only. Urashima and colleagues explain that lactose synthesis is contingent on the mammalian-specific protein alpha-lactalbumin (2012). Alpha-lactalbumin is very similar in amino-acid structure to C-type lysozyme, a more ancient protein found throughout vertebrates and insects. C-type lysozyme acts as an anti-bacterial agent. Oligosaccharides are predominant in the milks of marsupials and egg-laying monotremes (i.e. the platypus), but lactose is the most prevalent sugar in the milk of most placental (aka eutherian) mammals. Interestingly, the oligosaccharides in the milk of placental mammals are most similar to the oligosaccharides in the milk of monotremes. Unique oligosaccharides in marsupial milk emerged after the divergence of placental mammals.
Marsupial and monotreme young seemingly digest oligosaccharides. Among placental mammals, however, young do not have the requisite enzymes in their stomach and small intestine to utilize oligosaccharides themselves. Why do eutherian mothers synthesize oligosaccharides in milk, if infants don’t digest them?
In May, Anna Petherick’s post “Multi-tasking Milk Oligosaccharides” revealed that oligosaccharides serve a number of critical roles for supporting the healthy colonization and maintenance of the infant’s intestinal microbiome. Beneficial bacterial symbionts contribute to the digestion of nutrients from our food. Just as importantly, they are an essential component of the immune system, defending their host against many ingested pathogens. The structures of milk oligosaccharides have been described for a number of primates, including humans, and data are now available from all major primate clades; strepsirrhines (i.e. lemurs), New World monkey (i.e. capuchin), Old World monkey (i.e. rhesus), and apes (i.e. chimpanzee).
Among all non-human primates studied to date, Type II oligosaccharides are most prevalent (Type II oligosaccharides contain lacto-N-biose I). Type I oligosaccharides (containing N-acetyllactosamine) are absent, or in much lower concentrations than Type II(Taufik et al. 2012).
In human milk, there is a much greater diversity and higher abundance of milk oligosaccharides than found in the milk of other primates. Most primate taxa have between 5-30 milk oligosaccharides; humans have ~200. Even more astonishingly, humans predominantly produce Type I oligosaccharides, the preferred food of the most prevalent bacterium in the healthy human infant gut- Bifidobacteria (Urashima et al 2012, Taufik et al. 2012).
Human infants have bigger brains and an earlier age at weaning than do our closest ape relatives. Many anthropologists have hypothesized that constituents in mother’s milk, such as higher fat concentrations or unique fatty acids, underlie these differences in human development. But only oligosaccharides, a constituent that the human infant does not itself utilize, are demonstrably derived from our primate relatives (Hinde and Milligan 2011). At some point in human evolution there must have been strong selective pressure to optimize the symbiotic relationship between the infant microbiome and the milk mothers synthesize to support it. The human and Bifidobacteria genomes show signatures of co-evolution, but the selective pressures and their timing remain to be understood.
Vertical Transmission of Bacteria via Milk
In the womb, the infant is largely protected from maternal bacteria due to the placental barrier. But upon birth, the infant is confronted by a teeming microbial milieu that is both a challenge and an opportunity. The first inoculation of commensal bacteria occurs during delivery as the infant passes through the birth canal and is exposed to a broad array of maternal microbes. Infants born via C-section are instead, and unfortunately, colonized by the microbes “running around” the hospital. But exposure to the mother’s microbiome continues long after birth. Evidence for vertical transmission of maternal bacteria via milk has been shown in rodents, monkeys(Jin et al. 2011), humans(Martin et al. 2012), and… insects.
A number of insects have evolved the ability to rely on nutritionally incomplete food sources. They are able to do so because bacteria that live inside their cells provide what the food does not. These bacteria are known as endosymbionts and the specialized cells the host provides for them to live in are called bacteriocytes. For example, the tsetse fly has a bacterium, Wigglesworthia glossinidia,* that provides B vitamins not available from blood meals. Um, if you are squeamish, don’t read the previous sentence.
*I submit the tsetse fly and its bacterial symbiont (Wigglesworthia glossinidia) for consideration as the number one mutualism in which the common name of the host and the Latin name of the bacteria are awesome to say out loud! Bring on your challenger teams.
Hosokawa and colleagues recently revealed the Russian nesting dolls that are bats (Miniopterus fuliginosus), bat flies (Nycteribiidae), and endosymbiotic bacteria (proposed name Aschnera chenzii)(2012). Bat flies are the obligate ectoparasites of bats (Peterson et al. 2007). They feed on the blood of their bat hosts, and for nearly their entire lifespan, bat flies live in the fur of their bat hosts. Females briefly leave their host to deposit pupae on stationary surfaces within the bat roost.
Bat flies are even more crazy amazing because they have a uterus and provide MILK internally through the uterus to larva! Male and female bat flies have endosymbiotic bacteria living in bacteriocytes along the sides of their abdominal segments (revealed by 16S rRNA). Additionally, females host bacteria inside the milk gland tubules, “indicating the presence of endosymbiont cells in milk gland secretion”.
The authors are not yet certain of the specific nutritional role that these bacterial endosymbionts play in the bat fly host. The bacteria may provide B vitamins, as other bacterial symbionts of blood-consuming insects are known to do. My main question is what is the exact role of the bacteria in the milk gland tubules? Are they there to add nutritional value to the milk for the larva, to stowaway in milk for vertical transmission to larva, or both?
The studies described above represent new frontiers in lactation research. The capacity to secrete “milk” has been evolving since before the age of dinosaurs, but we still know relatively little about the diversity of milks produced by mammals today. Even less understood are the consequences and functions of various milk constituents in the developing neonate. Despite the many unknowns, it is increasingly evident that mother’s milk cultivates the infant’s gut bacterial communities in fascinating ways. A microbiome milk-ultivation, if you will, that has far reaching implications for human development, nutrition, and health. Integrating an evolutionary perspective into these newly discovered complexities of milk dynamics allows us to reimagine the world of “dairy” science.
Hosokawa et al. 2012. Reductive genome evolution, host-symbiont co-speciation, and uterine transmission of endosymbiotic bacteria in bat flies. ISME Journal. 6: 577-587
Jin et al. 2011. Species diversity and abundance of lactic acid bacteria in the milk of rhesus monkeys (Macaca mulatta). J Med Primatol. 40: 52-58
Martin et al. 2012. Sharing of Bacterial Strains Between Breast Milk and Infant Feces. J Hum Lact. 28: 36-44
Oftedal 2012. The evolution of milk secretion and its ancient origins. Animal. 6: 355-368.
Peterson et al. 2007. The phylogeny and evolution of hostchoice in the Hippoboscoidea(Diptera) as reconstructed using fourmolecularmarkers. Mol Phylogenet Evol. 45 :111-22
Taufik et al. 2012. Structural characterization of neutral and acidic oligosaccharides in the milks of strepsirrhine primates: greater galago, aye-aye, Coquerel’s sifaka, and mongoose lemur. Glycoconj J. 29: 119-134.
Urashima, Fukuda, & Messer. 2012. Evolution of milk oligosaccharides and lactose: a hypothesis. Animal. 6: 369-374.
A UK study finds that homebirth in specifically low-risk women carries no increased risk for women who have had children previously. They assessed data for 64,538 women and found, after a whole lot of statistical adjustment, that there were no increased odds of negative outcomes for women having birth at home or midwife-attended births in facilities. They did find an increased risk for women who were trying to have planned home births who were giving birth for the first time.
The FDA is thinking about lowering the standard it’s set for how much arsenic exposure is OK in apple and other juices. Cutoffs are usually set in what are known as “parts per billion” (ppb). That means what you think: if the cutoff is 3 ppb, that means, for example, three drops in a billion drops. Right now, the cutoff for arsenic in drinking water is 10 ppb, and consumer groups are asking the EPA to drop that to 3 ppb. Deborah Blum has addressed the fact that arsenic is present in food, water, and soil and that different forms of it have different effects. As always, it’s not as simple as hollering “toxic metal!” and calling for its removal.
Speaking of being like us, some dinosaurs cared for their young, as this fossilized nest of 15 baby dinosaurs seems to suggest.
Looking for the animal with the most amazing, the strangest, the most remarkable nose around? Look no more. It’s the star-nosed mole:
Need a break from the workaday world? Listen to some whale songs and help scientists translate the language of whales.
Speaking of whales, scientists have sunk a 67-foot fin whale carcass off of the San Diego coast. Why go to the trouble? Whale fall is an important contribution to ocean ecosystems, and the researchers plan to study how an entire ecosystem builds up around the sunken cetacean. Here’s a video of the community that forms around a whale fall:
And here’s a beautiful video via Radiolab that uses cutouts to illustrate how such a community builds.
Nicole Ostrowsky shares her love of science in her book, An Agenda of an Apprentice Scientist. She also shares her love of science–and inspires it in others–as a teacher. As she notes, to teach science well to non-scientists, “You have to master subject to explain it simply.”
With the holidays fast approaching, the Double X Science team has come up with a great list of science-themed gifts to help you in your quest for the perfect present. Not only are these gifts thoughtful, they are full of thought. So go forth and spread some nerd love this year!
I Love Science T-shirt, Amazon, $19.99 Let your loved ones tell the world around them that they are into science with this cool take on a T-shirt. I doubt that any of the Kardashians will be wearing this one.
The way we work by David Macaulay, Amazon, $23.10Recommended for ages 5 and above, this book elegantly demonstrates how and why our bodies function the way they do, from digestion, to respiration, to reproduction, all from the perspective of an engineer and illustrator.
Here Comes Science by They Might Be Giants, Amazon, $8.99 This is a seriously wonderful music/DVD combo that uses catchy tunes and big voices to turn science into singable fun! My personal favorite is “Electric Car.” Here is a video form this collection about why the sun shines (my kids know every word):
Hometown Puzzle, National Geographic, $39.95Forget those generic puzzles found on the shelves of cookie-cutter toy stores, this highly personalized jigsaw will tickle the fancy of puzzle-lovers anywhere. I’m probably going to get this for my mom. NOTE: You need to order this by 12/13 if you want it by 12/25.
DNA Gel Travel Mug, Cafe Press, $24.50What’s in your cup is not nearly as cool as what’s on your cup! I’ve always said that DNA is beautiful, and with this mug, everyone will be able to see it. I’d venture to say that it would be a great conversation piece as well.
Leah Gerber is an Associate Professor of ecology at Arizona State University. Her research is motivated by a desire to connect academic pursuits in conservation science to decision tools and effective conservation solutions. This approach includes a solid grounding in natural history and primary data collection, quantitative methods and an appreciation for the interactions between humans and the environment. She is keenly aware of the need for the communication of scientific results to the public and to government and non-governmental agencies. This communication is essential for the translation of scientific results into tenable conservation solutions.
DXS: First, can you give me a quick overview of what your scientific background is and your current connection to science?
LG: I learned about ecology and environmental conservation as an undergraduate and quickly became motivated to do science that impacted the real world of conservation. Learning about the impacts of humans on nature was a wake-up call for me, and inspired me to channel my feeling of concern for the demise of nature in a positive way.
From there, I have walked the tightrope between science and policy. After getting my undergraduate degree in environmental biology, I wanted to do more than just the science. So I enrolled in a masters program at the University of Washington – an interdisciplinary program called Marine Affairs. It was a great experience, but I wanted to have more substance to my science background – I wanted to know how to do the science in addition to how to apply the science.
This compelled me to enter a PhD at the University of Washington, which was largely funded by NOAA. My thesis involved trying to figure out how to make decisions about endangered species – how to determine which were endangered and which were threatened. This was a perfect project given my interest in developing tools to solve problems. After finishing my PhD, I did a postdoc at the National Center for Ecological Analysis and Synthesis (NCEAS) and developed approaches for marine reserve design and endangered species recovery. I was at NCEAS for three years before starting on the tenure track at Arizona State University. I’ve been at ASU for about 10 years now.
A major theme in my work has remained constant – that is, how to use the information we are generating in the natural and social sciences to better manage our natural world. Pre-tenure I focused a lot more on doing the science, publishing in good journals, and hoping that it made its way into good policy. Now that I am midcareer, meaning that I have a good amount of papers and tenure, I am enjoying the opportunity to work with practitioners outside of academia. For instance, I just got off the phone with someone from National Geographic regarding my recent publicationon seafood health and sustainability. In that study, we performed an analysis regarding seafood in the context of health and sustainability, to answer simple questions like, what to order when out to sushi? How do we educate about health benefits and risks? We will be organizing a workshop to help restaurant chains, grocery stores, as well as environmental NGOs identify a path forward in informing consumers about healthy and sustainable seafood choices. As a tenured professor, I feel fortunate to have the opportunity to work at the science-policy interface and to give society some science that is truly applicable.
DXS: It is too bad that you have to wait until you are more established and have tenure to go out and engage with the public, because this type of thing is just so important!
LG: Yes, I agree. There isn’t a clear path in academia when it comes to public engagement. But in recent years I have felt optimistic – the landscape within academia is starting to change, and at ASU this change is noticeable. We have a fabulous president, Michael Crow, who has really transformed ASU from just another state institution to a leader in sustainability. Part of this is the establishment of the Global Institute for Sustainability, and one of Michael Crow’s mantras is “community embeddedness.” He is really on board with this type of thing and I have seen evidence of his commitment trickle down throughout the University. For instance, when I first arrived, I had to justify and explain why I was serving on these federal recovery teams for endangered species. Now I feel that there is no justification needed. Developing solutions is not only so important for society, but should also be a key aspect of what we do at Universities.
DXS: We were introduced by another fantastic science communicator, Liz Neeley, who you met at a communications workshop. Why is it important to take part in this type of training?
LG: I met the Fantastic and Fashionable Liz through the Leopold Leadership Program, offered through the Woods Institute for the Environment at Stanford University. The Leopold Leadership training was the best professional development experience of my career, and has made me a better translator and communicator of science to policy. Pre-Leopold, I had little training in communications, and there I was, in a teaching position where I taught hundreds students. I thought to myself, well, how do I do this? The Leopold experience has solidified my commitment to teaching students about communication and engaging in policy.
One development emerging from this training is a science communication symposium at the AAAS meeting. Elena Bennett and I are giving a talk on overcoming institutional barriers for community engagement, and we will address the issues head on. We put out a survey asking others if they faced institutional barriers, and how they might work to engage more.
DXS: What ways do you express yourself creatively that may not have a single thing to do with science?
LG: I have 2 young kids, a 3yo and a 7yo. Being a mom helps me keep it real - I love that I get to enjoy the awe of discovering the world with my girls. We just got a puppy this weekend and we are having fun dressing her up and painting her nails (only partly joking). Other things that I do that are creative – truthfully, I am uninteresting – I don’t bake bread or go to the opera. I just work and take care of my kids. I practice yoga for my own sanity and also love to work in the garden. Doing these things gives me a reason to pause and step off the treadmill of keeping up with everything.
DXS: Do you find that your scientific background informs the creativity you have with your kids or your yoga practice, even though what you do may not specifically be scientific?
LG: I think there is synergy with my science and my kids and my yoga practice in helping me to accept things and be mindful – but not in any conscious way. For instance, when doing my science, the type A person that I am, I have an inclination to keep pushing, pushing, pushing. My kids and my yoga help me to shift gears and accept that things are going to happen when they happen. I try to let the kids be kids, including the associated chaos, and accept that this is a snapshot in time that they will be little. Now I find joy in that chaos. Having kids and yoga gives me a little more perspective, and the knowledge that things aren’t lined up and neatly placed in a box. It rounds me out.
DXS: Are your kids are major influencers in your career?
LG: My first child, Gabriella, was born just after I submitted my application for tenure – so it was good timing. And I was able to slow down. I quickly realized that I wasn’t able to work a 60+hour week. Before kids, I lived to work. Now, I work to live. I absolutely love my job and I feel so lucky that I have a career that I believe in and that I am actually paid to do it – it’s not just a hobby. But having kids made me chill out a little. If I get a paper rejected, I can let it go instead of lamenting about it for weeks. It has made me healthier. I don’t necessarily know if it has had positive impact on my career – time will tell. While my publication rate may be slightly smaller, I think my work now has different dimensions, and greater depth.
I am still pretty passionate about my work, and my kids know what I do and are proud of it. They share it with their classmates, and take every opportunity to wax poetic about how their mom saves animals in the ocean. They also have a built in conservation effort – my 7YO gets irritated when she can’t find a compost bin, and her new thing is to only fill her cup half way because she will only drink a little bit of water.
DXS: When you decided to have children, did your colleagues view you differently? Did they consider that you were sending your career down the tubes or was it a supportive environment?
LG: I honestly had a really positive experience. I can’t think of any negative sentiments from my colleagues, and they were actually really supportive. For instance, when I was pregnant with my first daughter, ASU did not have a maternity leave policy. Before that, you would have to take sick leave. So my colleague worked within the parameters of the unit to give me maternity leave. And then with my second daughter, our new president had established a maternity policy.
The support of my colleagues at ASU has made me feel loyal to my institution. Normally, I am loyal to people and not institutions, but overall, the support has been fabulous. Of course, with having the kids in each case, I did decline a lot of invitations – some pretty significant ones – but I did not have a desire to drag a newborn to give a talk, especially when I was nursing. And it was hard for me to do this at times, especially given my career driven nature, and I had to learn to accept that there would be other opportunities.
I had to shift it down a notch and realize that the world wasn’t going to freeze over, and that I could shift it back to high gear later. With “mommy brain”, I knew I wasn’t going to be at the top of my game at that point in my life. But I have incredible role models. Most notable is Jane Lubchenco, currently the Director of the National Oceanic and Atmospheric Administration. During the first part of her career, she shared a position with her husband – each did 50% – and they did that on purpose so they’d be able to enjoy having children and effectively take care of them. Now, she is in the National Academy, is having major scientific impacts, and she did it all despite having kids. If she can do it, why cant the rest of us?
DXS: Given your experiences as a researcher, as a mother, and now as a major science communicator, do you feel that your ability to talk to people has evolved?
LG: Absolutely. I think that the Leopold Training Program, which selects 20 academics from North America to participate in retreats to learn how to be better communicate and lead, has re-inspired all who attended. It has recharged our batteries and allowed us to make realizations that doing good science and putting it out there via scientific publication is just not enough. We also have to push it out there and make it available to a broader, more diverse population. As part of the training, we also learned about different thinking styles – super analytical or super emotional – and after I returned, I had my lab group participate in this type of exercise. And now I feel like I can better assess a persons thinking style and adjust the way I communicate accordingly.
DXS: Did you always have the ability to talk to the general public or does having kids help you to better understand some of the nuances associated with science communication?
LG: I think so. In fact, I am thinking back to when I had a paper in Sciencecome out around the time that I had my first child. It got a lot of news coverage and was featured in Time magazine. I thought it was so cool at the time, but looking back on it I realized that have come a long way. I said something to a journalist, who then asked me to translate it into “plain English.” It was a little bit of a jab.
Now, with kids, I can tell you a lot more about my research and can better see the broader impact. Talking to them helps me to do that. Here is a conversation about my research with my daughter:
L: Mama is working on figuring out how to help the whales that people like to eat. It’s a big problem because some people like to eat whales and some like to see them swimming in the ocean.
G: What we have to do is let the people eat the whales in the ocean, and buy some whales from the pet store to put back in the ocean. How much do whales cost?
L: Good idea. But you can’t buy whales at the store. They are too big. And if we take them all out of the ocean there will be none left.
G: Well instead we should ask the people to eat bad things like sharks.
L: Another good idea. But if we take sharks out there will be no predators to eat the big fish. And the whole ecosystem would collapse.=
G: Well then the people should eat other things like fish instead of whales. They should buy a fishing pole and catch a fish and eat those instead of whales.
L: What about chicken, shouldn’t people just eat chicken?
G: Mama, we can’t kill chickens. Chickens are nicer than fish, so that’s why we have to eat fish.
L: What about just eating vegetables?
G: Oh mama, some people are meat-eaters. And there are no more dinosaurs. They all got extinct. They should have saved some of the dinosaur meat in the freezer for the meat-eaters. When the dinosaurs come back, there will be enough meat to eat and people won’t want to eat whales.
The simplicity of taking myself out of my research bubble and engaging with a creative (and nonlinear?) 7YO has taught me how to be a better communicator – with the media, with my students, and with the general population.
DXS: Do you think these efforts in science communication are helping to shift other peoples perspectives about who a scientist actually is? For instance, are we changing the old crazy haired white guy stereotype?
LG: Well, I hope so. A couple of examples – again, as a mom, one of my daughters a Girl Scout and I get to help with the troop. One of the themes was to teach about environmental and conservations awareness. We did this Crayola molding experiment where we put our fingers into cold water. We then did the same thing except we put modeling clay over our fingers before putting them into the cold water and to learn about adaptations to extreme environments. Also, we play games where they simulate fishing – what if there is plastic? What happens to you if you eat that? My hope is that this shows these young girls that science is both interesting and fun.
Another thing that just happened today is that I was contacted by Martha Stewart’s office, and it seems that some of my research results will be featured in the October issue of Martha Stewart Living. The message here is that I happen to care about the ocean, but I also love sushi. I also I care about health. I am not just a nerd in a lab coat. I am a mom, I do yoga, I have wonderful friends, and here is the kind of science that I do. It seems to me that it is better to connect with others when I can give them something that is relevant to their lives instead of a more abstract ecological theory.
DXS: If you had something you could say to the younger you about getting on your chosen career path, what would you say?
LG: I feel like I have been very effective at figuring out how to get from point A to point B, but less successful at savoring the process. I think that I’d tell myself to make time to celebrate the small victories. I have also learned to identify what kind of research is most exciting, and I would tell myself to say “no” to everything that is only moderately interesting. I tell my grad students that if you don’t dive in head first, you won’t ever know. So why just not give it a try! And if it doesn’t work, move on. Also, if something isn’t making you happy, change! Academia isn’t for everyone, and there is a lot more to life than science.