DoubleXplainer: What is a vagina?

Development of the female (right) and male (left)
sex anatomy (now unreversed; thanks Peter Edmonds!). (Source)

By Emily Willingham, DXS managing editor

What is a vagina?

First, let’s just practice saying the word. Vagina. Vuh-ji-nuh. VAGINA!

OK. Why are we practicing this? So that we can avoid suffering from the fluttery sensibilities of one Rep. Mike Callton of Michigan who, upon hearing colleague Rep. Lisa Brown use the word vagina during a speech on the Michigan House floor, commented:

What she said was offensive. It was so offensive, I don’t even want to say it in front of women. I would not say that in mixed company.

So here we have a fellow who is so squeamish about female anatomy that he won’t even use the appropriate terminology for that anatomy in front of the people who have the body part. So beflustered are his tender feelings about the word vagina that he and the Republican leadership of the Michigan house of representatives refused to allow Rep. Brown speak again when discussing a bill about retirement of school employees. I assume they were concerned that somehow, she’d drag in the dreaded V-word again while talking about pensions.

All for the transgression of saying the word “vagina.” Vagina.

You know what? It’s not a mellifluous word. It has that giraffey g in it, an ugly “vuh” sound. It would probably be more palatable in general if we had decided to term this particular part of female anatomy something else, perhaps “hibiscus.” Unfortunately, as with so much in anatomy, we had to rely on Latin instead of flowers, and in Latin, vagina means “sheath” or “scabbard.” In other words, a place to put a sword… or a penis. Or, as I like to call them, “sperm delivery systems.”

The offensive body part is indicated. (Source)
People tend to have a misunderstanding about the vagina. They think that what they’re seeing on the outside of the woman is the vagina. Unless their viewpoint is very up close and personal, it isn’t. Those are the labia majora and labia minora, sometimes referred to crassly as “the lips,” and making up part of the vulva (actual vulva pic, fair warning). There’s a big pair (the majora) and a little pair (the minora). In men, the two sides of the big pair zip early in development to encase the testes (see top image). The little pair forms the shaft of the penis. In women, both pairs stay apart. No zipping (ETA: see good interactive explanation here). But that’s not the vagina. 
Behold the clitoris. (Source


For those who are unfamiliar, you can usually find the entrance to the vagina if you peek between the labia minora. If you’ve never poked around knowledgeably in the female anatomy, let’s orient ourselves a little. Up at the very top, tucked away under the labia majora, is the clitoral hood. Look under the hood–this is highly recommended on specific occasions–and you’ll find the clitoris. This fabulous body part has far more to it than first appearances might suggest. What you see there under the hood is a small fraction of what a woman gets (recommended reading!), and we have this clitoris to thank for a woman’s superior orgasmic capacities. Yes, I said “superior.” The male echo of the clitoris is the glans penis (actual penis pic, fair warning), and the two anatomical features share some commonalities, including the ability to become erect. Of course, if you have a clitoris, no one notices if you become aroused in algebra class. Clitoris FTW!

Just below all of those interesting bits is the urethral opening. Men have this opening at the tip of the penis, where it serves a double duty, releasing semen and urine, preferably not simultaneously. In women, this opening is dedicated to elimination only. Follow that sucker up a few inches, and you hit the bladder. Don’t go in there. That’s an “exit only” kind of orifice, like your nostrils.

Move down just a tad more and… that’s it! There between the labia minora, that’s the vaginal opening. That’s where the actual vagina is. The part of the female anatomy that got a female legislator blocked from speaking just for saying it.

There it is, the vagina, bridging the outside
and inside worlds and freaking out Michigan
legislators since time began. (Source)


The vagina is an amazing structure. Nothing else in human anatomy has the flexibility of this thing. It starts there at the opening and extends several inches into the body, leading to the cervix. Cervix means “neck” (think of cervical collars), and it is indeed the neck of the uterus. If you’ve given birth vaginally, you know that the baby exits the uterus through this neck, travels very quickly through the vagina, and enters the world through the vaginal opening. If you’ve seen the cervical or the vaginal opening, you will be astonished that an entire baby can fit through either. But the uterus, the most powerful muscle in the body, handles the cervical part, contracting and pulling and contracting and pulling until the cervix is juuuuuust wide enough for an infant human head to fit through… sort of. The vagina deals with the rest.

And once that infant–someone like you, Mike Callton–leaves the cervix, it is in the vagina. If you didn’t arrive here via C-section, you got here by making your first extended trip–through a vagina. The vagina is so accommodating and flexible that it can stretch to many times its usual diameter to allow an entire human infant to exit a woman’s body and enter the world. If you’ve never put a finger in a vagina, try it if you can find a willing partner or if you have a vagina of your own. Then imagine that cozy-feeling vagina stretching fairly effortlessly to accommodate an entire infant.

That flexibility isn’t relevant only to childbirth. When a woman becomes aroused during sex, the vagina elongates to facilitate the process of sperm delivery and penis accommodation. It also self lubricates and has a ton of nerves near the opening, all part of making sex that super fun thing that people with vaginas or penises tend to think it is. But it wouldn’t be so fun–or pragmatically useful–without the vagina. There. I said it. Thirty times in this single blog post. And you should, too.

These views are the opinion of the author and do not necessarily either reflect or disagree with those of the DXS editorial team. 

See also our Pregnancy 101 series, by Jeanne Garbarino, biology editor

To Cut or Not to Cut…Cirumcision Decision

“You wanna do WHAT?!” Photo courtesy of Justyna Furmanczyk at
By Tara Haelle, DXS contributor
[Tara Haelle ( is a health and science writer and a photojournalist based in Peoria, IL after years as a Texan, where she earned her undergraduate degrees and MA in journalism at UT-Austin. She’s the mental health editor for in addition to reporting on pediatrics, vaccines, sleep, parenting, prenatal care and obesity. This post first appeared on her blog, Red Wine & Apple Sauce focuses on health and science news for moms (, and you can follow her on Twitter at @health_reporter and @tarasue. She’s also swum with 9 different species of sharks, climbed Kilimanjaro and backpacked in over 40 countries, but that was in the years of B.C. (Before Children). She finds that two-year-olds are tougher to tussle with than tiger sharks.]
So you’ve likely heard by now that the American Academy of Pediatrics issued their updated policy statement on circumcision, the first since 1999. I’ve been sitting on the statement and the task force technical report for a week now, and even though I’ve written a news summary for dailyRx…  I have many mixed feelings.
I am grateful that their statement was issued with the sensitivity and caution needed for such a controversial practice and decision. Some of the headlines have been frustrating, implying that the AAP said “Circumcision is better.” Um, no. That’s not what they said. They said that the “preventive health benefits of elective circumcision of male newborns outweigh the risks of the procedure.” (To be fair, most headlines basically ran with “benefits trump risks” or some variation thereof.)
In other words, if you choose to do this procedure, the benefits you will gain are greater than the risks involved in the procedure. This is very different from saying “It’s better to be circumcised.” In fact, their policy explicitly points out that they do not officially “recommend” the procedure routinely: “Although health benefits are not great enough to recommend routine circumcision for all male newborns, the benefits of circumcision are sufficient to justify access to this procedure for families choosing it and to warrant third-party payment for circumcision of male newborns.” (That last part just means yes, insurance companies, you should pay for it.)
An analogy: A child with obstructive sleep apnea can have a tonsillectomy/adenoidectomy (called an adenotonsillectomy) to remove their tonsils and adenoids for treatment. The tonsils and adenoids (lumps of issue behind the nose) generally cause the blockage that interferes with a child’s breathing while asleep, so removing them can usually cure the sleep apnea (in 75 to 100 percent of the cases).
There are risks to adenotonsillectomy, namely infection and excessive bleeding. There are risks to sleep apnea, including obesity, heart disease, diabetes, depression and death. For a child with obstructive sleep apnea, the benefits generally outweigh the risks of the procedure. A parent can still elect not to give their child the surgery.
Is it better for the child with sleep apnea to have the surgery? Probably. But perhaps not. It depends on the situation and the child. Is it better for a child without obstructive sleep apnea to have the surgery? Of course not. Why take any risk when there’s no benefit?
Now consider the two primary benefits conferred by circumcision: lower risk of urinary tract infections during the first year and reduced risk of HIV and a several other sexually transmitted infections during heterosexual sex. The risks of circumcision are most commonly bleeding, infection or the wrong amount of tissue snipped off, and this happens in about 1 of every 500 newborn boys (0.2 percent). Other studies found the rates higher, up to 2 to 3 percent, but these complications were still just minor bleeding. They even offered a comparison of a similar surgery as the one I discussed above: complications involving severe bleeding from tonsillectomies occur about 1.9 percent of the time in kids age 4 and under.
For parents with wild imaginations about horror stories, fear not: “The majority of severe or even catastrophic injuries are so infrequent as to be reported as case reports (and were therefore excluded from this literature review). These rare complications include glans or penile amputation, transmission of herpes simplex after mouth-to-penis contact by a mohel (Jewish ritual circumcisers) after circumcision, methicillin-resistant Staphylococcus aureus infection, urethral cutaneous fistula, glans ischemia and death.” Basically, yea, there’s a bunch of really bad stuff that can happen, but it’s really, really, really, really rare. Probably rarer than being struck by lightning. Twice. But that happens too.
So, the risks are pretty low. How beneficial are the benefits? Here’s a condensed run-down from the AAP’s technical report:
  • Circumcision reduces the odds of contracting HIV during male-female sex by 40 to 60 percent… in Africa. When the CDC calculated that figure with the rate of contracting HIV by heterosexual sex in the U.S., they came up with a 15.7 percent reduction here. It’s something, but nowhere near as good as a condom. Plus, if your kids turns out to be gay, there’s not much evidence that circumcision helps him avoid contracting HIV. (And on the other side of the coin, circumcision can make it a little easier for women to contract HIV from a man, per one study cited in the AAP review.)
  • Circumcised men are about 30 to 40 percent less likely to get any type of human papillomavirus (HPV), including both the relatively harmless strains and the ones that can lead to cervical cancer or raise your risk of cancer of the mouth, throat, penis and anus. Now, the CDC has recommended that boys get the HPV vaccine, but the vaccines available do not cover all the strains. Gardasil takes care of four of them, including the two responsible for about 70 percent of cervical cancer (HPV-16 and HPV-18) and the two responsible for 90 percent of genital warts. Cervarix only takes care of HPV-16 and HPV-18. So, circumcision would offer some protection against getting the HPV strains that the vaccines don’t cover, most of which — but not all — are not linked to cancer or warts.
  • There’s some evidence that circumcision reduces risk of herpes (HSV-2) by about 28 to 34 percent, based on two studies in Africa.
  • Evidence for protection against syphilis is weak. There’s no evidence that circumcision decreases the risk of contracting gonorrhea or chlamydia.
  • There’s good evidence that uncircumcised boys get more urinary tract infections that circumcised boys, in part because bacteria can hang out in that moist area under the hood. The AAP estimates that 7 to 14 of every 1,000 uncircumcised boys will get a UTI before their first birthday, compared to 1 to 2 out of 1,000 circumcised boys. With such a low rate overall, in either population, the AAP notes that “the benefits of male circumcision are, therefore, likely to be greater in boys at higher risk of UTI, such as male infants with underlying anatomic defects such as reflux or recurrent UTIs.” (These are mostly the boys that get UTIs anyway.)
So, those are definitely some benefits to circumcision, especially if your little guy will have sex one day (which, presumably, you want him to do at some point in the far off, I-don’t-want-to-think-about-it future). It’s also fair to say that good sex education and condom use would make those benefits almost moot (not the UTIs, which are pretty low risk, and not all HPV strains, which sometimes infect even with condom use).
In any case, these two benefits, a lower risk for UTIs and some STIs, then become the risks of not being circumcised. The former is — usually — not very serious. There are some very serious urinary tract infections, and untreated ones can damage the kidneys. And they’re certainly not fun. They aren’t, however, usually life or death situations. HIV (somewhat still) is. Of course, boys are still at a pretty high risk for getting HIV if they sleep with someone who has it and don’t use a condom, circumcised or not. But every bit of protection helps, right?
Unless it requires lopping off part of a little boy’s penis. There. I said it. Because that’s what many parents are simply uneasy about, regardless of the health benefits, which are great or marginal, depending on your perspective. And that’s why the AAP stopped short of recommending circumcision as a routine procedure.
They did include in their review several studies related to sexual satisfaction and sensitivity, one of the complaints that “intactivists” bring up. The AAP summarizes it pretty nicely: “The literature review does not support the belief that male circumcision adversely affects penile sexual function or sensitivity, or sexual satisfaction, regardless of how these factors are defined.”
But it’s not possible to take into consideration, in scientific, mathematical terms, the primary complaint of those who oppose circumcision, which is that the man these little boys become may have wanted that little flap over the tip. And this is one of those gray areas that give parents pause. Once you cut that hood, you can’t put it back. How many circumcised men regret what their parents did? Well, probably not vast numbers, or circumcision rates would have plummeted.
Rates have, in fact, decreased, from somewhere around three-quarters of all boys in the 1960s to around 55 to 59 percent in 2010. (Here’s a nifty map to see where your states’ rates are.) But they haven’t plummeted.
So, this is where we end up. There are some decent benefits. There are very few and mostly minor risks to the procedure. And there’s big, giant, gray unknown area of “what if’s” and “could have been’s” for the boys who get snipped. It’s disingenuous to compare the practice to female circumcision, as some do, since neither its intent nor its effect is to influence sexual satisfaction. But whether it’s the right thing to do…? The AAP says it’s up to mom and dad. (Which, in many households, like mine, probably means mostly dad.)
“Parents ultimately should decide whether circumcision is in the best interests of their male child,” they wrote. “They will need to weigh medical information in the context of their own religious, ethical, and cultural beliefs and practices. The medical benefits alone may not outweigh these other considerations for individual families.”
What are those other considerations? Well, whether you want your little guy to have a foreskin. Or, whether you don’t know if he does or doesn’t want it and figure he should decide that in 18 years. Maybe daddy’s not circumcised and you both want him to look like daddy. (I know many people who circumcised for this reason alone.)
About the only certain thing that can be said about circumcision, based on the AAP’s policy statement and research and what we know about opposition to the practice, is that this controversy will be with us for years to come.
The opinions in this post do not necessarily reflect or disagree with the opinion of the DXS editorial team.

Biology Explainer: The big 4 building blocks of life–carbohydrates, fats, proteins, and nucleic acids

The short version
  • 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.

Nucleic Acids

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.

 By Emily Willingham, DXS managing editor 
This material originally appeared in similar form in Emily Willingham’s Complete Idiot’s Guide to College Biology