The Inhabited Body
Companion Primer

Primer Chapter 2: The Cell — Life's Basic Unit

This chapter is part of the companion primer to The Inhabited Body. It explains what cells are, how they work, and why the cells of bacteria, archaea, and humans are built on fundamentally different plans — differences that matter enormously for medicine and for understanding the microbiome.

The Idea That Changed Everything

In 1665, an English polymath named Robert Hooke pointed a crude microscope at a thin slice of cork and saw something no one had seen before: a lattice of tiny, empty compartments. They reminded him of the small rooms — cellae — in which monks lived, and so he called them cells [hooke1665].

Hooke did not know what he was looking at. The compartments in cork are the remains of dead plant tissue, and his microscope was far too weak to reveal anything of their internal machinery. But the name stuck, and over the next two centuries a series of increasingly powerful observations converged on a single, transformative insight.

In the late 1830s, the botanist Matthias Schleiden and the physiologist Theodor Schwann independently proposed that all living things are composed of cells — that the cell is the fundamental unit of life [schwann1839]. In 1855, the physician Rudolf Virchow added a crucial corollary: every cell arises from a pre-existing cell. Omnis cellula e cellula — "all cells from cells" [virchow1855].

This is cell theory, and it remains one of the foundational ideas in biology. It tells us that whether you are looking at a human being composed of roughly 37 trillion cells, or a bacterium that is a single cell, the same basic logic applies. Life is cellular. To understand living things — including the trillions of microbes that share your body — you need to understand cells.

What Every Cell Has in Common

Cells come in a staggering variety of sizes, shapes, and specialisations. A human nerve cell can be a metre long. A red blood cell is a flattened disc barely 7 micrometres across. A bacterium might be a hundred times smaller still. Yet despite this diversity, every cell on Earth shares a handful of features. Think of these as the non-negotiable requirements for membership in the club of life.

A boundary. Every cell is surrounded by a cell membrane (also called the plasma membrane) — a thin, flexible barrier that separates the inside of the cell from the outside world. This is not a passive wall. The membrane is studded with proteins that act as gates, sensors, and pumps, controlling what enters and what leaves. Without a membrane, a cell would simply dissolve into its surroundings, like a drop of ink in water.

The membrane is made of phospholipids — molecules with a water-loving (hydrophilic) head and two water-fearing (hydrophobic) tails. In water, these molecules spontaneously arrange themselves into a double layer — a lipid bilayer — with the tails pointing inward, away from the water on both sides. This arrangement is so energetically favourable that it forms automatically. You do not need a factory to build a cell membrane. You just need the right molecules and some water.

An interior. Inside the membrane is the cytoplasm — a gel-like substance made mostly of water, along with salts, organic molecules, and the molecular machinery the cell needs to function. Everything the cell does — breaking down food, building new components, responding to signals — happens in this crowded interior environment.

A set of instructions. Every cell carries its genetic information in the form of DNA — the long, double-stranded molecule we will explore in detail in Primer Chapter 3. DNA contains the instructions for building every protein the cell needs. It is the cell's blueprint, its recipe book, and its operating manual, all in one molecule.

Protein-building machinery. Instructions are useless without the equipment to follow them. Every cell contains ribosomes — tiny molecular machines that read the instructions encoded in DNA (via an intermediate molecule called messenger RNA) and assemble the corresponding proteins, one amino acid at a time. Ribosomes are so fundamental to life that their structure is nearly identical across all three domains. As we saw in Primer Chapter 1, it was precisely this deep conservation that allowed Carl Woese to build a universal family tree of life by comparing ribosomal RNA sequences.

These four features — a membrane, a cytoplasm, DNA, and ribosomes — are universal. Every cell that has ever lived on Earth possesses them. Beyond this shared foundation, however, cells diverge dramatically. The most important divergence, for the purposes of this book, is the one between prokaryotic cells and eukaryotic cells.

The Prokaryotic Cell: Streamlined and Successful

The word prokaryote comes from the Greek pro ("before") and karyon ("kernel" or "nut"). A prokaryotic cell is one that lacks a nucleus — a membrane-bound compartment to house its DNA. Instead, the DNA sits in an open region of the cytoplasm called the nucleoid, with no membrane separating it from the rest of the cell's contents.

All bacteria and all archaea are prokaryotes. They are, by a wide margin, the most numerous cells on Earth and the dominant organisms in the human microbiome. Understanding their architecture is essential.

[Figure P2.1: The prokaryotic cell — see diagram above]

Size

Prokaryotic cells are small. A typical bacterium is between 0.5 and 5 micrometres in length — roughly a thousandth the width of a pinhead. To put this in perspective: if a human cell were the size of a football stadium, a bacterium would be about the size of a car parked in the middle of the pitch. This is not just an interesting fact. Size constrains biology. A small cell has a large surface area relative to its volume, which means nutrients can diffuse in and waste products can diffuse out quickly, without the need for elaborate internal transport systems. This is one reason why prokaryotes can get away with a simpler internal organisation.

The Cell Envelope

Most prokaryotic cells are surrounded not only by a cell membrane but also by one or more additional layers collectively called the cell envelope. The most important of these is the cell wall — a rigid or semi-rigid structure that sits outside the membrane and gives the cell its shape. Without a cell wall, a bacterium would swell and burst under osmotic pressure, much as a balloon pops when you overfill it.

In bacteria, the cell wall is made of peptidoglycan — a mesh-like polymer of sugars and short amino acid chains that wraps around the cell like a chain-link fence wrapped around a water balloon. Peptidoglycan is unique to bacteria. It is not found in archaea, in eukaryotes, or in any other form of life. This uniqueness makes it an excellent target for antibiotics. Penicillin, the first antibiotic ever discovered, works by disrupting peptidoglycan synthesis — it weakens the cell wall so that the bacterium swells and bursts. Because human cells have no peptidoglycan, penicillin harms bacteria without harming us. This is the principle of selective toxicity, and it underpins much of antimicrobial medicine.

A Clinical Landmark: The Gram Stain

In 1884, the Danish bacteriologist Hans Christian Gram developed a staining technique that would become one of the most widely used tools in clinical microbiology [gram1884]. The procedure is simple: bacteria are stained with a violet dye, treated with iodine to fix the dye, washed with alcohol, and then counterstained with a pink dye. Bacteria that retain the violet dye are called Gram-positive; those that lose it and take up the pink counterstain are called Gram-negative.

The difference comes down to cell wall architecture. Gram-positive bacteria have a thick layer of peptidoglycan — up to 40 layers deep — that traps the violet dye and resists the alcohol wash. Gram-negative bacteria have only a thin peptidoglycan layer, but they compensate with an additional outer membrane — a second lipid bilayer sitting outside the cell wall. The alcohol wash penetrates the thin peptidoglycan of Gram-negatives, washes out the violet dye, and the cells pick up the pink counterstain instead.

This distinction is not merely academic. The outer membrane of Gram-negative bacteria contains molecules called lipopolysaccharides (LPS), which are potent triggers of the human immune system. When large quantities of LPS enter the bloodstream — as happens in severe Gram-negative infections — the immune response can spiral out of control, leading to sepsis. The outer membrane also acts as a barrier to many antibiotics, which is one reason why Gram-negative infections are generally harder to treat than Gram-positive ones.

For a clinician, knowing whether an infection is Gram-positive or Gram-negative is often the first step in choosing the right antibiotic. For the microbiome researcher, the Gram stain provides a quick shorthand for grouping bacteria and predicting how they interact with the immune system.

Inside the Prokaryotic Cell

The interior of a prokaryotic cell is, compared to a eukaryotic cell, relatively unstructured. There are no membrane-bound compartments, no elaborate internal transport systems. But "unstructured" does not mean "simple." The cytoplasm of a bacterium is densely packed with molecular machinery.

The nucleoid contains the cell's main chromosome — typically a single, circular molecule of DNA. Unlike eukaryotic chromosomes, bacterial chromosomes are not wrapped around histone proteins (with a few exceptions). The DNA is, however, tightly coiled and organised by specialised proteins that ensure it fits within the tiny cell.

Scattered throughout the cytoplasm are the cell's ribosomes — typically thousands of them in a rapidly growing cell. Prokaryotic ribosomes are slightly smaller than their eukaryotic counterparts (designated 70S versus 80S, where "S" stands for the Svedberg unit, a measure of sedimentation rate during centrifugation). This size difference is medically important: several classes of antibiotics — including tetracyclines, aminoglycosides, and macrolides — specifically target the 70S ribosome, blocking protein synthesis in bacteria without affecting the 80S ribosomes in human cells.

Many bacteria also carry plasmids — small, circular DNA molecules that are separate from the main chromosome. Plasmids replicate independently and often carry genes that provide advantages in particular environments: antibiotic resistance, the ability to break down unusual food sources, or toxin production. Critically, plasmids can be transferred between bacteria — even between different species — through a process called conjugation. This is one of the primary mechanisms of horizontal gene transfer, a phenomenon we explore in detail in Chapter 2 of the main book. It is also one of the main ways antibiotic resistance spreads through bacterial populations.

On the outside, many prokaryotes sport additional structures. Flagella (singular: flagellum) are long, whip-like appendages that spin like a propeller to drive the cell forward. Pili (singular: pilus) are shorter, hair-like projections used for attachment to surfaces or for transferring DNA during conjugation. Some bacteria also produce a capsule — a slimy outer layer of sugars that helps them evade the immune system and stick to surfaces, including human tissues.

The Eukaryotic Cell: Compartmentalised Complexity

The word eukaryote comes from the Greek eu ("true") and karyon ("kernel"). A eukaryotic cell is one that has a true nucleus — a membrane-enclosed compartment that houses its DNA, separating the genetic material from the rest of the cytoplasm.

All animals, plants, fungi, and protists are eukaryotes. Your body is made of eukaryotic cells. So are the fungi in your microbiome — Candida, Malassezia, Saccharomyces, and their relatives.

Eukaryotic cells are, on average, ten to a hundred times larger than prokaryotic cells. A typical animal cell is 10 to 30 micrometres in diameter — big enough that you could fit a dozen bacteria inside with room to spare. But the really significant difference is not size. It is internal organisation.

[Figure P2.2: The eukaryotic cell — see diagram above]

A Cell of Many Rooms

If a prokaryotic cell is like a studio apartment — one open room where everything happens — then a eukaryotic cell is like a house with many rooms, each dedicated to a particular function. These rooms are the cell's organelles — membrane-bound compartments that create specialised chemical environments within the cell.

The nucleus is the largest and most prominent organelle. It contains the cell's DNA, organised into structures called chromosomes — long DNA molecules wrapped around spool-like proteins called histones. The nucleus is enclosed by a double membrane perforated with nuclear pores, which act as gatekeepers controlling the flow of molecules between the nucleus and the cytoplasm. Inside the nucleus, a dense region called the nucleolus manufactures the ribosomal RNA that will be assembled into ribosomes.

Mitochondria are the cell's power stations. These oval-shaped organelles take in oxygen and nutrients and convert them into ATP (adenosine triphosphate) — the molecule that serves as the universal energy currency of the cell. Almost every energy-requiring process in your body — from contracting a muscle to firing a nerve impulse to copying DNA — is powered by ATP generated in your mitochondria.

As we discussed in Primer Chapter 1, mitochondria have a remarkable origin. They are descended from free-living bacteria — specifically, ancient alphaproteobacteria — that were engulfed by an ancestral cell roughly two billion years ago [margulis1967?]. Over vast stretches of evolutionary time, the engulfed bacterium lost most of its genes to the host cell's nucleus, but it retained its own small circular chromosome and its own ribosomes (which are bacterial-type 70S ribosomes, not eukaryotic 80S). This is the theory of endosymbiosis, first championed by Lynn Margulis in 1967, and it is now one of the best-supported ideas in biology [sagan1967].

The endosymbiotic origin of mitochondria has a practical consequence that matters for the microbiome story. Because mitochondria retain bacterial-type ribosomes, antibiotics that target bacterial ribosomes — such as aminoglycosides and chloramphenicol — can also damage mitochondria at high doses. This is one reason why certain antibiotics carry a risk of side effects involving tissues with high energy demands, such as the inner ear (hearing loss) and the kidneys.

The endoplasmic reticulum (ER) is a vast network of folded membranes that extends from the nucleus throughout the cytoplasm. The "rough" ER is studded with ribosomes and specialises in synthesising proteins destined for export from the cell or for insertion into membranes. The "smooth" ER lacks ribosomes and is involved in lipid synthesis and detoxification.

The Golgi apparatus (named after the Italian physician Camillo Golgi, who first described it in 1898) acts as the cell's post office. Proteins arriving from the ER are sorted, modified, packaged into small membrane-bound sacs called vesicles, and dispatched to their final destinations — other organelles, the cell membrane, or the world outside the cell.

Lysosomes are the cell's recycling centres. These small, acidic vesicles contain powerful digestive enzymes that break down worn-out organelles, invading bacteria, and cellular debris. When a white blood cell engulfs a bacterium, it is lysosomes that digest it. Defects in lysosomal enzymes cause a group of inherited diseases known as lysosomal storage disorders, in which undigested material accumulates within cells.

The Cytoskeleton

Eukaryotic cells have an internal scaffolding called the cytoskeleton — a network of protein filaments that gives the cell its shape, enables it to move, and provides tracks along which organelles and vesicles are transported. The cytoskeleton is a dynamic structure, constantly being assembled and disassembled as the cell's needs change. It has no equivalent in most prokaryotes (although some bacteria have simpler cytoskeletal proteins that were discovered more recently [shih2006]).

A Word About Plant Cells

The eukaryotic cell described above is an animal cell — the kind that makes up your body. Plant cells share all of these organelles but add two features of their own: a rigid cell wall made of cellulose (not to be confused with the peptidoglycan wall of bacteria), and chloroplasts, the organelles responsible for photosynthesis. Like mitochondria, chloroplasts are descended from ancient bacteria — in this case, photosynthetic cyanobacteria — acquired through endosymbiosis. Plant cells are not part of the human microbiome, so we will not dwell on them here, but it is worth knowing that the endosymbiotic trick of swallowing a bacterium and keeping it as a permanent internal partner happened at least twice in the history of life.

Archaea: Prokaryotic Architecture, Unique Chemistry

Archaea, as we noted in Primer Chapter 1, are prokaryotes — they lack a nucleus and membrane-bound organelles, and in size and general appearance they resemble bacteria. Under an ordinary microscope, you might not be able to tell them apart. But at the molecular level, archaea are profoundly different, particularly in their membranes.

Bacterial and eukaryotic cell membranes are built from phospholipids with ester-linked fatty acid tails — ordinary straight-chain fatty acids attached to a glycerol backbone by ester bonds. Archaeal membranes use ether-linked isoprenoid chains instead — branched hydrocarbon tails attached by ether bonds, which are more chemically stable. Some archaeal membranes go further: instead of a bilayer (two separate layers of lipids), they form a monolayer — a single sheet of lipids that spans the entire membrane. This monolayer is extraordinarily tough and is found in archaea that live in extreme environments, such as the boiling acidic springs of Yellowstone National Park.

For the human microbiome, the most important archaeal group is the methanogens — organisms that produce methane as a metabolic by-product. The dominant human-associated methanogen, Methanobrevibacter smithii, lives in the gut and plays a supporting role in the microbial ecosystem. It consumes hydrogen gas produced by bacterial fermentation, which prevents hydrogen from building up and inhibiting the very fermentation reactions that produce it. Think of M. smithii as a waste-removal service that keeps the production line running.

Archaea have no peptidoglycan — their cell walls, where present, are made of different materials, most commonly a protein lattice called an S-layer (surface layer). This means that antibiotics targeting peptidoglycan, such as penicillin, have no effect on archaea. It also means that the Gram stain — designed to detect differences in peptidoglycan thickness — does not apply to archaea in any meaningful way.

Why the Difference Matters: A Medical Perspective

The structural differences between prokaryotic and eukaryotic cells are not merely academic. They are the foundation of antimicrobial medicine. Almost every antibiotic in clinical use exploits a feature that bacterial cells have and human cells lack:

Peptidoglycan synthesis — targeted by penicillins, cephalosporins, carbapenems, and vancomycin. Human cells have no peptidoglycan, so these drugs are selectively toxic to bacteria.

70S ribosomes — targeted by tetracyclines, aminoglycosides, macrolides (such as erythromycin and azithromycin), and chloramphenicol. Human cytoplasmic ribosomes are 80S and are not affected. (Mitochondrial ribosomes, being bacterial in origin, are 70S — which is why some ribosome-targeting antibiotics carry mitochondrial toxicity at high doses.)

Unique metabolic pathways — for example, the folic acid synthesis pathway, targeted by trimethoprim and sulfonamides. Humans obtain folic acid from their diet and lack this pathway entirely.

This principle — find a target that the pathogen has and the patient does not — is the reason we can treat bacterial infections without poisoning the patient. It is also the reason viral infections are so much harder to treat: viruses hijack the host cell's own machinery, which means there are far fewer unique targets to aim at.

For the microbiome, the implication is equally important but often overlooked. Antibiotics do not distinguish between pathogenic bacteria and beneficial bacteria. A course of amoxicillin prescribed for a throat infection will also kill susceptible bacteria in the gut, the skin, and every other microbiome site. The structural similarity of all bacteria — the shared peptidoglycan, the shared ribosomes — means that our most powerful medical tools are also blunt instruments when it comes to the microbiome. We will return to this problem in Chapter 19 of the main book.

The Factory Analogy

It can help to pull these ideas together with a single analogy. Imagine two kinds of factory.

The prokaryotic factory is a single open warehouse. There is one large room. The blueprint archive (the DNA) sits on a table in the middle of the floor — anyone can walk up and read it. The assembly stations (ribosomes) are scattered around the room. Raw materials come in through the loading dock (the membrane), and finished products leave the same way. The whole operation is lean, fast, and efficient. It does not need much space. It can be duplicated quickly — some bacteria can divide every twenty minutes, effectively copying the entire factory in less time than it takes to eat lunch.

The eukaryotic factory is a large, multi-storey building. The blueprints are locked in a secure archive room (the nucleus), and only authorised copies (messenger RNA) are allowed to leave. Assembly stations sit on a dedicated production floor (the rough ER). Finished products are sent to a quality-control and packaging department (the Golgi apparatus) before being shipped out. The building has its own power plant (mitochondria), a recycling department (lysosomes), and a structural framework (the cytoskeleton) that keeps everything in place and moves materials between floors. It is slower, larger, and more expensive to run — but it can manufacture products of extraordinary complexity.

Neither design is "better" in an absolute sense. The prokaryotic model has dominated life on Earth for over three billion years. The eukaryotic model enabled the evolution of multicellular organisms — including you. Both are present in your microbiome, and understanding their architecture is the first step toward understanding how they interact with your body and with each other.

A Note on Scale

Numbers in biology can become abstract very quickly, so it is worth pausing to anchor a few of them.

A single bacterium is roughly 1 to 2 micrometres long. A micrometre is one millionth of a metre, or one thousandth of a millimetre. The full stop at the end of this sentence is about 500 micrometres across — large enough to fit several hundred bacteria side by side.

A typical human cell is about 10 to 30 micrometres in diameter. Your body contains roughly 37 trillion of them [bianconi2013].

The bacterium Escherichia coli, which lives in your gut and is probably the most studied organism on Earth, is about 2 micrometres long and 0.8 micrometres wide. Its entire genome — the complete set of genetic instructions for building and running the cell — is about 4.6 million base pairs of DNA, encoding roughly 4,300 genes [blattner1997].

Your genome, by comparison, is about 3.2 billion base pairs long and contains approximately 20,000 protein-coding genes [international2004]. Your genome is 700 times larger than E. coli's — but you have only about five times as many protein-coding genes. Much of the difference is accounted for by non-coding DNA, regulatory sequences, and the remnants of ancient transposable elements and endogenous retroviruses (as we discussed in Chapter 2 of the main book).

Where This Matters in The Inhabited Body

Chapter References

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