Cell: The Building Block of Life

Most scientists agree that life first appeared in water, though not necessarily the wide open ocean. Some researchers now think it more likely began in small, changeable pools rather than the sea, and hot springs offer a working model of what those pools might have been like. In Ladakh's Puga Valley, hot springs stay near boiling point even in a landscape that is otherwise bitterly cold, conditions thought to closely resemble Earth's environment roughly 3.5 billion years ago. Living inside these springs today are heat-loving bacteria called thermophiles, entirely single-celled and entirely at home in water hot enough to scald you. Researchers at the Birbal Sahni Institute of Palaeosciences in Lucknow studying these same springs found that calcium carbonate forms rapidly around them, and they suspect these mineral deposits may once have shielded early organic molecules from damaging radiation, and perhaps even helped the very first protective membrane, the boundary that turns a loose cluster of chemicals into an actual cell, come into being. Every living thing on Earth, from that first membrane onward, is built from cells. Some organisms, like bacteria or yeast, are made of only one; others, like you, are made of trillions working together. This chapter goes further inside a cell than any before it, then follows what happens once cells start grouping together into tissues.

Hold a piece of paper with two dots drawn close together at arm's length, and slowly bring the dots closer, and there is a precise point at which your eyes stop being able to tell them apart and see only one blurred point instead. This limit is called the limit of resolution of the human eye, and for most people it sits at around 0.1 millimetre, viewed from roughly 25 centimetres away. A typical cell is far smaller than that limit, which is exactly why cells stayed invisible to humanity for the vast majority of history. In 1665, Robert Hooke became the first person to break through that limit, building his own microscope, capable of magnifying roughly two to three hundred times, and pointing it at a thin slice of cork, where he spotted small box-like compartments he named cells. School microscopes today work on the same basic principle Hooke used, a system of lenses that magnifies an object, though scientists studying the very finest cell structures now reach for something far more powerful: an electron microscope, which uses a beam of electrons instead of light and can resolve detail at the scale of a nanometre, one billionth of a metre. You can actually estimate a real cell's true size using an ordinary school microscope and a little arithmetic: measure the diameter of the circular field of view you see through the eyepiece using a transparent ruler, count how many cells fit across that same diameter on a slide of onion peel, then divide the field's diameter by that cell count. If the field of view measures 5000 micrometres across and exactly 25 onion cells span that width, each individual cell works out to 5000 divided by 25, or 200 micrometres, invisible to your unaided eye but perfectly measurable once you know how to look.

Cut a potato into two equal pieces, weigh each one, then drop one piece into a beaker of plain water and the other into a strong salt solution, and leave them for an hour. The piece sitting in plain water gets noticeably heavier and firmer. The piece sitting in salt solution gets lighter and softer. Both pieces are made of the exact same living cells, so the only thing that changed is what surrounded them, and the explanation lies in the cell membrane, the thin boundary wrapping every cell, which is selectively permeable: it lets some substances cross freely while blocking others, water included among the substances it lets through, salt and sugar molecules generally not. Water naturally moves from wherever it is more concentrated (that is, less crowded with dissolved substances) toward wherever it is less concentrated (more crowded with dissolved substances), a specific case of diffusion called osmosis, and it keeps moving until the concentration evens out on both sides of the membrane. Plain water outside the potato cells is more dilute than the fluid inside those cells, so water flows in and the potato swells. Concentrated salt solution outside is far more crowded with dissolved salt than the fluid inside the cells, so water flows the opposite way, out of the cells, and the potato shrinks. Structurally, the cell membrane is built from a double layer of fat molecules only seven to ten nanometres thick, with proteins scattered through it that act like the membrane's own gatekeepers, physically escorting specific substances across. Because the molecules making up this membrane can slide, rotate and shift position rather than staying rigidly fixed in place, scientists call this the fluid mosaic model.

A tree cannot walk away from a strong wind, and a blade of grass cannot duck out of the way of a grazing goat, so plants have evolved something animal cells never bothered with: a rigid cell wall, sitting just outside the cell membrane, built mainly from a tough carbohydrate called cellulose. Place plant cells in a strong sugar solution and osmosis pulls water out of them exactly as it did the salt-soaked potato, yet the outer boundary of the cell barely changes shape at all. Only the softer material just inside, the cell membrane and everything it wraps, actually shrinks away from the rigid outer wall, leaving a visible gap between the two. Animal cells, lacking any such wall, simply shrink all over when they lose water the same way. This single structural difference explains a great deal about how plants and animals ended up looking and behaving so differently: a rigid wall around every cell is exactly what lets a stem stay upright without any internal skeleton at all, but that same rigidity is also why plant cells can never crawl or change shape the way an animal cell can. The cell wall is not sealed shut, either: it stays permeable enough to let water and dissolved minerals pass through, which combined with the cell membrane's own selective permeability just inside it, is exactly how a plant's roots manage to pull water and nutrients up out of the soil in the first place.

Every eukaryotic cell, meaning every cell with a properly enclosed nucleus, keeps its most important information locked inside that nucleus, wrapped in its own double-layered membrane, studded with pores that let specific material pass between the nucleus and the rest of the cell. Inside sits a dense round body called the nucleolus, which manufactures the components later assembled into ribosomes, and a tangled, thread-like mass called chromatin, which is really the cell's DNA loosely spread out and unwound. DNA is the actual molecule carrying genetic information, organised into functional stretches called genes, and every time a cell is about to divide, its chromatin coils and condenses tightly into the rod-shaped structures called chromosomes, visible under a microscope only at that specific moment. Step outside the nucleus and the rest of the cell's interior looks like a small, fully staffed factory floor. Tiny structures called ribosomes, some drifting free in the cytoplasm and others attached directly to a folded, membranous network called the endoplasmic reticulum, are where proteins actually get built, which is exactly why the ribosome-studded version of that network is called rough endoplasmic reticulum, while a smoother version with no ribosomes attached specialises in building fats and hormones instead. Whatever the endoplasmic reticulum manufactures still needs sorting, packaging and shipping, and that job falls to stacks of flattened sacs called the Golgi apparatus, first properly identified in 1898 by the Italian scientist Camillo Golgi studying nerve cells from a barn owl, though so many doubted his discovery at the time that decades passed before more powerful microscopes finally confirmed it was real. Cells also generate their own waste, which is where lysosomes come in: small sacs packed with enzymes capable of breaking down unwanted material and recycling the pieces back into the cytoplasm, quietly keeping the whole cell clean.

Every cellular activity, from building a protein to contracting a muscle, needs energy, and most of that energy is supplied by mitochondria, structures so central to keeping a cell running that they are often called its powerhouses. Each mitochondrion is wrapped in two separate membranes, an outer one that is smooth, and an inner one thrown into deep, finger-like folds called cristae, which dramatically increase the surface area available for the chemical reactions that actually release energy. Inside, glucose and other fuel molecules are broken down in a process called cellular respiration, and the energy that process releases gets stored in a molecule called ATP, effectively the cell's own currency, spent to pay for almost everything else the cell does. Plant cells have an equivalent set of energy-related organelles that animal cells simply do not need, called plastids, and the most familiar of these is the chloroplast, also wrapped in two membranes, containing a semi-fluid substance called the stroma packed with flattened, disc-shaped structures where the green pigment chlorophyll actually absorbs sunlight during photosynthesis. Chloroplasts are not the only kind of plastid. Chromoplasts, containing pigments other than chlorophyll, are responsible for the yellow, orange or red colours in flowers and fruit, while colourless plastids called leucoplasts, found in structures like a potato, specialise purely in storing food such as starch. Remarkably, both mitochondria and chloroplasts carry their own small circular DNA and their own ribosomes, entirely separate from the ones in the cell's nucleus, structural quirks that strongly suggest both organelles descend from free-living bacteria absorbed into far larger cells at some point deep in evolutionary history.

A mature plant cell usually contains one large, central vacuole, wrapped in its own selectively permeable membrane and filled with a watery fluid called cell sap, functioning as storage space for water, minerals, sugars and waste all at once. By holding a large volume of water, this single vacuole presses outward against the cell wall and keeps the whole cell firm, which is exactly why a plant wilts when it goes too long without water: its vacuoles lose their fluid, the pressure that kept every cell firm disappears, and the entire plant visibly droops. Animal cells sometimes contain vacuoles too, but theirs stay small, nothing close to the single dominant vacuole found in most plant cells. Zoom out from any single organelle and there turns out to be an even more fundamental split running through the whole of life, one that cuts right across the plant-versus-animal divide. Bacteria have no properly enclosed nucleus at all: their genetic material sits as a single circular DNA molecule in a region simply called the nucleoid, with no membrane sealing it off, and they lack membrane-bound organelles of any kind. Cells built this way are called prokaryotic, from Greek roots meaning before a nucleus. Plant and animal cells, by contrast, package their DNA inside a proper membrane-bound nucleus and build a whole range of membrane-bound organelles around it, which makes them eukaryotic, meaning a true nucleus. Prokaryotic cells also tend to be dramatically smaller, typically just one to ten micrometres across, compared to ten to one hundred micrometres for a typical eukaryotic cell.

You began life as a single fertilised cell, and that one cell divided, and divided again, until it produced the trillions of cells that make up your body today, almost all of them created through a type of division called mitosis, in which one parent cell splits into two daughter cells carrying an identical copy of the parent's DNA and the exact same number of chromosomes. Mitosis handles ordinary growth, replacing worn-out cells, and healing injuries, but it is not the only way a cell can divide. A second, more elaborate process called meiosis happens only inside reproductive organs, dividing a parent cell twice in a row to produce four daughter cells, each carrying only half the parent's usual number of chromosomes, exactly the right amount to combine with a partner's contribution during fertilisation and restore the full chromosome count in the next generation. Discoveries like these, made across the nineteenth century, eventually merged into a single unifying idea called cell theory. In 1838, the German botanist Matthias Schleiden concluded that every plant is built from cells. In 1839, the German zoologist Theodor Schwann reached the identical conclusion about animals. In 1855, Rudolf Virchow added the final piece: every new cell comes only from a pre-existing cell that divided, never spontaneously from nothing. Together, these three ideas tie together everything from a single bacterium to an entire human being under one framework. Cell division itself has to stay tightly controlled. In most animal tissue, cells stop dividing the moment they come into direct contact with their neighbours on all sides, a self-limiting safety mechanism called contact inhibition, and when that control mechanism fails, cells that should have stopped keep dividing anyway, forming the uncontrolled growths known as tumours.

A single amoeba has to do absolutely everything itself using one lone cell. You do not, because your body, like every multicellular organism, distributes those jobs across huge numbers of specialised cells, and cells performing the same specialised job, grouped together, are what biologists call a tissue. In a plant, active growth is entirely the job of meristematic tissue, made of small, thin-walled cells packed with a large nucleus and dense cytoplasm but almost no vacuole, permanently capable of dividing again and again. Meristematic tissue sitting right at the very tip of a root or shoot, called apical meristem, is what makes roots push deeper and shoots grow taller; cut off a root tip and that root simply stops lengthening. A ring of meristematic tissue running around the inside of a stem, called lateral meristem, is instead responsible for increasing girth, which is also why a cut tree trunk shows a pattern of concentric growth rings, one added per year. A third kind, intercalary meristem, sits at the base of structures like grass stems, which is precisely why grass, and a garden hedge, grow bushier after being cut or grazed rather than simply staying shorter forever. Not every plant cell keeps dividing throughout its life. Most eventually lose that ability and become permanent tissue instead, specialising into one particular job, a transformation called differentiation. Permanent tissue built from just one type of cell is called simple, and includes parenchyma (loosely packed living cells that store food or photosynthesise), collenchyma (living cells with flexible, thickened corners, letting stems bend without snapping), and sclerenchyma (dead cells with rigid, lignin-hardened walls, forming genuinely woody structures). Permanent tissue built from more than one type of cell is called complex, and the two most important examples, xylem and phloem, run through every plant like a two-way delivery system: xylem carries water and minerals upward from the roots, while phloem carries the food a plant's leaves make to wherever else it is needed.

Blink, clench your fist, take a deep breath, touch something cold, and four completely different kinds of tissue just did the actual work behind each of those simple actions. Epithelial tissue forms your skin and lines every internal surface exposed to the outside world, built from cells packed so tightly together that almost no space exists between them, which is exactly what keeps germs out and prevents fluid loss; depending on exactly where it sits, epithelial tissue can be built to allow rapid diffusion, to physically protect against friction and injury, to secrete substances like sweat or digestive juices, to sense smell or taste, or to efficiently absorb nutrients. Connective tissue does exactly what its name suggests, connecting and supporting other tissues, and it comes in some genuinely surprising forms: blood, with its watery, fluid matrix, is a connective tissue every bit as much as rigid bone, with its hard, mineral-packed matrix, and so are the flexible cartilage that cushions your joints, the tendons that anchor muscle to bone, and the ligaments that anchor bone to bone. Muscular tissue is what actually produces movement, and comes in three distinct types: skeletal muscle, attached to bone and under your conscious, voluntary control, built from long, banded fibres with many nuclei each; smooth muscle, working automatically inside organs like your stomach, spindle-shaped with a single nucleus and no visible banding; and cardiac muscle, found nowhere except the heart, branching and striped, uniquely built to contract rhythmically without ever fully resting for an entire lifetime. Nervous tissue, finally, is built from cells called neurons, each shaped with a central cell body, branching dendrites that receive incoming signals, and a long axon that carries an outgoing signal onward, together forming the network that lets your brain coordinate everything else happening inside you.

In 1958, a scientist named F. C. Steward took an almost absurdly small starting point, a handful of 2-milligram fragments cut from the phloem tissue of a carrot root, cells that had already finished dividing and had specialised permanently into their one adult job, transporting food. Grown in a nutrient-rich liquid medium containing the right sugars and hormones, those mature, supposedly finished phloem cells did something nobody had shown before: they reverted, losing their specialised identity and regaining the ability to divide, first forming a shapeless mass of unspecialised cells, then gradually redifferentiating all over again into roots, a shoot, and eventually an entire new carrot plant, genetically identical to the one the original fragment came from. Steward had just demonstrated something called totipotency: the astonishing ability certain plant cells retain to rebuild an entire organism from scratch, essentially replaying the exact same journey a single fertilised egg cell takes when it first develops into a full plant, except starting from a cell that had already grown up and specialised once already. It is a genuinely remarkable capability, and also, in its own way, the perfect place to end this chapter's story. Everything explored here, from a boundary made of fat molecules seven nanometres thick, to a control centre holding coiled threads of DNA, to power plants inherited from ancient bacteria, to whole armies of specialised tissues built from trillions of cooperating cells, all traces back to one thing: a single cell, following cell theory's oldest and simplest rule, arising only from a cell that came before it.