The Invisible World Inside Every Living Thing

In 1665, an English scientist named Robert Hooke pointed a homemade microscope, one powerful enough to magnify things two to three hundred times, at the thinnest sliver he could cut from a piece of cork. What he saw surprised him: the cork was not solid at all, but packed edge to edge with tiny, empty compartments, walled off from each other like the chambers of a honeycomb. Hooke sketched what he saw and published it in a book called Micrographia, and for these tiny box-like compartments he needed a word. He chose cell, from a Latin word for a small room, the same root that gives us words like a monk's 'cell' in a monastery. It was the first time that word had ever been used in science, and it stuck. Hooke had actually seen the leftover walls of dead plant cells, not living ones, but the discovery cracked open an entirely new way of looking at every living thing. A few years later, in the 1660s, a Dutch cloth merchant and lens-grinder named Antonie van Leeuwenhoek built even sharper microscopes of his own and became the first person in history to see living cells moving under a lens: bacteria, and the cells inside a drop of his own blood. Between the two of them, Hooke naming the structure and Leeuwenhoek discovering it alive and moving, biology gained both a word and a whole invisible world to point it at.

You do not need Hooke's original equipment to see cells for yourself. Peel the thin, almost transparent inner skin from a piece of onion, dab it with a red stain called safranin, and slide it under even a simple microscope, and neat, brick-like rectangles snap into focus, arranged edge to edge without a single gap between them, exactly like the bricks in a wall. Gently scrape the inside of your own cheek with the blunt end of a toothpick instead, stain the scraping blue this time, and a completely different picture appears: loose, irregularly shaped cells, more like flattened polygons than tidy bricks, each with its own round structure glowing near its centre. In both cases, look closely and the same three basic parts appear. An outer boundary called the cell membrane wraps around everything else, controlling what is allowed to enter and leave. A round structure called the nucleus sits inside, usually near the centre, and everything in between, filling the rest of the cell, is a jelly-like substance called cytoplasm, where most of a cell's actual work gets done. The onion cells have one extra feature the cheek cells lack entirely: a rigid outer layer called the cell wall, sitting just outside the cell membrane, which is exactly why onion cells look so much stiffer and more box-shaped than the soft, irregular cheek cells. A cell wall is something only plant cells, fungal cells, and bacterial cells have. Animal cells, including every cell in your own body, never have one.

Cut open a plant cell and look past the cell wall and cell membrane, and there is more going on inside than just a nucleus floating in cytoplasm. Small rod-shaped structures called plastids are scattered throughout, and some of them, called chloroplasts, are packed with a green substance called chlorophyll, the same pigment responsible for capturing sunlight during photosynthesis, which is exactly why they are green. Plastids in non-green parts of a plant, like a potato or a carrot root, do a different job entirely, storing starches and other substances instead. Plant cells also usually contain one large, almost empty-looking space called a vacuole, which acts as a kind of storage tank and rubbish bin combined, holding water and dissolved substances, getting rid of waste, and pressing outward against the cell wall to keep the whole cell firm and upright, the same way air pressure keeps a balloon from going floppy. Animal cells can have vacuoles too, but theirs tend to be small and easy to miss, nothing like the single large one that dominates most plant cells. None of this means a cell is just a bag of liquid with a nucleus floating in it. It is closer to a tiny, fully staffed workshop, with different structures handling different jobs, all cooperating to keep the whole cell, and by extension the whole organism built from cells like it, alive and functioning.

Line up a handful of different cells from your own body under a microscope and they would look like they belong to entirely different kinds of creatures. A muscle cell is long and pointed at both ends, spindle-shaped, built that way because that exact shape lets it contract and shorten efficiently, pulling on whatever it is attached to. A nerve cell looks nothing like it: astonishingly long and thin, branching out at both ends like a tree's roots and canopy combined, a shape built purely for one purpose, carrying electrical messages rapidly across long distances to precisely the right destination. Compare either of those to the cheek cells you looked at earlier, flat, thin, and loosely arranged, which is exactly the shape needed to form a smooth, protective lining across the inside of your mouth. This is not a coincidence repeated a few times. It is a genuine pattern: a cell's shape is built around the specific job that cell has to do. The muscle cells lining your food pipe are thin and flexible enough to contract in the wave-like squeezing motion that pushes food down toward your stomach, precisely the mechanism you may already have studied in Grade 7. The cells lining your stomach wall are shaped differently again, some built for churning muscle contraction, others specialised for oozing out digestive juices and acid. Different job, different shape, every time.

A single cell is the smallest unit that can be considered properly alive, but almost nothing you would call an animal or a plant is made of just one. The body of any complex living thing is built up in stages, like a set of nested building blocks. A single cell is the smallest unit, the way one brick is the smallest unit of a wall. A group of similar cells working together forms a tissue. Different tissues, arranged and combined, form an organ. Several organs working together toward one shared job form an organ system. And every organ system in the body, working together, makes up a complete organism. Cell, then tissue, then organ, then organ system, then organism: five steps from the smallest living unit to a complete plant or animal. Remarkably, every one of those complex, many-celled bodies, called multicellular organisms, actually begins life as a single cell too, an egg, which divides again and again until it eventually builds an entire organism out of the resulting cells. The single largest cell known in the entire living world is, in fact, an egg: the yolk of an ostrich egg, a solitary cell measuring somewhere between 130 and 170 millimetres across, roughly the size of a grapefruit, wrapped in a protective shell and a nourishing liquid that are not cells at all, just supporting material around that one enormous cell.

Fill a jar with water scooped from a stagnant pond, or stir a spoonful of garden soil into a beaker of water and let it settle, and a single drop from either one, placed under a microscope, turns out to be teeming with life invisible to your naked eye. These are microorganisms, or microbes, living things so small that seeing them at all requires magnifying them somewhere between one hundred and four hundred times their real size. Some, like Amoeba, are made of just a single cell with no fixed shape at all, constantly flowing and reshaping itself as it moves. Others, like Paramecium, are also single-celled but keep a consistent slipper-like shape, propelling themselves along using thousands of tiny hair-like structures beating in unison. Certain algae are single-celled too, but unlike Amoeba or Paramecium, they are green, packed with the same chlorophyll a plant leaf uses to make its own food. Moulds are built quite differently again, made of branching, thread-like filaments, sometimes ending in tiny sac-like or brush-like structures. And bacteria, smaller again than all of these, come in a handful of basic shapes, spherical, rod-shaped, spiral, or comma-shaped, sometimes fitted with thin hair-like structures that help them move or attach to surfaces. Even viruses turn up in this same invisible world, though they break the rule entirely: a virus is not even made of a cell, and can only multiply once it forces its way inside one that belongs to something else.

Leave a slice of lemon or tomato out in the open for a few days and it eventually grows a soft, powdery or cotton-like fuzz, evidence that microorganisms drifting invisibly through the air have landed on it and begun to grow. Pickles and murabbas made from the very same fruits somehow avoid this fate for months, and the trick has nothing to do with luck: the heavy dose of salt or sugar used to preserve them draws water out of any microorganism that lands there, stopping it from being able to grow at all. Bury a mix of fruit peels and dried leaves under a layer of soil, though, and leave it alone for a few weeks, and the exact opposite happens on purpose: the buried waste gradually darkens and crumbles into a rich, nutrient-dense material called manure. Microorganisms living in the soil, mostly various fungi and bacteria, are doing the same job here that they did on the abandoned lemon, breaking down complex organic material into simpler substances. Here, though, that breakdown is useful rather than wasteful, since it releases nutrients back into the soil where growing plants can use them. This process, called decomposition, is also exactly why a pile of fallen leaves left in a garden corner slowly vanishes over the following months rather than piling up forever, and it is the same process microorganisms use on the bodies of dead animals. Left to their work, decomposers quietly recycle practically every scrap of organic waste in nature back into a form life can use all over again.

Mix flour with a pinch of sugar and warm water, knead it into two identical balls of dough, but stir a small amount of yeast into only one of them, and within a few hours something strange happens to just that one: it swells up, turns visibly fluffy, and develops a faintly different smell, while the untouched dough sits exactly as dense and flat as it started. Yeast is a microorganism, a member of the fungus family, and like every other living thing this chapter has discussed, it respires, breaking down the sugar you mixed in to release energy for its own growth, exactly the same respiration process studied for animals in Grade 7. One of the waste products of that respiration is carbon dioxide gas, and as tiny bubbles of that gas form and get trapped throughout the sticky dough, they are what puffs it up and makes it soft, which is the entire secret behind bread, cake, and pastry. A second kitchen experiment reveals a different microbe entirely. Stir a spoonful of curd into a bowl of lukewarm milk, leave it in a warm spot for a few hours, and the milk transforms into curd itself, tangy and thickened, while an identical bowl left in the refrigerator barely changes at all. The bacterium responsible, called Lactobacillus, feeds on the natural sugar in milk and multiplies rapidly in warmth, but instead of producing carbon dioxide the way yeast does, it produces lactic acid instead, which is exactly what curdles the milk and gives curd its distinctive sour taste. Warmth speeds both microbes along; cold slows them down almost to a stop, which is exactly why the refrigerated bowl stayed as milk.

Pull up a bean, pea, or lentil plant and rinse the soil off its roots, and small round swellings, easy to mistake for disease at first glance, turn out to be something else entirely: nodules packed with a bacterium called Rhizobium, living inside the plant's own root tissue in a genuinely two-way partnership. Rhizobium has a rare and valuable talent: it can pull nitrogen gas directly from the air trapped in soil and convert it into a form the plant's roots can actually absorb and use to grow, something the plant itself cannot do on its own. In exchange, the plant simply houses and feeds its bacterial tenant. This is exactly why farmers deliberately grow legumes like beans and lentils in rotation with other crops: the Rhizobium left behind naturally enriches the soil with nitrogen, reducing how much chemical fertiliser the next crop needs. A completely different group of microorganisms does its most important work not underground but in water. Microalgae, microscopic plant-like organisms drifting through ponds, oceans, and even damp soil, capture sunlight to make their own food in exactly the same way a plant leaf does, and in the process release oxygen as a byproduct, on a scale large enough that microalgae are responsible for producing more than half of all the oxygen in Earth's atmosphere. Certain microalgae, such as Spirulina, are packed with enough protein and nutrients that people harvest and eat them directly as a health supplement, which means the very same tiny organisms quietly keeping the planet's air breathable are also, for some people, dinner.

Every single living thing this chapter has covered, from a towering tree down to a single drifting bacterium, is built from cells, whether that means trillions of them cooperating inside one multicellular body or just one cell doing every single job needed to stay alive on its own, the way a unicellular organism like Amoeba or a bacterium does. Look closely enough, though, and not every cell is built quite the same way. Peer inside a bacterium and you will search in vain for the tidy, membrane-wrapped nucleus that showed up so clearly in both the onion peel and the cheek cell earlier in this chapter. Bacteria have no true nucleus at all. Their genetic material instead sits loosely inside a region called the nucleoid, with no membrane sealing it off from the rest of the cell, a fundamental difference that sets bacterial cells apart from every plant, animal, fungal, and protozoan cell covered so far. Everything explored in this chapter, using nothing fancier than a simple microscope, is really only the beginning. An electron microscope, a far more powerful instrument than anything used in a school laboratory, can magnify a cell roughly one million times, revealing an entire universe of structures inside a single cell too small for an ordinary microscope to ever resolve.