How Living Things Get Their Fuel

A sunbird hovers at a hibiscus flower and drinks its nectar through a needle-thin beak, sipping sugary liquid the way you might drink through a straw. A python that has just swallowed a rat whole will not eat again for days, sometimes weeks, while its body slowly works through that one enormous meal. Filter-feeding fish barely seem to eat at all, simply gliding through water with their mouths open, straining out tiny food particles too small for you to see. Every one of these animals is doing the exact same job in a completely different way: taking in food from outside its body. But getting food in is really only step one. Whether you are a sunbird, a python, a filter-feeding fish, or a hungry twelve-year-old on your way home from school, that food still has to be broken down into something your body can actually use, and moved to wherever it is needed. That breaking-down process is called digestion, and this chapter follows it in careful detail, along with its equally important partner process, respiration, in animals first and then in plants.

Chew a mouthful of plain rice for a full minute without swallowing, and something strange happens: it starts tasting faintly sweet. Rice is packed with starch, and your saliva contains its own digestive juice that begins breaking that starch down into sugar the moment you start chewing, which is why the taste changes the longer you chew. This is the very first stop on a long journey. Swallowed food slides down a stretchy tube called the oesophagus, pushed along not by gravity but by the tube's own muscular walls, which squeeze in a slow wave from top to bottom, the same way toothpaste moves through a tube when you squeeze it from the bottom. That means you could technically swallow food while standing on your head and it would still reach your stomach. Inside the stomach, powerful muscles churn the food while a secretion containing acid and digestive juice breaks down proteins; the acid is strong enough to kill many of the harmful bacteria that arrive with your food, while a layer of protective mucus stops that same acid from damaging the stomach's own walls. What leaves the stomach next enters the small intestine, which, despite its name, is the longest section of the entire digestive tube, coiled up tightly enough to fit inside you even though stretched out flat it would measure roughly six metres, nearly the length of two classroom doors laid end to end. Its inner lining is covered in thousands of tiny finger-like projections, which exist purely to increase the surface area available for absorbing digested nutrients into the bloodstream, the same logic as crumpling a single sheet of paper into a ball with far more surface texture than a flat sheet. Whatever remains undigested moves on into the large intestine, which absorbs most of the leftover water, leaving a semi-solid waste that is finally expelled through the anus, a step called egestion.

Watch a resting cow for long enough and you will notice its jaw working steadily even when there is no fresh grass anywhere nearby, as if it is chewing gum that never runs out. Cows and buffaloes are ruminants: grazing animals that swallow grass after only a quick chew, let it soften and partly digest inside a special stomach chamber, then bring it back up to the mouth in small amounts to be chewed all over again, slowly and thoroughly this time. Grass is tough and difficult to break down, and a cow can spend as much as eight hours a day on this second round of chewing, called rumination, before the food is finally swallowed for good. Birds solve the same tough-food problem in a completely different way, because a beak has no teeth to chew with at all. Instead, swallowed food travels to a muscular pouch called the gizzard, which many birds deliberately fill with small swallowed stones called grit; as the gizzard's walls contract and relax, those stones grind against the food like millstones, doing mechanically what teeth would otherwise do. A chicken pecking at gravel on the ground is not confused about what counts as food. It is stocking up on grinding stones for its gizzard.

Place your hand flat on your chest and breathe in slowly. You should feel your chest rise and expand outward. That expansion is not incidental to breathing; it is the entire mechanism. Just below your lungs sits a dome-shaped sheet of muscle called the diaphragm, and when you inhale, it flattens and pulls downward while your ribs simultaneously move upward and outward, together making the space inside your chest noticeably larger. Air rushes in through your nostrils to fill that extra space, exactly the way air rushes into a bicycle pump's cylinder the moment you pull the handle back out. That incoming air passes through your windpipe, which branches into your two lungs and keeps branching into smaller and smaller tubes, finally ending in millions of tiny balloon-like sacs called alveoli. To breathe out, the whole sequence simply reverses: the diaphragm domes back upward, the ribs move down and inward, the chest space shrinks, and air is pushed back out, the way squeezing a pump's cylinder forces air out through its nozzle. You can build a rough working model of this entire system using nothing more than a plastic bottle, two balloons, and a stretched rubber sheet across the open base: pull the rubber sheet downward, mimicking the diaphragm, and the two balloons inside visibly inflate; release it, and they deflate again, on their own, with no air blown in by mouth at any point.

It is tempting to treat breathing and respiration as two words for the same thing, but they describe two entirely different processes that simply happen to be connected. Breathing is the physical, mechanical act of moving air in and out of your lungs, nothing more. Respiration is a chemical process that happens deep inside your body's cells, where oxygen is used to break down glucose, the simple sugar your digested food eventually becomes, releasing the energy that powers absolutely everything you do, from blinking to sprinting. The word equation for it is short: glucose plus oxygen yields carbon dioxide, water, and energy. Breathing exists purely to supply the oxygen respiration needs and to carry away the carbon dioxide respiration produces as waste, which is exactly why the air you exhale is measurably different from the air you inhale: breathe into a test tube of clear limewater through a straw, and it turns cloudy within seconds, because limewater reacts specifically with carbon dioxide, while simply pumping ordinary room air through a syringe into a second tube of limewater leaves it clear. Inhaled air is roughly twenty-one percent oxygen and a mere trace of carbon dioxide; exhaled air still carries plenty of oxygen your body did not use, but its carbon dioxide content has risen more than a hundredfold. Breathing, in short, is the delivery van. Respiration is the factory the van is delivering to.

A fish never once has to surface for air because it does not breathe air at all; it breathes the oxygen that is already dissolved in the water around it, using feathery, blood-rich structures called gills, tucked just behind its head. Water flows in through the mouth and out over the gills, and as it passes, oxygen from the water crosses directly into the blood while carbon dioxide crosses the opposite way, out into the water, all without a single lungful of air being involved anywhere in the process. A frog manages an even stranger trick: it effectively changes its breathing method partway through its own life. As a tadpole living entirely underwater, it breathes through gills, exactly like a fish. Once it grows into an adult frog capable of moving between land and water, those gills are gone, replaced by lungs for breathing air on land and, remarkably, by its own skin for absorbing oxygen directly whenever it is back in the water, which is one reason frog skin always needs to stay moist. Earthworms take that same skin-breathing trick even further, relying on their moist skin as essentially their entire respiratory system, with no lungs, no gills, and no windpipe anywhere in their bodies at all. Different animals, it turns out, have quietly evolved a whole toolkit of different solutions to one identical problem: how do you get oxygen from your surroundings into your own body?

Unlike you, a plant never has to go looking for its next meal, because it manufactures food from scratch inside its own leaves, using nothing but sunlight, air, and water as raw ingredients. This is why leaves are broad, flat, and usually green: their shape is built for catching as much sunlight as possible, and their colour comes from a pigment called chlorophyll, which is what actually captures that sunlight's energy. You can catch a leaf in the act of having made food. Boil a leaf briefly to soften it, then simmer it in alcohol until every trace of green has drained out of it, leaving it pale and colourless, and finally drip iodine solution onto it: any part of the leaf that has been making food will turn a deep, unmistakable blue-black, because iodine reacts specifically with starch, and starch is exactly the form in which plants store the food, or glucose, they have made. Compare a leaf kept in full sunlight against one kept in total darkness for a couple of days using this exact test, and only the sunlit leaf turns blue-black. Chlorophyll on its own is not enough; without sunlight, the food-making process this chapter is building toward simply refuses to start.

Sunlight and chlorophyll turn out to be only two ingredients out of four. Seal half of a leaf inside a container holding a chemical that absorbs carbon dioxide from the air, leaving the other half of the same leaf poking out into ordinary air, and after a few hours in sunlight, only the half that had access to real air tests positive for starch; the sealed half, starved of carbon dioxide even with all the sunlight and chlorophyll it could want, makes none at all. Carbon dioxide, it turns out, is just as essential as sunlight. This whole process, a plant capturing sunlight's energy to combine carbon dioxide and water into glucose, is called photosynthesis, and its word equation captures all four ingredients along with what comes out the other end: carbon dioxide plus water, in the presence of sunlight and chlorophyll, yields glucose plus oxygen. That released oxygen is not just a side detail. Trap the gas bubbling off a sunlit water plant in an upturned test tube, then plunge a glowing splint of wood into that collected gas the moment you remove the tube, and the splint bursts into a brief, vigorous flame, because concentrated oxygen makes things burn far more fiercely than the ordinary air around us. All of this gas exchange, carbon dioxide flowing in and oxygen flowing back out, happens through the very same kind of tiny pores you may already know from an earlier characteristics-of-life lesson: stomata, thousands of them scattered across a leaf's surface, each one barely visible even under a microscope.

Plants have no heart and no blood, yet water clearly reaches every leaf at the very top of even a tall tree, and food made in those same leaves clearly reaches roots buried deep underground. You can watch this happen with a simple, satisfying experiment: place the cut stem of a white flower into water dyed with red ink, and within a day the red colour has travelled up through the stem and appeared inside the white petals themselves, sometimes tracing delicate red veins across them. Slice across that same stem and look closely, and you will find the red dye concentrated inside a ring of thin, tube-like structures called xylem, which is exactly what carries water and dissolved minerals upward from the roots to every other part of a plant, in exactly the one-way direction that red dye just demonstrated. Food takes an entirely separate route. The glucose a leaf produces during photosynthesis needs to travel to parts of the plant that cannot make their own food at all, roots, developing fruit, and stored seeds among them, and it does so through a second, completely distinct set of tubes called phloem, running alongside the xylem but doing the opposite job, since phloem can carry food downward to the roots just as easily as upward to a growing shoot. Xylem moves water up. Phloem moves food wherever it is needed. Between the two of them, a plant manages to run its entire internal transport network without a single heartbeat.

It is easy to assume that a plant, busy making its own food all day, has no further use for respiration, the very process this chapter began by carefully distinguishing from mere breathing. That assumption is wrong. Soak a handful of seeds until they begin to germinate, then seal them inside a flask connected to a test tube of clear limewater, and within a day that limewater turns cloudy, the unmistakable sign of carbon dioxide, even though the seeds were left in complete darkness the entire time, with no sunlight available for photosynthesis to be happening at all. Every living part of a plant respires, constantly, whether it is green or not, in daylight or in darkness, using oxygen to break down glucose for energy in precisely the same word equation that applies to you: glucose plus oxygen yields carbon dioxide, water, and energy. This is where photosynthesis and respiration finally reveal themselves as two halves of the same loop rather than two unrelated topics. Photosynthesis takes in carbon dioxide and releases oxygen; respiration takes in oxygen and releases carbon dioxide. Seal a healthy green plant inside a large, otherwise airtight glass bottle, and it can carry on growing for months, because on a sunny day it produces more oxygen through photosynthesis than it consumes through respiration, and the leftover surplus is exactly enough to keep the plant's own respiration supplied in return, a small, self-sufficient world sealed inside a jar.