Digestion

• 1. Introduction

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  Digestion turns large insoluble food compounds into small water-soluble components that can slip into the blood plasma. That single sentence hides a sprawling story. The human gastrointestinal tract runs around 9 meters long, and a meal can take between 24 and 72 hours to pass through it. Along the way, food is torn, churned, soaked in acid, and dismantled by enzymes that each recognize a different kind of molecule. Why does the stomach not digest itself with the acid it makes? How does a sharkstomach turn inside out, and why do young pandas eat their mother's faeces? What does an earthworm have in common with a cow's four-chambered stomach? Digestion is a form of catabolism, and across living things it takes a startling variety of forms. This documentary follows food from the moment it enters the mouth to the moment waste leaves the body, and then steps outside the human frame entirely to see how spiders, bacteria, and birds solve the same problem.

• 2. Mechanical And Chemical

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  Mastication, the act of chewing, is where the human story begins. It is a form of mechanical digestion, the physical breakdown of large pieces of food into smaller pieces that digestive enzymes can then reach. Saliva, secreted by the salivary glands, wets the food and carries salivary amylase, an enzyme that starts the digestion of starch. Roughly 30% of starch is hydrolyzed into disaccharide right there in the oral cavity. Saliva also carries mucus to lubricate the food and the electrolyte hydrogencarbonate, which sets the ideal pH for amylase to work.

  The bolus, a small round slurry mass, forms once mastication and starch digestion have done their work. It travels down the esophagus into the stomach by the action of peristalsis, waves of muscular contraction that move along the wall. Gastric juice begins protein digestion, carrying mainly hydrochloric acid and pepsin. In infants and toddlers, that juice also contains rennin to digest milk proteins. Pepsin breaks proteins into peptides or proteoses, later cut into dipeptides and amino acids by enzymes in the small intestine. Studies suggest that more chews per bite raises relevant gut hormones and may lower self-reported hunger and food intake.

• 3. The Long Tube

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  The pyloric sphincter valve opens, and partially digested food, now called chyme, enters the duodenum. There it mixes with digestive enzymes from the pancreas and bile juice from the liver before passing through the small intestine, where digestion continues. The intestinal walls are lined with villi, and their epithelial cells are covered with microvilli that increase the surface area for absorption. Around 95% of nutrient absorption happens in the small intestine.

  Water and minerals are reabsorbed into the blood in the colon, the large intestine, where the pH is slightly acidic at about 5.6 to 6.9. Bacteria living in the colon produce some vitamins, including biotin and vitamin K, which are absorbed there too. The large intestine slows the passage of food so fermentation by gut flora can take place. Waste material, the feces, is finally eliminated from the rectum during defecation, passing out through the anal canal and anus.

• 4. Inside Or Outside

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  External digestion came first in evolutionary history, and most fungi still rely on it. Enzymes are secreted into the surrounding environment, where they break down organic material, and some of the products diffuse back to the organism. Nearly all spiders work this way too, secreting biotoxins and digestive chemicals into the extracellular space before ingesting the resulting soup.

  Animals took a different path: a tube, the gastrointestinal tract, in which internal digestion occurs. It is more efficient because more of the broken-down products can be captured and the internal chemical environment can be controlled more tightly. A simpler version is the gastrovascular cavity, which acts as a stomach for both digestion and the distribution of nutrients. Lined with the gastrodermis, this cavity has a single opening that serves as both mouth and anus, an arrangement described as an incomplete gut. Smaller still is the phagosome, a vacuole formed around a particle absorbed by phagocytosis, where pathogens can be killed and digested; in humans, Entamoeba histolytica can phagocytose red blood cells.

• 5. How Bacteria Feed

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  Bacteria use several systems to pull nutrients from other organisms. In a channel transport system, proteins form a contiguous channel crossing the inner and outer membranes, built from just three subunits: the ABC protein, the membrane fusion protein, and the outer membrane protein. It moves species ranging from the small Escherichia coli peptide colicin V at 10 kDa up to the Pseudomonas fluorescens adhesion protein LapA at 900 kDa.

  A type III secretion system acts as a molecular syringe. Certain Salmonella, Shigella, and Yersinia inject nutrients straight into protist cells, a mechanism first found in Y. pestis, where toxins moved directly from bacterial cytoplasm into the host's cells. Conjugation machinery, discovered in Agrobacterium tumefaciens, can transport both DNA and proteins; its VirB complex introduces the Ti plasmid into the host, which develops the crown gall tumor. Transferred genes turn plant cells into factories producing opines, which the bacteria use as carbon and energy sources. Gram-negative bacteria have one more route: portions of the outer membrane pinch off as outer membrane vesicles, spheres of lipid bilayer enclosing periplasmic material, some carrying virulence factors that adhere to and intoxicate host cells.

• 6. Tools Of The Mouth

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  Beaks, tongues, teeth, crops, and gizzards are organs animals evolved to handle their food. Macaws eat seeds, nuts, and fruit, first scratching a thin line with the sharp point of the beak, then shearing the seed open with the sides. The squid's mouth carries a sharp horny beak of cross-linked proteins that kills and tears prey; robust but mineral-free, it is the only indigestible part of the squid.

  Teeth are not made of bone but of enamel, dentine, and cementum, tissues of varying density and hardness. Human teeth carry a blood and nerve supply that enables proprioception, the sensation that warns us to stop chewing if we bite something too hard, like a chipped plate hidden in food. Herbivores carry molars to grind tough plant matter, while carnivores carry canines to kill and tear meat. The tongue, skeletal muscle on the floor of the mouth in most vertebrates, manipulates food and rolls it into a bolus. Beneath its front, the sublingual region has very thin oral mucosa over a plexus of veins, an ideal site for introducing medication straight into the cardiovascular system while bypassing the gastrointestinal tract.

• 7. Stomachs And Strategies

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  Ruminants carry a fore-stomach of four chambers: the rumen, reticulum, omasum, and abomasum. In the rumen and reticulum, food mixes with saliva and separates into solid and liquid layers, with solids clumping into the cud. The cud is regurgitated and chewed slowly to break down particle size. Microbes, including bacteria, protozoa, and fungi, break fibre such as cellulose and hemi-cellulose into the volatile fatty acids acetic acid, propionic acid, and butyric acid. The abomasum, the fourth and final compartment, works much like a monogastric stomach, handling acid hydrolysis of microbial and dietary protein.

  Many sharks can turn their stomachs inside out and evert them through the mouth to expel unwanted contents, perhaps to reduce exposure to toxins. Rabbits, hamsters, and capybaras lack a complex ruminant system, so they pass food through the gut a second time, excreting soft cecotropes that they consume immediately. Young elephants, pandas, koalas, and hippos eat their mother's faeces to obtain gut bacteria, because their intestines are completely sterile at birth. The earthworm runs food through a mouth, pharynx, esophagus, crop, and gizzard; its esophagus adds calcium carbonate to neutralize acids before the gizzard's powerful muscles churn the mass with soil.

• 8. Enzymes And Control

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  Zymogens are the inactive precursors in which most digestive enzymes are first secreted, a safeguard against self-digestion. Trypsin, for example, leaves the pancreas as trypsinogen and is activated in the duodenum by enterokinase. Pepsin, trypsin, and chymotrypsin break food proteins into polypeptides, which exopeptidases and dipeptidases reduce to amino acids. Salivary and pancreatic amylase cut starch into glucose and maltose, while lactase splits lactose into glucose and galactose. Roughly 65 percent of adults make only small amounts of lactase; more than 90 percent of people of east Asian descent are lactose intolerant, against about 5 percent of those of northern European descent.

  Hormones coordinate the whole sequence, and there are at least five in mammals. Gastrin stimulates the gastric glands to secrete pepsinogen and hydrochloric acid. Secretin signals sodium bicarbonate release in the pancreas, cholecystokinin triggers digestive enzymes and bile in response to fat, and gastric inhibitory peptide slows stomach emptying while inducing insulin secretion. pH governs it all. The mouth and esophagus sit near 6.8, gastric acid drops the stomach to a pH of 1 to 3, and the small intestine's alkaline mucosa reaches about 8.5. Vitamin B12 shows how delicate the system can be: haptocorrin in saliva shields the molecule from stomach acid, and only in the ileum do cubilin receptors allow B12-intrinsic factor complexes to enter the blood.