Trace food through the alimentary canal, describe the accessory digestive organs, distinguish mechanical from chemical digestion, explain enzyme action and absorption, and analyze digestive imbalances

Learning goal: Identify the organs of the gastrointestinal tract (alimentary canal) and the accessory organs (teeth, tongue, salivary glands, liver, gallbladder and pancreas); describe the six major functions of the digestive system; trace food from the mouth to the anus and describe what happens in each region, including mastication, peristalsis and segmentation; distinguish mechanical from chemical digestion; explain the role of specific enzymes, including where each is produced, where it enters, its best pH, what it digests and its products; explain absorption and the role of hydrochloric acid, bile and colonic bacteria; and analyze homeostatic imbalances such as gallstones, heartburn, ulcers, dehydration, diarrhea, cirrhosis and cancers of the digestive system.

What the system does. The digestive system processes food and supplies the small molecules needed to sustain the living tissues of the body through the absorption of nutrients. Six major functions describe its work: ingestion (taking food in), mechanical processing (tearing, grinding and mixing), secretion (releasing water, acid, enzymes, buffers and salts into the tract), enzymatic (chemical) digestion (breaking large molecules into small ones), absorption (moving the small molecules across the lining into blood or lymph) and excretion (removing indigestible material and some wastes as feces). The lining of the tract also protects surrounding tissues from the mechanical and enzymatic stresses of digestion: mucus and a constantly renewed epithelium keep the stomach from digesting itself.

Mechanical versus chemical digestion. Mechanical digestion breaks food into smaller pieces without changing its molecules: chewing (mastication), churning in the stomach and segmentation in the small intestine, where rings of smooth muscle contract and relax in alternate places to mix food with enzymes. Its purpose is to increase surface area so enzymes can work faster. Chemical digestion breaks the bonds within large molecules by hydrolysis, catalyzed by enzymes: starch to sugars, proteins to amino acids, fats (triglycerides) to fatty acids and monoglycerides. Peristalsis is different from both: waves of smooth muscle contraction that push food along the tract.

Tracing food, worked fully. Follow a bite of a cheese sandwich. In the mouth, teeth cut and grind the bread and cheese while the tongue shapes the food into a ball (bolus) and pushes it back; salivary glands add saliva, which moistens the food and contains salivary amylase that begins breaking starch into maltose. Swallowing moves the bolus through the pharynx into the esophagus, where peristalsis carries it down to the stomach; the esophagus secretes no digestive enzymes, though salivary amylase keeps working in the bolus on the way. The stomach has four regions: the cardia, where food enters from the esophagus; the dome-shaped fundus, which can hold food and swallowed gas; the large body, where most of the mixing with acid and pepsin happens; and the pylorus, which grinds the contents and pushes chyme toward the pyloric sphincter, which releases it into the intestine a little at a time. Stomach glands secrete hydrochloric acid (HCl), which keeps the contents at about pH 1.5 to 3.5 during digestion; the acid unfolds (denatures) proteins, converts inactive pepsinogen into active pepsin, which begins cutting proteins into shorter peptides, and kills most microbes swallowed with food, protecting the body from pathogens. Churning turns the food into a soupy mixture called chyme. In the small intestine (duodenum, jejunum, ileum), most chemical digestion and nearly all absorption take place. The pancreas, a gland lying behind the stomach, is made mostly of clusters of enzyme-secreting cells (acini) that drain into ducts joining the pancreatic duct, with scattered hormone-secreting islets. It releases pancreatic juice into the duodenum: bicarbonate neutralizes the acid chyme to near pH 7 to 8, and the juice carries pancreatic amylase, lipase and protein-digesting enzymes secreted in inactive form, such as trypsinogen, which an enzyme on the intestinal lining activates to trypsin only once it reaches the duodenum; that protects the pancreas from digesting itself. The liver, the largest organ inside the body, lies under the diaphragm on the right and receives blood from the gut through the hepatic portal vein; it produces bile, which the gallbladder, a small sac on the underside of the liver, stores and concentrates and releases into the duodenum after a fatty meal. Bile is not an enzyme: its bile salts emulsify fat, breaking large droplets into many small ones, which increases the surface on which lipase acts. Enzymes on the surface of the intestinal lining finish the job: lactase splits lactose into glucose and galactose, maltase splits maltose into glucose, and peptidases release amino acids. The cheese's fat, protein and lactose and the bread's starch are now small molecules. In the large intestine (cecum, colon, rectum), water and salts are absorbed, symbiotic colonic bacteria ferment fiber that human enzymes cannot digest and produce vitamin K and some B vitamins, and the remaining indigestible material is stored and eliminated through the anus as feces.

Hormones that coordinate digestion. The tract releases acid, bicarbonate, bile and enzymes at the right moment because hormones made by cells in its own lining signal when food arrives. Gastrin, made by G cells in the lower stomach and duodenum when protein digestion products arrive and the stomach is stretched, stimulates the stomach glands to secrete more acid. Secretin, made by S cells in the duodenum when acid chyme lowers the pH there, stimulates the pancreas (and bile ducts) to release bicarbonate-rich fluid that neutralizes the acid. Cholecystokinin (CCK), made by I cells in the duodenum and upper small intestine when fats and protein digestion products arrive, makes the gallbladder contract and release bile, stimulates the pancreas to secrete enzymes, and slows the emptying of the stomach so the intestine is not overloaded. Each hormone travels in the blood and responds to what has arrived where it is made (food in the stomach for gastrin, acid and fat in the duodenum for secretin and CCK); each works as negative feedback, because its signal fades once the acid is neutralized or the fat is digested.

Absorption and the surface-area design. The small intestine's lining is folded, the folds carry finger-like villi, and each epithelial cell carries microscopic microvilli, multiplying the absorptive surface many times. Each villus contains a capillary network and a lymphatic lacteal. Monosaccharides and amino acids enter the capillaries and travel through the hepatic portal vein to the liver, which processes them and helps maintain glucose levels in the blood by storing glucose as glycogen. Fatty acids and monoglycerides are reassembled into fats, packaged with protein, and enter the lacteals, reaching the blood by way of the lymph. The metabolic functions of the accessory organs, such as the liver's storage of glycogen and the pancreas's release of insulin and glucagon, play roles in regulating homeostasis far beyond digestion.

Tissues of the tract. Most of the tract has the same four layers: an inner mucosa (epithelium, a thin layer of connective tissue called the lamina propria and, in most of the tract, a thin layer of smooth muscle, with glands opening onto it), a submucosa with vessels and nerves, a muscular layer of smooth muscle for peristalsis and segmentation, and an outer serosa. The epithelium changes with function: stratified squamous in the mouth and esophagus to resist abrasion, simple columnar in the stomach and intestine for secretion and absorption.

Comparative and evolutionary view. Herbivores generally have a longer alimentary canal relative to body length than carnivores, because plant material, especially cellulose, is slow to break down and needs a long time in contact with microbes. Ruminants such as cattle have a four-compartment stomach in which microbes ferment cellulose before the food reaches the true stomach, so they extract energy from grass that humans cannot use. Lactase persistence, continuing to make lactase into adulthood, is common in populations with a long history of herding dairy animals and uncommon in many others, where lactase production falls after childhood: an example of recent evolution in human populations linked to culture.

Homeostatic imbalances. Gallstones are hardened deposits in the gallbladder that can block bile flow. Heartburn is burning pain when stomach acid moves back up into the esophagus, whose lining is not protected against it. Ulcers are sores through the lining of the stomach or duodenum, most commonly caused by infection with the bacterium Helicobacter pylori or by use of nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen and naproxen, both of which weaken the lining's defenses against acid and pepsin. Diarrhea is stool with too much water; it can happen when material moves through the large intestine too quickly for water to be absorbed, when the intestinal lining secretes extra fluid (as in some infections), or when unabsorbed solutes hold water in the gut (as with undigested lactose), and can lead to dehydration, a loss of body water and salts. Cirrhosis is scarring of the liver that replaces working tissue. Cancers can arise in several organs, including the colon, stomach, pancreas and esophagus.

Food choices and the whole system. Everything the tract absorbs comes from what is eaten, so one useful exercise is to journal daily food choices for several days and relate them to the USDA MyPlate recommendations, which group foods into fruits, vegetables, grains, protein foods and dairy. Sort each food into its group, then connect the groups to digestion: starches in grains are digested by amylases, proteins by pepsin and trypsin, fats by lipase with the help of bile, and the fiber in fruits, vegetables and whole grains passes to the colon, where bacteria ferment it. The journal is a data exercise about digestion, not a diet plan.

Investigation: models of mechanical and chemical digestion. (1) Place a whole sugar cube and an equal mass of crushed sugar in two cups of the same volume of room-temperature water and time how long each takes to dissolve without stirring. (2) Shake cooking oil with water in two sealed jars, one with a few drops of dish soap added as a bile model, and compare how long the oil stays in small droplets. Wear goggles and do not taste anything. Faster dissolving of crushed sugar supports the claim that breaking food into smaller pieces (mechanical digestion) speeds later processes by increasing surface area; smaller, longer-lasting droplets with soap support the claim that emulsification increases the surface for lipase. Both are physical changes: dissolving and emulsifying break no chemical bonds, so neither is chemical digestion. (3) To model chemical digestion itself, add a pinch of powdered amylase (sold as diastase for baking) to a dilute starch solution and test a drop on a white tile with iodine solution every two minutes; the blue-black color fading shows starch molecules being broken into sugars. Iodine stains, so wear goggles and gloves. The models cannot show pH effects or absorption.

Worked example 1 (one enzyme, all five facts): Pepsin. Produced as inactive pepsinogen by chief cells in the stomach lining; enters the alimentary canal in the stomach; activated by hydrochloric acid and works best at about pH 1.5 to 2; chemically digests proteins; products are shorter chains of amino acids (peptides). In the duodenum, where bicarbonate raises the pH toward 7 to 8, pepsin stops working and pancreatic enzymes take over.

Worked example 2 (pancreatic lipase): Produced by the pancreas; enters through the pancreatic duct into the duodenum; works best near pH 7 to 8; digests triglycerides (fats); products are fatty acids and monoglycerides. Because lipase is water-soluble and fat is not, lipase can act only at the surface of fat droplets, which is why bile's emulsification makes it far more effective.

Worked example 3 (investigation data): Hypothetical results: whole sugar cube dissolved in 6 minutes; crushed sugar of equal mass in 1.5 minutes. The ratio is 6 / 1.5 = 4, so the crushed sugar dissolved four times faster. The mass and water were the same, so the difference is due to surface area, the variable changed, which supports the role of mechanical digestion in speeding later processes by exposing more surface. Dissolving is a physical change used here only as a stand-in; chemical digestion breaks bonds, which the amylase and iodine test shows.

Worked example 4 (lactase and symptoms): An adult whose lactase production has fallen drinks milk. Lactose is not split in the small intestine, so it passes into the colon, where bacteria ferment it, producing gas, and the undigested sugar draws water into the colon by osmosis, which can cause diarrhea. The chain of cause is missing enzyme, undigested molecule, microbial fermentation and osmosis.

Representation task: Draw the alimentary canal and accessory organs on one outline and label every organ. For each region from mouth to anus, write whether mechanical digestion, chemical digestion, absorption or none of these occurs there, and name one enzyme or secretion that acts there. Label the four regions of the stomach.

Independent performance: Carry out both digestion models and write a report. Success criteria: (a) the dissolving times, droplet observations and iodine color changes are recorded with the controlled variables named; (b) the results are used to distinguish mechanical from chemical digestion and to explain why bile is not an enzyme; (c) one enzyme is described with all five facts: where produced, where it enters, best pH, molecules digested and products; (d) the role of hydrochloric acid in digestion and in protection from pathogens is explained; (e) the path of an absorbed amino acid and of an absorbed fatty acid is traced to the blood; (f) the limitations of the models are listed.