Every fact in the three digestion lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical or dietary advice; questions about a pupil's own tummy problems, allergies or diet go to a parent, the school nurse or a doctor. Pupils should not taste or eat anything in the experiments; check for food allergies (for example wheat or gluten) before handling food. Lengths and times are typical textbook values and vary from person to person: OpenStax gives about 3 m for the small intestine in a living person (about twice that in a cadaver, so other books quote longer figures), and describes the large intestine as about half as long as the small intestine. Timings such as 3–5 hours in the small intestine and 12–24 hours in the large intestine are averages. The bag and sugar-cube experiments are models: they show mechanical breakdown and surface area, not real digestive enzymes.
All sources
[1] Your Digestive System & How It Works
National Institute of Diabetes and Digestive and Kidney Diseases (NIH, USA) · Reliability: High
Type: National health research agency
The GI tract is the mouth, oesophagus, stomach, small intestine, large intestine and anus. Digestion starts in the mouth when you chew; saliva has an enzyme that begins to break down starches. Food moves by peristalsis. Stomach glands make acid and enzymes that break down food. The liver makes bile that helps digest fats; the gallbladder stores bile; the pancreas makes juice with enzymes that break down carbohydrates, fats and proteins. The walls of the small intestine absorb water and digested nutrients into the bloodstream; blood carries simple sugars, amino acids, glycerol and some vitamins and salts to the liver; the lymph system absorbs fatty acids and vitamins. The large intestine absorbs water and changes waste into stool; its bacteria help break down remaining nutrients and make vitamin K. Hormones from the stomach and small intestine and nerves in the gut wall control digestion.
Teeth break food into smaller particles (chewing); saliva contains mucus that moistens food, lysozyme and starch-digesting salivary amylase. Peristalsis of oesophagus muscles pushes food to the stomach; sphincters keep stomach contents from going back up. Stomach contents are extremely acidic, pH 1.5–2.5; pepsin digests protein; the mixture of food and gastric juice is chyme; a thick mucus lining protects the stomach. Villi and microvilli increase the surface area of the small intestine. Pancreatic juices break down starches, disaccharides, proteins and fats, and bicarbonate neutralises acidic chyme. The liver makes bile needed for fat breakdown. The large intestine reabsorbs water; the colon is home to many bacteria; the rectum stores faeces; the anus is the exit.
[3] Anatomy and Physiology, 23.5 The Small and Large Intestines
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The small intestine is about 3.05 m long in a living person (about twice as long in a cadaver) and about 2.54 cm wide; duodenum about 25 cm, jejunum about 0.9 m, ileum about 1.8 m. There are 20–40 villi and about 200 million microvilli per mm²; circular folds, villi and microvilli increase the absorptive surface more than 600-fold, to about 200 m². Enzymes fixed to the microvilli finish digesting carbohydrates and proteins. Chyme takes about 3–5 hours to leave the small intestine. The large intestine is about half as long as the small intestine; residue stays 12–24 hours; trillions of bacteria live there and make vitamins including vitamin K.
[4] Biology for AP Courses, 25.3 Digestive System Processes
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Salivary amylase begins breaking starch into maltose; pancreatic amylase continues in the duodenum. Pepsin breaks proteins into peptides; trypsin, elastase, carboxypeptidase and chymotrypsin come from the pancreas. Bile emulsifies fats into many small droplets, increasing the surface area for lipases, which break lipids into fatty acids and glycerides (monoglycerides). Maltases, sucrases and lactases split disaccharides into monosaccharides. Monosaccharides enter the blood; fats travel as chylomicrons into lymph capillaries. Water-soluble vitamins are absorbed directly into the blood.
[5] Investigating the effect of pH on amylase activity
Practical Biology (Royal Society of Biology, with Nuffield Foundation and CLEAPSS) · Reliability: High
Type: Professional society's school practical guide
Method: 2 cm³ of 1% amylase plus 1 cm³ of buffer, then 2 cm³ of 1% starch; every 10 seconds a drop of the mixture is added to a drop of 0.01 M iodine on a spotting tile. Blue-black means starch is still present; the end point is when the iodine stays orange. Rate = 1 ÷ time; plot against pH. About 60 s is expected at pH 6, the usual optimum. Safety: eye protection; iodine irritates eyes; amylase powder is harmful but solutions below 1% are low hazard; universal buffer is an irritant.
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz)