E. Animals: Structure and Function · Animals: Structure and FunctionThe Digestive Tract: Mouth, Stomach and Small Intestine
E3.2 E2.1 A1.12About 70 minutes
Learning goals- I can describe the anatomy of the alimentary canal and the accessory organs of digestion.
- I can explain the mechanical and chemical digestion that happens in the mouth, stomach and small intestine.
- I can explain how the stomach protects itself and how bile and pancreatic juice prepare chyme for the small intestine.
- I can trace a meal's carbohydrates, proteins and fats through the tract, naming each enzyme.
Words to know
alimentary canal (gastrointestinal tract)
The continuous tube from mouth to anus through which food passes.
accessory organs
Organs that add secretions to the tract but that food does not pass through: salivary glands, liver, gallbladder and pancreas.
bolus
A soft, rounded mass of chewed food mixed with saliva, ready to be swallowed.
peristalsis
Waves of smooth-muscle contraction that push food along the tract.
sphincter
A ring of muscle that closes an opening, such as the lower esophageal sphincter and the pyloric sphincter.
chyme
The thick, acidic liquid that leaves the stomach.
gastric juice
Stomach secretion containing hydrochloric acid, pepsinogen and mucus.
emulsification
Breaking large fat drops into tiny droplets, which increases the surface area for lipase. Bile salts do this; it is physical, not chemical, digestion.
A 9-metre journey
From mouth to anus, the adult human digestive tube is often quoted as about 9 m long (measured with its muscles relaxed), most of it the small intestine folded inside the abdomen. Food takes from about a day to a few days to make the trip. Along the way it is crushed, churned, soaked in acid, mixed with half a dozen enzymes, absorbed, and finally the leftovers are packaged for removal.
In Grade 10 you met the organs. Now we'll look at how each part does its job, including the cells that make the secretions and the muscles and valves that control the flow.
Two kinds of organs
The alimentary canal (also called the gastrointestinal or GI tract) is the tube food actually travels through: mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus.
The accessory organs never touch the food directly. They make secretions and send them into the canal through ducts: the salivary glands, the liver (makes bile), the gallbladder (stores and concentrates bile) and the pancreas (makes most of the digestive enzymes, plus bicarbonate).
The mouth: starting both kinds of digestion
- Mechanical digestion: teeth cut and grind. Incisors cut, canines tear, premolars and molars crush and grind. Chewing gives enzymes much more surface area to work on.
- Chemical digestion: three pairs of salivary glands release saliva (roughly 1 to 1.5 L a day). Saliva contains salivary amylase (starch → maltose), mucus to make food slippery, and some antibacterial substances.
- The tongue rolls food into a bolus and pushes it back. When you swallow, the soft palate closes off the nasal cavity and the epiglottis covers the opening of the larynx, so the bolus enters the esophagus, not the trachea.
The esophagus: a muscular conveyor
The esophagus moves the bolus to the stomach by peristalsis: a ring of smooth muscle contracts behind the bolus while the muscle ahead of it relaxes, squeezing it along like toothpaste in a tube. No digestion happens here.
At the bottom, the lower esophageal sphincter (also called the cardiac sphincter, because it is near the heart) opens to let food into the stomach and closes to keep acid out of the esophagus. When it does not close well, acid splashes up and causes heartburn (more in the disorders lesson).
The stomach: storage, churning and acid
The stomach is a J-shaped, stretchy bag. When empty, its lining lies in folds called rugae, which flatten as it fills.
- Mechanical: its wall has three layers of smooth muscle (longitudinal, circular and oblique), which churn food in different directions.
- Chemical: millions of tiny gastric glands in the lining release about 2 L of gastric juice a day:
- Parietal cells secrete hydrochloric acid (HCl), making the stomach contents about pH 1.5 to 3.5. Acid kills most microbes in food, denatures proteins (unfolding them so enzymes can reach their bonds) and converts pepsinogen to pepsin.
- Chief cells secrete pepsinogen.
- Mucous cells secrete a thick layer of alkaline mucus that protects the lining from acid and pepsin. Damaged lining cells are replaced every few days.
- The hormone gastrin (from the stomach lining) stimulates gastric juice when food arrives (more on hormones in the next lesson).
After a few hours, food has become chyme. The pyloric sphincter at the stomach's exit releases it into the small intestine a little at a time. Very little absorption happens in the stomach, although some water, alcohol and certain drugs such as ASA (Aspirin) are absorbed there.
The small intestine: where most digestion happens
The small intestine has three parts: the duodenum (about 25 cm, the first section, where secretions arrive), the jejunum and the ileum. It is "small" only in diameter.
When acidic chyme enters the duodenum, two secretions arrive through a shared duct:
- Pancreatic juice contains sodium bicarbonate, which neutralizes the acid (raising the pH to near neutral or slightly basic, close to the optimum for pancreatic enzymes), plus pancreatic amylase, trypsinogen (activated to trypsin), chymotrypsin, lipase and nucleases.
- Bile, made by the liver and stored in the gallbladder, contains bile salts. Bile salts are not enzymes. They emulsify fats: they break big fat drops into tiny droplets, increasing the surface area where water-soluble lipase can attack.
The lining's brush-border enzymes (maltase, sucrase, lactase and peptidases) then finish the job, producing monosaccharides, amino acids, fatty acids and monoglycerides small enough to be absorbed. Besides peristalsis, the small intestine also mixes chyme by segmentation: rings of muscle contract and relax in alternating places, sloshing the chyme back and forth against the lining.
Worked exampleExample 1: Tracing a grilled cheese sandwich
Bread is mostly starch; cheese has protein and fat. Trace each nutrient from mouth to absorption, naming the organ, the secretion and the enzyme at each step.
- Mouth: chewing breaks the sandwich up (mechanical). Salivary amylase starts hydrolyzing starch → maltose.
- Stomach: acid stops salivary amylase. HCl denatures the cheese proteins and activates pepsin, which cuts proteins → polypeptides. Churning turns everything into chyme. Fat is mostly unchanged.
- Duodenum: bicarbonate neutralizes the acid. Bile salts emulsify the fat. Pancreatic amylase continues starch → maltose; trypsin and chymotrypsin cut polypeptides → peptides; pancreatic lipase hydrolyzes triglycerides → fatty acids + monoglycerides.
- Brush border: maltase splits maltose → glucose; peptidases split peptides → amino acids.
- Absorption (next lesson): glucose, amino acids, fatty acids and monoglycerides cross the lining of the small intestine.
Starch: amylase (mouth, pancreas) → maltase. Protein: pepsin → trypsin/chymotrypsin → peptidases. Fat: bile emulsifies, pancreatic lipase hydrolyzes.
Worked exampleExample 2: How much does emulsification help?
A spherical fat drop has radius 1.0 mm. Bile salts break it into 1000 equal droplets of radius 0.10 mm (same total volume). Compare the total surface area before and after. (Sphere: SA = 4πr².)
- Before: SA = 4π(1.0 mm)² ≈ 12.6 mm².
- Check volume: each small droplet has 1/1000 of the volume, since (0.10 ÷ 1.0)³ = 0.001. ✓
- Each small droplet: SA = 4π(0.10 mm)² ≈ 0.126 mm².
- After: 1000 × 0.126 mm² ≈ 126 mm².
- 126 ÷ 12.6 = 10.
The surface area increases 10 times, so lipase can work about 10 times faster on the same fat. Emulsification is physical (mechanical) digestion: no bonds are broken.
Hands-on activityModel emulsification and peristalsis
Part A: Emulsification. You need: a clear jar with a lid, water, cooking oil, liquid dish soap, and a spoon.
- Fill the jar halfway with water and add 2 tablespoons of oil. Close and shake for 10 seconds. Watch the oil for 1 minute: it separates into a layer of big drops.
- Add a few drops of dish soap (playing the role of bile salts). Shake again for 10 seconds and watch.
- Record what you see. Why does the second mixture stay cloudy longer?
Part B: Peristalsis. You need: a smooth ball (such as a tennis ball) and a leg cut from old pantyhose or a long sock.
- Put the ball in the open end. Without pulling, squeeze the tube just behind the ball and move your squeeze along. That is peristalsis.
- Now try squeezing in front of the ball. What happens?
What to look for: with soap, the oil breaks into tiny droplets and the mixture stays milky, just as bile salts break fat into droplets with more surface area. In Part B, squeezing behind the ball moves it forward; squeezing in front blocks it, which is why the muscle ahead of the bolus must relax. Wash hands after; don't drink the soapy water.
Just for funYour stomach lining makes acid strong enough to help digest a steak, so it covers itself in a thick layer of mucus and keeps replacing its own lining cells every few days. Basically, your stomach is constantly redecorating so it doesn't digest itself.
Your turn
1Name the four accessory organs of digestion.
Salivary glands, liver, gallbladder, pancreas.
2What are the three jobs of hydrochloric acid in the stomach?
It kills most microbes, denatures proteins, and converts pepsinogen to pepsin (and gives pepsin its acidic working conditions).
3Which stomach cells make (a) HCl, (b) pepsinogen, (c) mucus?
(a) parietal cells; (b) chief cells; (c) mucous cells.
4What is the job of the pyloric sphincter?
It controls the release of chyme from the stomach into the duodenum, a little at a time.
5Explain why bile is needed for fat digestion even though it contains no enzymes.
Bile salts emulsify fat into tiny droplets, greatly increasing the surface area on which lipase can act.
6What neutralizes stomach acid in the duodenum, and where does it come from?
Sodium bicarbonate in pancreatic juice.
7Name the three sections of the small intestine in order.
Duodenum, jejunum, ileum.
More practice: warm-up
A ball of chewed food mixed with saliva.
Waves of smooth-muscle contraction that push food along the tract.
3What flap stops food from entering the trachea?
4Where is bile made, and where is it stored?
Made in the liver, stored in the gallbladder.
5What are the folds of the empty stomach called?
More practice: on your own
1Explain why the stomach does not digest itself.
Its lining is covered with a thick layer of alkaline mucus; pepsin is made as inactive pepsinogen and activated only in the stomach cavity; and damaged lining cells are replaced quickly.
2Distinguish between peristalsis and segmentation.
Peristalsis moves food forward (contraction behind, relaxation ahead). Segmentation mixes chyme back and forth by alternating contractions, increasing contact with enzymes and the lining.
3Explain why chewing food well speeds up chemical digestion.
Chewing breaks food into smaller pieces, increasing the surface area for enzymes, and mixes it with saliva.
4A person has had their gallbladder removed. Bile now drips slowly and continuously into the duodenum. Predict which meals might cause them trouble, and why.
High-fat meals. Without a store of concentrated bile to release at once, there may not be enough bile salts to emulsify a large amount of fat, so fat digestion is slower and less complete.
5Why does salivary amylase stop working in the stomach?
The stomach's low pH (about 2) is far from amylase's optimum (about 7), so it denatures; pepsin may also digest it.
Challenge
1The duodenum receives acidic chyme, but its lining has less protective mucus than the stomach's. Explain why the timing of the pyloric sphincter and the pancreatic bicarbonate release matter for protecting it.
The pyloric sphincter releases chyme in small amounts, so only a little acid enters at a time, and the pancreas releases bicarbonate (triggered by the hormone secretin, next lesson) that neutralizes it quickly. If too much acid entered too fast, the duodenal lining could be damaged; in fact, many peptic ulcers occur in the duodenum.
2A fat drop of radius 2.0 mm is emulsified into droplets of radius 0.020 mm. By what factor does surface area increase? Show your reasoning.
Number of droplets = (2.0 ÷ 0.020)³ = 100³ = 1 000 000. Each droplet's SA is (0.020 ÷ 2.0)² = 1/10 000 of the original. Total = 1 000 000 × 1/10 000 = 100 times the original. (Shortcut: SA increases by the ratio of radii, 2.0 ÷ 0.020 = 100.)
3Explain why digestion is organized as a sequence of compartments (mouth, stomach, small intestine) instead of all happening in one place.
Different enzymes need different conditions: pepsin needs strong acid; pancreatic enzymes need a slightly basic pH. Compartments separated by sphincters let the body keep very different pH levels, time each stage (storage in the stomach, slow release), and protect tissues (acid only where mucus protects the lining). Ordering also lets mechanical breakdown and denaturing come before the enzymes that need access to the molecules.
Quiz
1. Which organ is an accessory organ, not part of the alimentary canal?
2. Which cells of the gastric glands secrete hydrochloric acid?
4. Where does most chemical digestion take place?
5. What is the main job of pancreatic bicarbonate?
6. Peristalsis is caused by:
Big idea: E3.2 asks students to explain the anatomy of the digestive system and its mechanical and chemical processes. This lesson adds the cell-level detail (parietal and chief cells), the sphincters, emulsification by bile, and pancreatic juice.
How to help: At a meal, ask your teen to trace one food through the tract, naming each enzyme. Try the emulsification jar together; it is a vivid model of bile.
Common difficulties:
- Calling bile an enzyme. It emulsifies (physical digestion); lipase does the chemical digestion.
- Thinking most digestion happens in the stomach. The stomach starts protein digestion; the small intestine does most of the work.
- Mixing up accessory organs and the canal. Food never passes through the liver or pancreas.
Safety: in the activity, don't drink the soapy water; wash hands.