The digestive system breaks large food pieces into small molecules that cells can absorb and use for energy, growth, and repair. It includes a long digestive tract from the mouth to the anus, plus accessory organs that add important fluids and enzymes. Understanding digestion helps explain nutrition, metabolism, hydration, and many common health problems.
It also shows how several organ systems work together to maintain homeostasis.
Digestion begins with mechanical breakdown by chewing and churning, while chemical digestion uses enzymes and acids to split large molecules into smaller ones. Food moves through the tract by peristalsis, a wave-like squeezing of smooth muscle. Most nutrient absorption occurs in the small intestine, where villi and microvilli create a large surface area.
The large intestine absorbs water and salts, then stores and eliminates undigested waste.
Key Facts
- Path of food: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
- Mechanical digestion physically breaks food into smaller pieces without changing its chemical identity.
- Chemical digestion uses enzymes such as amylase, pepsin, and lipase to break chemical bonds in food molecules.
- Carbohydrate digestion begins in the mouth: starch + amylase -> smaller sugars.
- Protein digestion begins strongly in the stomach: proteins + pepsin + acid -> shorter peptides.
- Most absorption happens in the small intestine because villi and microvilli greatly increase surface area.
Vocabulary
- Peristalsis
- Peristalsis is the rhythmic squeezing of smooth muscle that moves food through the digestive tract.
- Enzyme
- An enzyme is a protein that speeds up a chemical reaction, such as breaking starch, protein, or fat into smaller molecules.
- Villi
- Villi are tiny finger-like projections in the small intestine that increase surface area for nutrient absorption.
- Bile
- Bile is a fluid made by the liver and stored in the gallbladder that helps break large fat droplets into smaller droplets.
- Accessory organ
- An accessory organ helps digestion by producing or storing digestive fluids, but food does not pass directly through it.
Common Mistakes to Avoid
- Thinking the stomach is where most nutrients are absorbed is wrong because most absorption occurs in the small intestine through villi and microvilli.
- Confusing mechanical digestion with chemical digestion is wrong because chewing and churning change food size, while enzymes change food molecules.
- Saying bile is an enzyme is wrong because bile emulsifies fats but does not chemically break bonds like lipase does.
- Forgetting the accessory organs is wrong because the liver, gallbladder, and pancreas provide bile and enzymes that are essential for digestion.
Practice Questions
- 1 A student eats a meal at 12:00 p.m. If food reaches the small intestine about 4 hours later and spends 5 hours there, at what time does most absorption finish?
- 2 The small intestine is about 6 m long. If a diagram uses a scale of 1 cm = 0.5 m, how long should the small intestine be drawn on the diagram?
- 3 Explain why damage to villi in the small intestine can cause poor nutrition even if a person eats enough food.