A slice of pizza is made of huge molecules: long chains of starch, folded proteins and fats. None of them can pass through the wall of your intestine into your blood, and your cells could not use them as they are. Digestion is the process that breaks food down into small molecules, such as glucose, amino acids, fatty acids and glycerol, that can be absorbed into the blood and carried to every cell. There the molecules are used for energy (in cellular respiration), for growth and repair (building proteins and membranes), and for regulating body processes. The digestive system is essentially a long tube, the alimentary canal or digestive tract, running from mouth to anus, with helper organs that add chemicals along the way.
What food provides: the nutrients
| Nutrient | Main role in the body | Digested into |
|---|---|---|
| Carbohydrates (starch, sugars) | Main fast energy source | Monosaccharides such as glucose |
| Proteins | Build and repair tissue (e.g. muscle); enzymes, antibodies, hemoglobin | Amino acids |
| Lipids (fats and oils) | Stored energy, cell membranes, insulation, some hormones | Fatty acids and glycerol |
| Vitamins and minerals | Regulate body processes (e.g. iron in hemoglobin, calcium in bone) | Absorbed without being broken down |
| Water | Solvent for reactions and transport; temperature regulation | Absorbed without being broken down |
| Fibre | Adds bulk so material moves through the intestine; helps prevent constipation | Not digested by human enzymes |
Mechanical and chemical digestion
- Physically breaks food into smaller pieces
- Does not change the molecules themselves
- Examples: chewing, churning by stomach muscles, bile breaking fat into droplets
- Purpose: increases surface area so enzymes can work faster
- Breaks chemical bonds to make smaller molecules
- Uses enzymes, and acid in the stomach
- Examples: amylase turning starch into maltose, pepsin cutting proteins
- Reactions are hydrolysis: water is used to split each bond
The journey, organ by organ
| Organ | What happens there |
|---|---|
| Mouth | Teeth chew (mechanical). Saliva moistens food and contains salivary amylase, which starts breaking starch into maltose (chemical). The tongue shapes a ball of food called a bolus. |
| Pharynx and esophagus | Swallowing pushes the bolus into the esophagus; the epiglottis covers the airway. Waves of muscle contraction called peristalsis move food to the stomach. No digestion happens here. |
| Stomach | Muscular walls churn food into a soupy mixture called chyme. Gastric juice contains hydrochloric acid (pH about 2) and pepsin, which begins protein digestion. A thick mucus layer protects the stomach lining from its own acid. Sphincter muscles at each end control flow. |
| Small intestine | Most chemical digestion and nearly all nutrient absorption. The first section, the duodenum, receives bile from the liver and gallbladder and pancreatic juice from the pancreas. The long remaining sections absorb nutrients through villi. |
| Liver and gallbladder | The liver makes bile; the gallbladder stores and concentrates it and releases it into the duodenum. The liver also processes absorbed nutrients arriving from the intestine. |
| Pancreas | Releases pancreatic juice containing amylase, trypsin and lipase, plus bicarbonate, which neutralizes stomach acid so intestinal enzymes can work. |
| Large intestine (colon) | Absorbs much of the water and some minerals that remain, forming solid feces. Helpful bacteria live here and make some vitamins, including vitamin K. |
| Rectum and anus | The rectum stores feces until they are eliminated through the anus. |
Enzymes: the chemical tools
An enzyme is a protein that speeds up a specific chemical reaction without being used up. Each enzyme has an active site shaped to fit one kind of molecule, its substrate, which is why amylase works on starch but does nothing to protein. Enzymes work best within a narrow range of conditions: human digestive enzymes work fastest near body temperature (37 °C), and each has an optimum pH. Pepsin works best in the strongly acidic stomach, while pancreatic enzymes work best in the nearly neutral small intestine. Very high temperatures or the wrong pH change the enzyme's shape (denaturation), so the substrate no longer fits.
| Enzyme | Made by | Works in | Substrate | Products |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Mouth | Starch | Maltose (a disaccharide) |
| Pepsin | Stomach lining | Stomach | Proteins | Shorter polypeptides |
| Pancreatic amylase | Pancreas | Small intestine | Starch | Maltose |
| Trypsin | Pancreas | Small intestine | Proteins and polypeptides | Shorter peptides |
| Lipase | Pancreas | Small intestine | Lipids (fats) | Fatty acids and glycerol |
| Maltase, sucrase, lactase | Small intestine lining | Small intestine | Disaccharides | Monosaccharides (e.g. glucose) |
| Peptidases | Small intestine lining | Small intestine | Peptides | Amino acids |
Absorption: getting nutrients into the blood
The inner wall of the small intestine is folded and covered in millions of finger-like projections called villi, and the cells on each villus carry even smaller projections called microvilli. Together they give an enormous surface area, which is the key to absorbing nutrients quickly. Each villus has a network of capillaries and a central lymph vessel called a lacteal. Glucose, amino acids, vitamins and minerals pass into the capillaries and travel through the hepatic portal vein to the liver, which processes and stores some of them. Fatty acids and glycerol are reassembled into fats and enter the lacteals, reaching the blood later through the lymphatic system. The small intestine absorbs most of the water we take in; the large intestine absorbs much of the water that remains. If material moves through the colon too slowly, too much water is removed and feces become hard and difficult to pass, which is constipation. Fibre and water help keep material moving.