Digestive System
The digestive system is the organ system that breaks food into nutrients the body can absorb, using the mouth, stomach, intestines, and helper organs.
Definition
The digestive system breaks the food you eat into tiny pieces your body can use, as food travels from your mouth to your stomach and through your intestines, where nutrients are absorbed. It performs ingestion, mechanical and chemical digestion, absorption, and elimination along a main pathway of mouth, esophagus, stomach, small intestine, and large intestine, with accessory organs including the salivary glands, liver, gallbladder, and pancreas adding enzymes and bile. Enzymes break carbohydrates into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol, absorbed mainly in the small intestine. Chemically, salivary and pancreatic amylases cleave starch, pepsin and pancreatic proteases cleave peptide bonds, and lipases act on fats emulsified by bile salts; absorption uses sodium-coupled cotransporters for glucose and amino acids and micelle-mediated diffusion for lipids, which are repackaged into chylomicrons and enter the lymph. Motility and secretion are coordinated by the enteric nervous system and hormones such as gastrin, secretin, and cholecystokinin, and the gut microbiome ferments fiber and synthesizes some vitamins.
Example
When you eat a sandwich, your teeth chop it, your stomach mixes it with acid, and your small intestine pulls out the sugars, proteins, and fats for your blood to carry away. The human digestive tract is about $9\ \text{m}$ long, food takes roughly $24$ to $72$ hours to pass through, and stomach acid at pH $1.5$ to $3.5$ kills many bacteria and activates the protein-digesting enzyme pepsin. Pancreatic bicarbonate then raises the pH of chyme from about $2$ to about $7$ to $8$ as it enters the duodenum, the range in which pancreatic enzymes work, and sucrose ($C_{12}H_{22}O_{11}$) is hydrolyzed at the brush border into glucose and fructose, with glucose pulled into the cell by the SGLT1 transporter using the Na$^+$ gradient.
Key Insight
Your body cannot use a slice of bread as it is; digestion turns it into glucose molecules small enough to pass into your blood and into each cell. Most absorption happens in the small intestine, not the stomach, since its folds, villi, and microvilli give it a surface area near $30\ \text{m}^2$, about the size of a small classroom floor. Digestion is controlled hydrolysis: the bonds formed by removing water during synthesis are broken by adding it back, and enzymes make that chemistry fast and selective at body temperature, whereas the same reactions in a test tube would need strong acid and heat.