Summary
Highlights
All living things consume food for two primary reasons: to obtain energy to survive and to acquire raw materials for building tissues. Both food and our bodies are made of matter with stored energy in chemical bonds. While oxygen is easily obtained, specific 'stuff' for building DNA or cells requires digestion. Nachos serve as an example, containing carbohydrates, fat, and protein, which store chemical energy measured in calories. This chemical energy needs to be converted into a form the body can use at a cellular level.
The digestive system is responsible for converting the 'stuff' in food into the 'stuff' that makes up our bodies. It acts as a disassembly line, breaking down food mechanically and chemically into its tiniest and most basic forms. Enzymes, which are proteins acting as catalysts, play a crucial role in breaking down large molecules into smaller building blocks that cells can absorb.
Food is made of biological molecules (macromolecules) such as lipids, carbohydrates, proteins, and nucleic acids, each with different caloric values. These are often polymers, which cells cannot use directly. The digestive system breaks these polymers into their individual components, called monomers (fatty acids, sugars, amino acids, and nucleotides), to build the body's own polymers and obtain energy. The body's needs are constantly changing, requiring a flexible digestive system.
The digestive system consists of a continuous tube called the alimentary canal (gastrointestinal tract), running from the mouth to the anus. This hollow canal includes the mouth, pharynx, esophagus, stomach, small and large intestines. The lining of the GI tract varies: stratified squamous epithelial tissue in the mouth, esophagus, and anus for resistance to abrasion, and simple columnar epithelial cells from the stomach onwards for secretion and absorption. The mucosal and submucosal layers provide blood supply and elasticity, while the muscularis externa layer contains muscles for moving food. Accessory organs like teeth, tongue, salivary glands, liver, gallbladder, and pancreas secrete enzymes to aid digestion.
Digestion occurs in six main steps. The first is ingestion, which is eating, introducing nutrients into the body. This begins mechanical and chemical breakdown in the mouth, with teeth pulverizing food and salivary glands releasing enzymes. The second step is propulsion, moving the food down the digestive tract. This starts with voluntary swallowing, quickly becoming involuntary peristalsis – wave-like muscle contractions that push food through the alimentary canal. This mechanical process continues in the stomach and small intestine, breaking food into increasingly smaller pieces to increase surface area.
The fourth step is chemical digestion, where accessory organs secrete enzymes into the alimentary canal to break down food into basic chemical building blocks (monomers). Step five is absorption, where these nutrients pass from the small intestine into the blood, either actively or passively. This is the main goal of digestion, allowing cells to use energy and build new tissues. The final step is defecation, the elimination of indigestible substances like fiber from the body. Many of these steps involve cooperation between multiple organs, with both mechanical and chemical processes occurring simultaneously in different parts of the tract.