Digestion is the process by which food is broken down so that it can be absorbed by an organism's cells and used to maintain bodily functions. In humans, digestion begins in the mouth, where food is chewed and mixed with saliva. The food then travels to the stomach and into the small intestine, where the greatest part of digestion takes place. The small intestine is the principal digestive organ where enzymes are produced that are essential for digestion, and most nutrients are absorbed. The large intestine primarily absorbs water and electrolytes from digestive residues.

Why do humans and other animals need to digest their food?

All organisms, including humans and other animals, must obtain nutrients from their environment to sustain themselves. These nutrients provide raw materials for building cellular material, regulate chemical reactions, and supply energy through oxidation. However, many nutrient molecules are too large and complex to be used directly by the organism. Digestion is the process of breaking down food into smaller, usable molecular particles. This enzymatic splitting of large molecules is effective because digestive enzyme molecules come into direct contact with the molecules of the material they are to digest. Once broken down, these smaller molecules can be absorbed and used for various biological processes, including energy production, cellular repair, and growth.

What are the main organs in the human digestive system?

The human digestive system consists of a series of organs that work together to break down food and absorb nutrients. The digestive tract begins at the lips and ends at the anus. The main organs include the mouth, where food is chewed and mixed with saliva, the pharynx (throat), the esophagus, which carries food to the stomach, the stomach, which mixes food with gastric juices, and the small intestine (duodenum, jejunum, and ileum), where most of the digestion and absorption take place. The large intestine (cecum, ascending colon, transverse colon, descending colon, and sigmoid colon) absorbs water and stores waste until it is eliminated. Other organs that contribute to digestion are the salivary glands , gastric glands , pancreas , and liver with its adjuncts (gallbladder and bile ducts).

How does the stomach help in the process of digestion?

The stomach is a sac-like organ that plays a vital role in digestion. It serves as a temporary storage and mechanical distribution center for food before it enters the small intestine. The stomach's inner walls contain gastric glands that secrete enzymes and hydrochloric acid to partially digest proteins and fats. The stomach muscles then contract to churn and knead the food into a semi-fluid mixture called chyme. Gastric juice, containing water, hydrochloric acid, electrolytes, and organic substances, helps to dissolve food particles and initiate protein digestion, converting the gastric contents into chyme for further digestion in the small intestine. The gastric mucosa, which lines the inner surface of the stomach, secretes thick mucus that lubricates food masses and protects the stomach lining from its own digestive enzymes.

What are digestive enzymes, and what role do they play during digestion?

Digestive enzymes facilitate the enzymatic splitting of large molecules into smaller ones, a process essential for digestion . These enzymes, such as proteolytic enzymes that degrade dietary proteins, are produced by the pancreas and other digestive organs. During digestion, food is broken down mechanically, and then digestive enzymes catalyze reactions by inserting a water molecule at the bond between two subunits of a molecule, effectively disconnecting them. This process, known as hydrolysis, is how enzymes like amylase, protease, and lipase break down carbohydrates, proteins, and fats, respectively.

How are nutrients absorbed in the small intestine after digestion?

After digestion, nutrients are absorbed in the small intestine through four key processes: active transport, passive diffusion, facilitated diffusion, and endocytosis. The small intestine's structure, especially the villi , significantly increases the surface area for absorption. Each villus contains blood vessels and a lymphatic capillary called a lacteal. Water absorption occurs throughout the small intestine, mainly in the jejunum. Fats are made soluble for absorption via micelles , which transport them to the cells lining the intestine. Once inside these cells, fats are processed and packaged into chylomicrons, which then enter the lymphatic system.

digestion

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Diagram of the human digestive system viewed from the front, showing and labeling organs including the mouth, teeth, tongue, salivary glands, pharynx, esophagus, liver, stomach, gallbladder, pancreas, large intestine, small intestine, appendix, rectum, and anus.

The chemistry of digestion: How food is broken down

Why do people feel full after a big meal?

Major organs of the human digestive system. Food taken in by the mouth is guided by the tongue as it is sheared and ground by the teeth. The bolus of chewed food is moistened and lubricated with saliva secreted by the salivary glands. Enzymes in the saliva begin the breakdown of starches. The epiglottis, a flap of tissue, prevents food from the pharynx from entering the larynx during swallowing. Muscles in the esophagus wall contract in waves to move the food to the stomach. Gastric juices secreted by the stomach contain a mixture of substances (including enzymes and hydrochloric acid) that break down the food into a semiliquid mass called chyme. The chyme passes into the small intestine, where food molecules are broken down into sugars, amino acids, and fatty acids. These useful substances are absorbed into the bloodstream as the food passes through the duodenum, jejunum, and ileum. The pancreas secretes digestive enzymes into the duodenum. The liver secretes bile salts that make insoluble fats entering the small intestine water-soluble and vulnerable to enzymatic action. Excess bile salts are stored in the gallbladder. The large intestine (colon) serves to remove water and electrolytes from the digested chyme and to compact and store undigestible material, called feces. Fecal matter is moved by muscular contractions into the rectum and expelled from the anus.

At a Glance

digestion summary

digestion

biology

Also known as: digestive process

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Nov. 1, 2026 •History

Nutrients and their breakdown

In order to sustain themselves, all organisms must obtain nutrients from the environment. Some nutrients serve as raw materials for the synthesis of cellular material; others (e.g., many vitamins) act as regulators of chemical reactions; and still others, upon oxidation, yield energy. Not all nutrients, however, are in a form suitable for immediate use by an organism; some must undergo physical and chemical changes before they can serve as energy or cell substance.

Through the act of eating, or ingestion, nutrients are taken from the environment. Many nutrient molecules are so large and complex that they must be split into smaller molecules before they can be used by the organism. This process of breaking down food into molecular particles of usable size and content is called digestion. Unusable components are expelled from the organism by a process called egestion, or excretion. Some plants, many microorganisms, and all animals perform these three functions—ingestion, digestion, and egestion (often grouped under the term alimentation)—but, as expected, the details differ considerably from group to group.

Modes of nutrition

The problems associated with nutrient intake and processing differ greatly depending on whether the organism is autotrophic or heterotrophic. Autotrophic organisms can manufacture the large energy-rich organic compounds necessary for life from simple inorganic raw materials; consequently, they require only simple nutrients from the environment. By contrast, heterotrophic organisms cannot manufacture complex organic compounds from simple inorganic ones, and so they must obtain preformed organic molecules directly from the environment.

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Green plants constitute by far the majority of the Earth’s autotrophic organisms. During the process of photosynthesis, plants use light energy to synthesize organic materials from carbon dioxide and water. Both compounds can be absorbed easily across the membranes of cells—in a typical land plant, carbon dioxide is absorbed from the air by leaf cells, and water is absorbed from the soil by root cells—and used directly in photosynthesis; i.e., neither of them requires digestion. The only other nutrients needed by most green plants are minerals such as nitrogen, phosphorus, and potassium, which also can be absorbed directly and require no digestion. There are, however, a few green plants (such as sundew, Venus flytrap, and pitcher plant) that supplement their inorganic diet with organic compounds (particularly protein) obtained by trapping and digesting insects and other small animals.

Heterotrophy characterizes all animals, most microorganisms, and plants and plantlike organisms (e.g., fungi) that lack the pigment chlorophyll, which is necessary for photosynthesis. These organisms must ingest organic nutrients—carbohydrates, proteins, and fats—and, by digestion, rearrange them into a form suitable for their own particular needs.

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Ingestion

As already explained, the nutrients obtained by most green plants are small inorganic molecules that can move with relative ease across cell membranes. Heterotrophic organisms such as bacteria and fungi, which require organic nutrients yet lack adaptations for ingesting bulk food, also rely on direct absorption of small nutrient molecules. Molecules of carbohydrates, proteins, or lipids, however, are too large and complex to move easily across cell membranes. Bacteria and fungi circumvent this by secreting digestive enzymes onto the food material; these enzymes catalyze the splitting of the large molecules into smaller units that are then absorbed into the cells. In other words, the bacteria and fungi perform extracellular digestion—digestion outside cells—before ingesting the food. This is often referred to as osmotrophic nutrition.

Like bacteria, protozoans are unicellular organisms, but their method of feeding is quite different. They ingest relatively large particles of food and carry out intracellular digestion (digestion inside cells) through a method of feeding called phagotrophic nutrition. Many protozoans also are osmotrophic to a lesser degree. Some organisms, such as amoebas, have pseudopodia (“false feet”) that flow around the food particle until it is completely enclosed in a membrane-bounded chamber called a food vacuole; this process is called phagocytosis.

figureOther protozoans, such as paramecia, pinch off food vacuoles from the end of a prominent oral groove into which food particles are drawn by the beating of numerous small hairlike projections called cilia. In still other cases of phagotrophic nutrition, tiny particles of food adhere to the membranous surface of the cell, which then folds inward and is pinched off as a vacuole; this process is called pinocytosis. The food particles contained in vacuoles formed through phagocytosis or pinocytosis have not entered the cell in the fullest sense until they have been digested into molecules able to cross the membrane of the vacuole and become incorporated into the cellular substance. This is accomplished by enzyme-containing organelles called lysosomes, which fuse with the vacuoles and convert food into simpler compounds.

Most multicellular animals possess some sort of digestive cavity—a chamber opening to the exterior via a mouth—in which digestion takes place. The higher animals, including the vertebrates, have more elaborate digestive tracts, or alimentary canals, through which food passes. In all of these systems large particles of food are broken down to units of more manageable size within the cavity before being taken into cells and reassembled (or assimilated) as cellular substance.

digestion

Nutrients and their breakdown

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Modes of nutrition

Ingestion