Digestion

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Heterotrophic

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Animals that are unable to synthesize their own nutrients. Food provides the raw material for energy, repair, and growth of tissues. The food must first be ingested.

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Digestion

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Consists of the degradation of large molecules into smaller molecules that can be absorbed into the bloodstream and used directly by cells.

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Intracellular Digestion

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Digestion that occurs within the cell, usually in the membrane-bound vesicles.

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Extracellular Digestion

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Digestion that occurs outside of the cell, within a lumen a tract.

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Phagocytosis

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Digestion process that involves surrounding and engulfing food and enclosing it in food vacuoles. This is the primary mechanism utilized by unicellular organisms. Lysosomes (containing digestive enzymes) fuse with the food vacuole and release their digestive enzymes that act upon the nutrients. The resulting simpler molecules diffuse into the cytoplasm. The unusable end-products are eliminated from the vacuole.

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Paramecium

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Cilia sweep food into the oral groove and cytopharynx. A food vacuole forms around food at the lower end of the cytopharynx. Eventually, the vacuole breaks off into the cytoplasm and progresses towards the anterior end of the cell. Enzymes are secreted into the vacuole and the products diffuse into the cytoplasm. Solid wastes are expelled at the anal pore.

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Invertebrates

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Multicellular organisms have developed numerous adaptations for food capture and ingestion, digestion, and absorption. In many animals, the physical breakdown of large particles of food into small particles begins by cutting and grinding in the mouth and churning in the digestive tract. The molecular composition is unchanged but the surface area of the substrates on which the enzymes act is increased. Chemical breakdown of molecules is accomplished by enzymatic hydrolysis. The smaller digested nutrients (glucose, amino acids, fatty acids, and glycerol) pass through the semi-permeable plasma membrane of the gut cells to be further metabolized or transported.

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Cnidarians

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The hydra uses intracellular and extracellular digestion. Tentacles bring food to the mouth (ingestion) and release the particles into a cup-like sac. The endodermal cells lining this gastrovascular cavity secrete enzymes into the cavity. Thus, digestion occurs outside the cells. However, once the food is reduced to small fragments the gastrodermal cells engulf the nutrients and digestion is completed intracellularly. Undigested food is expelled through the mouth. Every cell is exposed to the external environment, thereby facilitating intracellular digestion.

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Annelids

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Have a one-way digestive tract with both a mouth and an anus. This allows specialization of different parts of the digestive tract for different functions. These parts include the mouth, pharynx, esophagus, crop (to store the food), gizzard (to grind the food), intestine (which contains a large dorsal fold (typholosole) to provide increased surface area for digestion and absorption), and anus. Soluble food passes, by diffusion, through the walls of the small intestine into blood.

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Arthropods

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Insects have a digestive system similar to earthworms, but also have jaws for chewing and salivary glands that improve food digestion.

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Human Digestion

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Begins with the oral cavity, and continues with the pharynx, esophagus, stomach, small intestine, large intestine, and the anus. Accessory organs, such as the salivary glands, the pancreas, liver, and the gall bladder, also have vital functions.

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Oral Cavity

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AKA the mouth is where mechanical and chemical digestion of food begins. Mechanical digestion is the breakdown of large food particles into smaller particles through biting and chewing of teeth (mastication). Chemical digestion refers to the enzymatic breakdown of macromolecules into smaller molecules and begins in the mouth when the salivary glands secrete saliva. Saliva lubricates food to facilitate swallowing, and provides a solvent for food particles. Saliva is secreted in response to a nervous reflex triggered by the presence of food in the oral cavity and contains the enzyme salivary amylase (ptyalin), which hydrolyzes starch to maltose (a disaccharide).

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Esophagus

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The muscular tube leading from the mouth to the stomach. Food is moved down it by rhythmic waves of involuntary muscular contractions called peristalsis.

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Stomach

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A large, muscular organ located in the upper abdomen that stores and partially digests food. The walls of it are lined by the thick gastric mucosa, which contains the glands. These glands secrete mucus that protects the stomach lining from the harshly acidic juices (pH=2). They also secrete pepsin, which is a protein-hydrolyzing enzyme; hydrochloric acid (HCl), which kills bacteria, dissolves the intercellular “glue” holding food tissues together, and activates certain proteins. The churning of this organ produces an acidic, semifluid mixture of partially digested food known as chyme. The chyme passes into the first segment of the small intestine, the duodenum, through the pyloric sphincter.

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Small Intestine

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Chemical digestion is completed in this organ. It is divided into three sections: the duodenum, the jejunum, and the ileum. It is highly adapted to absorption. It is extremely long (<6meters) and highly coiled. Numerous finger-like projections (villi) extend out of the intestinal wall. Villi contain capillaries and lacteals (vessels of the lymphatic system). Amino acids and monosaccharides pas through the villi walls into the capillary system. Large fatty acids and glycerol pass into the lacteals and are then reconverted into fats (fatty acid + glycerol). Some nutrients are actively absorbed (e.g. requiring energy) such as glucose and amino acids, while others are passively absorbed. Most digestion occurs in the duodenum, where the secretions of the intestinal glands, pancreas, liver, and gall bladder mix together with the acidic chyme entering from the stomach. The intestinal mucosa secretes lipases (for fat digestion), aminopeptidases (for polypeptide digestion), and disaccharidases (digestion of maltose

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Liver

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Produces bile that is stored in the gall bladder prior to release into the small intestine. Bile contains no enzymes; it emulsifies fats, breaking down large globules into small droplets. Emulsification of fats exposes a greater surface area of the fat to the action of pancreatic lipase. Without bile, fats cannot be digested.

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Pancreas

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Produces enzymes such as amylase for carbohydrate digestion, trypsin for protein digestion, and lipase for fat digestion. It secretes a bicarbonate rich juice that neutralizes the acidic chyme arriving from the stomach. Its enzymes operate optimally at this higher pH.

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Large Intestine

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This organ is around 1.5 meters longs and functions in the absorption of salts and the absorption of any water not already absorbed by the small intestine. The rectum provides for transient storage of feces prior to elimination through the anus.

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Intracellular Digestion (Plants)

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Plants store insoluble polymers, starches, lipids, and proteins in the cells. The main storage food is starch (a glucose polysaccharide), found in large quantities in seeds, stems, and roots. When nutrients are required, the storage polymers are broken down to simpler molecules (glucose, fatty acid, glycerol, and amino acids) by enzyme hydrolysis. The simple products can be used in the storage cell itself or transported by diffusion to other cells.

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Extracellular Digestion (Plants)

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Some heterotrophic organisms such as fungi must obtain preformed organic molecules (nutrients) from the environment. Enzymes are secreted, hydrolyzing complex nutrients into simpler molecules that are then absorbed. Once inside, the simpler molecules can be used for energy or to synthesize larger molecules. E.g. rhizoids of bread mold, a typical saprophyte that lives on dead organic material, secrete enzymes into the external environment (the bread). Digestion produces simple soluble end-products (glucose, amino acids, fatty acids, and glycerol), which are absorbed by diffusion into the rhizoid and transported throughout the mold. In plants, the Venus flytrap is the closest to actual ingestion. When a fly arrives, certain sensitive tissues entrap the insect. Enzymes are secreted to digest the fly and absorb the soluble end products (extracellular digestion). It is still an autotroph – it photosynthesizes to produce glucose. It uses the insect as a nitrate source, as it grows in nitrogen-poor soils.