Digestion (comparative physiology)

The bulk of animal food consists of proteins, carbohydrates, and fats. In addition, smaller molecules that make up these complex molecules—vitamins, nucleic acids, and minerals—are essential components of animal food. Animals obtain their food in the form of solutions, suspensions, dry particles, aggregates, and masses of particles, or whole animals and plants and their parts. It is the selection of food and the feeding behavior of animals that distinguish different animal populations and allow them to live together in the same habitat without competing for the same resources. The organs that break down the food mechanically into small particles and particles into molecules by the processes called digestion constitute what is called a digestive system. The length of an animal’s digestive system is related to its diet. Carnivores have short digestive tracts, while those that eat fibrous plants have long digestive tracts. Usually, the fore (anterior) part of the digestive system of animals is adapted for capturing and breaking down food (like the bills and beaks of birds and the jaws of mammals), and the remaining system can become specialized to store, chemically digest, and absorb the digested food and eliminate the unabsorbed food.

The Digestive Sequence

A typical functional sequence of digestion can be summarized as follows: First is the mouth, its appendages, and the oral cavity. These are involved in selecting (by taste, smell, touch), capturing, ingesting, and initial breaking down of food. Secretions here can include lubricants (mucus coming from salivary glands as well as other fluids), anticoagulants (in bloodsuckers), paralyzing toxins (in carnivorous coelenterates, spiders, reptiles), proteases (in cephalopod mollusks), and carbohydrases (in plant eaters). In microphages, loco motory appendages, oral tentacles with cilia, can drive currents of water containing food toward the mouth. In macrophages, locomotory appendages can be modified to capture food and ingest it. Most small aquatic animals and some large ones strain small particulate material with the help of their body surface projections (cilia, setae, bristles, legs, mucus, or nets); these microphages are called filter-feeders. In vertebrates, movable jaws, and in invertebrates, hard structures or surfaces can be used for crushing food. The mouth leads into an oral cavity whose posterior chamber is a muscular pharynx that opens into the esophagus.

Second, the muscular, tubular esophagus transfers the food, in bits, to the stomach. Sometimes, a distension in this part of the digestive system (the crop found in cockroaches and birds, for example) is used to store food.

Third, the stomach, a muscular vessel into which the esophagus leads, mechanically breaks down the food through contractions and wavelike motions and begins the process of chemical digestion via enzymes. Sometimes, the stomach is equipped with hard projections (such as the gizzards and gastric mills of birds, cockroaches, earthworms, or alligators). The lining of the stomach or its diverticula (branches) secretes digestive enzymes and, in vertebrates, hormones and hydrochloric acid. The stomach opens into the next chamber, the intestine.

Fourth, the small intestine completes the digestive process. Its cells and the cells of its glands (the pancreas, liver, hepatic caecum) secrete digestive fluids containing enzymes and hormones that enable absorption of the resulting molecules and water into the cells of the intestine and from there into the blood. The inner lining of the intestine can be thrown into ridges and microridges, which greatly increase the surface area and, thus, the amount of absorption.

Fifth, the large intestine, or hindgut, reabsorbs water. The undigested food is evacuated in the form of feces through an opening to the exterior called the anus. This part of the digestive tract also stores colonies of microorganisms, especially in plant eaters, to digest cellulose, lignin, and other substances and to provide some vitamins that the animal cannot synthesize.

Continuous and Noncontinuous Feeders

In animals that feed on soluble or suspended particles, called continuous feeders, digestion is a continuous process. In these animals, which include sponges, coelenterates, and flatworms, the digestive system is in the form of a tube open at one end only, and the chemical digestion of particles takes place inside each cell lining this tube. Annelids, arthropods, mollusks, and echinoderms have digestive systems that are open at both ends. These animals have developed various other systems, and the digestive system has become independent of the circulatory system. The opening of the digestive tube has allowed these animals to specialize their parts into various regions for capturing, grinding, masticating, mechanically breaking down, chemically digesting, absorbing, and eliminating their food. That, in turn, has allowed them to conduct extracellular digestion in the digestive cavity to become discontinuous feeders.

Hence, with the evolution of a digestive tube dedicated to digestion only, animals started secreting their cellular enzymes into this cavity in response to food. This, then, constituted extracellular digestion. Extracellular digestion is present in small animals that feed on particles (microphages) or larger animals that feed on bulk food (macrophages). The animals with intracellular digestion and those microphages with extracellular digestion feed continuously and nonselectively. The evolution of a complete digestive tract, opening at both ends, and extracellular digestion have allowed the evolution of larger, more active, and more advanced animals. These macrophages feed discontinuously and select their food. The time that they have saved from feeding had been spent performing other activities and evolving complex behavior patterns. Also, ingestion of a large mass of food has enabled them to obtain the bulk of their energy from this food, which provides a tremendous amount of dependable power to move and even fly.

Digestive Specialization

The evolution of a complete digestive tube has resulted in the specialization of its parts for various digestive processes. The general pattern of functional sequence that was outlined above is evident: The digestive system is usually divided into foregut (mouth and its appendages), midgut (for chemical digestion and absorption), and hindgut (for absorption of water and elimination of undigested food). Within this general structure, however, are innumerable and complex variations in adaptation to the type of food and feeding mechanisms of different animals. Those animals that feed on solid food, for example, have appendages (such as jaws and teeth) to enable them to grind, crush, or masticate it. In addition, these may have parts of the stomach modified for storage (such as rumen in ruminants and the crops of birds) or for further grinding (the gizzards of various insects, birds, and alligators). Cows and goats, for example, have four-chambered stomachs, one of which stores colonies of bacteria. These ruminants swallow the food as a whole while grazing. Then, later, while resting, they bring the food and bacteria back to the mouth as cud to mix them together, subsequently swallowing the food. The bacteria then digest cellulose by fermentation. The microorganisms are then digested by the animal in the intestine. These animals also secrete copious amounts of saliva, which prevents abrasive damage by the solid food to the cells lining the foregut.

In fluid feeders, by contrast, the oral end is equipped with sucking apparatus containing piercing devices (as in moths, bees, flies, mosquitoes, and leeches). Some feeders on plant juices ingest large amounts of water with sugars. The last part of their foregut becomes connected with the anterior part of the hindgut, forming a filtering apparatus (as in insect leaf hoppers). Only water passes from the foregut to the hindgut, while food enters the midgut, which now does not have to process large amounts of water.

The Role of Enzymes

The chemical breakdown of food particles takes place by means of enzyme catalysts, which are proteins that are released into the stomach and intestine (midgut) from their cells or from cells of appendages (hepatic caecum in insects, hepatopancreas in crustaceans and mollusks, and pancreas and liver in vertebrates) opening into the intestine. These enzymes are secreted in response to the entry of food into the gut. Moreover, the presence and release of specific enzymes depend on the chemical nature of the food. In plant-eating herbivores, which eat an abundance of carbohydrates (sugars), these secretions are rich in carbohydrates (carbohydrate-hydrolyzing enzymes), while in animal eaters (carnivores), protein-digesting enzymes, proteases, and fat-hydrolyzing enzymes, lipases, are predominant. In omnivores (which feed on both plants and animals), all three groups of enzymes are present. In food specialists, such as sheep blow flies (which feed on wool keratin), head lice (which feed on hair keratin), cloth moths (which feed on textile fibers), wax moths (which feed on wax), or carpet and leather beetles (which feed on keratin), the digestive fluid is rich in specific enzymes for handling one kind of food. In wood-eating termites, snails (genus Helix), and ruminant mammals, the cellulose is digested by colonies of microorganisms that are carried in parts of these animals’ guts.

In addition, different enzymes are present in different stages of an animal’s life cycle. For example, maggots feeding on flesh have proteinases, while adult flies feeding on sugars have sucrases. The intestinal enzyme lactase, which breaks down the milk sugar lactose, is always present in land mammals at or before birth. It usually decreases after weaning. Among insects, certain leaf hoppers and moths that feed on soluble sugars (which do not require further breakdown) have no enzymes, while hoppers feeding on mesophyll cells and caterpillars actively chewing plant parts have carbohydrases and lipases. Among bees, nurses have more proteases than foragers; wax bees have no proteases; the carbohydrases are predominant in foragers, especially during midsummer, and lipase is found only in wax bees. The carnivorous turbellarians, coelenterates, cephalopod mollusks, crustaceans, scavenger insects, and starfish have more proteases and fewer carbohydrases.

In mammals, most of the enzymes (pepsinogen, trypsinogen, chymotrypsinogen, lipase) are secreted as zymogens (proenzymes) and are activated by other secretions. For example, hydrochloric acid converts pepsinogen into active pepsin in the stomach; enterokinase converts chymotrypsinogen into active chymotrypsin, which activates trypsinogen to trypsin. Secretion of zymogens and their activation are precisely controlled and occur when food is present in the gut. For example, when chyme leaves the stomach, the duodenal hormone enterogastrone inhibits the release of hydrochloric acid from parietal cells so that no activation of pepsinogen occurs; otherwise, pepsin could destroy the proteins in the membranes of cells lining the gastric cavity. The digestive epithelia of animals are thereby protected from damage by physical (solid food) and chemical (enzymes, acids) sources. This digestive strategy became necessary as discontinuous feeding evolved since the presence of food for digestion was intermittent.

Studying Digestion

A variety of observations and experiments have been performed to study the different types of digestive systems. Examination of the anterior (mouth) end of different animals, for example, reveals the broad range of strategies used to collect and initially break down food.

Soluble food feeders, for example, can be examined under the microscope. Observation of a microscopic slide of the head of a human tapeworm shows that it is equipped with hooks and suckers by means of which it attaches itself to the digestive tract of a person. The soluble, predigested food in the intestine needs no further breakdown and is absorbed through the flat body surface of the worm, which lacks any digestive organs. Observed under the microscope, the anterior end of a liver fluke has hooks and suckers to suck fluid; a lamprey has a round mouth and rasping tongue with which to suck the blood of its host fish; a mosquito has a piercing device to break the skin and suck blood; and the mouthparts of an adult moth include a long, coiled proboscis designed to suck nectar from flowers.

Intracellular digestion of food by a variety of microorganisms can be observed in progress under the microscope. Amoebas can be starved for one or two days and then transferred to drops of a culture on a shallow depression slide. The amoebas will exhibit phagocytosis (cell eating) with the help of their “feet” (pseudopodia), surrounding the food and ingesting it. A change in the color of the Blepherisma pigment (in the food vacuole of the amoeba) can be seen—from red (indicating acidic) to neutral or colorless (indicating alkaline pH). This indicates that earlier stages of digestion are acidic, later stages are alkaline.

Paramecium can also be observed feeding on starch solution with and without a drop of iodine (which turns starch blue and inhibits feeding). If this procedure is repeated using compressed yeast in a 3 percent solution of Congo red, one can observe the direction of movement of the yeast (which has taken on the red color) as it travels in the direction of the beating of the paramecium’s cilia and into the food vacuole, then as it circulates through the cytoplasm. The change in color from red to blue indicates digestion. Paramecia will also reject algae particles and ingest only yeast, indicating the presence of chemical sensory mechanisms.

Solid food eaters, which can be observed with the naked eye, reveal a variety of specially adapted parts: hard, strong mandibles for crushing leaves in the caterpillar; similar mandibles for handling solid food in the cockroach; the “Aristotle’s Lantern” of the sea urchin, used for grinding; the tentacles of the Hydra, which feeds on fine, suspended food particles; and the human jaw and teeth, with incisors, canines, and molars designed to break down a variety of food in a variety of ways.

The activity of various digestive enzymes can be determined by using appropriate substrates (the food molecules) and physiological conditions in a test tube. The source of the enzyme is the part of the digestive tract where it is produced and used. Tissue from this area is ground in a small blender or homogenizer using an appropriate buffer at about 4 degrees Celsius (39 degrees Fahrenheit). The homogenate of the tissue is either used as is or fractionated using a high-speed, refrigerated centrifuge, which can fractionate cell membranes, various organelles, and cytoplasm. Then, the subcell fraction, where the enzyme is located, can be used as the source of the enzyme. The enzyme is further purified by means of biochemical devices. The substrate is either natural or synthetic. The pH, temperature, and other conditions are controlled in the incubation mixture containing enzyme and substrate. Time-course aliquots (samples) are withdrawn, and the activity of the enzyme is measured by analyzing the hydrolysis product of the substrate using various spectrophotometric devices. The enzymes, from the same tissue, or its subcell fraction, of animals feeding on plants, meat, or both are compared to determine how active various enzymes are in these animals. The presence of certain enzymes can be related to the chemical nature of the food.

Digestion and Survival

Food selection, feeding behavior, and the structure and function of the digestive apparatus of animals form an important mechanism of survival by which animals in a population isolate themselves from other populations to avoid competition for the same source of food. The feeding behavior depends on the type of food available (soluble, suspended, aggregates, or large organisms), and the form of the feeding apparatus (shapes and sizes of bills of birds and jaws and teeth of mammals, for example) depends on the physical nature of the food. The anatomy of the digestive system is closely adapted to the physical nature of the food, while the chemical functioning (enzymes) of the digestive system depends on its chemical nature.

The adaptations of the digestive systems have enabled the evolution of larger and more active animals, which feed less frequently on greater bulks of food as compared with less active small animals, which may have to feed more often and even continuously. For example, birds have evolved to digest food more quickly than other animals their size. Scientists posit this evolutionary adaptation likely occurred to allow them to fly more easily, maximize efficiency while foraging, and escape predators. Smaller species have much faster digestion rates than larger birds However, all birds have smaller and shorter intestines with a simplified digestive tract that retains food for shorter periods than animals of comparable size. Their stomach and liver are muscular and are centrally located to maximize their maneuverability in flight. Their meals return to their stomach after being digested in the small intestine to allow the food to be digested twice, maximizing nutrient efficiency in a process called small intestinal reflux. Additionally, birds' digestive enzymes have evolved to process the foods they eat. Those that eat more grain produce more amylases and those that eat animal flesh produce more lipases and proteases.

The broad variety of different digestive systems and their enzymes has enabled animals to make the best use of the food resources available in their environment. Those animals able to exploit their environment more fully than others (such as omnivores, including humans) have a wide array of digestive enzymes that can chemically break down a wide variety of foods. They are more successful at survival than those confined to a particular type of food (food specialists with a limited ability to digest only one type of food) and, hence, are likely to survive longer as a group.

Principal Terms

Enzyme: A protein that acts as a catalyst under appropriate physiological conditions to break down bonds of a large protein, fat, or carbohydrate

Esophagus: The part of the oral cavity (pharynx) that transfers morsels to the stomach; it is usually a long, muscular tube with no digestive function other than transport

Hormone: A chemical released into the blood for transport to a specific site, where it will perform a specific function; many hormones stimulate chemical and mechanical aspects of digestion

Intestine: The part of the digestive system involved in completing the process of digestion and absorption of nutrients; usually divided into the small intestine and the large intestine, which opens to the exterior by way of the anus

Mouth: The anterior part of the digestive system, used for ingesting food; it leads into the oral cavity, which opens into the esophagus

Mucus: A secretion of the salivary glands and other parts of the digestive system which lubricates passages

Stomach: The part of the digestive system where mechanical breakdown of food is completed and chemical digestion begins

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