Animal Nutrition PDF

Animal Nutrition Plants and other photosynthetic organisms can produce their own food by converting inorganic components into organic molecules, and thus are called autotrophs or self-feeders. Animals, on the other hand, are called heterotrophs because they must consume organic molecules from other...

Animal Nutrition Plants and other photosynthetic organisms can produce their own food by converting inorganic components into organic molecules, and thus are called autotrophs or self-feeders. Animals, on the other hand, are called heterotrophs because they must consume organic molecules from other organisms for nutrition. Animals eat other organisms, either dead or alive, in pieces or whole, to provide them with the nutrients they need to maintain general health and well-being. Animals fall into three dietary categories: herbivores (exclusive to eating plants), carnivores (exclusive to eating flesh), and omnivores (eat both plants and animals). Detritivores, also known as detritus feeders or eaters, are heterotrophs that obtain nutrients from decaying bodies of plants and animal called detritus, usually by breaking them into smaller sizes before decomposers act on them. Typical soil-dwelling detritivores include earthworms, slugs, and some insects such as dung flies, woodlice, and springtails. In aquatic ecosystems, bottom dwellers such as crabs, sea cucumbers, and sea stars play an important role in breaking down organic matter. They differ from scavengers, which eat large chunk of organic matter. Types of Digestive Systems in Animals Animals have different mechanisms to prepare nutrients from food for use by cells for various purposes. Single-celled organisms process their food right inside their cells. This digestion process, known as intracellular digestion, which happens inside the cells through food vacuoles. This kind of digestion is common in single-celled organisms such as protists, Amoeba, and Paramecium. The Amoeba engulfs their food in the form of bacteria, plant cells, and microscopic protozoa, through the process known as phagocytosis. They eat by surrounding the food particle with their pseudopods or false feet that fork a bubble-like vacuole. The food vacuole will then break down the nutrients inside its cellular body. Wastes and other excess water will be released by a contractile vacuole. In most multicellular organisms, nutrients are absorbed into the bloodstream after they have been broken down within a digestive cavity while wastes are excreted out. This process is known as extracellular digestion. Extracellular digestion happens when food is broken down outside of cells, as observed in animals with an incomplete or complete digestive system. Incomplete digestive system or gastrovascular cavity is observed among simple animals such as the Hydra, flatworms, and coelenterates. These animals have a single opening where food and waste pass through. The cells lining the gastrovascular cavity have no specialization and are all involved in all stages of food digestion. The Hydra's tentacles have nematocyst that traps prey. The tentacles then lead the prey to the Hydra's widened mouth. Gland cells lining the Hydra's gastrovascular cavity secrete digestive enzymes into its cavity where extracellular digestion takes place. Smaller food particles are engulfed by nutritive cells to complete the digestion internally within the food vacuoles of each nutritive cell. Feeding Mechanisms in Animals Animals are also categorized based on how they obtain and ingest food. Marine invertebrates live in an environment with an abundance of floating microscopic food particles such as bacteria, phytoplankton, and zooplankton. Most of these organisms use their body parts to move water toward a feeding structure to sift through the food suspended in water in a feeding mechanism mode known as suspension feeding. Higher organisms obtain food through its subtype known as filter feeding. Animals, such as humpback whales that sift small shrimps known as krills, and small fishes through their baleen, are called filter feeders because they extract food particles suspended in surface water and sieve it to various filtering structures. Filter feeding is also the ingestion mode of clams and oysters as a film of mucus on their gills traps tiny bits of food suspended in water while their gills sweep this food into their mouth. Earthworms are known as substrate feeders because they eat their way through the soil while digesting and excreting food as they crawl. In this way, they also help the environment by aerating air and fertilizing the soil with their wastes. Fluid feeders ingest their food by sucking nutrient-rich fluid from a living host that is either a plant or an animal. Bloodsucking mosquitoes, headlice and dog ticks have needle-mouth structures that can pierce through the skin of their hosts. Female mosquitoes fill their abdomen with blood by sucking from their hosts through their proboscis. However, not all fluid feeders are bad as some provide desirable benefits to their hosts. Hummingbirds and bees transfer pollen between flowers while gathering nectar. Most higher forms in the animal kingdom are bulk feeders because they ingest large pieces of food into their mouth. The bulk feeders use a variety of means such as claws, pincers, venomous fangs, retractable jaws, and sharp teeth to tear the food source into pieces of meat so they could take mouthfuls of animal or plant body parts. Whatever type of feeding mechanism is practiced by animals, the same types of food processing are involved to supply the animals with essential nutrients. Nutrient Uptake in Cells among Animals Cells must take in nutrients, which may either be fluid or solid. Recall that macromolecules, like proteins and other large particles, cannot enter and exit the cell through carrier proteins, and this requires bulk transport mechanism. They enter the cell through different processes that also require energy. Large molecules enter the cell through the process called endocytosis, wherein the cell membrane bends inward (invaginates), forming a vesicle that contains the macromolecule to be transported. There are three types of endocytosis depending on the kind of macromolecule that needs to be transported: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis Phagocytosis or "cell eating" is a process wherein cells take in large particles or solids through the infolding of the cell membrane to form endocytic vesicles. The process starts with the formation and extension of the membrane pseudopodia (false feet). The membrane pseudopodia surround and engulfs the macromolecule, packaging them in a membrane-bound vesicle called a phagosome. When the endocytic vesicle fuses with lysosomes, digestion occurs. Lysosomes carry with them digestive enzymes that break down the ingested material. As you have learned, phagocytosis is used by many single-celled organisms as a means to capture food. However, multicellular organisms have cells that are specialized to perform phagocytosis. Phagocytosis is exhibited by white blood cells to capture and kill invading bacteria, viruses, or worn-out cells, and is thus crucial when fighting infections. Aside from pathogenic organisms, phagocytosis is also used to eliminate cell debris. Pinocytosis Pinocytosis or "cell drinking" is a process wherein a cell takes in fluids by the invagination of the cell membrane that forms a vesicle or vacuole. Any solute or small particles in the fluid will be moved into the cell. If the vesicle is small with a diameter of 0.1 um, it is called micropinocytosis, whereas a vesicle with a diameter of 1- 2 um is called macropinocytosis. Pinocytosis widely occurs in many types of cells of multicellular organisms, but there are specific situations where pinocytosis plays a major role. Pinocytosis is well observed in a human egg cell that matures in the ovary, where it is surrounded by other cells. These cells can pass nutrients to the egg cell through pinocytosis. Pinocytosis has also been observed in white blood cells (macrophages and leukocytes), kidney cells, in the uptake of nutrients by epithelial cells containing microvilli of the intestine, during the uptake of extracellular fluids such as hormones and enzymes by the body cells, and plant root cells to ingest liquid substances. Receptor-Mediated Endocytosis Specific molecules, on the other hand, can be transported into eukaryotic cells through receptor-mediated endocytosis. One type of molecule that is transported by the receptor-mediated endocytosis is low-density lipoprotein (LDL), which brings cholesterol in the body. For specific molecules to enter the cells, they must first bind to specific receptors on the plasma membrane, which then forms a pit. The pit is lined with receptor proteins that pick specific molecules from the surroundings. Only with the right fit of molecules against the shape of the receptors can plasma membrane begin to indent and form a pit. The pit will close and pinch off to form a vesicle, which will carry the molecules inside the cytoplasm. Metabolites, hormones, and other proteins enter through this process. Stages of Food Processing Animals follow the four stages of food processing. The first stage involves ingestion or the act of eating or taking in food via the mouth cavity. You have learned that the various feeding mechanisms that animals use to obtain and ingest food. The second stage is digestion, which involves the mechanical and chemical breakdown of large food molecules into soluble or diffusible molecules that can be absorbed by the cells. Mechanical digestion is used by animals by chewing or tearing food (through their teeth) to break apart large food particles. Likewise, you have learned the different endocytic ways by which nutrients are taken up by the cells. Once they are inside the cells, chemical reactions will occur as enzymes start to act on the food. Chemical digestion uses enzymes whereby water is added in hydrolysis to break the chemical bonds in food. As the molecules in food are large, in the form of proteins, fats, and carbohydrates, animals cannot use them directly. The molecules are too large to enter the plasma membrane of the cell so the animal must break down these macromolecules into building blocks (e.g., protein to amino acid) before it could be used to build new molecules of its own. Through the chemical digestion process, the macromolecule or polymer (e.g., protein) is broken down into its building block or monomer (e.g., amino acid). In the same way, carbohydrates are broken down into monosaccharides, nucleic acids into nucleotides, and fats into fatty acids and glycerol. The third stage involves absorption, where the small molecules, now in the form of building blocks, will be absorbed by the lining of the digestive tract. These nutrients will travel through the blood to the body cells, where they will be used by the cell to rebuild larger molecules (amino acids to a specific protein) or broken down further for energy. Since animals eat more than it needs, most of the nutrients are converted to fat for storage. The last stage is elimination, whereby undigested food is removed from the digestive tract. Human Digestive System: The human digestive system is composed of a tubular alimentary canal and its accessory glands. This tubular gut extends from the mouth and ends in the anus, with most of its length coiled in the digestive tract. Ingestion, the initial phase of food processing, starts in the mouth or oral cavity. Here, the mucin in saliva helps to soften the food while the teeth play an important role in physical digestion by masticating food and breaking it down into smaller pieces. The enzyme present in the saliva, known as salivary amylase, also begins the chemical digestion of food. The tongue assists by rolling the food into small, slippery masses of boli (singular: bolus). The food is then swallowed and led into the esophagus, a long tube connecting the mouth and the stomach. Muscle layers surrounding the esophagus cause rhythmic, wavelike contractions known as peristalsis, which move food along the gut. It takes 5- 10 seconds for the food to pass from the pharynx down to the esophagus and into the saclike stomach. Muscular rings like valves, called sphincters, regulate the passage of food into and out of the stomach. It controls the food to stay within the stomach for 2-6 - hours to allow stomach acids and enzymes to continue the digestive process. The partially digested food enters a J-shaped stomach, which lies beneath the diaphragm and partly covered by the liver. Its inner surface is convoluted, allowing the stomach to fold up when empty or expand when full. The stomach's thick muscular walls contract to mash the food into a sloppy soup while its lining filled with gastric glands release strong gastric juices. The gastric glands are composed of two types of secretory cells the parietal cells that secrete hydrochloric acid and the chief cells that secrete pepsinogen, a weak enzyme that, when activated is converted to a more powerful enzyme known as pepsin. Thus, gastric juice is a dilute solution of hydrochloric acid and pepsin with a pH between 1.5-2.5. Despite the presence of the acid, the thick slippery mucus coats and protects the stomach wall from eroding. The diluted hydrochloric acid stops the action of salivary amylase. It also provides a slightly acidic medium suitable for the action of the gastric enzymes, such as when pepsin breaks down some of the complex proteins even further into simpler peptides. In addition, the hydrochloric acid can kill germs and certain potential parasites. The food in the stomach stimulates the gastric glands to secrete gastric juice into the stomach cavity. While food is in the stomach, it undergoes both physical and chemical digestion. Peristalsis in the stomach wall churns the food and mixes it well with the gastric juice. These actions attack the food in a chemical way, thereby breaking down and dissolving its nutrients. The partly digested food becomes liquefied and, together with gastric juice, forms chyme, which passes in small amounts into the duodenum (first part of the small intestine). The stomach is connected to the small intestine through a muscular valve called the pyloric sphincter, which controls the food that passes into the small intestine. The small intestine is divided into three: duodenum, jejunum, and ileum. Further breakdown of food happens in the small intestine where the enzymes it produces aided by bile and pancreatic juices, continue the digestion process. The accessory organs, the liver and pancreas, help the digestive process. The pancreas produces hormones responsible for controlling the glucose level in the blood and releases bicarbonate that neutralizes the acidity of the chyme, whereas the liver aids in digestion by producing an alkaline, greenish-yellow liquid, which contains bile salts and bile pigments called bile. The gallbladder, a greenish-yellow bag, temporarily stores bile. As food moves into the small intestine, the gallbladder releases bile through a duct (bile duct) into the small intestine. Because bile is not an enzyme, it does not chemically digest food. However, it does help in breaking up large fat particles into smaller ones. These smaller fat particles can now be digested by enzymes easily in the small intestine. The small intestine is where terminal digestion of carbohydrates, lipids, and proteins occurs, as well as where the nutrients are absorbed by the blood. Here, proteins are broken down into individual amino acids; carbohydrates, such as starches and sugars, are broken down into simple sugars; and fats are broken down into fatty acids and glycerol. But before these nutrients can be used by the body for energy, they must first be absorbed or taken in by the bloodstream through the walls of the small intestine. The small intestine also has well-adapted structures for the absorption of digested food products. The surface wall contains numerous transverse folds and furrows, while its inner lining is covered with millions of tiny fingerlike structures called villi (singular: villus) that projects into the intestinal cavity. Microscopic examinations also show that the epithelial cells of the villi bear numerous cytoplasmic extensions called microvilli. In between the bases of the villi are minure openings of the intestinal glands char secrete intestina) juice. Both the villi and microvilli increase the surface absorption of the small intestine. The digested food is absorbed by the villi into a network of blood vessels that carry the nutrients to all parts of the body. By the time the food is ready to leave the small intestine, it is basically free of nutrients, except for water. All the nutrients have been absorbed. What remains are undigested substances that include water and cellulose from the food consumed. When cells have absorbed the nutrients, they will be transported to provide fuel and essential raw materials that the body needs. The process of transport and use of absorbed nutrients is known as assimilation. The simple sugars are converted into glycogen and stored in the liver while some glucose is carried into the bloodstream to be distributed throughout the body. Upon entering the cells, amino acids are converted as protoplasm and used for growth and repair of worn-out tissues of the body. They are also used for the formation of enzymes and hormones. Fats, before they are used, are brought to the liver where they are converted into forms that can be oxidized or stored. Under normal conditions when there is an adequate supply of glucose, fats are not oxidized. They are used to build protoplasm in cell membranes. Excess fats are stored in special tissues, called adipose tissues (fat storage and insulating tissues) located beneath the skin, around the heart, the kidneys, and in the mesenteries that bind the intestines. At the junction between the small intestine and the ascending colon (part of the large intestine) is a small sac, the cecum and the blindly ending appendix. The main function of the large intestine is to absorb water and mineral salts from the undigested food material. Its walls are not the same as the small intestines and have no villi. After about 18-24 hours in the large intestine, most of the water contained in undigested food is absorbed. Helpful resident bacteria of the large intestine produce certain vitamins, such as vitamin K and two B vitamins, which are needed by the body. Nutrients that are not absorbed in the large intestine form a solid waste known as feces. Feces, which is made up of dead bacteria and some fat and protein molecules, undigested food roughage, dried out digestive juices, mucus, and discarded intestinal cells, is stored temporarily in the rectum until defecation. When the rectum contracts, the feces is expelled through an opening called the anus. The process of removing undigested matter from the body is called egestion or defecation. Two sphincters (inner involuntary and outer voluntary sphincters) regulate the exit of feces. The voluntary sphincter is controlled by the brain, which permits one to have a conscious decision to delay defecation. You have observed that the gastrointestinal activities are coordinated by the nervous system and the endocrine system. The nervous system stimulates the production of salivary amylase and gastric secretions because of the sight and smell of food. The arrival of the food in the stomach stimulates the secretion of the hormones. Figure below shows the major hormones that play important roles in the digestive process. Other hormones play a role in the stimulation of the release of bile by the gallbladder and the secretion of bicarbonate by the pancreas.