Key Concepts Animals require an array of nutrients to stay healthy including specific amino acids vitamins and elements as well as organic compounds that act as building blocks in chemical synthesis or have high potential energy There is usually a close correspondence between the structure of an animal s mouthparts and the function of those mouthparts in capturing and processing food 2011 Pearson Education Inc Key Concepts Digestion occurs in the digestive tract which is compartmentalized into organs that have specialized functions in the ingestion and digestion of food absorption of nutrients and water or excretion of wastes Lack of homeostasis with respect to nutrients such as glucose and cholesterol can cause disease 2011 Pearson Education Inc Introduction Animals are heterotrophs they obtain the energy and nutrients they need from other organisms rather than making their own food as plants do The types of food that are available to different animals vary widely and animals have a variety of means for obtaining food Ingestion is the first of four processes needed to obtain energy from food and it must be followed by digestion absorption and elimination 2011 Pearson Education Inc 2011 Pearson Education Inc Nutritional Requirements Animals get the chemical energy and carbon containing building blocks they need from carbohydrates and fats Animals also require other nutrients substances an organism needs to remain alive Food is any material that contains nutrients Understanding which nutrients an individual needs and in what amounts are basic issues in research on animal nutrition 2011 Pearson Education Inc Meeting Basic Needs in Humans Recommended Dietary Allowances RDAs specify the amount of each essential nutrient that an individual must ingest to meet the needs of most healthy people In addition to obtaining building blocks for chemical synthesis and chemical energy from carbohydrates and other compounds humans require several other essential nutrients meaning nutrients that cannot be synthesized and must be obtained in the diet 2011 Pearson Education Inc Essential Nutrients Essential amino acids cannot be synthesized by humans and must be obtained from food Vitamins are organic compounds that are vital for health but are required only in minute amounts several function as coenzymes in critical reactions Electrolytes are inorganic ions that influence osmotic balance and are required for normal membrane function Inorganic substances fulfill a variety of functions not performed by electrolytes Often they are important components of cofactors or structural materials 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc How Are Nutrients Digested and Absorbed Four processes are necessary for an animal to obtain energy from its food ingestion digestion absorption and elimination Digestion is the breakdown of food into small enough pieces to allow for absorption the uptake of nutrients 2011 Pearson Education Inc An Introduction to the Digestive Tract Digestion takes place in the digestive tract also called the alimentary canal or gastrointestinal GI tract beginning at the mouth and ending at the anus Digestive tracts come in two general designs 1 Incomplete digestive tracts 2 Complete digestive tracts 2011 Pearson Education Inc An Introduction to the Digestive Tract Incomplete digestive tracts have a single opening through which food is ingested and wastes are eliminated The mouth opens into a chamber called a gastrovascular cavity where digestion takes place 2011 Pearson Education Inc 2011 Pearson Education Inc An Introduction to the Digestive Tract Complete digestive tracts have two openings they start at the mouth and end at the anus The interior of this tube communicates directly with the external environment via these openings Complete digestive tracts have three advantages 1 Animals can feed on large pieces of food 2 Chemical and physical processes can be separated within the canal so that they occur independently of each other and in a prescribed sequence 3 Material can be ingested and digested continuously 2011 Pearson Education Inc 2011 Pearson Education Inc An Overview of Digestive Processes Digestion begins in the mouth starting with the tearing and crushing activity of teeth during chewing Distinct chemical changes occur as food moves through each compartment in the digestive tract 2011 Pearson Education Inc An Overview of Digestive Processes Each of the three major types of macromolecules carbohydrates lipids and proteins must be broken down during digestion This chemical processing started in the mouth continues in the stomach and finishes in the small intestine The small molecules that result from this digestion are absorbed in the small intestine along with water vitamins and ions More water is absorbed in the large intestine producing feces that eventually exit the body at the anus 2011 Pearson Education Inc 2011 Pearson Education Inc The Mouth and Esophagus During chewing enzymes in saliva start to break down some components of the food Salivary amylase is the most important catalyst in the breakdown of carbohydrates Cells in the tongue also secrete lipase which begins the breakdown of lipids Salivary glands in the mouth release water and glycoproteins called mucins When mucins contact water they form the slimy substance called mucus 2011 Pearson Education Inc The Mouth and Esophagus The combination of water and mucus makes food soft and slippery enough to be swallowed Food then enters the esophagus a muscular tube connecting the mouth and stomach and is propelled to the stomach by a wave of muscle contractions called peristalsis The system is a reflex an automatic reaction to a stimulus that is stimulated by the act of swallowing 2011 Pearson Education Inc 2011 Pearson Education Inc A Modified Esophagus the Bird Crop In an array of bird species the esophagus has a prominent widened segment called the crop where food can be stored and in some cases processed In many groups the crop is a simple storage sac and is thought to be an adaptation that allows individuals to eat a large amount in a short time then retreat to a safe location while digestion occurs However two groups of leaf eating birds have an enlarged crop that contain bacteria capable of breaking down cellulose 2011 Pearson Education Inc The Stomach The stomach is a tough muscular pouch bracketed on both ends by valves called sphincters After eating muscular contractions in