Digestion and Nutrition Chapter 32
Obtaining Energy • All organisms require energy to maintain their complex structure. • The ultimate source of energy is the sun. • Green plants utilize energy in sunlight to make glucose. • Autotrophs (phototrophs) • A few autotrophs are chemotrophs, gaining energy from inorganic chemical reactions.
Animals are Heterotrophs! • Animals are heterotrophs, depending on other organisms for food. • Animals fall into one of three dietary categories: • Herbivores eat mainly autotrophs (plants and algae). • Carnivores eat other animals. • Omnivores regularly consume animals as well as plants or algal matter. • Saprophagous animals feed on decaying organic matter.
Why We Eat • Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs: • Fuel for all cellular work. • The organic raw materials for biosynthesis. • Essential nutrients, substances such as vitamins that the animal cannot make for itself.
Feeding Mechanisms • Very few animals absorb nutrients directly from the environment. • Exceptions include parasites that absorb nutrients that have been digested by the host: • Blood parasites • Protozoan parasites • Tapeworms • Acanthocephalans
Feeding Mechanisms – Particulate Matter • The upper portion of lakes and oceans contains very small animals and plants (plankton) that drift with the water currents. • Along with plankton, there is also organic debris floating in the water column and mixed in with the sediment. • Many organisms feed on this particulate matter.
Feeding Mechanisms – Particulate Matter • Suspension feeders use ciliated surfaces to create a current that draws drifting food particles into their mouths. • Many use mucous sheets to entrap food. • Tube dwelling polychaetes, bivalve molluscs, hemichordates, protochordates.
Feeding Mechanisms – Particulate Matter • Others use sweeping movements of setae-fringed legs to create currents. • Fairy shrimp, daphnia, barnacles.
Feeding Mechanisms – Particulate Matter • Filter feeding is a form of suspension feeding that involves straining food from the water as it passes through a filtering device. • Herring, menhaden, basking sharks, flamingos, baleen whales.
Feeding Mechanisms – Particulate Matter • Deposit feeders consume the organic matter (detritus) that accumulates on the substratum. • Many annelids simply eat the substrate, digesting organic matter. • Others use appendages to gather organic deposits and move them to the mouth. • Scaphopods, sedentary or tube-dwelling polychaetes, some bivalves, some annelids.
Feeding Mechanisms • Predators have evolved a variety of ways to capture, hold, and swallow prey. • Many swallow food items whole. • Some have specialized teeth, beaks, or tooth-like structures. • Some have highly elastic jaws and distensible stomachs to accommodate large meals. • Insects have 3 pairs of appendages specialized for feeding.
Feeding Mechanisms • Only mammals can actually chew their food. • Mammals have teeth that are specialized for different functions. • Incisors – biting, cutting, stripping leaves. • Canines – seizing, piercing, tearing. • Premolars & molars – grinding and crushing.
Feeding Mechanisms • Herbivorous animals have evolved special devices for crushing and cutting plant material. • Snails have a radula for scraping algae or plant material. • Insects have grinding & cutting mandibles. • Mammals have wide corrugated molars for grinding.
Feeding Mechanisms • Fluid feeders may bite and rasp at host tissues, suck blood, and feed on contents of a host’s intestines. • Many have specialized, tubelike mouthparts.
The Main Stages of Food Processing • Ingestion is the act of eating. • Digestion is the process of breaking food down into molecules small enough to absorb. • Involves enzymatic hydrolysis of polymers into their monomers.
The Main Stages of Food Processing • Absorption is the uptake of nutrients by body cells. • Elimination occurs as undigested material passes out of the digestive system.
Intracellular Digestion • In intracellular digestion, food particles are engulfed by endocytosis and digested within food vacuoles. • Protozoa, sponges.
Extracellular Digestion • Extracellular digestion is the breakdown of food particles outside cells. • Digestion occurs in the alimentary canal. • Cells lining the lumen of the alimentary canal are specialized for secreting enzymes or absorbing nutrients.
Extracellular Digestion • Radiates, flatworms, & ribbon worms practice both intracellular and extracellular digestion. • Extracellular digestion became emphasized with the appearance of a complete digestive tract. • Digestion is almost entirely extracellular in arthropods and vertebrates.
Digestive Systems • Animals with simple body plans have a gastrovascular cavity that functions in both digestion and distribution of nutrients.
Digestive Systems • Animals with a more complex body plan have a digestive tube with two openings, a mouth and an anus. • This digestive tube is called a complete digestive tract or an alimentary canal.
Digestive Systems • The digestive tube can be organized into specialized regions that carry out digestion and nutrient absorption in a stepwise fashion.
Mammalian Digestive System • The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices through ducts.
Digestive Enzymes • Enzymes are essential in the breakdown of food into small, absorbable units. • Digestive enzymes are hydrolytic enzymes. • Food molecules are split by hydrolysis. • R-R + H2O digestive enzyme R-OH + H-R
Digestive Enzymes • Proteins are broken down into individual amino acids. • Complex carbohydrates are broken down into simple sugars. • Fats are reduced to glycerol, fatty acids, and monoglycerides.
Motility in Alimentary Canal • Food moves through the alimentary canal by cilia, specialized musculature, or both. • Gut musculature is present in coelomates.
Motility in Alimentary Canal • The gut is lined with opposing layers of smooth muscle: a circular layer and a longitudinal layer.
Motility in Alimentary Canal • Two types of gut movement: • Segmentation involves alternate constriction of rings of smooth muscle that move the contents around, mixing with enzymes. • Peristalsis involves waves of contraction behind the food mass that move it through the gut.
Organization - Five Major Regions • Reception • Conduction & Storage • Grinding & early digestion • Terminal digestion and absorption • Water absorption and concentration of solids.
Receiving Region • The receiving region consists of devices for feeding and swallowing. • Mouthparts – mandibles, jaws, teeth, radula, bills. • Buccal cavity – mouth • Muscular pharynx – throat • Salivary glands – produce lubricating secretions that may also contain toxic enzymes or salivary enzymes to begin digestion. • Amylase begins hydrolysis of starches.
Receiving Region • The vertebrate tongue assists in food manipulation and swallowing. • Also used as a chemosensor.
Conduction and Storage Region • The esophagus transfers food to the digestive region. • In many invertebrates (annelids, insects, octopods) the esophagus is expanded into a crop used for storage. • Birds also have a crop that serves to store and soften food.
Region of Grinding & Early Digestion • The stomach provides initial digestion as well as storing and mixing food with gastric juice. • For further grinding of food, terrestrial oligochaetes and birds have a muscular gizzard that is assisted by stones or grit swallowed with food.
The Stomach • The lining of the stomach is coated with mucus, which prevents the gastric juice from destroying the cells. • Pepsin is a protease that splits specific peptide bonds.
The Stomach • Gastric ulcers, lesions in the lining, are caused mainly by the bacterium Helicobacter pylori.
Region of Terminal Digestion and Absorption • The small intestine is the longest section of the alimentary canal. • It is the major organ of digestion and absorption.
Region of Terminal Digestion and Absorption • Increasing the surface area of the intestine increases the area available for absorption. • Longer intestine • Villi – fingerlike projections of intestinal tissue in birds and mammals • Microvilli – tiny processes on intestinal cells.
The Small Intestine • The first portion of the small intestine is the duodenum, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself.
The Small Intestine • The pancreas produces: • Proteases, protein-digesting enzymes. • Lipases for breaking up fat. • Amylase for hydrolyzing starches. • Nucleases which degrade RNA & DNA into nucleotides.
The Small Intestine • The liver secretes bile into the bile duct which drains into the duodenum. • Bile is stored in the gallbladder between meals. • Bile salts are important for digestion of fats.
The Small Intestine • Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine.
Absorption of Nutrients • The small intestine has a huge surface area due to the presence of villi and microvilli that are exposed to the intestinal lumen.
Absorption of Nutrients • The enormous microvillar surface is an adaptation that greatly increases the rate of nutrient absorption.
Absorption of Nutrients • The core of each villus contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal.
Absorption of Nutrients • Amino acids and simple sugars pass through the epithelium of the small intestine and enter the bloodstream. • Initial absorption occurs by facilitated transport, later by active transport.
Absorption of Nutrients • Fats are emulsified by bile salts. • Micelles are tiny droplets consisting of fatty acids and monoglycerides complexed with bile salts. • Micelles diffuse into epithelial cells. • Resynthesized into triglycerides and pass into the lacteals of the lymphatic system.
Region of Water Absorption • The large intestine, or colon is connected to the small intestine.
Region of Water Absorption • A major function of the colon is to recover water that has entered the alimentary canal. • The wastes of the digestive tract, the feces, become more solid as they move through the colon. • The terminal portion of the colon is the rectum where feces are stored until they can be eliminated.
Region of Water Absorption • The colon houses various strains of the bacterium Escherichia coli. • Some produce various vitamins.