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Digestion Overview of Digestion Assimilation – processes of nutrient acquisition, digestion, and absorption Takes place along the gastrointestinal (GI) tract Contiguous with the external environment Composed of many cell types Secretory cells Absorptive cells Muscle cells Neurons Egestion – Excretion of undigested food Diets Provide Energy Energy content of diet must match the metabolic demands of the animal Caloric content – energy content of a marcomolecule, e.g., protein and carbohydrates – 4 kcal, fat – 9 kcal Not all food is digestible Energy is lost as feces Specific Dynamic Action (SDA) or heat increment – increase in metabolic rate during the digestive process; important source of thermal energy Figure 11.2 Nutrients are the external molecules that allow an animal to maintain and build cells. Vitamins and Minerals Group of unrelated molecules with diverse functions Many participate in catalysis as cofactors for enzymes Usually categorized based on solubility Fat-soluble – vitamins A, D, E, K Water soluble – vitamins B, C Obtained in diet or from bacteria living in the GI tract Metallic elements that participate in protein structure Calcium, Phosphorus, Iron, Copper, Zinc Most are absorbed along the GI tract by specific transporters Table 11.1 Vitamins Amino Acids Animals use 20 amino acids to build proteins Most can be produced by the animal Eight essential amino acids must be obtained in the diet Diets deficient in any essential amino acid lead to developmental defects and slow growth Protein quality The amino acid profile of dietary protein Animal tissue provides higher protein quality than plant tissue Some plants lack specific amino acids Fatty Acids Animals can make almost all fatty acids from acetyl CoA Animals cannot produce sufficient amounts of omega-3 or omega-6 fatty acids Omega-3 fatty acids must be ingested as linolenic acid Found in plant seeds Omega-6 fatty acids must be ingested as linoleic Found in cold-water fish Digestive Enzymes Enzymes convert macromolecules to forms that can be absorbed and processed Lipases Break down triglycerides and phospholipids into fatty acids Proteases Break down proteins into shorter polypeptides Amylases Break down polysaccharides into oligosaccharides Nucleases Break down DNA into nucleotides Most digestion takes place extracellularly For example, in the lumen of the GI tract Symbiotic Organisms and Digestion Digestion in many animals benefits from the assistance of symbiotic organisms For example, bacteria, fungi, and photosynthetic organisms Three main types of symbionts participate in digestion Enterosymbionts Live within the lumen of the GI tract Often in an enlarged region called the cecum Exosymbionts Actively cultivated outside the body Endosymbionts Grow in interstitial spaces or within host cells Nutrient Transport Across Plasma Membranes Some nutrients are transported by protein carriers Polar molecules require specific protein transporters Transport down a concentration gradient Facilitated diffusion Transport against a concentration gradient Active transport Via Na+-dependent cotransporters Some nutrients are transported in vesicles Uptake Pinocytosis or Phagocytosis Expulsion Exocytosis Carbohydrate Breakdown and Absorption Main types consumed by animals Polysaccharides Glycogen, starch, cellulose, chitin Disaccharides (sucrose, lactose, maltose) Poly- and disaccharides are hydrolyzed to monosaccharides For example, glucose, fructose, galactose Monosaccharides are absorbed by epithelial cells in the intestine (enterocytes) Active transport and facilitated diffusion Figure 11.4 Carbohydrate Transport Combination of active transport and facilitated diffusion Intestinal absorptive cells (enterocytes) Proteins Extracellular hydrolysis, starting in the stomach, to dipeptides and amino acids Amino acids are absorbed into epithelial cells by amino acid-Na+ cotransporters Some proteins are also carried into cells intact via endocytosis/exocytosis i.e. Infant mammalian antibodies Transferring immunoprotection Lipids Digestion and import of lipids is complicated by their hydrophobicity GI tract secretes bile that emulsifies lipids into small droplets (micelles) Dietary fats are broken down into fatty acids and monoglycerides Lipids diffuse across cell membrane into enterocyte Transport of lipids depend on physical properties Short chain fatty acids and glycerol in blood Triglycerides in the lymph as chylomicrons Table 11.2 Chylomicrons and Lipoproteins in the Blood Sensing, Finding, Consuming Food Basic dietary strategies Carnivory Herbivory Omnivory Physiology of digestion is matched to the chemical and physical nature of the diet Animals detect food with different sensory receptors Gustatory and olfactory receptors detect chemical stimuli Various receptors detect energy emitted by, or reflected from, the food source For example, light, sound, heat, or electricity Figure 11.8 Feeding in Simple Animals: Sponges Simple animals (e.g., sponges) ingest food by phagocytosis Digestion occurs intracellularly in endocytic vacuoles Feeding Structures Most animals have specialized mouthparts for obtaining and processing food Siphons Attachment organs Found in many parasitic organisms Tongues Radula – rasping tongue Proboscis – long tubelike tongue Beaks Jaws Bird Beaks Are composed of bone covered by overlapping epidermal scales Beak morphology is diverse and reflects various feeding strategies Evolutionary adaptation Serve purposes other than feeding Vocalization Defense Grooming Courtship Figure 11.12a Teeth Many vertebrates have toothlike structures Mammals have structurally unique bony teeth Three main parts: crown, neck, root Three main layers: enamel, dentin, pulp Four types of teeth: incisors, canines, premolars, molars The shape of the teeth reflects the type of diet Digestive Systems Evolutionary history shows increasing anatomical and functional specialization Two-way gut Simple internal sac Sac may have diverticula to increase surface area Food enters and wastes leave via the same opening Digestive Systems One-way gut (gastrointestinal tract) Specialized regions Mouth, pharynx, esophagus Mechanical breakdown of food Stomach Acidic compartment Upper or small intestines Digestion and absorption Lower or large intestines Absorption of water Anus Release of indigestible material Figure 11.14 Gastrointestinal Tract The Coelom Internal cavity between layers of mesoderm Linked to evolutionary and developmental origins of the one-way gut Acoelomates - No coelom Pseudocoelomates - Cavity between endoderm and mesoderm Coelomates - Cavity within mesoderm Figure 11.16 Coelom Formation Blastopore appears early in development, during gastrulation Protostomes Most invertebrates Blastopore becomes mouth Coelom forms when mesoderm splits (schizocoelous) Deuterostomes Chordates, hemichordates, echinoderms Blastopore becomes anus Coelom forms when mesoderm pinches off from the gut (enterocoelous) Schizocoelous process in chordates Gut Formation Gut is derived from endoderm Three regions Foregut Esophagus, stomach, and the anterior section of the duodenum Forms buds that become the pancreas and liver Midgut Posterior part of duodenum, jejunum, ileum, and large intestine Hindgut Colon and rectum Surface Area In most animals, nutrients are hydrolyzed in the lumen of the GI tract Nutrients are taken up by cells lining the gut Nutrient uptake is improved by increasing surface area in two ways Increasing gut length Increasing surface undulations Circular folds, Villi, and Microvilli Specialized Compartments Specialized compartments increase efficiency of digestion Compartments have functional specializations pH Enzymes Types of secretory and absorptive cells Muscular valves (sphincters) control passage of food from one compartment to the next Complexity of gut morphology varies across taxa Reflects complexity of the diet and ease of digestion Figure 11.20 Ruminants Some mammals possess modifications that improve the digestion of plant material (e.g., ruminants) Fermentative Bacteria Salivary Glands Multicellular exocrine glands Ducts open into mouth Saliva Lubricates food Dissolves food so nutrients can bind to gustatory receptors Cleanses the mouth with antimicrobial properties Contain enzymes that initiate digestion Salivation is controlled by nerve signals Parasympathetic nerves stimulate salivation Sympathetic nerves inhibit salivation Figure 11.22 Stomach Surface is composed of columnar epithelial cells Tight junctions Prevent leakage across epithelium Mucous neck cells Secrete mucus Parietal cells Secrete hydrochloric acid (HCl) Chief cells Secrete the protease pepsin Enteroendrocrine cells Secrete hormones into the blood Intestines Most nutrients are absorbed in the intestines Four main layers Mucosa, Submucosa (Blood and lymphatic vessels, nerves), Circular smooth muscle, Longitudinal smooth muscle Mucosal cells Enterocytes Absorptive cells with microvilli Goblet cells Secrete mucus Enteroendocrine cells Secrete hormones Paneth cells Secrete antimicrobial molecules Figure 11.24 Exocrine Secretions into the Intestine Bile Solution of digestive chemicals and liver waste products Produced in the liver and stored in the gallbladder Phospholipids Aid in the uptake of lipids Bile salts Emulsify fats Bile duct opens into small intestine Exocrine Secretions into the Intestine Enzymes secreted by the pancreas Proteases break down protein Secreted as inactive proenzymes Activated in the intestine Amylase breaks down glycogen and starch Lipases break down triglycerides Nucleases break down nucleic acids Exocrine Secretions of the Pancreas Regulating Feeding and Digestion Digestive functions are controlled by nerve and hormone signals Nerve signals From central nervous system (CNS) Within the gastrointestinal tract Hormone signals Wide variety of endocrine tissues and hormones Control of Appetite Three hormones control appetite by binding to receptors in the hypothalamus Leptin Secreted by white adipose tissue when lipid content is high Suppresses appetite Ghrelin Secreted by stomach when empty Stimulates appetite Peptide YY Secreted by colon when full Suppresses appetite Control of Appetite Hypothalamic neurons release neurotransmitters in response to hormones from the gut Some neurotransmitters stimulate appetite Neuropeptide Y (NPY) Agouti-related peptide Gamma amino butyric acid (GABA) Some neurotransmitters inhibit appetite Proopiomelanocortin (POMC) Figure 11.26 Hormonal Control of Appetite Control of Secretions Secretions of saliva, acid, mucus, bile, bicarbonate, digestive enzymes Respond to both the anticipation of food and its presence in the digestive system Nerve and hormonal signals Figure 11.28 Control of Intestinal Secretions Control of Gut Motility Food moves along the GI tract by contractions of smooth muscle Controlled by nerves and hormones Optimal speed Fast enough to minimize amount of indigestible material in the GI tract Slow enough to allow time for digestion and assimilation Rate will vary according to diet Control of Gut Motility Smooth muscle layers Longitudinal – controls length Circular – controls diameter Between muscle layers is a nerve network Myenteric plexus Receives signals from CNS Peristalsis Contractile waves that move food down the GI tract Controlled by intrinsic myogenic activity and interstitial cells of Cajal that act as pacemaker cells Resting muscle tone Controls lumen diameter and is regulated by intrinsic pathways (myogenic) and neurotransmitter release Figure 11.30 Control of Smooth Muscle Motility Metabolic Transitions Between Meals Postprandial period Period immediately after feeding Duration can vary from seconds to months Nutrients are absorbed into the blood Some nutrients are utilized and others are stored Hormones control postprandial levels of nutrients Insulin from ?-cells of the pancreas Stimulates glucose uptake and storage Promotes conversion of glucose to fat Glucagon from ?-cells of the pancreas Stimulates glucose release by liver Glucocorticoids (e.g., cortisol) from adrenal cortex Stimulates gluconeogenesis by the liver Mobilize triglycerides from adipose tissue Starvation Response Reorganization of metabolism to ensure long-term survival Conserve glucose to protect glucose-dependent tissues, for example, nervous tissue Muscles shift to lipid metabolism After lipid and glucose stores are depleted protein breakdown accelerates Amino acids are converted to fatty acids and carbohydrates Structural degradation occurs because there is no protein store in the body Degradation of skeletal muscle Figure 11.32 Starvation Response Digestive System Degradation and Rebuilding Reduce energetic costs in between meals Pythons that eat infrequently Mucosa and submucosa degrade Gut becomes thinner and the brush border decreases Smooth muscles and nerves are maintained After a meal, the snake rebuilds the GI tract just ahead of the bolus of food High SDA; ? body temperature Figure 11.33 Dormancy Hypometabolism Reduction in metabolic rate Allows the animal to survive adverse environmental conditions Types of hypometabolism Torpor Hibernation Estivation Most dormant mammals accumulate urea and urine because of protein breakdown Bears recycle the urea nitrogen to regenerate amino acids and proteins
