Digestion and Nutrition Chapter 34

Digestion mechanical/chemical breakdown of food  nutrients  absorbed into blood mechanical initial, physical breakdown of food into small pieces does not change chemical make-up of food mouth and stomach chemical breakdown of food into smaller chemical units (nutrients) changes chemical make-up of food mostly small intestine nutrition refers to the properties and functions of nutrients absorption nutrients transported into bloodstream across inner wall of small intestine extracellular digestion occurs outside of cells may be within or outside of organism’s body if inside, then in a digestive system most common (incl. humans) intracellular digestion occurs within cells less common single-celled organisms

Intracellular digestion in Paramecium

Digestive Systems incomplete (sac-like) lack separate openings for food entering and waste exiting gastrovascular cavity An incomplete digestive system in a gastrovascular cavity

Fig. 34.1 Incomplete digestive tract of a planarian

Fig. 34.2 Complete digestive tract of an earthworm complete (tube-within-a-tube) found in most animals (incl. humans) separate openings for food entering and waste exiting invertebrate examples earthworm mouth  pharynx  esophagus  crop  gizzard  long intestine (w/typhlosole)  anus insects mouthparts  foregut  midgut (w/glands)  hindgut  anus Fig. 34.2 Complete digestive tract of an earthworm

Dentition among mammals vertebrate examples sharks and bony fishes often very effective teeth spiral valve and lengthy intestine amphibians and reptiles socketed teeth or fangs, Jacobson's organ birds beaks, feet, crop two-part stomach gizzard and proventriculus mammals teeth types temporary and permanent incisors, canines, premolars, molars ruminants animals with a 4-part stomach rumen, reticulum, omasum, abomasum able to digest cellulose Fig. 34.4 Dentition among mammals

Ruminant digestion

Fig. 34.6 Swallowing Human Digestive System oral cavity (mouth and throat) taste buds, tongue, lips, teeth salivary glands secrete saliva and amylase pharynx back of throat also contains nasopharynx and larynx (contains voice box) swallowing hard palate, soft palate (with uvula), epiglottis, and glottis lips seal mouth  tongue presses food against soft palate larynx raised  bends epiglottis over glottis  forces food down esophagus Fig. 34.6 Swallowing

esophagus stomach transport tube with no digestive function voluntary muscle at top mostly involuntary smooth muscle muscle contractions called peristalsis lined with mucous stomach mostly mechanical breakdown stores and mixes food very high acidity (pH 1 or less) rugae  mucin lining muscular valves at both ends pyloric and cardiac sphincters some chemical breakdown pepsin gastric lipase Fig. 34.7 Peristalsis in the digestive tract

Fig. 34.8 Anatomy of the stomach

small intestine functions three portions, all composed of smooth muscle duodenum  jejunum  ileum functions most chemical digestion secretes and receives many digestive enzymes all absorption increased surface area for absorption length rugae villi finger-like projections lining the rugae microvilli fine hairs (cellular extensions) lining the villi sends undigested waste material to large intestine

Fig. 34.9 Anatomy of the small intestine

large intestine (colon, bowel) consists of several portions, mostly composed of smooth muscle ascending colon  transverse colon  descending colon  sigmoid colon  rectum  anal canal  anus much voluntary skeletal muscle from rectum  anus rectum contains rectal valves to hold waste in place ileocecal valve muscular valve at junction of small and large intestine wider and shorter than small intestine caecum “dead end” pouch of tissue where small and large intestine meet appendix small extension of tissue projecting off of caecum vestigial structure functions reabsorption of water concentration of waste material (feces) provide suitable habitat for beneficial, symbiotic bacteria

Fig. 34.5 The human digestive tract

Fig. 34.10 Some structures of the large intestine

three accessory organs liver secretes products into duodenum many functions detoxifies blood stores glucose as glycogen secretes many enzymes produces bile emulsification of fats gall bladder stores and secretes bile through the bile duct pancreas secretes many digestive enzymes sodium bicarbonate and others pancreatic ducts  duodenum produces insulin and other hormones

Fig. 34.11 The liver, gall bladder, and pancreas

Chemistry of Digestion mechanical breakdown mouth (chewing) and stomach chemical breakdown mouth, stomach, and small intestine hydrolysis reactions absorption into the bloodstream small intestine nutrients distributed to body tissues circulatory system carbohydrate digestion (esp., starches and other polysaccharides) salivary amylase  pancreatic amylase  disaccharides  maltase  monosaccharides (glucose) lipid (fat) digestion gastric lipase  emulsification (bile)  pancreatic lipase  fatty acids/glycerol  reform as triglycerides  join w/cholesterol  chylomicrons protein digestion proteases pepsin in stomach  shorter peptides  pancreatic proteases (trypsin)  even smaller peptides  peptidases  individual amino acids nucleic acid digestion nucleases and nucleosidases individual nucleotides absorbed into bloodstream

Fig. 34.12 Digestion and absorption of nutrients