Digestion and Nutrition

Chapter 34

I. DigestionA. mechanical/chemical breakdown of food nutrients absorbed into blood

1. mechanicala. initial, physical breakdown of food into small piecesb. does not change chemical make-up of foodc. mouth and stomach

2. chemicala. breakdown of food into smaller chemical units (nutrients)b. changes chemical make-up of foodc. mostly small intestine

B. nutrition• refers to the properties and functions of nutrients

C. absorption • nutrients transported into bloodstream across inner wall of small intestine

D. extracellular digestion1. occurs outside of cells

• may be within or outside of organism’s bodyo if inside, then in a digestive system

2. most common (incl. humans)E. intracellular digestion

1. occurs within cells2. less common

• single-celled organisms

Intracellular digestion in Paramecium

II. Digestive SystemsA. incomplete (sac-like)

• lack separate openings for food entering and waste exitingo gastrovascular cavity

An incomplete digestive system in a gastrovascular cavity

Fig. 34.1 Incomplete digestive tract of a planarian

B. complete (tube-within-a-tube) 1. found in most animals (incl. humans)2. separate openings for food entering and waste exiting3. invertebrate examples

a. earthworm• mouth pharynx esophagus crop gizzard long intestine

(w/typhlosole) anusb. insects

• mouthparts foregut midgut (w/glands) hindgut anus

Fig. 34.2 Complete digestive tract of an earthworm

4. vertebrate examplesa. sharks and bony fishes

i. often very effective teethii. spiral valve and lengthy intestine

b. amphibians and reptiles• socketed teeth or fangs, Jacobson's organ

c. birdsi. beaks, feet, cropii. two-part stomach

• gizzard and proventriculusd. mammals

i. teeth types • temporary and permanent• incisors, canines, premolars, molars

ii. ruminants • animals with a 4-part stomach

o rumen, reticulum, omasum, abomasum• able to digest cellulose

Fig. 34.4 Dentition among mammals

Ruminant digestion

III. Human Digestive System A. oral cavity (mouth and throat)

1. taste buds, tongue, lips, teeth2. salivary glands

• secrete saliva and amylase3. pharynx

a. back of throatb. also contains nasopharynx and larynx (contains voice box)

4. swallowinga. hard palate, soft palate (with uvula), epiglottis, and glottisb. lips seal mouth tongue presses food against soft palatec. larynx raised bends epiglottis over glottis forces food down esophagus

Fig. 34.6 Swallowing

B. esophagus1. transport tube with no digestive function2. voluntary muscle at top3. mostly involuntary smooth muscle

• muscle contractions called peristalsis4. lined with mucous

C. stomach1. mostly mechanical breakdown

• stores and mixes food2. very high acidity (pH 1 or less)3. rugae mucin lining4. muscular valves at both ends

• pyloric and cardiac sphincters5. some chemical breakdown

a. pepsinb. gastric lipase

Fig. 34.7 Peristalsis in the digestive tract

Fig. 34.8 Anatomy of the stomach

D. small intestine1. three portions, all composed of smooth muscle

• duodenum jejunum ileum

2. functionsa. most chemical digestion

• secretes and receives many digestive enzymesb. all absorptionc. increased surface area for absorption

i. lengthii. rugaeiii. villi

• finger-like projections lining the rugaeiv. microvilli

• fine hairs (cellular extensions) lining the villid. sends undigested waste material to large intestine

Fig. 34.9 Anatomy of the small intestine

E. large intestine (colon, bowel)1. consists of several portions, mostly composed of smooth muscle

a. ascending colon transverse colon descending colon sigmoid colon rectum anal canal anus

b. much voluntary skeletal muscle from rectum anus• rectum contains rectal valves to hold waste in place

2. ileocecal valve• muscular valve at junction of small and large intestine

3. wider and shorter than small intestine4. caecum

• “dead end” pouch of tissue where small and large intestine meet5. appendix

a. small extension of tissue projecting off of caecumb. vestigial structure

6. functionsa. reabsorption of waterb. concentration of waste material (feces)c. provide suitable habitat for beneficial, symbiotic bacteria

Fig. 34.5 The human digestive tract

Fig. 34.10 Some structures of the large intestine

F. three accessory organs1. liver

a. secretes products into duodenumb. many functions

i. detoxifies bloodii. stores glucose as glycogeniii. secretes many enzymesiv. produces bile

• emulsification of fats2. gall bladder

• stores and secretes bile through the bile duct3. pancreas

a. secretes many digestive enzymes• sodium bicarbonate and others

b. pancreatic ducts duodenumc. produces insulin and other hormones

Fig. 34.11 The liver, gall bladder, and pancreas

IV. Chemistry of DigestionA. mechanical breakdown

• mouth (chewing) and stomachB. chemical breakdown

a. mouth, stomach, and small intestineb. hydrolysis reactions

C. absorption into the bloodstream • small intestine

D. nutrients distributed to body tissues • circulatory system

E. carbohydrate digestion (esp., starches and other polysaccharides)• salivary amylase pancreatic amylase disaccharides maltase

monosaccharides (glucose)F. lipid (fat) digestion

• gastric lipase emulsification (bile) pancreatic lipase fatty acids/glycerol reform as triglycerides join w/cholesterol chylomicrons

G. protein digestiona. proteasesb. pepsin in stomach shorter peptides pancreatic proteases (trypsin)

even smaller peptides peptidases individual amino acids H. nucleic acid digestion

a. nucleases and nucleosidasesb. individual nucleotides absorbed into bloodstream

Fig. 34.12 Digestion and absorption of nutrients