Biology 112

Chapter 15: Digestion & Nutrition

Introduction

digestion: mechanical & chemical breakdown of food & absorption of nutrients
alimentary canal: continuous passageway (8m long) created by organs of digestion

mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

accessory organs: organs that are not part of the GI tract, but participate in digestion

salivary glands, liver, gallbladder, pancreas

General Characteristics of Alimentary Canal

wall of GI tract consists of 4 layers

mucosa: innermost layer, lines lumen; stratified squamous or simple columnar epithelium with mucus-secreting glands; folded in stomach & intestine to increase surface area for absorption; lined with thin layers of connective tissue & smooth muscle
submucosa: loose CT, glands, blood vessels, lymphatics & nerves (plexuses)
muscularis externa: 2 layers of smooth muscle (inner circular & outer longitudinal layers) that produce movements of tube to propel food
serosa: visceral peritoneum; fibrous CT; secretes serous fluid to reduce friction

mixing movements by rhythmic contractions of smooth muscle to crush food & mix with enzymes
propelling movements: wavelike contraction (peristalsis) of alternate smooth muscle layers to propel food along tube

Mouth: receives food & begins mechanical & chemical digestion; includes oral cavity & vestibule (between teeth & cheeks/lips)

cheeks: outer layers of skin, pads of fat, muscles associated with expression & chewing & inner lining epithelium
lips: highly mobile with skeletal muscle, sensory receptors & rich blood supply
tongue: rough surface for gripping food & mixing with saliva

surface contains papillae with taste buds
lingual tonsils (immune cells) at root of tongue

palate: roof of oral cavity has hard palate formed from bone & mucosal soft palate

uvula at tip of soft palate closes off nasal cavity during swallowing
palatine tonsils: at either side of opening to throat at back of oral cavity

teeth: begin mechanical digestion by breaking food into smaller pieces to increase surface area for chemical digestion

20 primary & 32 secondary teeth
each tooth consists of crown (above gums) & root (below gums) & composed of enamel (calcium phosphate coating over crown), dentin (bulk of tooth), pulp, nerves & blood vessels
periodontal ligament attaches tooth to alveolar process in maxilla or mandible

Salivary Glands: secrete saliva, which moistens food & begins chemical digestion of carbohydrates, dissolves food chemicals for taste & cleanses mouth

salivary glands include serous glands that secrete digestive enzymes & mucous glands that secrete mucus
parotid glands (anterior/inferior to ear) secrete amylase, an enzyme that digests carbohydrates
submandibular gland secrete viscous saliva (serous fluid & mucus)
sublingual gland secretes mucus

Pharynx & Esophagus: food passageways to stomach

pharynx is divided into nasopharynx, oropharynx & laryngopharynx
swallowing stages: food mixed with saliva & forced into pharynx; involuntary reflexes move food into esophagus; peristalsis transports food to stomach
esophagus: passes through diaphragm & joins stomach at cardiac sphincter
cardiac sphincter: smooth muscle valve that prevents backflow of food from stomach

Stomach: receives food, mixes it with acidic gastric juice, begins chemical digestion of proteins, absorbs limited materials, & moves food into small intestine

stomach divided into cardiac, fundic, body & pyloric regions
pyloric sphincter: valve to prevent backflow between stomach & small intestine
gastric glands secrete gastric juice containing pepsin (protease; digests proteins), hydrochloric acid (HCl) and intrinsic factor (required for vitamin B12 absorption)
parasympathetic impulses & the hormone gastrin enhance gastric secretion
reflexes due to food in small intestine inhibit gastric secretion
a few substances (water, small molecules) may be absorbed from the stomach wall
mixing movements help produce chyme; peristalsis moves chyme into pyloric region & then small intestine
rate of emptying depends on fluidity of chyme & type of food present

Pancreas: produces pancreatic juice containing digestive enzymes that is moved into the duodenum of the small intestine through the pancreatic duct

pancreatic juice contains enzymes that digest carbohydrates (amylase), proteins, fats & nucleic acids
pancreatic juice has a high bicarbonate ion concentration (alkaline) that helps neutralize chyme
the hormone secretin stimulates release of pancreatic juice with bicarbonate ions
the hormone cholecystokinin (CCK) stimulate release of pancreatic juice with enzymes

Liver: metabolizes carbohydrates, lipids and proteins; stores some substances; filters blood; destroys toxins & secretes bile

left & right lobes contain hepatic lobules that secrete bile; bile is carried to hepatic ducts & flows into either cystic duct into gallbladder (storage) or common bile duct into duodenum of small intestine
in small intestine, bile salts act as an emulsifier (mix lipids & water) to help digestion & absorption of lipids (fatty acids & cholesterol) & fat-soluble vitamins
bile contains bile salts made from cholesterol, bile pigments (bilirubin), & electrolytes
gallbladder stores bile between meals; gallbladder located under right lobe of liver
the hormone cholecystokinin (CCK) stimulate release of bile
sphincter muscle at base of common bile duct controls bile flow into duodenum

Small Intestine: receives pancreatic juice from pancreas & bile from liver &/or gallbladder, completes digestion of food, absorbs nutrients, and transports residues to large intestine

small intestine consists of duodenum, jejunum & ileum
wall is lined with villi (folds) that increase surface area & aid in mixing & absorption
intestinal glands located between villi
secretes mucus & digestive enzymes (break down sugars, proteins & fats)
gastric juice, chyme & stretch reflexes stimulate secretions
enzymes on microvilli of mucosal cells complete digestion
villi absorb monosaccharides, amino acids, fatty acids & glycerol; nutrients move through mucosal cells & into capillaries
long-chain fatty acids absorbed by lacteals (lymphatic capillaries); short-chain fatty acids absorbed by blood capillaries
mixing movements & peristalsis propel material through small intestine
ileocecal sphinctercontrols movement of materials into large intestine

Large Intestine: reabsorbs water & electrolytes; forms, stores & eliminates fecal waste

large intestine consists of cecum, colon, rectum & anal canal
colon divided into ascending, transverse, descending & transverse colon
tissue of wall similar to stomach & small intestine, but longitudinal muscle layer arranged in distinct bands (teniae coli); muscle tension forms pouches (haustra)
forms mucus; little to no digestive function
movements similar to small intestine; mass movements 2-3 times daily
defecation reflex stimulates defecation to remove waste
internal anal sphincter (smooth muscle) & external anal sphincter (skeletal muscle) control removal of waste
feces consist of water, undigested material, electrolytes, mucus & bacteria
color of feces due to bilirubin pigment of bile converted to dark pigment by bacteria

Nutrition: study of nutrients & how the body utilizes them

macronutrients (carbohydrates, lipids & proteins) required in large amounts; micronutrients (vitamins & minerals) required in smaller amounts
calories measure potential energy in foods

calorie is amount of energy needed to raise temperature of 1g of water by 1˚C
kilocalories (Cal) are used to measure potential energy in macronutrients
carbohydrates & proteins yield ~ 4 Cal/gram; lipids yield ~ 9 Cal/gram

carbohydrates: used primarily to supply energy for cellular processes

carbohydrates include sugars (monosaccharides & disaccharides) & complex carbohydrates (starch, glycogen & cellulose)
starch from grains & vegetables; glycogen from meats; disaccharides from cane sugar; monosaccharides from honey & fruit
cellulose (in plant cell walls) cannot be digested by humans ® fiber; provides bulk that assists movement through GI tract
energy is released from glucose through oxidation (aerobic cellular respiration)
lipids & proteins can be utilized for energy in most cells if glucose levels low
neurons require constant supply of glucose
excess glucose is stored as glycogen or converted to fat (stored in adipose tissue)
carbohydrates required to synthesize nucleic acids & breast milk
carbohydrate requirements vary for different people depending on activity

lipids: supply energy for cellular processes & aid in building structures such as cell membrane & molecules such as steroids

lipids include fats (mostly triglycerides), phospholipids & cholesterol
saturated fats found in meats, eggs, milk & some oils; unsaturated fats found in seeds, nuts & plant oils (monounsaturated fats in olive, peanut & canola oils considered healthiest); cholesterol in egg yolk, milk, butter, cheese & meats
digestion breaks down triglycerides to fatty acids & glycerol
beta oxidation releases acetyl coA from fatty acids; acetyl coA can be used for energy or converted to ketone bodies (acetone)
excess fatty acids & glycerol stored in adipose tissue
linoleic acid, linolenic acid & arachidonic acid are essential fatty acids (required in diet)
liver synthesizes triglycerides, phospholipids, lipoproteins (transport lipids in blood) & cholesterol
cholesterol used in synthesis of bile salts, steroid hormones, membranes & vitamin D
fat intake requirements also vary for individuals; must be sufficient to carry fat-soluble vitamins (A, D, E & K)

proteins: wide variety of functions... enzymes control metabolism in cells; clotting factors clot blood; structural proteins include keratin of skin & collagen of connective tissue; plasma proteins regulate water balance; actin & myosin in muscle cells; hormones & antibodies in blood

proteins can be broken down for energy; must be converted to amino acids & amino acids deaminated (removes amino group; converted to urea waste) & converted to acetyl coA
proteins found in meats, fish, poultry, cheese, milk, nuts, eggs, cereals & legumes
8 amino acids (10 in children) are essential amino acids (required from diet)
complete proteins provide all essential amino acids; meats provide complete proteins; vegetables must be combined for complete proteins (grain & legume)
protein requirements: must have sufficient protein to supply all essential amino acids & to provide nitrogen for synthesis of nitrogen-containing compounds

vitamins: organic molecules that are essential nutrients required for metabolism

fat-soluble vitamins (vitamins A, D, E & K) are absorbed with lipids
water-soluble vitamins (vitamin C & B vitamins) absorbed with water
vitamin A can be synthesized from beta-carotene (antioxidant in orange vegetables) required for synthesis of visual pigments, normal development of bones, teeth & maintenance of epithelia
vitamin D required for calcium & phosphorus absorption during digestion
vitamin E is antioxidant (prevents oxidation of vitamin A & polyunsaturated fatty acids)
vitamin K required for blood clotting
B vitamins (B1-B12) required for cellular metabolism
vitamin C (ascorbic acid) required for collagen production, storage of folic acid, & metabolism of some amino acids; promotes iron absorption & synthesis of steroid hormones

adequate diets: provide sufficient energy & essential nutrients to support growth, maintenance & repair of tissues

food guide pyramids can help to personalize diets
malnutrition is poor nutrition due to lack of food or failure to utilize food