Download TOPIC - Digestion I. Introduction A. Breakdown of foods into small

TOPIC - Digestion
I. Introduction
A. Breakdown of foods into small molecules to deliver to cells
B. Feeding mechanisms (herbivores, carnivores, omnivores)
C. Types:
1. hydrolysis (enzymatic addition of water)
2. food vacuoles (intracellular digestion)
3. gastrovascular cavities (digestive sacs with a single opening)
4. alimentary canals (digestive tube with two openings)
D. Mechanisms
1. peristalsis (smooth muscle contractions)
2. sphincters (occur at junctions; regulate passage of food)
II. Vertebrates
A. Introduction
1. Parts
a. "Alimentary canal " (digestive tract-->mouth to anus)
b. Accessory organs -secrete enzymes
1. Salivary glands, pancreas, liver, gall bladder
2. 4 layers
a. Mucosa (innermost layer)
1. epithelial tissue, basement membrane, connective tissue, smooth
muscle
b. Submucosa
1. connective tissue, nerve fibers
c. Muscle layer (2 layers of smooth; circular & longitudinal)
1. peristalsis
B. Mouth (oral cavity)
1. Mechanical breakdown by teeth
2. Chemical breakdown by enzymes in salivary glands
a. Amylase (breakdown of starch by hydrolysis)
C. Pharynx and Esophagus
1. Epiglottis in pharynx blocks food (bolus) from entering trachea
2. Tube to stomach (25cm long) by peristalsis
3. Striated in the top portion and smooth in the bottom portion (voluntary to
involuntary)
D. Stomach
1. Food storage and liquification
2. Muscular sac (2L of food) below diaphragm
3. Mucosal layer secretes gastric juice:
a. mucus (protects lining)
b. HCl (pH of 2)
1. kills bacteria, loosens fibrous components, converts pepsinogen to
pepsin
c. pepsinogen
1. at low pH, converted to pepsin which breaks down proteins into
peptides
d. water
e. rennin (milk digestion)
4. Gastrin hormone secreted (stimulates gastric juices and muscle contraction of
stomach)
5. Food (chyme) moves through pyloric sphincter to small intestine
E. Small Intestine
1. Completes digestion (6m long)
2. Contains villi and microvilli that absorb nutrients into circulatory system (high
surface area)
3. Parts: Duodenum (digestion: 1st 25 cm; jejunum & ileum: reabsorption of water)
4. Substances:
a. mucus from mucosa
b. digestive enzymes produced by mucosa and pancreas (trypsin, lipase,
amylase)
c. alkaline fluids to neutralize stomach acid
d. bile (produced by liver, stored in gall bladder) fat-emulsifying substance
e. digestive enzymes:
1. amylases hydrolyze starch (maltase, sucrase, lactase)
2. lipases hydrolyze fat
3. peptidase hydrolyze protein
5. Hormonal action:
a. secretin: stimulates pancreas and liver to release alkaline fluids (duodenum)
b. cholecystokinin (CCK): signals the presence of fat in duodenum; triggers
release of bile
6. Absorption:
a. Directly into bloodstream1. water and nutrient molecules
2. disaccharides cleaved to monosaccharides; absorbed by active
transport
3. amino acids and dipeptides by active transport
b. Indirectly into bloodstream:
1. fatty acids and glycerol, resynthesized into fats, secreted into lymph
vessels, then to bloodstream (lacteals)
2. cholesterol, packaged into lipoprotein complexes (LDL & HDL),
secreted into lymph, then to bloodstream
F. Large intestine (colon-1.5m long))
1. Absorbs water, sodium, and other minerals
2. Symbiotic bacteria (E. coli) to break down food and synthesize vitamin K
G. Vitamin D
1. synthesized in skin by the action of UV light on cholesterol
III. Regulation of Blood Glucose
A. glucose passes from intestine to liver
B. liver converts some glucose to glycogen and fat
1. glycogen stored in liver
2. fat stored in fat cells
C. liver converts excess amino acids to glucose, then glycogen, releases nitrogen as urea
D. uptake and release of glucose by liver depends on blood glucose levels (insulin-->lowers
and glucagon->increases levels; from the pancreas)
Quick Notes:
Digestion: breakdown of foods into molecules that can be delivered to and used by the individuals
cells, involves breakdown of food and absorption into body, provides organic molecules that
can be used as energy and raw materials for each cell of the body
Digestive system: includes salivary glands, pancreas, liver, and gallbladder
Digestive Tract (gut)
•Long complex tube from mouth to anus
•Inner surface is continuous with the outer surface
•Mouth ( pharynx ( esophagus ( stomach ( small intestines ( large intestines ( anus
Four layers: mucosa, submucosa, muscle layer, serosa
Mucosa: inner layer made of epithelial tissue, basement membrane, and connective tissue
Submucosa: second layer made of connective tissue, nerve fibers, and blood and lymph vessels
Muscle layer: third layer made of two layers of smooth muscle
Serosa: outer layer of connective tissue
Peristalsis: wavelike contractions that moves food down digestive tract
Sphincters: heavy bands where the layer of muscle thickens, act as valves to control the passage
of food in digestive tract
The Mouth
•Starts the mechanical breakdown of food
•Includes teeth, gizzards (birds), tongues, sticky tongues (frogs), scrapers
•Secretion of saliva controlled by autonomic nervous system
•Humans produce 1 - 1.5 liters of saliva every 24 hours
Saliva: watery secretion that lubricates food so it can be swallowed easily, contains amylase
Salivary glands: secretes saliva
Amylase: digestive enzyme in saliva that breaks down starches, works by hydrolysis (addition of
water)
The Esophagus
•25 centimeter muscular tube
•Upper part striated, lower part smooth
•Allows for swallowing (passing of food through esophagus to stomach)
The Stomach
•Collapsible, muscular bag that lies in folds
•Human stomach holds 2 - 4 liters of food
•Mucosal layer is thick and has lots of pits
•Epithelial cells secrete mucus, HCl, and pepsinogen
•HCl kills bacteria, loosens fibers, and converts pepsinogen to pepsin
•Stomach is empty 4 hours after meal
Gastric juice: secretions of mucus, HCl, pepsinogen, and water - has pH of 1.5 - 2.5
Pepsin: enzyme that breaks proteins into peptides
Gastrin: hormone that increases secretion of gastric juices in epithelial cells
Pyloric sphincter: separates the stomach and small intestines
Small Intestines
•Completes the breakdown of food
•Nutrient molecules absorbed from digestive tract into circulatory system
•Fully extended small intestine of adult is 6 meters long, total surface area = 300 square
meters
Mucus secreted by cells of intestinal mucosa
•Digestive enzymes produced by epithelial cells of intestinal mucosa and pancreas
•Alkaline fluid from the pancreas - neutralizes stomach acid
•Amylases breakdown starches, producing disaccharides
Three types of enzymes break down proteins: breaks bond linking particular amino acids, splits of
dipeptides at ends of a chain, breaks dipeptides into single amino acids
•Digestive activities regulate by hormones and autonomic nervous system
Villi: microscopic fingerlike projections of the mucosa that increase surface area
Microvilli: tiny cytoplasmic projections that increase surface area
Duodenum: upper 25 centimeters of the small intestine, most active indigestive process
Bile: mixture of salts that emulsify fat, produced in liver
Lipases: hydrolyze fats into glycerol and fatty acids
Secretin: hormone that stimulate the pancreas and liver to secrete alkaline fluids
Cholecystokinin: hormone that triggers release of enzymes from pancreas and bile from
gallbladder
Absorption of Nutrients
•Water and nutrient molecules absorbed through epithelial cells of the intestinal mucosa
•Disaccharides are made into monosaccharides by enzymes in epithelial plasma membrane
and are absorbed by active transport and facilitated diffusion
•Amino acids and dipeptides absorbed by active transport and enter blood by capillaries of
the villi
•Large fatty acids, glycerol, and cholesterol enter epithelial cells by passive diffusion
•Fatty acids and glycerol are made into fats and packaged into protein-coated droplets
•Protein-coated droplets and LDLs secreted into lymph vessels and enter bloodstream
Large Intestine
•Continuation of absorption of water, sodium, and other minerals
•7 liters of water enter stomach and small intestines
•Lots of bacteria in large intestines (e. Coli) - bacteria breaks down food and chief source of
vitamin K
•Fecal matter consists of water, bacteria, cellulose fibers, and mucus - stored in rectum and
eliminated through anus as feces
Regulation of Blood Glucose
•Vertebrate have relatively constant blood glucose
•Liver helps to keep things constant by converting monosaccharides to glycogen and fat
(storage stuff for body) and breaking down excess amino acids and converting them
to glycogen
•Amount of glycogen that liver releases based on concentration of glucose in blood
Nutritional Requirements
•Energy requirements met by carbohydrates, proteins, and fats
•Carbohydrates and proteins supply same number of calories per gram, fats supply twice as
much
•Body cells need 20 different amino acids to make proteins - humans only make 12, other 8
must be gotten from diet
•Vitamins cannot be synthesized by animal cells and function as coenzymes
•Body needs calcium and phosphorus for bone formation, iodine for thyroid hormone, iron
for hemoglobin