Lecture Notes - Chapter 51
Homework - Review questions
I. The Nature of Digestion
Autotrophs - Plants and other organisms which contain photosynthetic machinery
to create their own food, and are self-sustaining.
Heterotrophs – organisms that must consume the materials they need for survival.
Digestion – the degradation of complex organic molecules into simpler
compounds (through hydrolysis reactions) for use by the body.
II. Vertebrate Digestive Systems
The digestive system consists of a tubular gastrointestinal tract and accessory
digestive organs. The tubular gastrointestinal tract has a characteristic layered
structure.
- Mucosa – innermost layer - an epithelial layer that separates the lumen
from the structures in the wall of the tract.
- Submucosa - connective tissue layer that contains blood vessels and
nerves.
- Muscularis - consists of two layers of smooth muscle oriented in different
directions. The first layer is circular, the outer layer is longitudinal.
- Serosa (or adventitia) - outermost layer - a connective tissue covering
III. Food Enters the Digestive Tract Through the Mouth
A. Mechanical Digestion – teeth and tongue.
- Teeth - specialized for cutting, tearing and grinding.
 Incisors (8 total) - used for biting.
 Cuspids/canines (4 total) - used in tearing food.
 Premolars (8 total) and molars (12 total) - all suited for grinding and
crushing food.
 In childhood, there are 20 teeth (deciduous) which are all eventually
replaced with 32 permanent teeth.
-
Tongue – moves food around to mix it with saliva; mashes softer foods.
B. Chemical Digestion –
- Saliva - a mucous secretion produced by three pairs of salivary glands
(exocrine). Contains salivary amylase.
IV. Esophagus and Stomach
A. Esophagus –
Oropharynx – esophagus – stomach.
Food is prevented from entering the trachea by the presence of the epiglottis.
Upper 1/3 of esophagus has skeletal muscle in muscularis; lower 2/3 has smooth
muscle.
Entry of food into stomach is controlled by the esophageal sphincter – prevent
movement of food from stomach back into esophagus.
B. Preliminary Digestion Occurs in the Stomach –
1) Mechanical Digestion –
- Smooth muscle of the stomach functions to churn the food and mix it with
the secretions of the gastric glands located in the mucosa.
2) Chemical Digestion- Stomach contains gastric glands (exocrine) - contain two types of secretory
cells - parietal cells and chief cells. T
 Parietal cells secrete HCl and intrinsic factor.

Chief cells secrete pepsinogen, a weak protein-digesting enzyme.
3) Absorption of Materials Minimal in the stomach.
VI. The Small Intestine
Pyloric sphincter – controls entry of food (chyme) from stomach into duodenum.
Chyme must enter slowly to allow maximum digestion and absorption to occur.
The small intestine consists of three parts – duodenum, jejunum and ileum.
A. Chemical Digestion of Food in the Intestine –
- Brush border enzymes -
-
Pancreatic enzymes –
B. Absorption of Food in the Intestine
- amino acid and monosaccharide subunits –
-
triglycerides (broken down into fatty acids and monoglycerides) –
VII. Accessory Organs
A. The Liver and Gall Bladder –
- Acts as an exocrine gland, secreting bile (bile pigments and bile salts) into
the duodenum.
- Bile is constantly being produced, and it is stored in the gallbladder until
fatty food arrives in the duodenum, triggering its release.
B. The Pancreas –
- Exocrine role – produce pancreatic enzymes and bicarbonate for use in the
duodenum
-
Endocrine role - from clusters of cells called islets of Langerhans - secrete
several hormones into the bloodstream, the most important of which are
insulin and glucagon.
IX. The Large Intestine Concentrates Solids
- No digestion
- Minimal absorption of fluids
- Shorter than the small intestine, the colon performs no digestion and only
minimal absorption of fluids.
- Compacts and stores undigested materials until expelled from the body feces.
- Consists of five parts – ascending colon, transverse colon, descending
colon, sigmoid colon, rectum.
X. Hormonal Regulation of Digestion
1) Gastrin –
2) Gastric inhibitory peptide –
3) Cholecystokinin –
4) Secretin -
XI. Nutrition
- Proteins – provide amino acids used to make new proteins and nucleic acids.
- Carbohydrates – most common source of fuel – used for production of ATP.
- Fats – provide lipids for renewal of cell membranes and production of hormones.
A. Essential Nutrients - substances that an organism cannot manufacture for itself
but are necessary for its health and must be obtained from the diet.
1) Essential vitamins - there are at least 13 vitamins needed by humans,
all of which are available in a well-balanced diet.
2) Essential amino acids - there are 8-10 amino acids required by humans
for the production of proteins which are not made within the body.
These must be obtained from the proteins in food.
3) Essential minerals - includes several trace elements and inorganic
substances. These are usually required in very small amounts, and are
obtained from plants or from animals that have eaten plants.