Digestion and Absorption

Digestion

The main stages of food processing are ingestion, digestion, absorption, and elimination

1. Ingestion is the act of eating
2. Digestion is the process of breaking food down into molecules small enough to absorb
3. Absorption is uptake of nutrients by body cells
4. Elimination is the passage of undigested material out of the digestive compartment

Digestive Compartments

1. Most animals process food in specialized compartments
2. These compartments reduce risk of self-digestion

Intracellular Digestion

In intracellular digestion, food particles are engulfed by endocytosis and digested within food vacuoles

Extracellular Digestion

1. Extracellular digestion is the breakdown of food particles outside of cells
2. It occurs in compartments that are continuous with the outside of the animal’s body
3. Animals with simple body plans have a gastrovascular cavity that functions in both digestion and distribution of nutrients
4. Digestion in the gastrovascular cavity of a hydra : Gland cells lining the gastrovascular cavity secrete digestive enzymes that break down the soft tissues of the prey, Other cells engulf small food particles, which are broken down in food vacuoles, Undigested materials are expelled through the mouth
5. More complex animals have a digestive tube with two openings, a mouth and an anus
6. This digestive tube is called a complete digestive tract or an alimentary canal
7. It can have specialized regions that carry out digestion and absorption in a stepwise fashion

Each organ of the mammalian digestive system has specialized food-processing functions

1. The mammalian digestive system consists of an alimentary canal and accessory glands that secrete digestive juices through ducts
2. Mammalian accessory glands are the salivary glands, the pancreas, the liver, and the gallbladder
3. Food is pushed along by peristalsis, rhythmic contractions of muscles in the wall of the canal

The Oral Cavity, Pharynx, and Esophagus

1. In the oral cavity, food is lubricated and digestion begins
2. Teeth chew food into smaller particles that are exposed to salivary amylase, initiating breakdown of glucose polymers
3. The region we call our throat is the pharynx, a junction that opens to both the esophagus and the windpipe (trachea)
4. The esophagus conducts food from the pharynx down to the stomach by peristalsis

The Stomach

1. The stomach stores food and secretes gastric juice, which converts a meal to acid chyme
2. Gastric juice is made up of hydrochloric acid and the enzyme pepsin
3. The secretory parietal cells of the stomach are responsible for producing hydrochloric acid
4. Pepsin is secreted as inactive pepsinogen; pepsin is activated when mixed with hydrochloric acid in the stomach
5. Mucus protects the stomach lining from gastric juice
6. Heartburn is usually caused by the reflux of chyme from the stomach into the lower esophagus
7. Regulation of the passage of food from the stomach is accomplished by sphincters
8. Gastric ulcers, lesions in the lining, are caused mainly by the bacterium Helicobacter pylori (spiral-shaped bacteria)

The Small Intestine

1. The small intestine is the longest section of the alimentary canal
2. It is the major organ of digestion and absorption

Enzymatic Action in the Small Intestine

1. The first portion of the small intestine is the duodenum, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and the small intestine itself
2. The pancreas produces proteases, protein-digesting enzymes that are activated after entering the duodenum
3. Pancreatic juices containing hydrolytic enzymes (eg. amylase, trypsin, chymotrypsin, lipase, nuclease) : digest nutrients into simplest molecules to be absorbed by small intestine
4. Intestinal juices containing enteropeptidase : convert trypsinogen to trypsin so that trypsin can activates other protease
5. The liver produces bile, which aids in digestion and absorption of fats (emulsify fat into smaller fat droplets )
6. The epithelial lining of the duodenum, called the brush border, produces several digestive enzymes
7. Enzymatic digestion is completed as peristalsis moves the chyme and digestive juices along the small intestine

The Large Intestine

1. The large intestine, or colon, is connected to the small intestine
2. Its major function is to recover water that has entered the alimentary canal
3. Wastes of the digestive tract, the feces, become more solid as they move through the colon
4. Feces pass through the rectum and exit via the anus
5. The colon houses strains of the bacterium Escherichia coli, some of which produce vitamins

Absorption of Nutrients

1. The small intestine has a huge surface area, due to villi and microvilli that are exposed to the intestinal lumen
2. The enormous microvillar surface greatly increases the rate of nutrient absorption
3. Each villus contains a network of blood vessels and a small lymphatic vessel called a lacteal
4. Amino acids and sugars pass through the epithelium of the small intestine and enter the bloodstream
5. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells
6. These fats are mixed with cholesterol and coated with protein, forming molecules called chylomicrons, which are transported into lacteals

Evolutionary adaptations of vertebrate digestive systems are often associated with diet

1. Digestive systems of vertebrates are variations on a common plan
2. However, there are intriguing adaptations, often related to diet

Some Dental Adaptations

1. Dentition, an animal’s assortment of teeth, is one example of structural variation reflecting diet
2. Mammals have specialized dentition that best enables them to ingest their usual diet

Stomach and Intestinal Adaptations

Herbivores generally have longer alimentary canals than carnivores, reflecting the longer time needed to digest vegetation

Symbiotic Adaptations

1. Many herbivores have fermentation chambers, where symbiotic microorganisms digest cellulose
2. The most elaborate adaptations for an herbivorous diet have evolved in the animals called ruminants