ASCI 220: Digestion and Absorption Lecture Notes

Flashcards covering the vocabulary and key concepts of digestion, absorption, enzymatic processes, and microbial fermentation in various animal species.

Digestion

The breakdown of complex food substances into constituent parts for use by the animal in maintenance processes and production.

Autoenzymatic digestion

Digestion carried out with the animal’s own set of digestive enzymes.

Alloenzymatic digestion

Digestion resulting from enzymes of microbial origin.

Prehension

The intake of food into the mouth.

Mastication

The process of chewing that subdivides food and increases surface area for enzyme attack.

Deglutition

The act of swallowing.

$\alpha$-Amylase

A salivary enzyme that converts starch or glycogen into maltose.

Lysozyme

An enzyme found in saliva that possesses antiseptic properties.

Gastric parietal cells

Cells that produce Hydrochloric acid ($HCl$) to denature proteins and activate enzymes.

Pepsinogen

An inactive proteolytic zymogen produced by gastric chief cells; it is converted to active pepsin by $HCl$ at a $pH$ of $1.6-2.4$.

Pepsin

An active enzyme that initiates protein digestion into polypeptides by targeting $TYR$, $PHE$, and $TRP$.

Rennin

A proteolytic enzyme in young mammals that coagulates milk to reduce the rate of passage.

Gastrin

A hormone released from pyloric $G$ cells that stimulates $HCl$ and pepsinogen secretion and gastric motility.

Cephalic phase

First phase of gastric secretion regulation initiated by brain impulses (anticipation of eating), accounting for $25-30\%$ of secretions.

Gastric phase

Second phase of gastric regulation initiated by ingesta in the stomach, maximizing activity and accounting for $70-75\%$ of secretions.

Intestinal phase

Third phase of gastric regulation where chyme or low $pH$ ($< 2$) in the duodenum triggers negative feedback to decrease secretion.

Secretin

A hormone from the duodenum wall that stimulates pancreatic juice production for buffering and decreases stomach motility.

Cholecystokinin (CCK)

A hormone released in response to lipid and protein in the duodenum that inhibits stomach acid and stimulates pancreatic and bile secretions.

Gastric Inhibitory Peptide (GIP)

A hormone that inhibits stomach acid, enzymes, and motility in response to lipid and protein in the duodenum.

Crypts of Lieberk\u00fchn

Intestinal crypts in the mucosal layer containing Paneth cells, Goblet cells, and enterocytes that produce specific enzymes.

Brunner\u2019s glands

Duodenal glands in the submucosa that secrete a mucus-rich alkaline secretion to protect against stomach acid.

Enterokinase

An enzyme that activates trypsinogen into trypsin.

Trypsin

An active pancreatic enzyme that targets $LYS$ and $ARG$; it also activates other zymogens like chymotrypsinogen and procarboxypeptidase.

Bile

A substance synthesized in the liver and stored in the gallbladder that emulsifies fats and neutralizes acidic chyme.

Chymotrypsin

A pancreatic enzyme that hydrolyzes peptide bonds of $TRP$, $PHE$, and $TYR$.

Carboxypeptidase

An enzyme that hydrolyzes peptide bonds at the terminal free carboxyl group.

Pancreatic amylase

An enzyme that breaks down oligosaccharides and starch into maltose.

Brush border enzymes

Enzymes located in the small intestine (e.g., dextrinase, lactase, maltase, sucrase) that complete carbohydrate digestion into monosaccharides.

Micelles

Lipid emulsions formed with bile salts that allow transport of lipid products through the intestinal lumen aqueous environment.

Paracellular pathway

An unmediated, passive absorption route through the intercellular space and tight junctions between cells.

Facilitated diffusion

Carrier-mediated passive transport that transfers a substance down its electrochemical gradient.

Active transport

The transfer of a substance against its electrochemical gradient requiring a direct or indirect source of cellular energy.

Symport

A mechanism of cotransport where two substances (such as glucose and $Na^+$) move in the same direction.

Chylomicron

Lipid globules combined with proteins inside the Golgi apparatus, extruded from epithelial cells to enter the lacteals.

Vitamin $B_{12}$

A vitamin that binds to intrinsic factor and is absorbed via endocytosis in the ileum; its microbial synthesis requires Cobalt ($Co$).

Volatile Fatty Acids (VFA)

Also known as short-chain fatty acids (SCFA); includes Acetate, Propionate, and Butyrate produced by microbial fermentation.

Escape protein

Dietary protein that bypasses ruminal degradation and reaches the lower digestive tract.

Urea-nitrogen recycling

A nitrogen-conserving mechanism where urea is recycled via saliva or blood back to the rumen or hindgut for microbial protein synthesis.

Uric acid-nitrogen recycling

A nitrogen-conserving mechanism in birds and reptiles where uric acid is refluxed from the cloaca to the ceca for microbial degradation.

Coprophagy

The ingestion of feces, practiced by some animals to recover nutrients or establish gut bacteria.

Cecotrophy

The selective ingestion of highly nutritious soft feces (cecal pellets) derived from cecal content, typically at night.

Colonic separation mechanism

A process in some herbivores that allows rapid passage of large particles while refluxing fluid and fine particles back into the ceca.