LEC 1-2

Vocabulary flashcards covering GI tract anatomy, cell types, gastric acid regulation, digestive enzymes, and carbohydrate chemistry.

Anomeric Carbon

The carbonyl carbon atom in a carbohydrate molecule that becomes a new chiral center upon ring closure, forming alpha or beta anomers.

$\alpha$-Anomer

A cyclic sugar conformation in which the hydroxyl group ($-\text{OH}$) attached to the anomeric carbon points down, trans to the reference carbon's terminal group.

$\beta$-Anomer

A cyclic sugar conformation in which the hydroxyl group ($-\text{OH}$) attached to the anomeric carbon points up, cis to the reference carbon's terminal group.

Sucrose

A digestible disaccharide composed of glucose and fructose joined by an $\alpha 1 \leftrightarrow \beta 2$ glycosidic bond.

Lactose

A disaccharide found in milk and dairy products composed of galactose and glucose joined by a $\beta(1\rightarrow 4)$ glycosidic bond.

Maltose

A disaccharide produced during starch breakdown composed of two glucose units joined by an $\alpha(1\rightarrow 4)$ glycosidic bond.

Amylose

A linear plant starch polymer composed of approximately $100$ glucose units joined strictly by $\alpha(1\rightarrow 4)$ glycosidic bonds.

Amylopectin

A branched plant starch polymer composed of approximately $1000$ glucose units with $\alpha(1\rightarrow 4)$ linear linkages and $\alpha(1\rightarrow 6)$ branch points every $20\,\text{to}\,25$ glucose units.

Glycogen

The primary animal storage polysaccharide composed of glucose molecules linked by $\alpha(1\rightarrow 4)$ glycosidic bonds with $\alpha(1\rightarrow 6)$ branch points every $10\,\text{to}\,12$ glucose units.

Resistant Starch

Starch that escapes enzymatic digestion in the small intestine due to physical protection or retrogradation and reaches the large intestine for bacterial fermentation.

Retrogradation

The process wherein cooked, gelatinized starch is cooled, causing linear amylose molecules to re-crystallize and pack tightly via hydrogen bonding, making the starch resistant to digestion.

Maillard Reaction

A non-enzymatic reaction between aldose sugars and amines that produces a Schiff base intermediate, leading to protein cross-linking, browning pigments, and complex flavors.

Glycation

The non-enzymatic covalent attachment of a sugar molecule to a protein, elevated levels of which occur in diabetes and are measured via glycated hemoglobin ($\text{HbA}_{1c}$).

Glycosylation

The enzyme-catalyzed addition of a sugar molecule to a protein or lipid.

Salivary Amylase

A soluble endoglucosidase secreted by salivary glands that initiates starch digestion in the mouth by cleaving internal $\alpha(1\rightarrow 4)$ glycosidic bonds.

Pancreatic Amylase

The principal enzyme responsible for luminal starch breakdown in the small intestine, hydrolyzing internal $\alpha(1\rightarrow 4)$ bonds to yield maltose, maltotriose, and $\alpha$-limit dextrins.

Isomaltase ($\alpha$-Dextrinase)

A brush border enzyme located on the apical membrane of jejunal enterocytes that hydrolyzes $\alpha(1\rightarrow 6)$ glycosidic bonds at the branch points of $\alpha$-limit dextrins.

Parietal Cells

Epithelial cells in gastric glands that secrete hydrochloric acid ($\text{HCl}$) via apical $\text{H}^+/\text{K}^+\text{ ATPase}$ pumps and produce intrinsic factor required for vitamin $\text{B}_{12}$ absorption.

Chief Cells

Gastric mucosa cells that synthesize and secrete pepsinogen (an inactive zymogen activated by $\text{HCl}$ to pepsin) and gastric lipase.

Enterochromaffin-like (ECL) Cells

Gastric endocrine cells that produce histamine in response to gastrin to stimulate $\text{H}_2$ receptors on parietal cells, increasing intracellular $cAMP$ and acid secretion.

G-Cells

Gastric enteroendocrine cells that release the hormone gastrin into the bloodstream to stimulate parietal cell $\text{HCl}$ production and ECL cell histamine synthesis.

D-Cells

Endocrine cells in the stomach, duodenum, and pancreas that release somatostatin when gastric pH drops, inhibiting gastrin release from G-cells and $\text{HCl}$ secretion from parietal cells.

Ghrelin

A peptide hormone secreted by P/D1-like (X/A-like) cells in the stomach fundus during fasting that acts on the hypothalamus to increase hunger and stimulate gastric motility.

Proton Pump Inhibitors (PPIs)

A class of acid-reducing medications (e.g., omeprazole) that irreversibly block the $\text{H}^+/\text{K}^+\text{ ATPase}$ enzyme on the parietal cell apical membrane.

$\text{H}_2$ Blockers

Medications (e.g., cimetidine, ranitidine, famotidine) that competitively inhibit histamine binding at $\text{H}_2$ receptors on parietal cells to reduce gastric acid production.

Cephalic Phase

The anticipatory phase of digestion initiated by the sight, smell, taste, or thought of food, sending parasympathetic signals via cranial nerves VII and IX to stimulate salivary secretion.

Parotid Glands

Pair of salivary glands producing approximately $25\%$ of total saliva, secreting a watery, serous fluid rich in salivary amylase via cranial nerve IX stimulation.

Submandibular Glands

Salivary glands responsible for producing the majority ($60\,\text{to}\,70\%$) of saliva, providing a mixture of serous and mucous secretions via cranial nerve VII.

Sublingual Glands

Salivary glands producing about $5\%$ of total saliva, secreting a mucous-rich, viscous fluid that lubricates food boluses and oral mucosa.

Duodenum

The C-shaped first $25\,\text{cm}$ section of the small intestine that neutralizes acidic chyme using Brunner's gland secretions and receives bile and pancreatic juice.

Brunner's Glands

Duodenal submucosal glands that secrete an alkaline, bicarbonate-rich mucus to protect the intestinal epithelium from acidic gastric chyme.

Jejunum

The middle $2.5\,\text{m}$ region of the small intestine with tall villi and prominent plicae circulares, serving as the primary location for macronutrient digestion and absorption.

Ileum

The final $3.5\,\text{m}$ segment of the small intestine specialized for reabsorbing bile salts and vitamin $\text{B}_{12}$, containing numerous Peyer's patches.

Peyer's Patches

Large aggregated lymphoid nodules concentrated in the ileum mucosal and submucosal layers that monitor intestinal lumen pathogens and provide mucosal immunity.

Microvilli

Microscopic folded extensions on the apical surface of enterocytes that form the brush border, contributing to an overall $600\text{-fold}$ surface area expansion in the small intestine.

Paneth Cells

Specialized epithelial cells located at the base of intestinal crypts that secrete antimicrobial peptides and lysozymes to protect intestinal stem cells and maintain mucosal defense.

Tuft Cells

Chemosensory epithelial cells present in the intestinal mucosa that act as environmental sensors to detect luminal contents.

Cholecystokinin (CCK)

An enteroendocrine hormone secreted by I cells in the duodenum and jejunum in response to fat and protein, triggering gallbladder contraction, pancreatic enzyme release, and slowed gastric emptying.

Secretin

A hormone released by S cells in the upper small intestine when acidic chyme enters the lumen, stimulating the pancreas to secrete bicarbonate-rich fluid.

Gastric Inhibitory Polypeptide (GIP)

An incretin hormone released by K cells in the duodenum and jejunum that enhances insulin secretion from pancreatic beta cells in response to absorbed nutrients.

GLP-1 (Glucagon-Like Peptide-1)

An incretin hormone secreted by L cells in the distal small intestine that stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and promotes satiety.

Enterohepatic Circulation

The physiological recycling pathway wherein $95\%$ of bile acids secreted into the intestine are reabsorbed in the ileum and returned to the liver via the portal vein.

Cholestasis

A condition resulting from disrupted bile acid transport or accumulation of detergent bile acids in hepatocytes, leading to toxic liver injury.

Pancreatic Acini

Exocrine clusters in the pancreas that synthesize and secrete digestive proenzymes (trypsinogen, chymotrypsinogen) and enzymes (amylase, lipase) into pancreatic ducts.

Pancreatic Islets

Endocrine tissue clusters within the pancreas containing alpha cells (producing glucagon) and beta cells (producing insulin) to regulate blood glucose homeostasis.