Macromolecule Digestion, Endocrine Control, and Cellular Metabolism

A comprehensive collection of vocabulary flashcards covering macromolecule digestion and absorption, pancreatic zymogen activation, pituitary-adrenal-thyroid endocrine axes, and cellular ATP synthesis pathways.

Hydrolysis

The enzymatic cleavage of chemical bonds by the addition of $H_2O$, used by the digestive system to break down polymers into monomer building blocks.

Salivary Amylase

An enzyme secreted in saliva with an optimum pH of $6.7$ that initiates carbohydrate digestion in the mouth by cleaving starch into shorter polysaccharide chains and maltose.

Pancreatic Amylase

An enzyme in pancreatic juice with an optimum pH of $6.7\text{--}7.0$ that continues starch digestion in the duodenum, producing maltose, maltriose, and short oligosaccharides.

Brush Border Carbohydrate Enzymes

Microvilli-bound enzymes of small intestine epithelial cells—including maltase, sucrase, and lactase—that hydrolyze disaccharides into monosaccharides (glucose, fructose, and galactose).

SGLT Transporters

Sodium-glucose cotransporters that perform symport secondary active transport, moving $Na^+$ down its concentration gradient and glucose against its concentration gradient; includes SGLT1 in the small intestine ($1\text{ glucose}/2\text{ }Na^+$) and SGLT2 in the kidney ($1\text{ glucose}/1\text{ }Na^+$).

GLUT Transporters

Uniport carriers (GLUT1-5) that move glucose across membranes via facilitated diffusion down its concentration gradient; includes insulin-independent GLUT2 (pancreas, liver, small intestine) and insulin-dependent GLUT4 (skeletal muscle, adipocytes).

Intestinal Glucose Absorption Mechanism

The process where apical SGLT1 transports glucose into enterocytes via $Na^+$ secondary active transport driven by basolateral $Na^+/K^+$ ATPase, followed by glucose exit into interstitial fluid and blood capillaries via GLUT2 facilitated diffusion.

Pepsin

An enzyme secreted by gastric glands in the stomach with an optimum pH of $1.6\text{--}2.4$ that works with hydrochloric acid to digest denatured proteins into shorter polypeptides.

Pancreatic Proteases

Enzymes in pancreatic juice—including trypsin, chymotrypsin, and carboxypeptidase—that act in the small intestine at an optimum pH of $8.0$ to convert polypeptides into amino acids, dipeptides, and tripeptides.

Aminopeptidase

A brush border enzyme of epithelial cells in the small intestine with an optimum pH of $8.0$ that hydrolyzes polypeptides into amino acids, dipeptides, and tripeptides.

Pancreatic Lipase

An enzyme in pancreatic juice with an optimum pH of $8.0$ that digests triglycerides in the small intestine into two free fatty acids and one monoglyceride.

Micelles (Mixed Micelles)

Vesicle-like structures formed by bile salts, phospholipids, cholesterol, and lecithin that dissolve hydrophobic free fatty acids and monoglycerides to transport them to the intestinal brush border surface.

Chylomicrons

Large lipoprotein complexes ($100\text{--}500\text{ nm}$) assembled inside enterocytes from re-synthesized triglycerides, cholesterol, phospholipids, and apolipoproteins (such as ApoB48), which exit via exocytosis into the lymphatic lacteals.

Zymogen

An inactive enzyme precursor (or proenzyme) that requires a specific biochemical change, such as hydrolysis revealing the active site, to become an active enzyme.

Enteropeptidase (Enterokinase)

An enzyme located on the intestinal mucosa that activates pancreatic trypsinogen into trypsin, initiating the activation cascade for other pancreatic zymogens.

Pancreatic Zymogen Activation Cascade

The pathway where enteropeptidase cleaves trypsinogen to form trypsin, which then cleaves and activates proelastase, procarboxypeptidase, chymotrypsinogen, and prolipase into their active enzymatic forms.

Adenohypophysis

The anterior lobe of the pituitary gland composed of glandular epithelium (pars tuberalis, pars distalis, and fetal pars intermedia) that secretes six classic trophic peptide hormones under hypothalamic control.

Neurohypophysis

The posterior lobe of the pituitary gland composed of neural tissue that stores and releases antidiuretic hormone (ADH/arginine vasopressin) and oxytocin synthesized in the hypothalamus.

Hypothalamo-Hypophyseal Portal System

A specialized vascular arrangement of capillaries and portal vessels that transports hypothalamic trophic neurohormones directly to the anterior pituitary endocrine cells.

Pituitary Gland Anatomy

Anatomical structure attached to the hypothalamus via the infundibulum, consisting of an anterior glandular lobe (adenohypophysis) and a posterior neural lobe (neurohypophysis).

Adrenal Medulla

The inner neural tissue layer of the adrenal gland that secretes epinephrine and norepinephrine into the bloodstream in response to sympathetic neural stimulation.

Zona Glomerulosa

The outer layer of the adrenal cortex that synthesizes mineralocorticoids (such as aldosterone) to regulate $Na^+$ and $K^+$ balance.

Zona Fasciculata

The middle layer of the adrenal cortex that secretes glucocorticoids (such as cortisol) in response to ACTH to regulate glucose metabolism.

Zona Reticularis

The inner layer of the adrenal cortex that secretes adrenal androgens, primarily dehydroepiandrosterone (DHEA).

Adrenal Cortex Histological Zones

Histological layout of the adrenal cortex beneath the connective tissue capsule, structured into three distinct glandular layers: zona glomerulosa, zona fasciculata, and zona reticularis.

Cortisol (Hydrocortisone)

A glucocorticoid steroid hormone from the zona fasciculata that opposes insulin by stimulating gluconeogenesis, inhibiting peripheral glucose uptake, increasing lipolysis, and promoting muscle protein degradation.

Cortisol Diurnal Rhythm

The circadian secretory profile of plasma cortisol, characterized by lowest levels during sleep and a peak shortly after awakening (around $9\text{:00 AM}$).

Thyroid Follicle

The functional unit of the thyroid gland, consisting of a central colloid fluid reservoir surrounded by simple cuboidal follicular cells that synthesize thyroglobulin.

Triiodothyronine ($T_3$)

The active form of thyroid hormone containing three iodine atoms; has a rapid onset of action ($2\text{--}3\text{ hours}$) and a plasma half-life of $3\text{ days}$.

Thyroxine ($T_4$)

The major circulating prohormone secreted by the thyroid gland ($10\times$ more abundant than $T_3$) with a half-life of $\text{7 days}$, converted into active $T_3$ inside target cells by iodinases.

Hypothyroidism vs. Hyperthyroidism

Pathological thyroid states: Hypothyroidism presents with decreased plasma $T_4$, lethargy, cold intolerance, dry skin, constipation, weight gain, and slow reflexes; Hyperthyroidism presents with increased plasma $T_4$, hyperactivity, heat intolerance, excessive sweating, weight loss, and rapid reflexes.

Aerobic Respiration Overall Equation

The complete oxidation equation for glucose in cellular respiration: $C_6H_{12}O_6 + 6\text{ }O_2 \rightarrow 6\text{ }CO_2 + 6\text{ }H_2O + \text{ATP}$.

Glycolysis

An anaerobic metabolic pathway in the cytoplasm that splits one $6\text{-carbon}$ glucose molecule into two $3\text{-carbon}$ pyruvic acid molecules ($C_3H_4O_3$), consuming $2\text{ ATP}$ to produce $4\text{ ATP}$ (net $2\text{ ATP}$) and $2\text{ NADH}$.

Lactic Acid Pathway

An anaerobic process (lactic acid fermentation) in which pyruvic acid accepts electrons from NADH to yield lactic acid and regenerate $\text{NAD}^+$, allowing net $2\text{ ATP}$ production in cells lacking oxygen or mitochondria.

The Cori Cycle

The inter-organ pathway where lactic acid produced by skeletal muscle during exercise travels via the blood to the liver, where it is converted back to pyruvic acid, glucose 6-phosphate, and free glucose via gluconeogenesis.

Citric Acid Cycle (Krebs Cycle)

An aerobic pathway in the mitochondrial matrix where acetyl CoA combines with oxaloacetic acid to form citric acid; per spin (two per glucose), it yields $3\text{ NADH}$, $1\text{ }FADH_2$, $1\text{ ATP}$, and $2\text{ }CO_2$.

Oxidative Phosphorylation

ATP synthesis driven by electron transport pumps pushing $H^+$ into the intermembrane space, creating a concentration gradient that powers ATP synthase as $H^+$ flows back into the matrix.

Oxygen as Final Electron Acceptor

The essential role of $O_2$ at the end of the electron transport chain to receive electrons and hydrogen ions, forming water according to $O_2 + 4\text{ }e^- + 4\text{ }H^+ \rightarrow 2\text{ }H_2O$.

Lipogenesis

The formation of triglycerides (fat) occurring primarily in adipose tissue and the liver from excess acetyl CoA, fatty acids, and glycerol.

Lipolysis

The hydrolysis of stored triglycerides in adipose tissue into glycerol (which enters respiration as 3-phosphoglyceraldehyde) and free fatty acids.

$\beta$-Oxidation

A pathway in the mitochondrial matrix that sequentially removes $2\text{-carbon}$ acetic acid units from fatty acids to form acetyl CoA, NADH, and $FADH_2$, yielding roughly $100\text{ ATP}$ per $16\text{-carbon}$ fatty acid.

Ketogenesis

The liver pathway synthesizing water-soluble ketone bodies (acetoacetate, $\beta$-hydroxybutyrate, acetone) from fatty acid-derived acetyl CoA via thiolase, HMG-CoA synthase, and HMG-CoA lyase during fasting or restricted carbohydrate diets.

Essential Amino Acids

The 8 amino acids (9 in children) out of 20 total amino acids that cannot be synthesized by human cells and must be supplied by the diet.

Urea Cycle

An energy-requiring liver pathway that converts toxic ammonia ($NH_3$ / $NH_4^+$) generated by amino acid deamination into urea for safe renal excretion.

Amino Acid Catabolism Pathways

The conversion routes of deaminated amino acid carbon skeletons into metabolic intermediates such as pyruvic acid, acetyl CoA, oxaloacetic acid, $\text{α-ketoglutaric acid}$, succinic acid, and fumaric acid.

Glycogenesis

The production of glycogen storage polymers from glucose, occurring primarily in skeletal muscles and the liver.

Glycogenolysis

The hydrolysis of stored glycogen into glucose 6-phosphate for glycolysis, which can be converted to free blood glucose exclusively in the liver.

Gluconeogenesis

The synthesis of new glucose from non-carbohydrate molecules (including lactic acid, glycerol, and amino acids), occurring primarily in the liver.