Smooth Endoplasmic Reticulum Functions

Comprehensive vocabulary flashcards reviewing the structure, cellular roles, biochemical pathways, and specialized functions of the Smooth Endoplasmic Reticulum (SER) based on Topic 4 Cell Biology lecture notes.

Endomembrane System

An intracellular membrane network consisting of the endoplasmic reticulum, the Golgi apparatus, and the nuclear envelope.

Smooth Endoplasmic Reticulum (SER)

A meshwork of tubular membranes lacking ribosomes that carries out lipid metabolism, detoxification, glycogenolysis, calcium storage, autophagy membrane donation, and vesicular transport.

Scramblase

An ER membrane enzyme that facilitates the non-specific, bidirectional transfer of random phospholipids between monolayers, ensuring symmetric expansion of the lipid bilayer.

Phospholipid Synthesis in the SER

A multi-step enzymatic pathway on the cytosolic leaflet of the ER membrane where fatty acids are conjugated to CoA, transferred to glycerol 3-phosphate to generate phosphatidic acid, dephosphorylated into diacylglycerol, and linked to CDP-choline to produce phosphatidylcholine.

$7\alpha$-Hydroxylase

A hepatic enzyme involved in the biosynthesis of bile acids, catalyzing the initial conversion step from cholesterol into bile acid precursors such as cholic acid.

Steroid Hormone Synthesis

The biochemical pathway in the smooth ER and mitochondria of adrenal cortex cells, testes, and ovaries that converts cholesterol into steroid hormones such as pregnenolone, cortisol, aldosterone, testosterone, and estradiol.

Chylomicrons

Lipoprotein complexes synthesized in enterocytes by combining smooth ER-derived triglycerides with apolipoproteins (including apoB-48) in the Golgi apparatus before exocytosis into lymphatic lacteals.

Cytochrome P450 (CYP450)

A superfamily of heme-containing monooxygenase enzymes located in the ER membrane of hepatocytes that detoxifies liposoluble endogenous compounds, xenobiotics, and drugs using $O_2$ and NADPH.

Microsomal Ethanol Oxidizing System (MEOS)

An inducible ER-associated pathway in hepatocytes that metabolizes excess ethanol into acetaldehyde, generating reactive oxygen species (ROS) and accumulating toxics upon enzyme saturation.

Clopidogrel Activation

The multi-step hepatic biotransformation of the inactive prodrug clopidogrel into its active thiol metabolite by cytochrome P450 enzymes (such as CYP2C19, CYP1A2, CYP2B6, CYP2C9, and CYP3A) to block platelet P2RY12 receptors.

Glucose-6-Phosphatase ($\text{G6Pase-}\alpha$)

An enzyme anchored in the ER membrane of hepatocytes that catalyzes the terminal step of glycogenolysis by dephosphorylating imported glucose-6-phosphate ($\text{G6P}$) into free glucose and inorganic phosphate ($P_i$).

Glucose-6-Phosphate Translocase (G6PT)

A transport protein embedded in the ER membrane that imports cytosolic glucose-6-phosphate ($\text{G6P}$) into the ER lumen for enzymatic cleavage by glucose-6-phosphatase.

SERCA (Sarco/Endoplasmic Reticulum $\text{Ca}^{2+}$-ATPase)

An ATP-driven calcium pump situated in the ER and sarcoplasmic reticulum membranes that sequesters cytosolic $\text{Ca}^{2+}$ back into the lumen against its concentration gradient.

Ryanodine Receptor (RyR)

A large intracellular channel protein located in the sarcoplasmic/endoplasmic reticulum membrane that mediates the rapid efflux of $\text{Ca}^{2+}$ into the cytosol via calcium-induced calcium release (CICR).

Inositol 1,4,5-Trisphosphate Receptor ($\text{IP}_3\text{R}$)

A ligand-gated calcium release channel on the ER membrane that opens upon binding second messenger $\text{IP}_3$, releasing luminal $\text{Ca}^{2+}$ into the cytoplasm.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum found in muscle fibers dedicated to storing and releasing $\text{Ca}^{2+}$ to regulate myofibril contraction and relaxation cycles.

Phagophore

An isolation cup-shaped double membrane, derived in part from the endoplasmic reticulum, that encloses targeted organelles and cytoplasmic components to form an autophagosome.

ULK1 Complex

A protein kinase assembly (consisting of ULK1, FIP200, and ATG13) that is negatively regulated by mTOR and translocates to the ER membrane to initiate phagophore nucleation during autophagy.

COPII-Coated Vesicles

Transport vesicles that bud from the transitional endoplasmic reticulum to carry cargo in an anterograde direction toward the ERGIC and cis-Golgi network.

COPI-Coated Vesicles

Coated vesicles responsible for retrograde transport, retrieving ER-resident proteins and lipids from the Golgi cisternae back to the rough and smooth ER.

Transitional ER

The specialized exit domain of the endoplasmic reticulum where export cargo is packaged into COPII transport vesicles destined for the Golgi apparatus.