Cellular Structures & Organelles
What is the membrane-bound organelle that houses the cell's genetic material (DNA) in eukaryotes?
The nucleus
What molecule serves as the primary direct energy currency of the cell?
ATP (Adenosine triphosphate)
What are chemical messengers released by cells that travel through extracellular fluid or blood to target cells?
Hormones
What is the resting phase of the cell cycle where cells spend time performing normal functions without actively dividing?
Interphase
Which organelle is studded with ribosomes and is the primary site for synthesizing membrane and secretory proteins?
Rough Endoplasmic Reticulum (RER)
What folded internal membrane structures inside mitochondria maximize the surface area for the electron transport chain?
Cristae
G protein-coupled receptors (GPCRs) interact with heterotrimeric G proteins. In the inactive resting state, what molecule is bound to the a subunit?
GDP
Which cyclin-CDK complex is primarily responsible for triggering the transition from the G2 phase into mitosis ($M$ phase)?
Cyclin B - CDK1 (MPF)
Which cytoskeletal filament is a non-polarized structural rope assembled from tetramers that provides mechanical strength and nuclear envelope stability?
Intermediate filaments
The classical active transport enzyme NA/K -ATPase pumps how many sodium and potassium ions per molecule of ATP hydrolyzed?
Which secondary messenger is synthesized directly from ATP by the membrane-bound enzyme adenylyl cyclase?
Cyclic AMP (cAMP)
During the $G_1$ phase checkpoint, unphosphorylated Retinoblastoma (Rb) protein binds to and inhibits which critical transcription factor?
E2F
Which targeting signal sequence is located at the C-terminus of resident endoplasmic reticulum proteins and ensures their retrieval from the Golgi via COP I vesicles?
KDEL sequence
How do chemical uncouplers like 2,4-dinitrophenol (DNP) disrupt mitochondrial oxidative phosphorylation?
They collapse the proton gradient ($\Delta p$) across the inner membrane, uncoupling electron transport from ATP synthesis.
Receptor tyrosine kinases (RTKs) signal by forming dimers upon ligand binding. What is the immediate enzymatic consequence of this dimerization?
Autophosphorylation of specific intracellular tyrosine residues
How does the tumor suppressor protein p53 respond to severe DNA damage in a mammalian cell?
It activates transcription of the p21 CDK inhibitor, halting the cell cycle and promoting DNA repair or apoptosis.
Peroxisomes import fully folded proteins using cytosolic receptors like Pex5p that recognize what specific C-terminal tripeptide signal?
Ser-Lys-Leu (SKL / PTS1)
What complex at the oxygen-evolving center of Photosystem II is responsible for splitting water molecules into oxygen, protons, and electrons?
Tyrosine-Z ($Y_Z$) and the manganese cluster (Mn4CaO5)
Which signaling pathway is directly activated by nitric oxide (NO) gas diffusing across vascular endothelial cell membranes into smooth muscle?
Activation of soluble guanylyl cyclase to produce cyclic GMP (cGMP)
What is the key regulatory role of the Anaphase-Promoting Complex / Cyclosome (APC/C) during cell division?
It acts as a ubiquitin ligase that targets securin and mitotic cyclins for proteasomal degradation.