(a) Structure of receptor for epidermal growth factor (EGF) -a single-pass transmembrane protein with an exodomain (outside of cell) that is EGF binding site, and an endodomain (inside of cell). The endodmain functions as the enzyme tyrosine kinase (TK), hence, EGF receptor is, in fact, receptozyme. It belongs to a large group of tyrosine receptor kinase (Trkpronounced "truck"). (b) Binding of two EGF molecules leads to EGF receptor dimerization and tyrosine (Tyr) phosphorylation, followed by signal transduction and the corresponding cell effect. Dephosphorylation by protein phosphatase leads to receptor deactivation. Drugs that inhibit (downregulate) the activity of Trk have anticancer effects. (•Brown C, Gralla RJ, et al. Impact of EGFR inhibitor in non-small cell lung cancer on progression free and overall survival: a meta-analysis. J Natl Cancer Inst 2013;105:595-605. DOI:10.1093/ jnci/djt072) • Life at intracellular level is mediated by membrane compartmentalization and membrane flows supported by cytoskeletal structures (AF/AAP*, MT/MAP, and myosin V-VIII) Two intracellular membrane flows operate inside the cell: (i) endocytotic membrane flow (endocytic pathway) starting from plasmalemma and moves in the following order: coated pits, coated vesicles, caveolae, caveolosomes, early endosomes, late endosomes, dynein [MT-based retrograde (-) motor protein], MVB, and lysosomes, (ii) protein secretory membrane flow (secretory pathway) starting on free polyribosomes, RER, COP II vesicles, Golgi complex, COP I vesicles, Golgi-derived coated vesicles and secretory vacuoles, kinesin [a MT-based anterograde (+) motor protein] and, finally, plasmalemma where the release of vacuole-stored proteins through porosomes occurred. *Q-1: In which cells are there actin and myosin? A-1: In all cells of a multicellular organism. Q-2: In which cells is there the highest amount of actin and myosin? A-2: In the myocytes -smooth muscles and striated muscle (cardiomyocytes and skeletal muscle cells). • There are two lysosomal and one nonlysosomal pathway for degradation of exhausted proteins and organelles