SOVEREIGN · INQ·12

M2 · The Gate & the Trap

Glucose enters by facilitated diffusion — no ATP at the door. Then hexokinase phosphorylates it to G6P, so it can never leave. The trap keeps cytosolic glucose near zero, which keeps the gradient steep, which keeps glucose pouring in.

GLUT ISOFORMKm 2 mM

INSULIN SIGNAL (GLUT4 → surface)OFF

EXTERNAL [GLUCOSE] — the gradient5.0 mM

EXTRACELLULAR [GLC]5.0 mM

CYTOSOLIC FREE [GLC]≈0.0 mM

INWARD FLUX0 glc/s

ACTIVE ISOFORM · KmGLUT1 · 2mM

LEGEND

Glucose — amber hexagon, high outside, ~0 inside.

Glucose-6-phosphate — the trapped product; cannot fit back through GLUT.

Phosphate — added by hexokinase (the trap enzyme).

Facilitated diffusion: passive, down-gradient, 0 ATP. Contrast: active transport pumps up a gradient and burns ATP.