In Golgi apparatus, the maturing face is:

Concept: The Golgi apparatus is a stack of flattened, curved membranous sacs (cisternae) with two structurally distinct faces defined by their curvature and role in vesicle traffic — one face receives incoming material and is convex, while the opposite face dispatches processed material onward and is concave.

Application: The convex face is the cis (forming) face, positioned closest to the rough endoplasmic reticulum; it receives newly synthesized proteins and lipids arriving in transport vesicles, marking the immature end of the stack. As cisternae mature and move across the stack, they reach the opposite face, the trans (maturing) face, from which vesicles bud off toward the plasma membrane, secretory granules, or lysosomes. Because of this curvature, the trans, or maturing, face is concave.

Cross-check — the near-miss options, by value:

• spherical does not describe either face of the Golgi stack; the cisternae are flattened and curved, not spherical, at either end.

• convex describes the cis (forming) face — the immature end nearer the rough endoplasmic reticulum, not the maturing end.

• bi-concave describes the shape of a mature red blood cell (erythrocyte), an unrelated structure, not a face of the Golgi apparatus.

Result: the maturing (trans) face of the Golgi apparatus is concave.