skin / the surface / a type

to keep up; to retain / to maintain

collagen (a glycoprotein)

flexibility / elasticity

ingredient / (mathematics) entity

skin

important, significant

importance, consequence, significance

elasticity (property by which a material can regain its original dimensions)

songbun (a system of ascribed status in North Korea, based on one's political, social, and economic background and the behaviour of relatives)