Given a scientific question, generate a correct answer to it.

[EX Q]: What are required when the damage from trauma or infection cannot be closed with sutures or staples?
[EX A]: skin grafts

[EX Q]: What physical property of matter reflects how closely packed the particles are?
[EX A]: density

[EX Q]: What are the active transport mechanisms by which molecules enter and leave the cell inside vesicles?
[EX A]:
endocytosis and exocytosis