Why is a cell considered an open system?

Question

Why is a cell considered an open system?

Answer 1

An open system is one that exchanges things like nutrients, energy, water, etc. with its surrounding environment.  In other words, it is not self-sustaining and entirely self-contained.

A cell creates proteins, many of which are shipped out of the cell to go do some function elsewhere.  Cells also excrete their waste products.  Cells bring in biological molecules like sugars, and keep ionic and osmotic balance by exchanging salts and water with its surroundings.