Why do cells need some things to go in and others out

Why do cells need to regulate what goes in and out of them?

The plasma membrane is selectively permeable; it regulates what passes from one side to the other.

A selectively permeable membrane will allow water to flow freely while limiting the passage of molecules, especially large molecules or those with a charge.

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.