Answer:
As a cell grows bigger, its internal volume enlarges and the cell membrane expands. Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases.
Answer:
If you measure by edge of the Sun's magnetic fields, the end is the heliosphere. If you judge by the stopping point of Sun's gravitational influence, the solar system would end at the Oort Cloud.
