Cell divison is necessary to replace cells that are worn and aging. Cells do not have an indefinite life span. Fo example, the life span of a red blood cell is about 120 days.
A volcanic winter is a reduction in global temperatures caused by volcanic ash and droplets of sulfuric acid and water obscuring the Sun and raising Earth's albedo (increasing the reflection of solar radiation) after a large, particularly explosive volcanic eruption.
Endocytosis and exocytosis are similar in that they are both forms of cell transport. They differ because in endocytosis, a cell transports molecules into a cell (think: endo - in), and in exocytosis, the cell's molecules are transported by being expelled.
