Cells are sacs of fluid surrounded by membranes. Inside the fluid float chemicals and organelles. An organism contains parts that are smaller than a cell, but the cell is the smallest part of the organism that retains characteristics of the entire organism. For example, a cell can take in fuel, convert it to energy, and eliminate wastes, just like the organism as a whole can. But, the structures inside the cell cannot perform these functions on their own, so the cell is considered the lowest level.
Each cell is capable of converting fuel to useable energy. Therefore, cells not only make up living things; they are living things. Cells are found in all plants, animals, and bacteria. Many of the basic structures found inside all types of cells, as well as the way those structures work, fundamentally are very similar, so the cell is said to be the fundamental unit of life.
The most important characteristic of a cell is that it can reproduce by dividing. If cells did not reproduce, you or any other living thing would not continue to live. Cell division is the process by which cells duplicate and replace themselves. If you did not replace your red blood cells, for example, you would have a life span only as long as that of red blood cells — a mere 120 days.
Increasingly more complex organisms are made up of increasingly more groups of cells (for example, in humans, groups of cells make up each organ and muscle tissue), and the organisms survive based on products that the cells make. For example, cells in the pancreas make insulin, which is necessary to ensure that the blood glucose level doesn’t skyrocket. Without insulin, the blood glucose can reach a level that is lethal. So, without that cellular product, you would die.
