A cell is like a city because it has walls, a transportation system, streets, a recycling center, a power source and a library. Cells also have factories and industrial parks.
The outer membrane of a cell, much like a city wall, surrounds the cell and creates a boundary between it and another cell. Fatty molecules, called lipids, form this thin and flexible membrane, which creates a strong cellular cover.
A cell's cytoskeleton consists of several networks of micro-filaments, intermediate filaments and microtubules. This system defines the center of the cell and distinguishes one end of the cell from the other end. Cell components move along this network, which is made up of water, ions, molecules and proteins, much like cars along a highway. This network can propel the entire cell if necessary.
Lysosomes are specialized compartments within a cell that are responsible for disposing of waste, such as carbon dioxide and urea. Lysosomes also break down and recycle parts of the cell. Mitochondria provide cells with a sugar-based power source, which it converts into energy. Most cells have multiple mitochondria. A cell's nucleus functions like a library, storing chromosomes, or the instructions the cell must follow to develop and function.
In a cell, ribosomes interpret the instructions and assemble amino acids into the correct sequences. A cell uses amino acids to make protein. A cell can use this protein or export it elsewhere, much like goods manufactured in a factory. If the ribosomes are a cell's factories, then a cell's endoplasmic reticulum is its industrial park. Ribosomes operate in this part of the cell, building proteins and determining where to send them.