As you travel along the endoplasmic reticulum, you will notice that stuck to the sides of this tube are several small balls called ribosomes. Ribosomes are like small factories, that use available materials to build proteins. These proteins can then be used by the cell for other purposes, such as to build new structures, repair damage, and direct chemical reactions.
Why do you think ribosomes are found on the walls of the endoplasmic reticulum? Think about it. Remember, the endoplasmic reticulum is the cell's transportation system. When a ribosome is done building a protein, it can release it directly into the endoplasmic reticulum where it can then be transported to wherever it is needed.
Not all ribosomes end up attached to the inside of the endoplasmic reticulum. Many simply float around in the cytoplasm. Ribosomes are created in the nucleolus, which is found inside the cell's nucleus.
