Ribosomes are seen at numerous locations within the cell. These ultramicroscopic bodies are the “workbenches” of the cells; they are the sites of protein synthesis from amino acid subunits (monomers). Ribosomes are especially numerous in cells that synthesize proteins, such as pancreatic cells, muscle cells, and epidermal (skin) cells.
An important organelle of the cytoplasm is the mitochondrion (mitochondria). The mitochondrion is a double-membrane enclosed organelle that produces ATP, which is the energy currency of the cell. Cells that require large amounts of energy, such as muscle cells and sperm cells, contain many mitochondria, while fewer of these organelles exist in less active cells.
The center of genetic activity in the cell is the nucleus. With the exception of red blood cells and gametes (sex cells), all human cells have forty-six chromosomes (DNA + specialized proteins) in their nucleus. A body of RNA called the nucleolus is suspended in the fluid-like nucleoplasm (similar to the cytoplasm) in the nucleus. The genes within the nucleus are small sections of DNA that are specific nucleotide sequences that contain the instructions for the synthesis of particular proteins (one gene - one protein).
The endoplasmic reticulum, also called the ER, is a system of interconnected membrane channels in the cytoplasm. These membranes may or may not have ribosomes associated with them. If ribosomes are associated with the ER, it is referred to as a rough ER. Rough ER predominates in cells that are actively synthesizing proteins for export (sent to other cells). Where the endoplasmic reticulum has few or no ribosomes, it is known as smooth ER. After proteins have been manufactured, they are generally stored in a series of flattened membranes called the Golgi body. The Golgi body sorts and packages proteins for secretion from the cell.
The cell stores digestive enzymes in organelles known as lysosomes. Enzymes in lysosomes help break down organic molecules into components that are useful to the cell in protein synthesis and energy metabolism. Enzymes are also stored in peroxisomes. This is the site in which toxic compounds are neutralized. For this reason, peroxisomes are abundant in liver cells where they participate in the breakdown of alcohol, among other toxins.