Every cell in your body contains organelles (structures that have specific functions). Just like organs in the body, each organelle contributes in its own way to helping the cell function well as a whole. The nucleus, mitochondria and chloroplasts are all organelles.
Despite their central importance to cell function (and therefore to all life), organelles have only been studied closely following the invention of the transmission electron microscope, which allowed them to be seen in detail for the first time.
Core organelles
Core organelles are found in virtually all eukaryotic cells. They carry out essential functions that are necessary for the survival of cells – harvesting energy, making new proteins, getting rid of waste and so on. Core organelles include the nucleus, mitochondria, endoplasmic reticulum and several others. The primary cilium (which has recently been shown to help cells sense their surroundings) may also be a core organelle because it seems to be present on most cells.
Different types of cells have different amounts of some organelles. For instance, cells that use a lot of energy tend to contain large numbers of mitochondria (the organelle responsible for harvesting energy from food). That’s why very active muscle cells are often full of mitochondria.
Specialised organelles
Some cell types have their own specialised organelles that carry out functions that aren’t required by all cells. Here are just a few of the specialised organelles that we know about:
Chloroplasts are found in plant cells and other organisms that conduct photosynthesis (such as algae). They are the site where photosynthesis occurs.
Storage granules are found in cells that produce a lot of material for secretion (release from the cell). For instance, some pancreas cells (which make insulin for release into the bloodstream) contain large numbers of storage granules that store insulin until the cell receives a signal to release it.
Microvilli are tiny finger-like protrusions on the surface of a cell. Their main function is to increase the surface area of the part of the cell in which they’re found. Cells in the intestinal wall have many microvilli so they can absorb as many nutrients as possible from the gut.