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 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.
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.
Location, location, location
Within cells, organelles tend to cluster close to where they do their job. In sperm cells, for instance, mitochondria are concentrated around the base of the tail, where they provide energy for the sperm’s rapid ‘swim’ towards the ovum during fertilisation. In intestinal wall cells, microvilli are clustered on the side of the cell that faces the intestinal space so that the cells maximise their surface area for absorbing nutrients.
Zooming in on organelles
Microscopes have been crucial for understanding organelles. In fact, without microscopes, we wouldn’t even know that organelles existed! However, most organelles are not clearly visible by light microscopy, and those that can be seen (such as the nucleus, mitochondria and Golgi) can’t be studied in detail because their size is close to the limit of resolution of the light microscope. The detailed structure of organelles only became clear after the development of the transmission electron microscope (TEM), which made it possible to look at individual organelles at high resolution.
Having detailed information about organelle structure has been very important for understanding how they work. For instance, the TEM showed that mitochondria contained two membranes and that the inner one was highly folded inside the outer one. This helped scientists to understand how mitochondria harvest energy from food.