Every cell1 in your body contains organelles (structures that have specific functions). Just like organs in the body, each organelle2 contributes in its own way to helping the cell function well as a whole. The nucleus3, mitochondria and chloroplasts4 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 eukaryotic5 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, mitochondria6, endoplasmic reticulum and several others. The primary cilium7 (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 muscle8 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 photosynthesis9 (such as algae10). 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 pancreas11 cells (which make insulin12 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 area13 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 nutrients14 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 fertilisation15. 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 resolution16 of the light microscope17. The detailed structure of organelles only became clear after the development of the transmission electron microscope18 (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.
Related content
Use these articles to explore investigating what is inside our bodies and how they work, whilst also understanding the important role microscopes play.
- Animal cells and their shapes
- A closer look at the cell’s antenna
- Mitochondria – cell powerhouses
- The human digestive system
- Preparing samples for the electron microscope
- The microscopic scale
Activity ideas
In Modelling animal cells in 3D, students make 3D models of specialised animal cells, imitating what can be seen under high-resolution microscopes.
In the Inside a cell activity, students learn about the contents of a cell. They explore some of the main organelles within a cell using the analogy19 of a school, an online game and/or by making something edible.
- cell: 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly. 2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.
- organelle: Structure within a cell that has a specific function such as the nucleus, mitochondria and ribosomes.
- nucleus: 1. The very small, very dense, positively charged centre of an atom containing protons and neutrons. 2. Part of the cell that contains the cell’s hereditary information (DNA) and controls the cell’s processes.
- chloroplasts: The organelles present in green plant cells where photosynthesis takes place.
- eukaryote: An organism whose genetic material is contained within a nuclear membrane. Examples include fungi, plants, animals and many single-celled organisms.
- mitochondria: Organelles where energy is generated. Found in all eukaryotic cells. They contain a small amount of genetic material, allowing them to make some of their own proteins.
- primary cilium: A single protrusion from the surface of eukaryotic cells. Primary cilia are thought to sense the extracellular surroundings by detecting mechanical and biochemical stimuli.
- muscle: The tissue that makes it possible for an animal to move and to maintain its posture. Muscles also make the heart beat, force blood to circulate and move food along the digestive system. The human body has more than 600 muscles.
- photosynthesis: A process that uses the energy from sunlight to convert carbon dioxide and water into carbohydrates, releasing oxygen as a byproduct. Photosynthesis occurs in the green parts of plants, in algae and in some microorganisms.
- algae: A large, diverse group of photosynthetic eukaryotic organisms. Algae have no stems or leaves and grow in water or on damp surfaces.
- pancreas: A greyish-pink organ, about 15 cm long, that stretches across the back of the abdomen, behind the stomach. It produces hormones such as insulin and glucagon (endocrine function) as well as pancreatic juice that contains digestive enzymes (exocrine function).
- insulin: A hormone produced in the pancreas that controls blood sugar levels.
- surface area: The total area of an object or surface.
- nutrient: A substance that provides nourishment for growth or metabolism.
- fertilisation: (Reproduction) The joining of male and female sex cells (gametes), resulting in combining genetic material.
- resolution: In microscopy, the ability to distinguish two separate points or objects as independent. The resolution of a microscope provides a measure of the level of detail it can be used to reveal. In an image, the degree of sharpness. Resolution is measured by the number of dots per linear inch in a hard-copy printout, and the number of pixels on a display screen.
- light microscope: A microscope that uses a glass lens (or lenses) to magnify small objects that are illuminated with visible light.
- electron microscope: A microscope that uses a focused beam of electrons, rather than visible light, to magnify objects. Electron microscopes use electromagnetic coils to focus the electron beam (instead of the glass lenses used to focus light in optical microscopes). Traditional light microscopes magnify images 1000-2000 times, electron microscopes can magnify 300,000 times or more.
- analogy: A comparison between two things, usually using something that is easy to understand to explain something that is more complicated.