Wellcome Trust Centre For Mitochondrial Research

Mitochondria under the Microscope

What do mitochondria look like?


How Mitochondria look


For many decades mitochondria were believed to be shaped like a baked bean, and when we look under high powered microscopes at the mitochondria they often do. However mitochondria are often not found like this and the majority of the time they join together to form beautiful branched networks that fill the cell they are in (see picture above) These networks are constantly changing and reshaping.

The structure of mitochondria and mitochondrial networks

Mitochondria have two membranes (protective coverings) one surrounding the other, called the inner and outer mitochondrial membranes. The inner membrane is highly folded and forms structures called cristae, the machinery for energy generation can be found on these cristae. Between the two membranes is a gap called the inter membrane space, while the space at surrounded by the inner mitochondrial membrane is called the matrix. The matrix contains the mitochondrial DNA, the components for the mitochondria to carry out their functions and the machinery needed to make new copies of the mitochondrial DNA.

Image and diagram of Mitochondria

We think that the mitochondria form networks to allow the contents of the matrix to mix. In cells that are dying or that contain mitochondria which are not functioning properly then the network breaks down and the mitochondria once again take on their ‘bean’ like appearance. The network reaches to the limits of the cell in all directions, but it varies in appearance in different cells. In brain cells for example around the nucleus the mitochondria form a network but in order to be transported to the end of the nerve which might be far away the mitochondria return to being ‘bean’ shaped and are transported along a long fibre called an axon. We can record movies of this happening and use measurements of how fast the mitochondria move, how many are moving and in which direction they travel in our research.

For a printable, quick summary of what mitochondria look like and their structure please click here.
The structure of mitochondria and their networks

Where do mitochondria originate from?

Mitochondria were not always resident within another cell; they were once organisms in their own right. When survival became tough they formed a relationship with another organism and they both benefitted. Without this relationship, these mitochondria and their energy generation, organisms would not have been able to grow bigger and become more complicated. In short we would not be here today.

Our story begins 2 Billion years ago. . . . . . . .

The earth was still developing and changing, life was becoming more complex. The earliest life on earth survived by breaking down the compounds around them into something less complicated (by a process of fermentation) which generated energy that they could use to survive and reproduce. Fermentation can only occur when there is no oxygen present.

However, a new type of cell was developing that could use light to generate energy (by a process of photosynthesis), in the same manner that plants do today. This process uses carbon dioxide and produces oxygen. These cells were increasing in number at an alarming rate and because of this the concentration of oxygen in the atmosphere was increasing. This was bad news for the organisms that relied on fermentation, or that could not tolerate high oxygen levels.

It was good news however for organisms such as Rickettsia prowazekii (let’s call them Ricke for short!). These bacteria belong to a family of bacteria called the α-proteobacteria and are the bacteria responsible for human Typhus, a disease estimated to have killed 20-30 million people in the years following the First World War. Billions of years ago, Ricke had the advantage, they could use oxygen to produce energy to survive and reproduce. However Ricke is a small bacterium (less than one thousandth of a millimetre in size) and was probably consumed by other organisms. All life on earth had to adapt to survive and exist within the changing atmosphere.

It is likely that at some point Ricke were taken up by a larger host cell but was not broken down. The host cell could now survive since Ricke provided a way to produce energy that used the increasing levels of oxygen within the atmosphere.  The benefit for Ricke was the protection from other predatory cells. This mutually beneficial relationship has lasted for millennia.

Over the past billion years Ricke has changed and become what we now call mitochondria. It has lost most of its independent function and much of it’s DNA to the nucleus of the host cell. A small amount of DNA however has been retained by  mitochondria and it uses this DNA to make the components necessary to produce energy. The mitochondria could now no longer survive as an independent organism and relies entirely on the host cell to carry out its functions.This process is illustrated in the diagram below:

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There is a lot of evidence to support this theory for the origin of mitochondria, this evidence includes;

  • The fact that the DNA contained within mitochondria is circular which is different to the DNA on our chromosomes and is similar to the DNA found within bacteria.
  • Mitochondria have two membranes.
  • The DNA contained within the mitochondria is very similar to the DNA of Ricke but is much smaller, due to the donation of information to the nucleus.

Without this relationship, these mitochondria and their energy generation, organisms would not have been able to grow bigger and become more complicated. In short we would not be here today.

For a printable, quick summary of the information on this page please click here.
The origins of mitochondria