Life begins and ends with many questions in between – that about sums it up, right? But what is life? That may be a pretty big question and depending on who you ask, the answer will vary. But if we had to consider it scientifically, there is a fairly simple answer. Mitochondria.
This may seem fairly literal, but this little organelle is at the centre of all multicellular life. They produce energy which is used in various other processes in cells. Considering that everything that has life is made up of cells, mitochondria are therefore the driving force behind all life. Does this sound too far fetched to describe the secret of life? After this book summary, you might have a different view.
In this summary readers will discover:
- The beginning of life
- What it means to not have mitochondria
- How mitochondria power us
- Suicide, Sex and Origins
- Ageing and Death
Key lesson one: The beginning of life
To understand why mitochondria are considered the secret to life, let’s go back 4 billion years ago. The world as we know it did not exist and all that existed were algae and single-cell bacteria. It was only three billion four hundred million years later that more complex life forms developed. Multicellular organisms arose with more cells and more importantly, different types of cells as well.
These different types of cells were capable of performing a range of functions within the cell. Each one of their cells contains a nucleus which is what makes them eukaryotes. Humans and animals are made up of these eukaryotic cells. The cells of bacteria, on the other hand, do not contain a nucleus and are known as prokaryotes. Originally, biologists assumed that eukaryotes evolved from prokaryotes but this has been disproved. The two types of cells are completely distinct from each other. This is the reason why eukaryotes can be so much bigger than prokaryotes. And guess what multicellular organisms have? Mitochondria. This leads us to the fact that if all life forms are eukaryotes, made up of eukaryotic cells which contain mitochondria; then mitochondria are integral to all multicellular life.
Key lesson two: What it means to not have mitochondria
Bacteria have come a long way in four billion years. They are present in almost all harsh and extreme environments which have deemed them extremophiles. They are able to survive these extreme environments due to their ability to adapt. But even with these adaptations, they have remained single-celled organisms. In contrast, eukaryotes have evolved into complex and larger organisms. Why have bacteria not evolved in the same manner or transformed into something else?
Well, firstly, a bacteria’s genome is much smaller than eukaryotes. To be able to adapt to extreme environments, bacteria have to be able to replicate quickly. To replicate quickly their physical size and the amount of DNA that needs to be replicated has to be small. If the genomes were larger, quick replication and therefore adaptation would not be possible. So the ability of bacteria to replicate quickly is also the reason why they could not become more complex, multicellular organisms. They simply do not have the time and energy for the larger genomes needed to produce multicellular organisms.
Bacteria also do not have mitochondria which are responsible for energy production. They rely on the outer cell membrane for respiration. If a bacterial cell were any bigger, this would result in an increased surface area and this would require more energy for respiration. Since they do not have mitochondria, they cannot produce the energy required for this so they stay small and save whatever energy they can for reproduction.
Mitochondria, therefore, allow eukaryotes to grow as much as they can. They can generate as much energy as the cell requires. Eukaryotes also have the ability to acquire more mitochondria which means that they have enough energy to keep growing.
Key lesson three: How mitochondria power us
Fun fact. The sun generates 0.2 microjoules of energy per gram per second. In contrast, humans produce ten thousand times more than this and they do it without even moving a muscle. This is the power of mitochondria. In humans, the act of breathing and thus supplying cells with oxygen for the breakdown of glucose is called cellular respiration. The majority of the chemical reactions associated with cellular respiration happens inside the mitochondria. It is through these reactions that energy is produced.
To describe this in more detail, mitochondria produce energy by pushing protons through the cell membrane. This creates an electrical charge. During cellular respiration, these protons accumulate within the membrane and thus store energy within the cell. These protons are then released slowly to produce energy. The energy produced is in the form of adenosine triphosphate or ATP. The ATP generated is used to fuel other cellular processes.
Key lesson four: Suicide, Sex and Origins
It takes millions and millions of cells to make up multicellular organisms. Every cell has a unique role to play within the organism and contributes to its survival and overall well-being. Now if cells continued to function like this indefinitely, there would be no reason for cells to stop multiplying or for organisms to stop growing. To prevent this from happening, a process called apoptosis exists.
Apoptosis refers to programmed cell death or cell suicide as it is sometimes called. This process is controlled by the mitochondria meaning that mitochondria ultimately determine when it’s time for a cell to die. Scientists have theorised that this ability of mitochondria might be a trait that evolved from a somewhat parasitic relationship. Maybe eukaryotes were formed because parasitic mitochondria took control of host cells and they used apoptosis to kill the host cell before moving on. But as eukaryotes evolved, a chemical dependency grew between the mitochondria and its host cell. Basically, this means that the mitochondria could no longer kill their host cells without killing themselves in the process. However, if cells are healthy and continue to divide, this means that the mitochondria would also benefit from being able to now over into the new cell. If a cell does not divide, the mitochondria would remain trapped in a single cell and they can’t kill this cell without killing themselves.
This brings us to sexual reproduction. In essence, mitochondria would only survive if their host cell ended up merging with another cell. And what is sexual reproduction besides the merging of cells which allows DNA to recombine and cells to rapidly divide? Interestingly enough, the chemical signals used by mitochondria to control apoptosis are the same chemical signals that trigger genes to produce male and female sex cells. The male sex cells or sperm, contain approximately 100 mitochondria. In contrast, female sex cells or eggs contain a thousand times more at 100 000. The difference in number between the two cells indicates that it is highly unlikely that any of the male mitochondria will be passed on to the offspring. This is actually rather important. If the offspring were to receive mitochondria from both parents, the two types would compete. This competition would potentially harm the host cell. Therefore, receiving just one set of mitochondria from a female sex cell ensures that all mitochondria are identical in the offspring.
Since it is only maternal mitochondria present in offspring, mitochondrial DNA between offspring and mother is largely identical. This is true for all maternal generations before and means that ancestry can be traced along the maternal line. It is with this knowledge that scientists were able to trace the ancestry of all humans to Mitochondrial Eve or African Eve. As her name states, she lived in Africa and is basically mother to the human race. The fact that we now know this is all thanks, once again, to mitochondria.
Key lesson five: Aging and Death
In biology, there is a simple rule. The larger the organism, the slower its metabolic rate and the longer its lifespan. With a few exceptions, this rule has remained true. Now, as metabolic rates are a measure of how fast our body’s consume energy, it is safe to assume that mitochondria contribute to determining the lifespan of an organism. More accurately, mitochondria cause ageing and death.
This reasoning was further used to consider free radicals. Free radicals were theorised to be the cause of ageing almost 50 years ago. They possess one unpaired electron which makes them unstable and harmful to the cell. Free radicals are a byproduct of metabolic activity and thus pose a direct threat to mitochondria. When mitochondria are damaged, cells begin to degenerate and this triggers the ageing process. This theory also suggests that the faster the metabolic rate, the more free radicals that are produced and the faster the pace of ageing and age-related illness. This would ultimately lead to a shorter lifespan. This is what became known as the mitochondrial theory of ageing. This theory is not without its flaws but for the most part, the link between ageing and mitochondrial leakage of free radicals is true.
Mitochondria, therefore, play a central role in the life and death of the cells they inhabit.
The key takeaway from Power, Sex, Suicide is:
Mitochondria are, without a doubt, the driving force between the life and death of all multicellular organisms. They allowed cells to develop into complex, multicellular organisms. These organelles produce the energy that is required for all cellular processes including respiration, reproduction, ageing and cell death. They may be tiny, but mitochondria can provide valuable insight into both our pasts and future which is why research in this area has expanded drastically.
How can I implement the lessons learned in Power, Sex, Suicide:
Remember that no matter who we are, what we do and what we look like, we are all ultimately complex, multicellular organisms. As much as we long for control over our health, the way we age and our appearance it is sometimes worth remembering that it is all controlled by a tiny little organelle that we got from our mothers. The sooner we accept that some things are beyond our control, the sooner we begin to appreciate and live our lives to the fullest.