Feeling tired today? Most of us grow more weary as the day goes on and as the years pass. We say we are out of energy or getting low on energy, which is a fact. The energy we expend is generated inside our cells by a symbiotic organelle called mitochondria.

Scientists once did not understand how bumblebees could fly because of their small wings in comparison with body weight. It just did not seem possible to expend energy fast enough to provide the lift needed for flight.

Then the facts were revealed that solved the problem and opened a new door into the study of the cell. Bumblebees, as it turns out, have an incredibly high density of mitochondria in their tiny wing muscles.

Inside eukaryotic cells, the complex kind like ours, are a number of organelles, each of which is an analog to a working member of an industrial park. Mitochondria extract energy from chemical bonds as they chemically change food and oxygen into water and carbon dioxide.

Mitochondria then store the energy as electrons in an excited energy state in a molecule of ATP (adenosine triphosphate). This way they generate more than 90% of all cellular energy in the body. Cells use the stored electrons to do what cells do, depending on the type of cell.

Another important function of mitochondria is unique among organelles: They contain their own genes. All cells contain DNA in the nucleus with two copies of each of more than 20,000 genes. One gene from mother and one from father pass on hereditary traits in infinite combinations that ensure that no two organisms are identical (notwithstanding identical twins).

DNA in mitochondria organelles is outside the nucleus, is much smaller and is inherited only from mother.

We now know that mutations cause changes in mitochondria over time at a constant rate. Thus, it is possible not only to trace lineage, but also to estimate timelines whenever mitochondrial DNA is present.

No two individuals have identical mitochondrial DNA, so everyone has different efficiencies in mitochondrial function as well as the potential for dysfunctional mutations. This diversity is great for the long-term adaptation of a given species to different environments, but not so great for rapid changes in environment and susceptibility to disease.

Recent studies suggest that mitochondrial dysfunctions are behind the causes and symptoms of complex diseases such as autism, cancer, inflammation and neurodegenerative diseases such as multiple sclerosis.

Mitochondria are also important in aging. As we age we accumulate mitochondrial DNA mutations that slowly diminish the energy- producing abilities of the organelle. This is why we do not have as much energy as we used to, and it just keeps getting worse as we age.

It also means that the cells and organs do not have the energy to function and maintain themselves, which some scientists think might lead to age-related diseases such as Alzheimer’s and Parkinson’s disease. For the brain, which uses one-fifth of the body’s energy, a change in energy can be crucial if not critical.

If mitochondrial dysfunction is the sprite underlying the chronic diseases, then globalization could be responsible for many of them. Human mitochondria are often mismatched to the current climate and diet, and people with a given mitochondrial lineage might thus be more susceptible to certain diseases.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.