Mitochondria – cell powerhouses
Mitochondria are tiny organelles inside cells that are involved in releasing energy from food.
This process is known as cellular respiration. It is for this reason that mitochondria are often referred to as the powerhouses of the cell. Cells that need a lot of energy, like muscle cells, can contain thousands of mitochondria.
Cell featuring mitochondria
Mitochondria are tiny organelles inside cells that are involved in releasing energy from food. This process is known as cellular respiration.
Apart from cellular respiration, mitochondria also play a key role in the ageing process as well as in the onset of degenerative disease.
The powerhouse function
When the breakdown products from the digestion of food find their way into the cell, a series of chemical reactions occur in the cytoplasm. This allows some of the energy locked up in these products to be released and incorporated into the universal energy supplier in cells known as ATP (adenosine triphosphate).
Remaining molecular fragments from this process then enter the mitochondria, and in a complex series of steps, they are finally converted into carbon dioxide and water. The energy locked up in these fragments is incorporated into more ATP.
The ATP molecules produced in this way can then be used by the cell to supply the energy needed to function. ATP → ADP + P + energy to function.
Breakdown of glucose
In the cell cytoplasm, glucose is broken down to pyruvate. On entry to the mitochondria, pyruvate is converted to carbon dioxide and water. Its chemical potential energy is transferred to ATP.
The overall chemical reaction that occurs when glucose is broken down is:
C6H12O6 glucose+6O2 oxygen→6CO2 carbon dioxide+6H2O water+energy in the form of ATP molecules
It has been estimated that, in an average person, the turnover rate (the rate at which ATP is produced and consumed) is a massive 65 kg per day.
The human body is a fantastically energetic machine. It has been estimated that, kilogram for kilogram, the human body, when sitting comfortably, is converting 10,000 times more energy than the Sun in every second!
Free radicals: a byproduct of respiration
During cellular respiration, highly reactive molecules called free radicals are formed within mitochondria. Perhaps the best known free radical produced in this way is the superoxide radical, O2-.
Free radicals are potentially very damaging to cell components such as proteins and genetic material like DNA and RNA. If too many free radicals are released in the mitochondria, the damage can be severe, resulting ultimately in the death of the cell. To protect against free radical damage, mitochondria produce their own antioxidant enzymes. One such enzyme is known as superoxide dismutase or SOD.
Free radicals and antioxidants
Plant & Food Research’s Dr David Stevenson explains what free radicals are and how they are produced. He outlines the role of cellular structures called mitochondria in the production of free radicals. He also describes the negative and positive effects of free radicals. The term ‘antioxidant’ is defined and common examples given.
Point of interest
One of the problems that David faces in his research is that some phytochemicals show antioxidant activity in cells outside of the body but not necessarily in cells inside the body. A large number of health foods claim to have antioxidant activity, but is there evidence to show that the activity is in the living body (in vivo) or in a cell line outside the body (in vitro)?
Although free radicals are damaging, they have an important signalling role. Scientists now believe that mitochondria operate a sensitive feedback mechanism in which some of the free radicals themselves act as signals to the cell, causing it to calibrate and adjust cellular respiration, so to remove them completely is not good for the cell.
Antioxidants in mitochondria
Chemicals present in some fruits and vegetables have been shown to have antioxidant activity. This means that, in laboratory tests, they can neutralise free radicals. It was thought that consuming these foods, or extracts made from them, would help the body to remove damaging free radicals.
Recent research suggests that antioxidants work differently in the body than in the laboratory. It is now thought that some antioxidants, in particular, a class of plant chemicals known as polyphenols, have a direct effect on the mitochondria. It appears that they stimulate the mitochondria to become more efficient in generating energy from food, so they generate fewer free radicals and neutralise them more quickly. It is as if the functioning of the mitochondria is being ‘tuned up’ by these polyphenols – an effect similar to that induced in the mitochondria by exercise.
Find out more about Antioxidants.
Maintaining healthy mitochondria
If mitochondria are not functioning efficiently, their energy-producing capacity is reduced, more free radicals escape causing damage to the cell and early cell death may follow.
Cyanidin.
Cyanidin is a plant polyphenol found in the skins of red-coloured fruits and vegetables. It is now thought that polyphenols can have a direct effect on mitochondria.
Research over recent years is indicating that the health of mitochondria is very much lifestyle and diet dependent. Excessive consumption of sugary foods and beverages reduces mitochondrial efficiency. Lack of exercise reduces the number of mitochondria in active cells such as muscle, and they become inefficient, leaking out more free radicals into the cell.
By choosing a lifestyle that includes regular exercise, daily consumption of fresh fruits and vegetables, avoidance of sugary foods, control of appetite and avoiding smoking, anyone can tune up their mitochondria, which should help to promote a long and very healthy life.
Nature of science
One of the habits of scientists is open-mindedness. Scientists need to be receptive to new ideas and suggestions. As new evidence is discovered, new ways of interpreting and understanding it may have to be considered.
Useful link
Etch A Cell ran the Powerhouse Hunt project, in which users were asked to draw around each mitochondria seen in images. The goal was to significantly improve understanding of mitochondrial biology and, by assisting with the process of manual segmentation, this helped train computers to automatically segment. The results can be see here. You could use the iamges in your class.
