Role of mitochondria in the process of cellular respiration and aging
All living organisms on this earth require energy in order to carry out important
physiological activities like digestion, respiration, excretion etc. Energy is universally obtained from oxidation of food (carbohydrates), which is mainly synthesized by plants through a process known as photosynthesis. These carbohydrate molecules serve as a source of energy for most of the organisms on surface of earth, which employ cellular respiration and convert carbohydrate molecules into ATP (energy molecules of adenosine triphosphate) and CO2.Now I shall be comparing photosynthesis and cellular respiration.
Photosynthesis is a photochemical reaction through which the green plants synthesize their own food (in form of glucose) using water and carbon-dioxide in presence of sunlight and chlorophyll pigments (Farbee, 2001). This is summarized in the following equation:
6 CO2 (carbon dioxide) + 12 H2 O (water) sunlight & chlorophyll? C6H12O6 (glucose) + 6H2O (water) + 6O2 (oxygen)
The process of photosynthesis takes place in two main stages namely light reaction (which is a light dependent process) and dark reaction (which is a light independent process). These processes as explained by Farbee (2001) are described below and also shown in figure 1.
Light reaction. During light reaction, photolysis of water takes place, resulting in
production of oxygen and hydrogen ions. The hydrogen ions released during this process reduce NADP (Nicotinamide Adenine Dinucleotide Phosphate) into NADPH. Also, the sunlight striking the chlorophyll pigments causes excitation of electrons, which after being channeled through electron transport chain (ETC) in the chloroplast, result in the formation of ATP.
The dark reaction. In dark reaction, a cyclic series of reactions called Calvin-Benson
cycle take place in which, ATP and NADPH (previously synthesized during the light reaction) reduce the trapped carbon dioxide into carbohydrates.
Figure 1. Figure Showing the Process of Photosynthesis
Source: Farbee, M.J. (2001). Photosynthesis. Online Biology Book. [online] Available from:
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookPS.html [accessed 7 August 2007]
Most of the living organisms including human beings derive their energy from food
obtained through plants by a process known as cellular or internal respiration, which involves oxidation of food molecules inside small organelles called mitochondria (present inside the cells) (Farbee, 2001). The complete process of internal respiration as described by Farbee (2001) can be summarized in the following equation:
C6H12O6 (carbohydrates) + 6O2 (oxygen) ? 6CO2 (carbon dioxide) + H2O (water) +36 ATP (energy molecules)
Most animals respire in presence of oxygen i.e. aerobic respiration, in which glucose
undergoes complete oxidation by undergoing the process of glycolysis, Kreb’s cycle and channeling of electrons through electron transport chain (as shown in figure 2). The end result of aerobic respiration is production of 36 molecules of ATP, water and carbon dioxide.
Figure 2. Process of Aerobic Respiration
Source: Longman, A.W. (2001). Cellular respiration. Biology corner website. [online] Available from:
http://www.biologycorner.com/bio3/notes-respiration.html [accessed 7 August 2007]
Certain bacteria and yeast perform respiration in absence of oxygen; this is known as
anaerobic respiration. In anaerobic respiration, the end products are ethyl alcohol, carbon dioxide and some energy (two molecules of ATP). Sometimes the process of anaerobic respiration also takes place inside our skeletal muscle cells in absence of oxygen. This anaerobic respiration occurring in skeletal muscle cells is similar to that occurring in the yeast except that in the muscles, end products are lactic acid and energy. The difference between aerobic and anaerobic respirations as described by Farbee (2001) has been summarized in table1.
Table 1. Comparison between aerobic respiration and anaerobic respiration
Requirement of oxygen
Takes place in the presence of oxygen
Takes place in the absence of oxygen
Place of occurrence of reaction
Occurs in the mitochondria
Occurs in the cytoplasm
End products are CO2 and H2O
End products are alcohol and carbon dioxide, or lactic acid.
Amount of energy released
One glucose molecule gives out 36 molecules of energy.
One glucose molecule gives out two molecules of ATP.
From the above discussion it becomes clear that both cellular Respiration and photosynthesis co-exist as paired processes in the energy cycle on the surface of this earth. The differences between these two processes have been summarized in table 2.
Table 2. Comparison between Photosynthesis and Cellular Respiration
Green plants and some bacteria.
All animals including human beings
Mitochondria (aerobic respiration)
Cytoplasm (anaerobic respiration)
CO2 and H2O in presence of sunlight and chlorophyll
Carbohydrates, water and oxygen.
Carbon dioxide, water and ATP in case of aerobic respiration.
Ethanol and carbon dioxide, or lactic acid, and energy in case of anaerobic respiration.
full and balanced chemical reaction
6 CO2 + 12 H2 O sunlight & chlorophyll? C6H12O6 + 6H2O + 6O2
For aerobic respiration C6H12O6 + 6O2 ? 6CO2 + 6H2O +36ATP
Apart from its essential role in cellular respiration, the mitochondria have also been
implicated in the process of aging. In addition to the nuclear DNA, mitochondria are controlled by their own DNA, known as mitochondrial DNA (mtDNA). Recently, through a study conducted by Trifunovic et al (2004, as cited by Portland, 2004) on laboratory mice it was shown that accumulation of tiny mutations in the mtDNA was responsible for producing number of changes related to ageing. I shall now be describing an experiment, which I would design to test the relationship between mtDNA and human aging.
Of the various theories which explain the process of aging, “Mitochondrial theory” is
gaining increasing amount of importance (Alexeyev et al, 2004; Trifunovic et al, 2005; Portland, 2004). According to this theory, the electrons leaking from the ETC of cellular respiration reduce molecular oxygen resulting in formation of highly reactive substances called ROS (reactive oxygen species). Increased production of ROS not only produces mutations in mtDNA but also results in dysfunction of respiratory chain and further increased free radicals production. Thus a vicious cycle is set up leading to an exponential increase in mtDNA mutations and damage to ETC over time, resulting in the eventual loss of cellular and tissue function, which is related with development of age resulted changes like weight loss, reduced subcutaneous fat, hair loss, osteoporosis, anemia, reduced fertility etc (Alexeyev et al, 2004)
The hypothesis which would be tested by my experiment is that mutations of mtDNA would be responsible for producing age-related changes in the laboratory mice.
The mice considered in this study would be divided into two groups, experimental group
and control group. Mice in the experimental group would be bred with a defective version of an enzyme (mtDNA polymerase). Under normal circumstances this enzyme is responsible for proof-reading mtDNA which helps in correction of mutations (Trifunovic et al, 2005). The control group would comprise of normal lab mice in which would be brought up with normal version of the enzyme (mtDNA polymerase). I would then evaluate the life histories of the mice in the experimental group in comparison with those in the control group until the time of their death by studying the development of age related changes (previously described). I would also calculate the life span of animals in both the groups.
The available evidence (Portland, 2004; Alexeyev et al, 2004; Trifunovic et al, 2004)
strongly suggests that mutations in mtDNA play an important role in the process of aging. Thus it is expected that the mice in the experimental group would show age related changes at an early age in comparison to that of control group in which there were no mutations of mtDNA. Similar results were found in the study by Trifunovic et al (2004, as cited by Portland, 2004) in which the mice belonging to the experimental group started showing the previously described age related degenerative changes at 25 weeks of age, whereas the normal mice showed these changes at 40 weeks of life. The lifespan of the mice in the experimental group was also reduced, with the median age of death being 48 weeks in the experimental group whereas that in the normal group was two years. Similar kinds of results are expected to occur in the experiment which would be performed by me.
Future Research and Drug Development
Though the available evidence does suggest positive association between somatic
mutations in mtDNA and age related degenerative changes, the exact process in which damage to mtDNA results in aging related changes have yet not been defined. More research in future regarding the processes involved in replication, transcription and translation of the mitochondrial genome in animals as well as humans is required in order to provide a major breakthrough by producing medicines which might be able to prevent or reduce age related degenerative changes. In future, development of anti-oxidant drugs targeted towards reducing the levels of free radicals and ROS would probably prove useful in reducing the levels of somatic mutations in mtDNA and thus age related changes.
Alexeyev, M.F., LeDoux, S.P., &Wilson, G.L. (2004) Mitochondrial DNA and aging. Clinical
Science, 107, 355–364.
Farbee, M.J. (2001). Photosynthesis. Online Biology Book. [online] Available from:
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookPS.html [accessed 7 August 2007]
Farbee, M.J. (2001). Cellular metabolism and fermentation. Online biology Book. [online]
Available from: http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookGlyc.html#Aerobic [Accessed 7 August 2007]
Portland, O. (2004). Key to aging: Mitochondrial DNA: Study could help explain the mechanics
of human aging process. [online] Available from: http://www.cbsnews.com/stories/2004/05/26/health/main619735.shtml [Accessed 7 August 2007]
Trifunovic,, A., Hansson, A., Wredenberg, A, Rovio, A.T., Dufour, E., Khorostov, I., et al.
(2005). Somatic mtDNA mutations cause aging phenotypes without affecting reactive oxygen species production. Proceedings of National Academy of Sciences U S A, 102 (50), 17993-8.