Cells are the basic structural, functional and biological unit of living organisms. They are the smallest unit of life that is classified as a living thing and building block of life. However just like all units of life cells also have a limited life span before they die. It is important to understand the processes leading to and involved in cell death, because it is a critical process of life and is linked to the dreaded process of aging itself. Cells go through the process known as cellular aging before they experience cell death via the process of apoptosis. This essay will look at; the process of telomere shortening, free radical theory, apoptosis, necrosis, and autophagy, in order to provide an overview of the basis of cell aging and the processes that lead to cell death.
Cell aging is known as the decrease in the cell's ability to multiply as time progresses or a decline in vital functions. Aging may occur as a result of continued damage to the cell or due to the genetic structures of the cell (Blagosklonny 2008). There is known to be a gradual shortening of telomeres during the process of cell aging. The free radical theory of aging acknowledges that oxidative damage drives the aging process (Wickens 2001). But even with these theories we must consider the fact that there may be no single cause of aging, proposed theories may jointly contribute to the final process of aging (Scoggin 1981).
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Telomeres are the physical ends of eukaryotic chromosomes; they are vital for the process of cell multiplication and survival as they protect chromosome ends from nucleolytic degradation (Chen et al. 2009). It has been suggested that cells lose telomeres with age and are therefore considered to be a basis of cell aging, plus the fact that as telomeres become shorter the cells ability to multiply decreases which is known as the process of cell aging (Levy et al. 1992). Some studies have proposed that telomere recombination leads to the acceleration of cell aging (Chen et al. 2009). Although telomere loss contributes to aging, the free radical theory is known to drive the process of aging. It is also proposed that increased ROS causes an increase in telomere shortening.
A free radical is any chemical species that contains an unpaired or odd number of electrons (Wickens 2001). The most common source of free radicals is oxygen, and generally living things that induce the formation of free radicals produced changes that resembled aging and lead to a shortened life span (Wickens 2001). Knowing this it can be said that the 2-3% of oxygen consumed by a cell that is converted into free radicals is a basis of aging in cells (Wickens 2001). During the process of cell aging, damage caused by free radicals may damage a cell's functionality eventually leading to cell death.
Cell death occurs when a cell is no longer needed or has become damaged. Cells self-destruct by activating an intercellular suicide programme known as apoptosis that leads to cell death. Other than the process of apoptosis cell death may also occur due to necrosis or autophagy.
It has been found that oxidative stress leads to the collapse of membrane potential leading to the activation of apoptosis (Ozawa 1999). Apoptosis is an essential process during development because without it cells would accumulate. Apoptosis is an intentional process of self-destructive cell death that involves activation of mechanisms encoded in the genomic information of eukaryotes, via the plasma membrane receptors on the cell's surface (White et al. 1998). The key morphologic feature of apoptosis is the shrinking of the cell and its nucleus (Hotchkiss at al. 2009). There are five groups of genes involved in the process of apoptosis, which include those involved in; triggering cell death, the cell death process itself, engulfment of the dying cell, the final disposal of the cellular corpse, suppression of cell death (White et al. 1998). Even though apoptosis is the most common mode of cell death it is important to take note of the other forms of cell death.
Other than apoptosis, cell death is recognized in two other modes: necrosis and autophagy. Necrosis is known as the death of cells as a result of external trauma, unlike apoptosis it is not a programmed form of cell death. The key morphologic features of necrosis are; a gain in cell volume, swelling of organelles, rapture of plasma membrane and loss of intracellular contents leading to cell death (Kroemer et al. 2009). Whereas autophagy is a process in which cells generate energy and metabolism by digesting their own organelles and macromolecules, allowing a cell that is; starving or deprived of growth factors to survive (Hotchkiss et al. 2009). Although autophagy is seen as a strategy for survival not death, when cells do not receive nutrients for an extended period of time they ultimately digest all available substances and die (Hotchkiss et al. 2009). However unlike apoptosis neither of; necrosis or autophagy have not been proven to be directly linked to the process of cell aging.
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While telomere loss has been found to contribute to cell aging, it isn't considered to drive the aging process; instead it is believed that the free radical theory is the basis of aging in cells. And damage caused by the process of aging may damage a cell's functionality leading to cell death via the activation of apoptosis. Even though necrosis and autophagy are also processes of cell death, apoptosis is the only one that is directly linked to cell aging. Overall through the exploration of cell aging and death, we can gather that the process of cell aging is linked to the critical process of programmed cell death.