Ageing
On the other hand, the dramatic increase in life expectancy after Second World War illustrated that ageing could be affected by another factor, most likely to be environmental, as it is the biggest change to our life from the 20 th century to the 21st century. For instance, the average life expectancy of women outlived men in the 20 th century due to medical advances in the hygiene of hospitals. This greatly reduces the death caused by infection after childbirth. There are a lot of different theories which suggest various environmental factors. Figure 2: The average life expectancy across different continents from 1770 to 2021
One of the most widely accepted and developed theories is the ‘ oxidative stress hypothesis ’ , 11 which advanced and modified the free radical theory of ageing. 12 A free radical is a type of reactive oxygen species (ROS), including highly reactive chemicals which contain oxygen formed from diatomic oxygen (O 2 ). These ROS can be produced endogenously and exogenously. The most common production of ROS in our body is oxidative phosphorylation, which happens in the mitochondrion. During this stage, electrons leak from the electron transport chain and react with the oxygen molecule to form a superoxide molecule (a type of free
Figure 3: Process of lipid peroxidation
radical). These ROS can also be generated from external factors outside the body, for instance, due to overexposure to UV light and cigarette smoking. The increased production of ROS could cause a number of different deleterious effects on macromolecules in our body, such as phospholipids, which make up the cell membrane through lipid peroxidation. One hydroxyl molecule reacts with the unsaturated fatty acid tail of phospholipid and causes a series of reactions which lead to a rupture in the cell membrane (figure 3); this leads to cell lysis and death. 13 Another significant damage caused by 11 Ghezzi, P. et al. (2017) ’The oxidative stress theory of disease’, British Journal of Pharmacology 174.12: 1784- 1796. 12 Harman, D (1956). ‘Aging: a theory based on free radical and radiation chemistry’, Journal of Gerontology 11.3: 298 – 300. 13 Ray, P. et al. (2012) ‘Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling’, Cell Signal 24.5:981-990. doi:10.1016/j.cellsig.2012.01.008.
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