12th Class Biology Organisms and population Ageing

Definition : Ageing is the slow deterioration in the structure and function of body cells tissues and organs of an animal and starts after the adulthood.

Gerontology : The field of developmental biology that deal with the process and problems of ageing is known as gerontology – (Gr. geron = old man; logos = discourse). The scientists involved in the science of ageing are called gerontologist.

Life cycle and life span : In all metazoan animals, the life cycle includes two developmental period; embryonic period (pre-natal developmental period) which extends from zygote to offspring till hatching or birth, and post embryonic period (post-natal developmental period)- which includes growth, adulthood, reproduction, ageing. Thus, the life cycle comprises five main events : birth, growth, maturity, old age and death, that follow in the sequence named. Maximum life span is the maximum number of years survived by any member of a species, while average life span is the number of years survived by members of a population. Life expectancy is the age at which half the population still survives.

Life span in different organisms

Why old age ends in a natural death : Though it is difficult to give a categorical answer, certain factors that lead to death in old age are known. During the growth period, new cells are formed faster than the rate of death of old cells. But after the maximum growth, the metabolic rate declines and rate of formation of new cells is lower than the rate of death of body cells. So the repair of damaged cells is not complete and a slow deteriorative process starts. In human beings, it starts after the age of 30 years. But in certain cases, deterioration in structure and function of cells may start during childhood or even during prenatal life e.g., hearing efficiency of ear and atresia in ovary.  As the organism grows older.

Deterioration data of an old person : A 75 years old man, for instance, has, as compared to a 30 years old person, about 64% less taste buds, about 44% less renal glomeruli about 20% less nerve cells in the brain, and about 37% less axons in the spinal nerves. His heart pumps 35% less blood and sends 20% and 58% less blood to the brain and the kidneys respectively. His lungs have 44% less vital capacity and provide about one-third less oxygen to the blood per minute. His kidneys have 31% lower rate of glomerular filtration. His nerve impulses are propagated at a rate about 10% slower. At the age of 30, the height starts decreasing indetectibly, it decreases by 0.3 cm. at 40, by 2 cm. at 50 and by 2.5 cm. at 70.

Changes in ageing or symptoms of ageing : Gerontologist have worked out a large number of changes that accompany ageing. These are discussed below under three heads -

(1) Changes at organ level : During ageing, different organs and organ systems show different rates of decline e.g.,

(i) Heart : With increasing age the efficiency of heart decreases. In a man of 70 years, the heart pumps only 65 per cent blood per minute as compared to a 30 years old man. Consequently, the blood going to the brain and kidney is reduced to 80 percent and 40 percent respectively.

(ii) Oxygen uptake by blood : At the age of 20 blood takes about 4 litres of oxygen per minute, while in a man of 75 years, it takes only about 1.5 litres of oxygen in the same period.

(iii) Decrease of blood volume : The production of new RBCs from the bone marrow declines and consequently the volume of blood also decreases.

(iv) Kidney : The number of kidney tubules is found to reduce to half in the old age. As a result the volume of urine decreases. This creates lots of other urinary troubles and also causes body ache, low back and difficulty in passing urine.

(v) Lungs : The capacity of lungs for intake of air decreases. This leads to reduction in the oxygen supply to different tissues. Therefore, old persons suffer from breathlessness and inflammation of mucous membrane.

(vi) Digestive system : The number of taste buds on tongue reduces to about one-third. The secretion of digestive juices also decreases with old age. This may result in indigestion, loss of appetite, dyspepsia, constipation and gas formation.

(vii) Retention of water : The capacity of body cells to retain water also decreases with the result, the skin in old persons is dry and wrinkled.

(viii) Nerve impulse : The rate of nerve impulse propagation reduces with age. The decline is about ten percent in man of 75 years as compared to that of 50 years old person.

(ix) Eyes : Accommodation power of eye starts declining in the 40s; ability to distinguish fine details may begin to decline in the 70s while there is increased susceptibility of eyes to glare and more difficulty in detecting moving objects from 50 years onward.

(2) Cellular changes : Cellular changes are of two types

(i) Morphological changes

(a) Accumulation of exhaustion pigments : The exhaustion pigment lipofuscin, yellow pigment and brown deposits are byproducts of unsaturated lipid oxidation. It is especially obvious in nerve and heart muscle cells but is present in almost all other cells of the body, though to a lesser degree.

(b) Appearance of lipid vacuoles : Small lipid vacuoles appear in the cytoplasm.

(c) Decline in cell volume : The cells showing ageing exhibit hypertrophy or decrease in cell volume.

(3) Physiological changes :

(i) Accumulation of chromosomal aberrations and gene mutation in the nuclei with advancing age. These change the transcribed RNA which leads to the synthesis of defective proteins. These also retard the replication of DNA.

(ii) Decrease in semipermeability of cell membrane due to deposition of calcium in the peripheral part of cytoplasm.

(iii) Decrease in the rate of metabolism due to decreased number of mitochondria with advancing age.

(iv) Decreased rate of protein synthesis is due to decrease in RER in cells.

(v) Increased inactivity of aldolase enzyme in the liver cells with advancing age.

(vi) Decreased rate of cell mitosis. The non-dividing nerve cells and muscle cells start ageing earlier than the dividing cells of spleen and liver.

(vii) Size of the nucleus decreases.

(viii) Breakdown of cellular membrane. With advancing age, the lipids of biological membranes, that surround the cells and certain cell organelles, breakdown, forming a fatty, brown pigment called lipofuscin. The lipofuscin granules accumulate in ageing muscle and nerve cells, and interfere in their functioning.

Theories of ageing : Biological phenomena leading to  ageing are not fully known. Several theories have been proposed to explain various aspects of ageing. These are following types –

(1) Immunity theory : This theory suggests a link between ageing and disappearance of the thymus gland by late middle age in man. Thymus stimulates the proliferation of lymphocytes, increasing resistance to infection. Absence of this gland affects in two ways -

(i) It weakens the body's natural defence against foreign germs.

(ii) It increases the number of abnormal (defective or harmful) cells formed in the body itself. This destroys the tissues. The neuro-hormonal and immunity theories are collectively called pacemaker theories of ageing.

(2) Wear-and-Tear or stress theory : It states that the cells and tissues of the body continuously wear out due to internal and external stress factors. This coupled with the fact that the regenerative capacity progressively declines with age, causes ageing and finally death.

(3) Cross linkage theory : According to this theory ageing is caused by the increase of bonds between protein and nucleic acid molecules in the cell. These bonds alter the functional characteristic of these important cellular components leading to non-availability of certain functional proteins and resulting in malfunctioning of the cell.

(4) Waste product theory : According to this theory the accumulation of waste products are considered to poison the cell gradually, resulting in their ageing and death.

(5) Clinker's theory : It states that ageing is due to accumulation of metabolic wastes inside the body cells. These wastes, beyond some limit, poison the body cells and decline the metabolic rate and induce ageing.       

(6) Error catastrophy theory : It was proposed by Orgel (1963 A.D.). It states that errors in reading genetic code results in defective proteins which form defective enzyme leading to catastrophic damage to cells, tissues and organs, so induce senescence.

(7) Free radical theory : A free radical is a molecule with an unpaired and highly reactive electron. The oxygen-free radicals are formed as the by-product of normal cellular respiration. These free radicals take electrons from other molecules of a biological system so making them unstable and combine readily with other molecules. These free radicals initiate a chemical instability.

• Brain and Ageing : The brain consumes more oxygen than any other organ so more free radicals of oxygen are generated. Free radicals cause damage of cell membranes and DNA with each collision. These damages cause ageing skin, ageing of brain and even cancer. Vitamin E and vitamin A are natural anti-oxidants of the body. Many antioxidants are found in fruits and vegetables such as - lycopene in tomatoes, isoflavones in soybean, sulphoraphane in broccoli, ellagic acid in strawberries and polyphenols found naturally in leaves of green and black tea.

(8) Metabolic or living theory : It states that those organisms (e.g., rats, mice, insects, birds, etc.) which have higher metabolic rate, mature, age and die earlier than those organisms (e.g., human beings) which have low metabolic rate and take years to mature and years to age.

(9) Collagen theory : It was proposed by F. Verzar (1964). It states that ageing is induced by changes in the collagen protein in the interstitial fluid surrounding the body cells.