COMPOSITION OF BLOOD

 Origin of life :

(Protobiogenesis)

• The living matter shows attributes or characters like responsiveness, growth, metabolism, energy transformations and reproduction.
• As far as origin of life is considered, it has remained an enigma for intellectuals at all times.
• Despite of advancements in various fields like biochemistry, astrobiology, geography, molecular biology, etc. scientists are unable to ascertain the truth.
• Various theories and hypotheses have been proposed to find the probable answer to this question.

a.Theory of special creation :

•It is the oldest theory and is based on religious belief without any scientific proof.

•All living organisms are created by a super-natural power.

b. Cosmozoictheory/Theory of Panspermia:

• This theory advocates that life did not arise on the planet Earth.
• It may have descended to the earth from other planets in the form of spores or micro-organisms, called cosmozoa/ panspermia.
• Recently, NASA has reported fossils of bacteria-like organisms on a piece of Martian rock recovered from Antarctica. 

c. Theory of spontaneous generation (Abiogenesis) :

• According to this theory, life originated from non-living material spontaneously.
• This theory was disproved by Louis Pasteur.

d. Theory of biogenesis :

• According to this theory, living organisms are always produced from pre-existing living forms, by process called reproduction.
• Theory of biogenesis however could not explain origin of first life on earth but could explain only the continuity of life.

Chemical Evolution of Life (Self assembly theory of origin of life ) :

• According to this theory, life originated on earth by combinations of several chemicals through constant chemical reactions over a long period of time.
• This theory is also called self assembly theory of origin of life or biochemical origin of life.
• This theory was first formulated by Haeckel but later developed by the Russian scientist Alexander I. Oparin (1924) and British biologist J. B. S. Haldane (1929).
• The process of chemical evolution can be divided into following steps : 

a. Origin of Earth and Primitive atmosphere:

• The origin of universe was explained by the Big-Bang theory of Georges Lemaitre (1931).

• According to this theory the Universe originated about 20 billion years ago by a single huge titanic explosion.

• As the universe expanded, the temperature decreased and various galaxies of solid objects were formed.

• Milky Way is one such galaxy of which our solar system is one small part.

• Earth is one of the planets of solar system and originated about 4.6 billion year ago.

• When formed, it was a rotating cloud of hot gases and cosmic dust called Nebula.

• The condensation and cooling resulted in stratification with heavier elements like nickel and iron passing to the core and lighter ones like helium, hydrogen, nitrogen, oxygen, carbon, etc. remaining on the surface.

• They formed the atmosphere of the earth. The primitive atmosphere of the earth was quite different from the present one.

b. Formation of ammonia, water and methane:

• Primitive atmosphere was very hot. As it slowly cooled, the lighter elements started to react with each other.

• The early atmosphere was rich in H, C, N and S of which hydrogen being more active, it reacted with other elements to form chemicals on earth like CH4, NH3, H2O and H2S. 

c. Formation of simple organic molecules :

• As temperature of the earth decreased, steam condensed into water that resulted in heavy rain fall and the earth gradually cooled.

• Rain water got accumulated on the land to form water bodys.

• The atmosphere then did not contain ozone layer and thus ultra-violet radiations reached the surface of earth directly.

• Under the influence of available energy sources such as ultra-violet rays, radiations, lightening and volcanic activities, the early molecules of hydrocarbons, ammonia, methane and water underwent reactions like condensation, polymerisation, oxidation and reduction.

• These reactions resulted in formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc.

• All these simple organic molecules accumulated at the bottom of water bodies.

• Haldane described it as the ‘‘hot dilute soup’’ or ‘‘primitive broth’’. It did not show any degradation due to absence of free oxygen and enzymes.

d. Formation of complex organic molecules:

• The primitive broth was neutral and free from oxygen.

• Polymerisationtook place and simple organic molecules aggregated to form new complex organic molecules like polysaccharides, fats, proteins, nucleosides and nucleotides.

• Polymerisationof amino acids formed protoproteinswhich later formedproteins.

• Formation of protein molecules is considered as landmark in the origin of life. Proteins (enzymes) accelerated the rate of other chemical reactions. 

e. Formation of Nucleic acids :

• Nucleotides may have been formed by the reaction between phosphoric acid, sugar and nitrogenous bases.
• Number of nucleotides join together to form nucleic acids.
• Nucleic acids acquired self-replicating ability which is a fundamental property of living form.

f. Formation of Protobiontsor Procells:

• Nucleic acids along with inorganic and organic molecules formed the first form of life called protobionts.

• Protobiontsare the prebioticchemical aggregates having some properties of living system.

• Protobiontsare formed due to coacervationi.e. aggregation of organic molecules.

• Oparin(1924) called them coacervatesand Sidney Fox called protenoidsor microspheres.

• Coacervatesare colloidal aggregations of hydrophobic proteins and lipoid bubbles.

• Coacervatesgrew in size by taking up material from surrounding aqueous medium.

• As they grew, they became thermodynamically unstable and split into smaller units, comparable to daughter cells of budding organisms.

• Microspheres are protenoidsformed from colloidal hydrophilic complexes surrounded by water molecules.

• These bodies may have outer double-membrane, likeprimitive cell.

• Diffusion and osmosis may have occurred across the membrane.

• Coacervatesand microspheres were non-living colloidal aggregations of lipids and proteinoidsrespectively.

• They had some basic properties of living cells, such as growth and division.

• These colloidal aggregations turned into first primitive living system called eobiontsor protocell.

g. Formation of first cell :

• When RNA or DNA system developed within protocells, they look like bacteria or viruses.

• They regulated various metabolic activities.

• First cell was anaerobic, heterotrophic and obtained energy by chemoheterotrophic processes.

Urey and Miller’s Experiment :

Stanley and his teacher Urey provided the first experimental evidence in support of chemical evolution theory of Oparin.

• They designed a glass-apparatus called spark-discharge apparatus.

• The apparatus was first sterilized and evacuated.

• Methane, ammonia and hydrogen gases were pumped in the proportion of 1:2:2 into the glass chamber.

• A tube carrying water vapourwas also connected to the chamber.

• Lightning effect was mimicked by electric discharge carbon arc spark in the chamber.

• Process of evaporation and precipitation was also simulated by the use of heating mantle and condenser respectively.

• The mixture of CH4, NH3, H2 was exposed continuously to electric discharge for several days causing the gases to interact, after which these were condensed.

• The liquid collected in the U-tube turned brown.

• Chemical analysis of this liquid reported the presence of simple organic compounds. (urea, amino acids, lactic acid).

• This experiment strongly supports that the simple molecules present in the earth’s early atmosphere combined to form the organic building blocks of life.