The eye is a complex sensory organ that allows us to see the world around us. The eye works by gathering light from the environment and converting it into electrical signals that can be interpreted by the brain. The basic structure of the eye includes the cornea, iris, pupil, lens, retina, and optic nerve. The cornea is the transparent outer layer of the eye that helps to focus incoming light. The iris is the colored part of the eye that regulates the amount of light that enters the eye through the pupil, which is the black circular opening in the center of the iris. The lens is a clear, flexible structure located behind the iris that helps to focus light onto the retina. The retina is a thin layer of tissue that lines the back of the eye and contains photoreceptor cells called rods and cones. These cells convert light into electrical signals that are sent to the brain via the optic nerve. The rods are responsible for the low light vision and detecting motion, while the cones are ...
Biological evolution on primitive earth
In previous post we talk about chemical evolution on primitive earth that how the protoplasmic constituents were formed and where? Now today we move to the biological evolution and start talking about how these protoplasmic constituents aggregate and form the first living cell and after that eukaryotic cell, so lets start.
Biological evolution also known as biogeny. We will study the biological evolution in four steps:
(1) Origin of protobionts:
Macromolecules which were synthesized abiotically in primitive oceans later came together and formed large colloidal drop like structures named as protobionts. It is believed that they were the clusters of proteins, polysaccharides, lipids, nucleic acids, etc. In these protobionts all prtoplasmic constituents were present but yet it is not reproduce or exhibit metabolism due to inactive proteins and nucleic acids but they exhibit growth by absorbing molecules from their surroundings.
Protobionts were also synthesized by some scientists in laboratory. Oparin prepared some protobionts without a lipid membrane and he called the coacervates. Similarly Sydney fox also synthesized some microsporic protenoid bodies with a lipid coat and called them microspheres.
(2) Origin of protocells (Eobionts):
After some time Nucleic acid developed the ability of self duplication due to sudden change called mutation. Nucleic acid and proteins combined to form nucleoproteins which were the first sign of life. And after that these nucleoproteins combined and form cluster of nucleoproteins and then these clusters were surrounded by lipid coat and the structure form is called the protocell, which were the first form of life. These first non-cellular forms of life could have originated around 3 billion years ago.
The protocell is the first form of life but it is not a cell because these are only the aggregation of giant molecules which were surrounded by capsule and these molecules reproduce their molecules perhaps.
(3) Origin of first cellular form (Prokaryotes):
As a result of mutation protocells became more complex and efficient to use the materials available
in the surrounding medium and evolved into prokaryotic cells. This is the first cellular form of life originated around 2 billion years ago.They were probably the chemoheterotrophs and because at that time the oceans were full of organic compounds so there is no need of synthesizing there own food and they are also anaerobic because atmosphere is still reducing. After some time some of these prokaryotes evolved into chemoautotrophs where synthesize their organic food by breaking down the inorganic compounds this process is called chemosynthesis. e.g. Iron Bacteria, Nitrifying bacteria etc.
When bacteriochlorophyll was developed in some Chemoautotrophic bacteria, they started to convert light energy into chemical energy. This mode of nutrition is photosynthesis but it is non oxygenic photosynthesis because they used H2S as a source of hydrogen instead of H2O. After some time some molecular changes occurred in bacteriochlorophyll, and it transformed into true Chlorophyll. Such organisms used H2O as source of hydrogen and released oxygen in the environment, they were oxygenic photosynthetic bacteria such as Cynobacteria (Blue Green algae).
After the liberation of free oxygen by cyanobacteria there was a revolutionary change in the history of earth. It includes some major changes like:
(a) Atmosphere of earth changes from reducing to oxidizing, hence possibilities of further chemical evolution finished, because chemical evolution always takes place in reducing environment. It is also said that the another reason of becoming oxidising environment is the escaping of hydrogen into space from earth's atmosphere.
(b) Accumulation of free oxygen formed a layer of ozone outside the atmosphere of earth which started to absorb most of the UV rays of sunlight.
(c) Some prokaryotes adapted themselves for aerobic mode of respiration which provides approx 20 times more energy than anaerobic respiration.
So, this is the end of Modern theory of evolution hope you will like it. In next post we will discuss about the evidences in favour of modern theory.
After the liberation of free oxygen by cyanobacteria there was a revolutionary change in the history of earth. It includes some major changes like:
(a) Atmosphere of earth changes from reducing to oxidizing, hence possibilities of further chemical evolution finished, because chemical evolution always takes place in reducing environment. It is also said that the another reason of becoming oxidising environment is the escaping of hydrogen into space from earth's atmosphere.
(b) Accumulation of free oxygen formed a layer of ozone outside the atmosphere of earth which started to absorb most of the UV rays of sunlight.
(c) Some prokaryotes adapted themselves for aerobic mode of respiration which provides approx 20 times more energy than anaerobic respiration.
(4) Origin of Eukaryotic cell:
Nucleus, mitochondria and other cell organelles developed in the cell and metabolically it became more active. These free living unicellular eukaryotic organisms originated about 1.5 billion years ago in the primitive ocean. And after that evolution further proceeds various organisms were evolved and our present day earth was formed.So, this is the end of Modern theory of evolution hope you will like it. In next post we will discuss about the evidences in favour of modern theory.
Comments
Post a Comment