Unicellular to multicellular organisms


The principle of organic evolution reveals that the earliest organisms were unicellular, some of them later evolved into multicellular organisms, many of which finally became very complex .This change in the organization of the organisms raises a few important questions.

Why didn't later formed higher organisms become enormous cells?


Why do UNICELLULAR organisms divide on attaining a characteristic size?

Unicellular organisms divide to maintain a small size for multiple reasons:

1):Efficient nutrient uptake:Unicellular organisms have a high surface-to-volume ratio, which allows for efficient exchange of nutrients and waste materials with their immediate environment. As they grow larger, their volume increases faster than their surface area, limiting their ability to efficiently obtain nutrients. By dividing into two smaller daughter cells, the surface-to-volume ratio is restored, ensuring efficient nutrient uptake.

2):DNA replication and distribution:Unicellular organisms need to replicate their DNA before cell division. As cells grow larger, it becomes more challenging to replicate the entire genome accurately and distribute the DNA equally between the daughter cells. Dividing into two smaller cells ensures that each daughter cell receives an identical copy of the genetic material.

3): Energy efficiency:In small unicellular organisms, energy requirements are relatively low. As they grow larger, their energy demands increase, and it becomes harder to sustain their metabolic activities. Dividing into smaller cells allows each daughter cell to have a lower energy demand, making it easier for the organism to meet its energy requirements.

4): Environmental adaptability:Unicellular organisms rely on their ability to adapt quickly to changing environmental conditions. Through cell division, they can increase their population size rapidly, enhancing their chances of survival. Additionally, dividing into smaller cells allows them to disperse to different locations and explore new habitats, maximizing their chances of finding a suitable environment.

In summary, unicellular organisms divide upon reaching a certain size to maintain efficient nutrient uptake, ensure accurate DNA replication and distribution, enhance energy efficiency, and increase environmental adaptability. This process allows them to maximize their chances of survival and reproduction.

Why did  further development lead to multicellularity?

Answer to all these question lies in the ratio of surface area to volume.A cell receives food and oxygen and turns out waste materials through its membrane. A cell membrane of specific area can serve the cell contents of a particular volume. Increase in the volume increases the requirements of the cell and this needs greater membrane area. The surface area of a sphere ( a general cell shape) increases as the square of the radius and the volume increase as the cube of the radius.Hence, as a cell grows, its surface area, in due course of time, becomes insufficient to move the required amounts of materials in and out.The cell, by dividing, forms two cells having more favourable surface area to volume ratio.Thus, the cells must maintain their size so that their requirements of nutrition,respiration and excretion are properly met.This explain why the later formed higher organisms could not become enormous cells.The other alternative for them was to become multicellular and attain large size.

Way to Multicellularity 

Multicellular organization probably became possible by remaining together of the cells formed by division of a parent cell as occurs at present in the segmentation of the zygote.As the evolution of multicellular organisms progressed, the number of cells per individual increased. Soon came a new change: layers of cells.Cells began to differentiate ( specialized), each performing a specific function in addition to its universal metabolic functions. With an increases in the number of cells and the consequent increase in the size of the organisms, several groups of cells started to cooperate in performing a particular set of functions. Such groups of cells are called the tissues. Cooperation among the tissues led to the formation of higher functional units termed organs,and the cooperation among the organs produced the largest functional units known as  the organ_ system.Thus, once evolved, the multicellular organisms gradually become complex. 


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