CELL:THE BASIC UNIT OF LIFE

What are organisms made up of?

 We all are familiar just like a building is made up of bricks similarly the body of all the living organisms is made up of cells defined as “the basic unit of life. It is called so as a cell is the smallest unit that can perform all the processes needed for life or it is also called “the structural and functional unit of life”. Cells are usually not visible to an unaided eye except for amoeba, a paramecium and a human egg that are visible to the human eye.

 Why cells were named so?

 Robert Hooke in 1665 coined the term cell ((L., Cella = hollow space) while observing the cork cells (bark of the tree) under the microscope. The box-like cells (honeycomb-like structures) reminded him of the cells (small rooms) in which monks lived and so he named them as cells (the word cell means a small room in a prison). Robert Hooke observed dead cell walls of plant cells, it was Anton van Leeuwenhoek who in 1674 first observed a single living cell (animalcules) under the microscope. In 1674 he likely observed a protozoan and later saw bacteria. Anton van Leeuwenhoek is also known as the “Father of Microbiology”.

Cells shape, size, and number.

 Cells differ considerably in their shape and size, and this difference exists according to the function they perform. The smallest known cell is PPLO-Pleuropneumonia-like organisms or mycoplasma which is only 10 micrometer in size (1 micrometer is equal to 0.001 mm) while the largest cell is the egg cell of an ostrich (170 mm x 130 mm). In humans, the largest cell is a female ovum, while the smallest is the male gamete (sperm). The longest cell in the human body is the nerve cell.

Animal cells are usually round (spherical) or irregular (e.g., cheek cells) while the plant cells are more rigid and rectangular due to the presence of a cell wall. The shape and size of a cell depend upon:

(a) surface-volume ratio (if the cell is large it has a small surface-area-to-volume ratio, thus diffusion of substances takes a longer time. Thus most cells are small to overcome issues related to the diffusion of substances.)

(b) nucleocytoplasmic ratio(the cytoplasmic area should be just enough which a nucleus can control)

(c)  rate of cellular activity (metabolically active cells are usually smaller)

(d)  cell association (it is important in multicellular organisms that provide cells some amount of rigidity).

 Last, based on the number of cells, organisms are classified as unicellular or multicellular. Unicellular organisms like an amoeba, bacteria, and paramecium comprise one cell, while multicellular organisms are those whose body is made up of over two cells. 

Cell theory

 Cell theory was formulated in the year 1839 by Matthias Schleiden (German botanist) and Theodor Schwann. (German physician and physiologist). Besides other scientists, Rudolf Virchow, the “father of modern pathology” also contributes to this theory. Cell theory proposed that:

1. All living organisms comprise one or more cells.

2. The cell is the structural and functional unit of all living organisms.

3. All cells arise from pre-existing cells.

 Rudolf Virchow (1855) first explained the concept that new cells arise from pre-existing cells (Omnis cellula-e-cellula). He modified the hypothesis given by Schleiden and Schwann that Cells form by free-cell formation, similar to the formation of crystals and this how the cell theory got its final shape.

Amazing facts

 1. Not all organisms comprise cells, like a virus, virions and viroids are commonly referred to as acellular microbes or infectious particles (these are exceptions to the cell theory)

2. The average human body contains approximately 37.2 trillion cells!

3. Some neurons are less than one millimeter, while others may be about a meter long.

4. The largest single-celled organism is Caulerpa taxifolia (an aquatic alga), the single cell is 6-12 inches long.

5. Amongst the plants, the ovule of a cycas is the largest cell.