What is a cell?

  • Print
Although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction.
The Columbia Encyclopaedia, First Edition, first sentence of the article on “life”, 1935

A cell is the smallest structure capable of basic life processes, such as absorbing nutrients, expelling waste, and reproducing.

Examining a thin section of cork through a homemade microscope Robert Hooke, in 1665, bcame the first to observe a cell. And the first thing that stuck him was how the rows of tiny boxes making up the dead wood’s tissue looked the rows of “cells” occupied by monks in a monastery; hence the name!

onion_cellscolluseum 

Some microscopic organisms, such as bacteria, consist of a single cell.

E_choli_Gram

Multicellular organisms, such as ourselves, consist of many different varieties of cells working in concert.

Kupffer_cells_high_mag_cropped

The human body is made up of around 20-30 trillion cells of over 200 different varieties. One of the largest cells is the female egg cell which is about one millimetre in diameter; one of the smallest human cells is the male sperm.

Sperm-egg

There is great variation in cell shapes. One of the longest cells are the nerve cells connecting the end of our toes to the spinal cord and so spanning half the human body. A much quoted contender for the longest cell in the animal kingdom are the nerve cells running down a giraffe’s neck which can be as long as 3 meters!

Giraffe

Cells that make up multicellular organisms are called eukaryotic cells. These have a nucleus containing DNA. This is in contrast to the simpler prokaryotic cells, found only in bacteria, where the DNA mingles freely inside the cell’s interior. Eukaryotic cells also contain other organelles such as mitochondria (in animal cells) and chloroplasts (in plant cells).

cell_diagram 

It is believed that eukaryotic cells evolved from the more primitive prokaryotic cells around 2 billion years ago when animal mitochondria and plant chloroplasts, that were once free-living bacteria, were engulfed and maintained inside other cells for their ability to produce energy.

chloroplasts

This idea of, once free, living cells living symbiotically inside our cells was used in the Star Wars movies and given the name of midi-chlorians (a derivative of mitochondria and chloroplasts) – Jedi warriers were said to have unusually high numbers of these!

star_wars