WHAT IS A CELL?

All organisms are composed of cells. Some organisms are made up of a single cell and these are called Unicellular organisms. Some organisms composed by many cells are called multi-cellular organisms. Unicellular organisms are capable of :

i) independent existence

 ii) performing the essential functions of life.

iii) Cell is the fundamental structural and functional unit of all living organisms.

CELL THEORY

Cell theory was put forward by Schleiden and Schwann. Which states that 'the bodies of all living beings are formed of cells and their products and that the cells are structural and functional units of living beings'. M.J.Schleiden was a German Botanist published his findings in 1838.

i) All types of plant tissues are made of one or other type of cells.

ii) Cells are structural units of all plant tissues.

Theodore Schwann was a German Zoologist who found that animal tissues were made of cells and that animal cells differ from plant cells in lacking cell wall. He therefore defined cell as membrane enclosed, nucleus containing structure. This theory however did not explain as to how new cells were formed. Rudolf Virchow first explained that cells divide and new cells are formed from pre-existing cells (Omnis Cellula-e-Cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Two basic aspects of cell theory are

i)Body of all living organisms made of cells and their products.

 ii) Cells develop from pre-existing cells.

 AN OVERVIEW OF CELL

The Onion cell which is a typical plant cell has a distinct cell wall. The cells of the human cheek have an outer plasma membrane as the delimiting structure of the cell. Membrane bound nuclei are called eukaryotes. whereas cells that lack a membrane nucleus are called Prokaryotes. In both the cells cytoplasm occupies the volume ( the cell, It is a semi-fluid matrix). Ex In plants and animal cells cytoplasm is the main site of cellular activities. Various chemical reactions occur in cytoplasm and keep the cell in the 'living state'. Eukaryotic cells have membrane bound organelles like endoplasmic reticulum, golgi complex, lysosomes, mitochondria, plastids(in plants), microbodies and vacuoles. In Prokaryotic cells membrane bound organelles are absent. Ribosomes are non-membrane bound organelles found in both eukaryotics as well as prokaryotics. These ribosomes not only present in cytoplasm but also found in chloroplast(in plants), mitochondria and on rough ER. Prokaryotes contain 70s types of ribosome, where as in eukaryotes 80s (in cytoplasm, ER) 70s(in chloroplast and mitochondria). Animal cells contain another non-membrane bound organelle called centriole help in cell division.