Tag: cell as structural and functional unit

• The generally accepted parts of the cell theory include: All known living things are made up of one or more cells. 
• All living cells arise from pre-existing cells by the division and cell is the basic unit of life.

Nehmiah Grew initiate cell concept when he found that plant tissues contain minute elementary structures or cells. Thus, it was a 'basic generalization' extended to all the plants by Schleiden. Schleiden by himself neither discovered cell nor described their structure but extended the cell concept to all plants.

Rudolf Virchow explained that cells divided and new cells are formed from pre-existing cells. "Omnis cellula e cellula" is also attributed to the Cell Theory, that works on a similar principle. The cell theory asserts that every organism is formed of the same units of the organisation, known as cells.

Leeuwenhoek was the first to see 'free-living cells' under his microscope. In 1809, Lamarck concluded that all living beings are formed of cells. Living substance was first observed by Corti in 1772 and was named sarcode by Dujardin in 1835. Dutrochet believed in the individuality of cells, i.e., living organisms are made up of a number of individual cells. The nucleus was discovered by Robert Brown and Schleiden and Schwann are credited with the formulation of the cell theory. 

New cells generate from pre-existing cells. Rudolf Virchow (1985) 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 cell theory a final shape.
Cell theory now is understood as:

1. All living organisms are composed of cells and the product of cells.
2. All cells arise from pre-existing cells.
   

According to the cell theory all organisms (unicellular or multicellular) are formed of cells. It also states that all cells arise from pre-existing cells by cell division and do not originate spontaneously. Bacteria and other organisms therefore cannot arise spontaneously. So, the correct answer is 'It states that bacteria and other small organisms can arise spontaneously'.

• Reproduction is based on the phenomenon that new cells arise from preexisting cells. 
• Spontaneous generation is not possible.