2.1.1 Outline Cell Theory 

  • All living things are made of cells.
  • Cells are the smallest unit of life.
  • Existing cells have come from other cells.
  • Stated in this way Cell Theory might be attributed to Schleiden and Schwann (1838).
  • Robert Hooke first coined the term 'cell' after observing the structure of cork in 1655.
  • The first observation of living cells was by Anton van Leeuwenhoek in 1674.
  • Retrospectively we regard the observations of Hooke and Leeuwenhoek as important contributions in the development of cell biology. However at the time the the microscope was regarded by many as a toy and not as a serious scientific instrument. The philosophy of science at this time was dominated by the physical sciences and 'reductionism' which held the view that all natural phenomena could ultimately all be best explained in terms of its constituent parts, namely molecules and atoms. Biology at this time was not established as a serious autonomous science, rather, physics and chemistry were regarded as 'science'. The dutch were regarded as the best lens makers of the time and it is there that the best telescopes and microscopes were being made. Newton himself was not appointed to the Royal Society until (December 21, 1671 ) the invention of his reflecting telescope. The work for which Newtonian became more famous followed this time and began with his discussion on how light is composed of a spectrum. The context and historical development of  has been covered by in the outstanding BBC broadcast In Our Time.   

 

2.1.2 Discuss the evidence of cell theory

a. All living things are made of cells:

When living things are observed under the microscope they consistently appear to be composed of cells. However, there are a number of examples of cells that do not conform to the standard notion of what a cell looks like at the microscopic level. This is a typical example of how variation in biological systems means that 'biological theories' are based on concepts or regularities.

Muscle cells: 

Muscle cells have more than one nucleus per cell

Muscle Cells called fibres can be very long (300mm).

They are surrounded by a single plasma membrane but they are multi-nucleated.(many nuclei).

This does not conform to the standard view of a small single nuclei within a cell

Fungal Cells
Fungal Hyphae: again very large with many nuclei and a continuous cytoplasm

The tubular system of hyphae form dense networks called mycelium.

Like muscle cells they are multi-nucleated

They have cell walls composed of chitin

The cytoplasm is continuous along the hyphae with no end cell wall or membrane

Protoctista:


A cell capable of all necessary functions Amoeba Single celled organisms have one region of cytoplasm surrounded by a cell membrane.

The protoctista cell is unusual in that it performs all functions. Such cells are usually much larger than other cells such that some biologist consider them 'acellular', that is, non-cellular.

This is an image of an amoeba. A single cell protoctista capable of all essential functions. What cell organelles can you see?

b. Cells are the smallest unit of life.

  • The cell is the smallest unit of organisation that can show all the characteristics of living processes.
  • Organelles often require the cooperation of other organelles for their successful function.
  • An interesting example is the mitochondria which can replicate and carry out metabolism. However the mitochondria needs metabolic product from the cytoplasm to begin its respiratory processes.

c. Cells come only from other cells.

  • Cells carry out a form of cell division to form new cells. This process of cell replication in eukaryotes is called mitosis and in prokaryotes is called binary fission. The parental cell divides to produce identical daughter cells.
  • This aspect of cell theory suggests that all cells therefore have a common ancestor, the original ancestoral cell form which all other cells have arisen by descent. (origin of cellular life).
  • This relationship of common ancestor therefore suggest that all organisms are related.
  • Cell theory replaces the former ideas of  or  in which inanimate matter assembles itself into living forms. This was particularly believed to be the case in out breaks of diseases. These ideas are then replaced by the work of , ,  and . There still remains the necessary idea however that at some point cellular life developed from non-cellular form. No doubt this pre-cellular form was a replicating macromolecule perhaps like RNA. Much of this discussion is based on the notion that life takes cellular form and that it is possible to define the exact boundary between living and non-living. There are scientist who suggest that too much importance is attached to the resolution of the question and that the definition of 'life' is of little value.
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2.1.3 Unicellular organisms carry out all the functions of life.

Unicellular organisms (Protoctista) evolved some 3-4 Billion (109 ) years ago. These unicellular life forms remained the dominant life form until the Precambrian period 600million years ago after which multi-cellular life forms proliferated. Other unicellular groups include the prokaryotes (Bacteria) which lack a true nucleus.

These organisms are able to carry out all the processes which are characteristic of living things such as:

a. metabolism which includes respiration the synthesis of ATP.
b.
response to a change in the environment
c.
homeostasis the maintenance and regulation of internal cell conditions.
d.
growth which ...

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