3B Scientific Plant cell model, magnified 500,000-1,000,000 times - Manual (Page 2)

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Model of a Plant Cell
(Magnification approx. 500,000-1,000,000)
The history of cytology
Cytology is an independent science in botany and deals with the structure and function of plant cells. The
term cell (Latin: cellula = chamber, compartment, cell) was coined in 1665 by Robert Hooke, after he had
discovered and recorded the cells within the tissue of a bottle cork with the help of one of the earliest light
microscopes. At the beginning of the 19th Century, Franz Meyen (1804-1840) recognised cells as the ele-
mentary units of plant organs. In 1838/1839, Matthias Jacob Schleiden and Theodor Schwann establish cell
theory: “Cells are the basis for all plants and animals.” In 1845, Karl Theodor Ernst von Siebold, based on
his observations on protozoa (unicellular organisms), wrote that cells can exist independently and represent
the smallest unit of life. At the same time, Louis Pasteur and other scientists refuted the prevailing theory
of the time which stated that cells can originate spontaneously out of dead organic matter (generatio
spontanea). In 1855, Rudolf Virchow confirmed Meyen’s theory which stated that every cell originates from
another cell (“omnis cellula ex cellula”). In 1879, Eduard Strasburger discovered the division of the nucleus
in plants. An important breakthrough in understanding the structure and function of cells was achieved by
E. Ruska and H. Mahl in 1940, thanks to the development of the transmission electron microscope.
As in the animal world, plant cells, too, are characterised by the following:
• They have a more complex structure than their environment
• They react to inner and outer stimuli
• They have the ability to reproduce
Differences between animal and plant cells
In spite of the consistency regarding the cellular structure of animal and plant cells – which had been
detected by Schleiden and Schwann in 1838 – there still are important differences in their basic structural
plan. The following three features characterise the differences between most plant cells and animal cells:
1. Plant cells are enclosed by a cell wall which is responsible for resisting the inner osmotic pressure of the
cell (turgor pressure), thereby giving it rigidity and increased stability.
2. As organelles, only plant cells possess plastids. These include, for example, the green chloroplasts, the
scene of photosynthesis.
3. They possess the sap vacuoles characteristic of plants, in which dissolved substances are stored and
macromolecules broken down.
Plant cells have an average size of 10-100 µm and can be observed by using a simple light microscope. A
tree is made up of 10
13 (= 10 trillion) cells! In multicellular organisms, they form groups of homogenous,
in part strongly differentiated and specialised cells (= tissues).
Structure and function of a plant cell
(For numbering see diagram)
Note: Unlike the model presented, all components of a living cell are in a state of constant motion!!!
The individual cell components have differences in their composition, e.g. proteins/enzymes, ionic milieu,
etc., and can best be classified according to their functions. An important term in plant cytology is proto-
plast, which refers to a cell surrounded by a plasma membrane in which the cell wall has been removed.
Cytoplasm with cytoskeleton (1)
In the course of evolution, a kind of division of work originated in a cell. This division of work is called
compartmentation. It is achieved when special reaction complexes, the organelles (Greek: organon =
tool), are surrounded and defined by membranes. These organelles can be detected, with the help of a
light microscope, in the fluid and colourless cytoplasm of protoplasts (60 to 90% water, proteins, lipids,
nucleic acids). The cell membrane (2) forms the boundary of the cell, marking it off from its exterior sur-
roundings. The cell membrane consists of monomolecular layers of phospholipids and proteins which can
move in the lipid matrix (“fluid mosaic” – model). Incidentally, all plant and animal membranes are based
on the same elementary principle (unit membrane).