Botany online 1996-2004. No further update, only historical document of botanical science!
Even 100 years ago were intracellular movements known. They were distinguished into random and directed movements. Random movements are caused by the Brownian movements of molecules. Depending on what is moving is it until today spoken of plasma currents, chloroplast movements, saltatory motions, chromosome distribution during mitosis and meiosis or of movements of whole cells.
Among the latter are amoeboid and flagellar movements. Many movements of whole cells are answers to an extern stimulus like light, warmth, touch, etc. Directed intracellular movements of particles within the plasma seem often to occur along established tracks.
Structure and cellular shape seem to be topics unrelated to cellular or intracellular movements. The assumption that the shape of plant cells is determined by the cell wall becomes increasingly dissatisfying. In this context is the previously discussed plasmolysis experiment once more of interest.
After addition of calcium ions becomes the shape of the protoplast clearly structured, 'Hecht's threads' (in German: Hechtsche Fäden) appear and the vacuole is often interspersed with plasma cords. But the protoplast and all vacuoles would have to look spherical if only the laws of hydrodynamics would apply. The membrane properties alone are not enough to explain the deviant shapes.
So where do the movements come in ?
The answer was found in studies on animal cells performed during the last twenty years. It was subsequently verified for plant cells, too. The most significant result was that all eucaryotic cells have a cytosceleton. Up to one third of the total cell protein of animal cells is used for this purpose.
The term cytosceleton is slightly misleading since no rigid building plan for cytosceletons with a defined place for each bone exists. The cytosceleton is actually a multi-functional structure more accurate described by the terms cytobones and cytomuscles that were suggested by C. de DUVE (1984) but never found their way into scientific literature. One of its main features is the constant restructuring. It does not only determine the shape of cells but is the basis for all intracellular and whole-cell movements, too. It consists of three independent systems, the microtubuli system, the microfilament system and the system of intermediate filaments.
The P-proteins, a peculiarity of some plant cells (mainly sieve cells) and clathrin, a protein surrounding coated vesicles should be mentioned, too. Coated vesicles develop during endocytosis. Their membrane stems either from the plasmalemma or from intracellular membranes.
Microfilaments and microtubuli have a great deal in common. Both are polymers consisting of globular protein subunits (actin and tubuline, respectively). Polymerisation is reversible and is balanced by depolymerisation. Monomerous actin is called G-actin (globular actin), polymerized actin is termed F-actin (fibrillar actin).
Tubuline is a dimerous protein out of two similar subunits (alpha and beta). On the protein level could no homologies of actin and tubuline be detected. Both systems are associated with numerous further proteins that guarantee on one hand the movements and participate in the required energy transformation (the splitting of ATP) and that help on the other hand regulating the movements. In this context becomes the question how the eliciting signal is recognized (for example in light induced movements) and the subsequent transformation into a movement interesting. The recognition will be discussed elsewhere. And about the signal transduction to elements of the cytosceleton is so far not much known.
© Peter v. Sengbusch - Impressum