One of the fun things to observe using a light microscope is the movement of chloroplasts around the cell, especially in the plant Elodea. This movement is referred to as cyclosis or cytoplasmic streaming.
Why do this? The streaming is thought to facilitate the transport of materials within, as well as between, cells. This “stirs the pot”, so to speak, so that the cell doesn’t “stew in its own juices”.
How does it work? This movement is on intracellular tracks called microfilaments, composed of actin proteins. The organelles are attached to the actin filaments by myosin, which is a member of a group of proteins called motor proteins. These proteins are able to transform the chemical energy in ATP into mechanical energy. Thus, myosin uses the energy released during the breakdown of ATP to change its conformation and “walk” down the actin filament.
Avoiding the Sun
In leaf cells under bright sunlight, chloroplasts may have the ability to “move into the shade” of other chloroplasts, a phenomenon called photorelocation.
“Chloroplasts gather in areas irradiated with weak light to maximize photosynthesis (the accumulation response). They move away from areas irradiated with strong light to minimize damage of the photosynthetic apparatus (the avoidance response). The processes underlying these chloroplast movements can be divided into three parts: photoperception, signal transduction, and chloroplast movement.” (from Ref 1 below)
Photoperception: Evidence presented in ref. 1 supports the hypothesis that plant blue-light photoreceptors phototropins perceive the light.
Signal transduction: This probably involves calcium signaling.
Chloroplast movement: A recent paper (see Ref. 2 below) implicates other motor proteins in this photorelocation movement that are similar to kinesins. Though kinesins normally interact with other cellular filaments called microtubules, the authors suggest that, in plants, kinesin-like proteins may be interacting with actin filaments.
1. Suetsugua, N. and M. Wada (2007) “Chloroplast photorelocation movement mediated by phototropin family proteins in green plants.” Biological Chemistry Vol. 388, pp.927-935. (Abstract)
2. Suetsugu, N., N. Yamada, T. Kagawa, H. Yonekura, T.Q.P. Uyeda, A. Kadota, and M. Wada (2010) “Two kinesin-like proteins mediate actin-based chloroplast movement in Arabidopsis thaliana.” Proceedings of the National Academy of Sciences (USA) Vol. 107, pp. 8860-8865 (Abstract)
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