thumbnail|Chloroplasts can be seen travelling around the central vacuole of a cell in Rhizomnium punctatum
thumb|Cytoplasmic streaming in [[onion epidermal cell]]
Cytoplasmic streaming, also called protoplasmic streaming and cyclosis, is the flow of the cytoplasm inside the cell, driven by forces from the cytoskeleton. It is likely that its function is, at least in part, to speed up the transport of molecules and organelles around the cell. It is usually observed in large plant and animal cells, as well as amebae, fungi and slime molds. It is seen in cells greater than approximately 0.1 mm. In smaller cells, the diffusion of molecules is more rapid, but diffusion slows as the size of the cell increases, so larger cells may need cytoplasmic streaming for efficient function. This process is complicated, with temperature alterations in the system increasing its efficiency, with other factors such as the transport of ions across the membrane being simultaneously affected. This is due to cells homeostasis depending upon active transport which may be affected at some critical temperatures.
In plant cells, chloroplasts are transported within the cytoplasmic stream to optimize their exposure to light for photosynthesis. This rate of motion is influenced by several factors including light intensity, temperature, and pH levels. Cytoplasmic streaming is most efficient at a neutral pH and tends to decrease in efficiency under conditions of both low and high pH.
Cyoplasmic streaming was first discovered by Italian scientist Bonaventura Corti in 1774, within the algae genera Nitella and Chara but it is still not fully understood how it comes about.
Mechanism
What is clearly visible in plants cells which exhibit cytoplasmic streaming is the motion of the chloroplasts moving with the cytoplasmic flow. This motion results from fluid being entrained by moving motor molecules of the plant cell. Myosin filaments connect cell organelles to actin filaments. These actin filaments are generally attached to the chloroplasts and/or membranes of plant cells.
In Chara corallina
Chara corallina exhibits cyclic cytoplasmic flow around a large central vacuole. In Chara coralina, cells can grow up to 10 cm long and 1 mm in diameter. Thus for a 1 mm diameter cell, the vacuole can have a diameter of 0.8 mm, leaving only a path width of about 0.1 mm around the vacuole for cytoplasm to flow. The cytoplasm flows at a rate of 100 microns/sec, the fastest of all known cytoplasmic streaming phenomena. Second, Raymond Goldstein and others developed a mathematical fluid model for the cytoplasmic flow which not only predicts the behavior noted by Kamiya and Kuroda, Thus, the unique flow trajectories of the cytoplasmic flow in Chara coralina lead to enhanced nutrient transport by diffusion into the storage vacuole. This allows for higher concentrations of nutrients inside the vacuole than would be allowed by strictly longitudinal cytoplasmic flows. Goldstein also demonstrated the faster the cytoplasmic flow along these trajectories, the larger the concentration gradient that arises, and the larger diffusive nutrient transport into the storage vacuole that occurs. The enhanced nutrient transport into the vacuole leads to striking differences in growth rate and overall growth size. This occurs in the chloroplasts of plants cells. Light photons interact with various intermembrane proteins of the cholorplast to accomplish this. However, these proteins can become saturated with photons, making them unable to function until the saturation is alleviated. This is known as the Kautsky effect and is a cause of inefficiency on the ATP production mechanism. Cytoplasmic streaming in Chara corallina, however, enables chloroplasts to move around the stem of the plant. Thus, the chloroplasts move into lighted regions and shaded regions. The barber pole chloroplast motion resulting from cytoplasmic streaming has one flow upward and another downward. It has been demonstrated that while the molecules are similar to those in humans, the molecule blocking the binding site of myosin to actin is different. While, in humans, tropomyosin covers the site, only allowing contraction when calcium ions are present, in this amoeboid, a different molecule known as calmodulin blocks the site, allowing relaxation in the presence of high calcium ion levels. Small holes in the septum allow cytoplasm and cytoplasmic contents to flow from cell to cell. Osmotic pressure gradients occur through the length of the cell to drive this cytoplasmic flow. Flows contribute to growth and the formation of cellular subcompartments.
Contribution to growth
Cytoplasmic flows created through osmotic pressure gradients flow longitudinally along the fungal hyphae and crash into the end causing growth. It has been demonstrated that the greater pressure at the hyphal tip corresponds to faster growth rates. Longer hyphae have greater pressure differences along their length allowing for faster cytoplasmic flow rates and larger pressures at the hyphal tip. However, eddies only form before the septum in Neurospora crassa. This is because when microtubules enter the septal hole, they are arranged parallel to flow and contribute very little to flow characteristics, however, as the exit the septal hole, the orient themselves perpendicular to flow, slowing acceleration, and preventing eddy formation.
Microfilaments, independent of microtubules and myosin 2, form a mesh network throughout the cell. Nuclei, positioned in non-centered cell locations, have been demonstrated to migrate distances greater than 25 microns to the cell center. They will do this without going off course by more than 6 microns when the network is present.