FIGURE 17-38 Cytoplasmic streaming in cylindrical giant algae. (a) Cells of Nitella, a freshwater alga commonly found in ponds in the summer. The cytoplasmic movement, described below, can be readily observed with a simple microscope, so go find some Nitella (or related algae) and watch this amazing phenomenon! (b) The center of a Nitella cell is filled with a single large water-filled vacuole, which is surrounded by a layer of moving cytoplasm (blue arrows). A nonmoving layer of cortical cytoplasm filled with chloroplasts lies just under the plasma membrane (enlarged in bottom figure). On the inner side of this layer are bundles of stationary actin filaments (red), all oriented with the same polarity. A motor protein (blue), a plant myosin V, carries parts of the endoplasmic reticulum (ER) along the actin filaments. The movement of the ER network propels the entire viscous cytoplasm, including organelles that are enmeshed in the ER network. (c) Electron micrograph of the cortical cytoplasm showing a large vesicle connected to an underlying bundle of actin filaments.

[Part (a) courtesy of James C. French; part (c) courtesy of Bechara Kachar.]