Explain cyclosis and in which organism it is absent.

Cyclosis, also known as cytoplasmic streaming, refers to the movement of the cytoplasm within a cell. It is a dynamic process where the cytoplasmic contents, including organelles, cytoskeletal components, and other substances, undergo directed streaming or circular flow along defined paths within the cell. Cyclosis plays a vital role in various cellular processes, including the transport of nutrients, distribution of organelles, and the movement of substances within the cell.

Cyclosis is particularly prominent in plant cells, where it occurs in various cell types, including parenchyma cells, which are responsible for many fundamental functions such as storage, photosynthesis, and secretion. In plant cells, cyclosis is facilitated by a network of actin filaments and myosin proteins, which generate the necessary force for the movement of cytoplasmic contents.

In contrast, cyclosis is generally absent in animal cells. Although some animal cells may exhibit limited cytoplasmic movement, it is typically not as pronounced or widespread as observed in plant cells. Instead, animal cells rely on other mechanisms for intracellular transport, such as vesicular transport mediated by the endoplasmic reticulum and the Golgi apparatus, as well as molecular motors like dyneins and kinesins that move along microtubules.

It's important to note that while cyclosis is absent in most animal cells, there are exceptions. For instance, certain specialized cells in invertebrates, such as the amoeboid cells of sponges or the coelomocytes of some annelids, exhibit cyclosis-like movements.

In summary, cyclosis is a prominent phenomenon of cytoplasmic streaming observed in plant cells, driven by actin-myosin interactions. It is generally absent in animal cells, which rely on other mechanisms for intracellular transport.