5 - Membranes and Organelles of Dictyostelium

Summary

Embryonic cells are spared some of the tasks that confront soil amoebae. Multicellular organisms are protected from direct contact with the environment because they are part of a large mass with specialized external epithelia. The plasma membrane of soil amoebae offers none of the protection of a multicellular organism. Every time it rains, the osmotic shock to the cells will be severe, because rainwater is essentially distilled water and like other single-celled organisms, D. discoideum and its relatives must be equipped to handle sudden changes in osmolarity. Other conditions, for example hyperosmotic mud, lead to hypertonic shock, for which the cells also have adaptive mechanisms. The internal membranes and organelles resemble those of higher organisms. Many of the membrane compartments of the amoebae can be marked by fusing the green fluorescent protein (GFP) gene to genes that code for appropriately targeted proteins. This useful property is illustrated in Plate 1.

The plasma membrane

The plasma membrane must be capable of movement on a variety of surfaces that constitute the matrix of the soil, whether the cellulose of decayed leaves, minerals, decaying vegetable matter, or films of bacterial growth. These cells must have substrate adhesion systems that are more versatile than those of embryonic cells, which move on a defined extracellular matrix. The amoebae eat all manner of bacteria and yeasts, and even undertake the occasional act of cannibalism, so several mechanisms of cell recognition must be deployed on the membranes.