Algae are the plants with the simplest organization. Many of them are single-celled, some have no cell wall, others do though its composition and structure differ strongly from that of higher plants. They are good specimen for tracing back the evolution of the cell wall. Primitive cell walls do not fulfil the same requirements as that of higher plants.
It seems quite likely that a structure like that of the cell wall has developed several times in the course of evolution. All archaebacteria, eubacteria and blue-green algae (cyanobacteria or blue-green algae) have complex walls with an energetically rather costly biosynthesis. Neither in composition nor in biosynthesis do they have any common ground with the cell walls of plants.
Although the evolution of plants from early eucaryotic cells is not known in detail, is it commonly agreed on that primitive algae are flagellates closely related to the non-green flagellates. Many, though not all species of this stage of evolution, among which the euglenophyta are typical green representatives, have no cell wall. It is not only a simple membrane, but by a pellicle of already quite complex organization, that separates them from the surrounding. It consists mainly of glycoproteins organized in regular patterns the way two-dimensional crystals are. Helical ribs wind round the cell's surface.
Most single-celled algae like the Volvocales possess real cell walls. The most-studied species is Chlamydomonas reinhardii. Its wall lacks long, fibrillary carbohydrates. Most of it is made up by glycoproteins, and even here can an extensin-like protein rich in hydroxyproline be found. Among the identified sugar residues are arabinosyl-, galactosyl- and mannosyl residues. In the electron microscope does it seem as if the wall consisted of seven layers. The middle layer contains an extensive grid-shaped framework of polygonal plates consisting mainly of the mentioned glycoproteins, while the layers above and below display fibre-like structures. The thickness of the outer layer varies since it includes components that the cell takes up from its surrounding.
This indicates a main function of the cell wall of simple, single-celled algae: it mediates between the cell and its surrounding. It protects not only the cell but serves, too, communication with cells of the same or other types. It has to be permeable for metabolites and regulators and / or to carry receptor molecules with which it may contact other cells. The diversity of these functions (and their specificity) caused the evolution of a variety of differently structured cell walls.
In many-celled plants is the communication via the whole cell surface largely restricted. Contact with neighbouring cells develops in the course of tissue formation. Strength is in this respect a decisive and limiting criteria. The exchange of compounds between cells occurs via specific openings in the wall (pits, plasmodesmata). The functions originally performed by one structure are now distributed onto two different structures.