Cell Wall

Cytomorphology and Ultrastructure
  Outside the Cell
    - Type 1: Simple Cell Membrane
    - Type 2: Cell Surface with Additional Extracellular Material
      - Mucilages and Sheaths
      - Scales
      - Frustule
      - Cell Wall
      - Lorica
      - Skeleton
    - Type 3: Cell Surface with Additional Intracellular Material in Vesicles
    - Type 4: Cell Surface with Additional Extracellular and Intracellular Material
      - First Level
      - Second Level
      - Third Level

A cell wall, defined as a rigid, homogeneous and often multilayered structure, is present in both prokaryotic and eukaryotic algae.

In the Cyanophyta the cell wall lies between the plasma membrane and the mucilaginous sheath; the fine structure of the cell wall is of Gram-negative type. The innermost layer, the electron-opaque layer or peptidoglycan layer, overlays the plasma membrane, and in most cyanobacteria its width varies between 1 and 10 nm, but can reach 200 nm in some Oscillatoria species. Regularly arranged discontinuities are present in the peptidoglycan layer of many cyanobacteria; pores are located in single rows on either side of every cross wall, and are also uniformly distributed over the cell surface. The outer membrane of the cell wall appears as a double track structure tightly connected with the peptidoglycan layer; this membrane exhibits a number of evaginations representing sites of extrusion of material from the cytoplasm through the wall into the slime. The cell wall of Prochlorophyta is comparable to that of the cyanobacteria in structure and contains muramic acid.

Eukaryotic algal cell wall is always formed outside the plasmalemma, and is in many respects comparable to that of higher plants. It is present in the Rhodophyta, Eustigmatophyceae (Figure 2.15a and 2.15b), Phaeophyceae (Heterokontophyta), Xanthophyceae (Heterokontophyta), Chlorophyceae, and Charophyceae (Chlorophyta). Generally, cell walls are made up of two components, a microfibrillar framework embedded in an amorphous mucilaginous material composed of polysaccharides, lipids, and proteins. Encrusting substances such as silica, calcium carbonate, or sporopollenin may be also present. In the formation of algal cell walls the materials required are mainly collected into Golgi vesicles that then pass it through the plasma membrane, where enzyme complexes are responsible for the synthesis of microfibrils, in a pre-determinate direction.
Transmission electron microscopy image of Nannochloropsis sp. in transversal section (a). Arrows point to the cell wall. Negative staining of the shed cell walls (b)
FIGURE 2.15 Transmission electron microscopy image of Nannochloropsis sp. in transversal section (a). Arrows point to the cell wall. Negative staining of the shed cell walls (b). (Bar: 0.5 µm.)

In the Floridophyceae (Rhodophyta) the cell wall consists of more than 70% of water-soluble sulfated galactans such as agars and carrageenans, commercially very important in food and pharmaceutical industry, for their ability to form gels. In the Phaeophyceae (Heterokontophyta) cell wall mucilagine is primarily composed of alginic acid; the salts of this acid have valuable emulsifying and stabilizing properties. In the Xanthophyceae (Heterokontophyta) the composition of the wall is mainly cellulosic, while in the Chlorophyceae (Chlorophyta) xylose, mannose, and chitin may be present in addition to cellulose. Some members of the Chlorophyceae (Chlorophyta) and Charophyceae (Chlorophyta) have calcified walls.