FREEZE-FRACTURE STUDIES IN THE BROWN ALGA ASTERONEMA RHODOCHORTONOIDES

The gross structure of the cell wall and the organization of the plasm alemma of the filamentous brown alga Asteronema rhodochortonoides were examined in replicas of freeze-fractured cells. The protoplasmic frac ture face (PF) of the plasmalemma, apart from the single particles, ex hibits two particular particle complexes, i.e., single linear arrays o f closely packed particles, and well defined particle pentads. The for mer display a consistent relationship with the ends of microfibril imp rints and therefore are considered as terminal complexes (TCs). They s eem to be composed of subunits, each one consisting of two particles. The average diameter of the particles is 7 nm. The number of the subun its forming the TCs varies between 2 and 40. Short TCs, consisting of 3-5 subunits were also found on the PF of dictyosome vesicles, a fact suggesting the involvement of the Golgi apparatus in exocytosis of pre formed TC portions. The occurrence, distribution and size of the TCs a ppear to be related to the developmental stage of the cell. A large nu mber of TCs occur in actively growing cells, while a few or no TCs are found in differentiated cells. The pentads are rectangular structures consisting of five particles, four in the corners and one in the cent re. Their dimensions are very constant, but their occurrence and distr ibution varies. They occur in young developing cells where TCs are few or absent, but were also observed in areas showing many TCs. In diffe rentiated cells no pentads were found. Pentad-like structures were rar ely observed on the PF of dictyosome vesicles or cisternae. The observ ations support the hypothesis that pentads are involved in the synthes is of matrix polysaccharides, which are the major components of brown algal cell wall and their synthesis begins before that of cellulose.