49
Anatomy
CYTOMORPHOLOGY AND ULTRASTRUCTURE
The description of the algal cell will proceed from the outside structures to the inside components.
Details will be given only for those structures that are not comparable with analog structures found
in most animals and plants. The reader is referred to a general cell biology textbook for the structure
not described in the following.
oUTside The cell
Cell surface forms the border between the external world and the inside of the cell. It serves a number of basic functions, including species identification, uptake and excretion/secretion of various
compounds, protection against desiccation, pathogens, and predators, cell signaling, and cell–cell
interaction. It serves as an osmotic barrier, preventing free flow of material, and as a selective barrier for the specific transport of molecules. Algae, besides naked membranes more typical of animal
cells and cell walls similar to those of higher plant cells, possess a wide variety of cell surfaces. The
terminology used to describe surface structure of cells of algae is sometimes confusing; to avoid
this confusion, or at least to reduce it, we will adopt a terminology mainly based on that of Preisig
et al. (1994).
Cell surface structures can be grouped into four different basic types:
• Simple cell membrane (Type 1)
• Cell membrane with additional extracellular material (Type 2)
• Cell membrane with additional intracellular material in vesicles (Type 3)
• Cell membrane with additional intracellular and extracellular material (Type 4)
Type 1—Simple Cell Membrane
This cell surface consists of a simple or modified plasma membrane. The unit membrane is a lipid
bilayer, 7–8-nm thick, rich with integral and peripheral proteins. Several domains exist in the membrane, each distinguished by its own molecular structure. Some domains have characteristic carbohydrate coat enveloping the unit membrane. The carbohydrate side chains of the membrane,
glycolipids, and glycoproteins form the carbohydrate coat. Difference in the thickness of plasma
membrane may reflect differences in the distribution of phospholipids, glycolipids, and glycoproteins (Figure 2.1).
A simple plasma membrane is present in the zoospores and gametes of Chlorophyceae
(Chlorophyta), Xanthophyceae (Ochrophyta), and Phaeophyceae (Ochrophyta), in the zoospores of
the Eustigmatophyceae (Ochrophyta), and in the spermatozoids of Bacillariophyceae (Ochrophyta).
This type of cell surface usually characterizes very short-lived stages and, in this transitory naked
phase, the naked condition is usually rapidly lost once zoospores or gametes have ceased swimming
and have become attached to the substrate, since wall formation rapidly ensues. A simple cell membrane covers the uninucleate cells that form the net-like plasmodium of the Chlorarachniophyceae
(Cercozoa) during all their life history. Most Chrysophyceae occur as naked cells, whose plasma
membrane is in direct contact with water, but in Ochromonas, the membrane is covered with both
2
Anatomy
CYTOMORPHOLOGY AND ULTRASTRUCTURE
The description of the algal cell will proceed from the outside structures to the inside components.
Details will be given only for those structures that are not comparable with analog structures found
in most animals and plants. The reader is referred to a general cell biology textbook for the structure
not described in the following.
oUTside The cell
Cell surface forms the border between the external world and the inside of the cell. It serves a number of basic functions, including species identification, uptake and excretion/secretion of various
compounds, protection against desiccation, pathogens, and predators, cell signaling, and cell–cell
interaction. It serves as an osmotic barrier, preventing free flow of material, and as a selective barrier for the specific transport of molecules. Algae, besides naked membranes more typical of animal
cells and cell walls similar to those of higher plant cells, possess a wide variety of cell surfaces. The
terminology used to describe surface structure of cells of algae is sometimes confusing; to avoid
this confusion, or at least to reduce it, we will adopt a terminology mainly based on that of Preisig
et al. (1994).
Cell surface structures can be grouped into four different basic types:
• Simple cell membrane (Type 1)
• Cell membrane with additional extracellular material (Type 2)
• Cell membrane with additional intracellular material in vesicles (Type 3)
• Cell membrane with additional intracellular and extracellular material (Type 4)
Type 1—Simple Cell Membrane
This cell surface consists of a simple or modified plasma membrane. The unit membrane is a lipid
bilayer, 7–8-nm thick, rich with integral and peripheral proteins. Several domains exist in the membrane, each distinguished by its own molecular structure. Some domains have characteristic carbohydrate coat enveloping the unit membrane. The carbohydrate side chains of the membrane,
glycolipids, and glycoproteins form the carbohydrate coat. Difference in the thickness of plasma
membrane may reflect differences in the distribution of phospholipids, glycolipids, and glycoproteins (Figure 2.1).
A simple plasma membrane is present in the zoospores and gametes of Chlorophyceae
(Chlorophyta), Xanthophyceae (Ochrophyta), and Phaeophyceae (Ochrophyta), in the zoospores of
the Eustigmatophyceae (Ochrophyta), and in the spermatozoids of Bacillariophyceae (Ochrophyta).
This type of cell surface usually characterizes very short-lived stages and, in this transitory naked
phase, the naked condition is usually rapidly lost once zoospores or gametes have ceased swimming
and have become attached to the substrate, since wall formation rapidly ensues. A simple cell membrane covers the uninucleate cells that form the net-like plasmodium of the Chlorarachniophyceae
(Cercozoa) during all their life history. Most Chrysophyceae occur as naked cells, whose plasma
membrane is in direct contact with water, but in Ochromonas, the membrane is covered with both
2