Page 52 - Algae

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 analogue 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 word 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 selec-
tive 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 sur-
faces. The terminology used to describe cell surface structures 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
Presig 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 of integral and peripheral proteins. Several domains exist in the mem-
brane, each distinguished by its own molecular structure. Some domains have characteristic carbo-
hydrate coat enveloping the unit membrane. The carbohydrate side chains of the membrane
glycolipids and glycoproteins form the carbohydrate coat. Difference in thickness of plasma mem-
brane 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, Xantho-
phyceae (Heterokontophyta), and Phaeophyceae (Heterokontophyta), in the zoospores of the
Eustigmatophyceae (Heterokontophyta), and in the spermatozoids of Bacillariophyceae (Hetero-
kontophyta). 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, as wall formation rapidly
ensues. A simple cell membrane covers the uninucleate cells that form the net-like plasmodium
of the Chlorarachniophyta 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 a carbohydrate coat and surface blebs and vesicles, which may serve to trap
bacteria and other particles that are subsequently engulfed as food. The properties of the membrane
or its domains may change from one stage in the life cycle to the next.

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