Hello,
David Hagerberg (mik_daha at luecology.ecol.lu.se) wrote:
: Now I would like to release for discussion:
: What is a normal eukaryotic cell?
: What is the most common eukaryotic cell?
Nature doesn't work like that. There are a lot of different lifeforms on
planet earth and additionally in multicellular organisms, cells are
specialized to perform different tasks.
Some unicellular parasites don't have mitochondria, their ancestor
possibly separated from the evolutionary tree before the endocytobiosis of
the mitochondrial ancestor happened.
Some lifeforms do photosynthesis:
In the green algae and their derivatives, the plants, a green chloroplast
is present.
In red algae a red chloroplast ("rhodoplast") is present.
Glaucocystophytes contain a blue plastid called a "cyanelle" with a
mureinsacculus still surrounding the cyanelle.
These three lineages seem to be derived from a primary endocytobiosis of a
cyanobacterium.
Other photosynthetic organelles are surrounded by three and more membranes
sometimes even with a second vestigial nucleus in the periplastidial
space as a result of secondary endocytobiosis (= four genomes in one
cell).
Most of these organisms seem to be descendants of protozoa who engulfed an
algal cell of the green, red or cyanelle evolutionary line.
Things start getting really complicated when cell division is included in
the discussion: simple bifission, symmetric/asymmetric cell division,
constriction through the middle of the cell, phragmoplast, phycoplast,
pole reversal with complete reconstruction of cell polarity, cells with or
without rigid surrounding, which might be intra- or extracellular.
Now which cell is the most common?
Animal type? Perhaps embryonic cells, since after differentiation there
are only highly specialized cells left with features not exactly typical
for eukaryotic cells in general.
Plant cells? Not all eukaryotic cells contain a plastid, glyoxysomes
and are surrounded by a cell wall. And even plants are organisms with
cells performing different tasks.
Unicellular lifeforms? They are even more complex since they can't do job
sharing like cells of an organism. They may be phagocytotic, heterotrophic
or mixotrophic, nonmotile, creeping by rhizopodia, mucus or raphes,
swimming with flagella or cilia, they may transform to gametes or resting
stages.
To my opinion there is no such thing as a "common eukaryotic cell". But
one may say: Nearly all eukaryotic cells have a basic set of organelles in
common, comprising nucleus, mitochondria (except for those
before-mentioned parasites), endoplasmic reticulum, golgi
apparatus/dictyosomes.
But this is only a model of a cell reduced to the features common to most
eukaryotic cells. To be able to survive, every cell has to perform
additional tasks which show a great variety and cannot be summarized in a
description of "the most common eukaryotic cell".
To discuss a specific cell, you have to compare its additional features
with the additional features of other cells. Plant and algal cells are
different from animal cells in that they contain plastids. Green plastids
and red plastids show a different set of photosynthetic pigments. A
mammalian muscle cell contains a highly developped actin-myosin-system, a
T-lymphocyte of that same organism produces antibodies.
To become aware of the typical features of a fungal cell compare it to an
animal cell or a plant cell.
Best regards,
Kerstin Hoef-Emden
