From owner-celegans@net.bio.net Tue Sep 30 23:00:00 1997 Path: biosci!biosci!not-for-mail From: Kerstin Hoef-Emden Newsgroups: bionet.celegans Subject: Re: normal eukaryotic cell Date: 1 Oct 1997 06:27:07 -0700 Organization: Regional Computing Center, University of Cologne Lines: 97 Sender: daemon@net.bio.net Approved: wmorgan@acs.wooster.edu Distribution: world Message-ID: <60tj3b$ltb@net.bio.net> NNTP-Posting-Host: net.bio.net Hello, David Hagerberg (mik_daha@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 .