Talk:Chloroplast
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I just created a page for plastids. The chloroplast is a type of plastid, and many of the comments about chloroplasts are true for all plastids, so I am tempted to change chloroplast to plastid in those situations.
Anyway, we need to consider how much we want chloroplast to repeat information found on plastid, and how much it should just contain facts unique to chloroplasts. Since the chloroplast is by far the most well known plastid, we might want to include a lot of the plastid information here. Any thoughts?
Josh Grosse asked if there are photosynthetic bacteria that don't use the Calvin cycle. Off the top of my head, yes. They use a reverse Citric acid cycle. This process was worked out by Evans, Buchanan and Arnon in 1966. "Chlorobium tepidum" and "Chlorobium thiosulfatophilum" use this cycle. I don't recall if they also use the Calvin cycle.
Lucky I just had photosynthesis in Biology, else I wouldn't understand a word of what that article says... ^_^. ugen64 21:14, Jan 4, 2004 (UTC)
I am correcting "...two lipid bilayer membranes, now thought to correspond to the outer and inner membranes of the ancestral cyanobacterium." to "...two lipid bilayer membranes, the inner of which is now thought to correspond to the plasma membrane of the ancestral cyanobacterium." Free-swimming cyanobacterium are single-membraned - the outer membrane of the chloroplast is contributed by the engulfing eukaryote.
-Peter
- Free-living cyanobacteria have a second membrane, as is typical of Gram-negative bacteria. For a long while, it was believed the second membrane of chloroplasts came from the eukaryotic host, but evidence now points to it corresponding to the second cyanobacterial membrane. Thus, I've reverted this change. Josh 21:41, 20 Jul 2004 (UTC)
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- I'll defer to you if you're a biologist, but it is my understanding that the inner membrane of the cyanobacterium would correspond to the chloroplast's thylakoid membrane system and the outer membrane to the chloroplast's inner membrane.
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- -Peter
I'm not a biologist, but I've been reading a number of papers on the origins of eukaryotes, and this is what they say. I don't remember a specific reference; The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa by Cavalier-Smith surely mentions it, but I don't think it's available online. Thylakoids aren't something unique to chloroplasts, as the few cyanobacteria that lack phycobilins (e.g. Prochloron) have them as well. Josh 00:09, 22 Jul 2004 (UTC)
- Can you please provide a quote attesting to the prokaryotic origin of the chloroplast's outer membrane? Cavalier-Smith's work sounds fascinating, and I'll endeavour to give it a read when I'm back at college, but in the meantime perhaps you can assuage my curiosity.
- -Peter
I don't remember the particular references, but a quick web search gives results like:
Origin of a Chloroplast Protein Importer: These findings suggest that a component of the chloroplast protein import system, Toc75, was recruited from a preexisting channel-forming protein of the cyanobacterial outer membrane. Furthermore, the presence of a protein in the chloroplastic outer envelope homologous to a cyanobacterial protein provides support for the prokaryotic nature of this chloroplastic membrane.
Importance of the Plasma Membrane in the Cellular Origin and Evolution: That is, the outer membrane as the host's plasma membrane contains a large channel-forming protein (called porin), which is a constituent of the cell wall of prokaryotes, e.g., Gram-negative bacteria, and cannot be found in the plasma membrane...the outer membranes of these organelles are not the plasma membrane origin of the host cell.
- Thanks for that. The abstract of the first paper seems compelling. The second paper is interesting, but the author's "recent evidence" is not cited, and the he seems to be in favor of a non-endosymbiotic model that contradicts Cavalier-Smith's own claims of a "Neomura" clade [1] linking the Eukaryota and the Achaea, which the cyanobacteria would fall outside of. Perhaps we should document competing theories of plastid evolution in separate write-ups and link them from here.
- -Peter
I'd also like to refer you here [2] in defense of a eukaryotic origin for the plastid outer membrane. I really think Cavalier-Smith is kind of a lone gun on this hypothesis.
-Peter
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[edit] request for more detail
I'd like to see the discussion of chloroplast DNA expanded a bit--a bit more detail on its origin, its similarity to (& differences from) prokaryotic DNA, how it differs from nuclear DNA, and how it has become increasingly significant in constructing molecular phylogenies. MrDarwin 02:26, 10 January 2006 (UTC)
[edit] A Useful Offline Source
One source that could be consulted if anyone here has access to it is this:
Green Plants : Their Origin and Diversity by Peter R. Bell and Alan. R. Helmsley, published by Cambridge University Press. Hardback edition : ISBN 0-521-64109-8 Paperback edition : ISBN 0-521-64673-1 . I recall that this work contains a great deal of information regarding the possible origins of the various membranes within the chloroplast, and includes electron micrographs and a range of other illustrative material that covers the structures of interest. For example, there is a diagram on page 4 which illustrates the distribution of important molecule complexes across the various thylakoid membranes - Photosystem I complexes are confined to the outer membranes of the grana and the stroma thylakoids, while Photosystem II complexes are absent from the stroma thylakoids. Page 6 contains a fairly detailed diagram of the membrane arrangement of a thylakoid in a higher plant. It also cites possible mechanisms for the creation of certain structures in the early evolutionary history of the cell (such gems among phrases as 'invagination of the plasmalemma' abound, so it is not light reading by any stretch of the imagination!). Hopefully this source will prove of use in dealing with the above discussion on the origin of various membraneous layers. Calilasseia 09:04, 10 January 2006 (UTC)
[edit] cytosol or cytoplasm
Is the stroma the equivalent of cytoplasm or cytosol? The article says cytoplasm, but according to professors, as well as the cytoplasm article, cytoplasm is cytosol plus the organelles. This article says that stroma is a fluid that contains DNA and ribosomes, so wouldn't that make it more like cytosol? I may be wrong, but I thought someone should look into that.
[edit] Protection Status
This article has been protected since October 2006. Is this still necessary? 75.17.192.215 08:16, 11 February 2007 (UTC)
