Pectin polysmers are integral part of plant cell wall forming gel matrix in which are embedded cellulose and hemicellulose.
It is well known that pectin polymers form gel at low acidic pH less than 3.5.
1) Is it the same case under physiological cell wall biosynthesis conditions in plants (in planta) ?
2) Can we say that the plant cell wall per se (excluding the apoplast) has a pH=3.5 ?
Dear Mohammad,
Pectin in plant cells is not in the same (gel-)state as when extracted and dissolved in an acidic aqueous solution. Pectin can be considered a complex polyanion, in the plant cell it may be partly covalently bound to cellulose, it may be bridged by bivalent metal ions and it can interact with (oppositely charged) species by electrostatic, van der Waals and hydrogen-bridge interactions. The exact organization of pectin is complex and still not fully elucidated but together with other cell wall polymers it forms intricate structures. Once extracted from cell walls, any covalent bonds are broken and without the other polymers present, pectin behaves like a polyanion. This means it may start to gel from pH values of 5 and lower, or by the addition of certain metal counter-ions. The gel obtained here is significantly different from pectin's organization under physiological conditions in the living cell.
Generally speaking, most living cells have an endoplasmatic pH of about 7 (reported ranges depend on location and vary from 6.5 to 8.5). You may check for yourself on http://book.bionumbers.org/what-is-the-ph-of-a-cell/
So i short, no the cell does not have a pH of 3.5 and its functionality is not derived from gelling as observed from the isolated species in an acidic aqueous environment.
Kind regards, Leo
Pectins are located in the cell walls and bathed by apoplastic fluid, not in the cells themselves, and the pH of the apoplasts appears to be i) different from that inside the cell (lower) and ii) tightly regulated in function of growth, hence the "acid growth" hypothesis). So pectins could very well be in conditions (water activity, pH, presence of calcium) allowing their gelation in muro.
There's nice pictures of pH variability in muro in http://dev.biologists.org/content/develop/125/15/2925.full.pdf
thank you Mrs. Renard for your answer and for providing that valuable article.
Do you mean that the cell wall is constantly soaked with the apoplastic fluid and depends on its composition and pH ?
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