The effect of intracellular geometry on auxin transport - II. Geotropism in shoots

1981 ◽  
Vol 214 (1194) ◽  
pp. 69-83 ◽  
Keyword(s):  
Thin Layer ◽  
Auxin Transport ◽  
Permeability Barrier ◽  
Starch Granules

When a shoot is tilted, the transport of auxin along its axis acquires a downward component. It has been conjectured that the sinking of dense starch granules, called statoliths, to the lowest part of cells somehow causes this downward diversion of auxin flow, but it is not known how thîs is brought about. I propose a mechanism by which statolith movement could influence auxin transport. Suppose that cytoplasm accumulates in the lower part of the cell around the statoliths, displacing the vacuole upwards and leaving a comparatively thin layer of cytoplasm elsewhere. If auxin moves through cells by diffusion, and if the tonoplast is a substantial permeability barrier, then more auxin will move through the cytoplasm in the lower part of the cell, and there will be a downward diversion of auxin, as required. I give calculations to support this argument, and show that a large enough downward diversion of auxin could be achieved if the permeability of the tonoplast is not too large.

The effect of intracellular geometry on auxin transport. I. Centrifugation experiments

1981 ◽  
Vol 214 (1194) ◽  
pp. 53-67 ◽  
Keyword(s):  
Thin Layer ◽  
High Velocity ◽  
Plant Hormone ◽  
Auxin Transport ◽  
The Other ◽  
Permeability Barrier ◽  
Cell Geometry ◽  
Internal Cell ◽  
A Cell ◽  
High Concentrations

When coleoptiles are centrifuged, the velocity of transport of the plant hormone auxin is dramatically altered. I show here that this may be due to changes in internal cell geometry. The tonoplast, the membrane surrounding the vacuole, may present a substantial permeability barrier for the diffusion of auxin. After centrifugation, the cytoplasm sediments to one end of the cell, displacing the vacuole to the other. If auxin, on entering the cell, must first accumulate in a mass of cytoplasm before crossing the tonoplast, the velocity will be lowered. If, on the other hand, there is only a thin layer of cytoplasm where auxin enters, high concentrations will quickly build up and enable auxin to cross the tonoplast, giving a high velocity. This would explain why centrifugation in a basal direction increases velocity, while apical centrifugation de­creases it. If this explanation is correct, and if the tonoplast constitutes an appreciable permeability barrier, then the position of the vacuole may strongly influence the flux of auxin inside a cell. I show in the adjoining paper that this can explain the changed transport pattern seen during the geotropic response.

Microanatomy of the Periplasm of Bacillus Licheniformis

1971 ◽  
Vol 29 ◽  
pp. 262-263
Author(s):  
B.K. Ghosh
Keyword(s):  
Cell Wall ◽  
Plasma Membrane ◽  
Bacillus Licheniformis ◽  
External Environment ◽  
Permeability Barrier ◽  
Periplasmic Space ◽  
Modified Technique ◽  
Gram Positive ◽  
Gram Negative ◽  
Gram Positive Bacteria

Periplasm of bacteria is the space outside the permeability barrier of plasma membrane but enclosed by the cell wall. The contents of this special milieu exterior could be regulated by the plasma membrane from the internal, and by the cell wall from the external environment of the cell. Unlike the gram-negative organism, the presence of this space in gram-positive bacteria is still controversial because it cannot be clearly demonstrated. We have shown the importance of some periplasmic bodies in the secretion of penicillinase from Bacillus licheniformis.In negatively stained specimens prepared by a modified technique (Figs. 1 and 2), periplasmic space (PS) contained two kinds of structures: (i) fibrils (F, 100 Å) running perpendicular to the cell wall from the protoplast and (ii) an array of vesicles of various sizes (V), which seem to have evaginated from the protoplast.

Applications of Photoelectron Microscopy

1976 ◽  
Vol 34 ◽  
pp. 506-507
Author(s):  
William J. Baxter
Keyword(s):  
Electron Microscopy ◽  
Thermal Decomposition ◽  
Chemical Composition ◽  
Ultraviolet Radiation ◽  
Thin Layer ◽  
Edge Effects ◽  
Electron Optics ◽  
Surface Layers ◽  
Crystalline Orientation ◽  
Fluorescent Screen

In this form of electron microscopy, photoelectrons emitted from a metal by ultraviolet radiation are accelerated and imaged onto a fluorescent screen by conventional electron optics. image contrast is determined by spatial variations in the intensity of the photoemission. The dominant source of contrast is due to changes in the photoelectric work function, between surfaces of different crystalline orientation, or different chemical composition. Topographical variations produce a relatively weak contrast due to shadowing and edge effects.Since the photoelectrons originate from the surface layers (e.g. ∼5-10 nm for metals), photoelectron microscopy is surface sensitive. Thus to see the microstructure of a metal the thin layer (∼3 nm) of surface oxide must be removed, either by ion bombardment or by thermal decomposition in the vacuum of the microscope.

An Electron Diffraction Study on Litnerized Potato Starch

1990 ◽  
Vol 48 (1) ◽  
pp. 580-581
Author(s):  
Jean-Claude Jésior ◽  
Roger Vuong ◽  
Henri Chanzy
Keyword(s):  
Electron Microscope ◽  
Electron Diffraction ◽  
Signal To Noise Ratio ◽  
Potato Starch ◽  
Image Intensifier ◽  
Electron Diffraction Study ◽  
Starch Granules ◽  
X Ray ◽  
Individual Grains ◽  
Diamond Knife

Starch is arranged in a crystalline manner within its storage granules and should thus give sharp X-ray diagrams. Unfortunately most of the common starch granules have sizes between 1 and 100μm, making them too small for an X-ray study on individual grains. There is only one instance where an oriented X-ray diagram could be obtained on one sector of an individual giant starch granule. Despite their small size, starch granules are still too thick to be studied by electron diffraction with a transmission electron microscope. The only reported study on starch ultrastructure using electron diffraction on frozen hydrated material was made on small fragments. The present study has been realized on thin sectioned granules previously litnerized to improve the signal to noise ratio.Potato starch was hydrolyzed for 10 days in 2.2N HCl at 35°C, dialyzed against water until neutrality and embedded in Nanoplast. Sectioning was achieved with a commercially available low-angle “35°” diamond knife (Diatome) after a very carefull trimming and a pre-sectioning with a classical “45°” diamond knife. Sections obtained at a final sectioning angle of 42.2° (compared with the usual 55-60°) and at a nominal thickness of 900Å were collected on a Formvar-carbon coated grid. The exact location of the starch granules in their sections was recorded by optical microscopy on a Zeiss Universal polarizing microscope (Fig. 1a). After rehydration at a relative humidity of 95% for 24 hours they were mounted on a Philips cryoholder and quench frozen in liquid nitrogen before being inserted under frozen conditions in a Philips EM 400T electron microscope equipped with a Gatan anticontaminator and a Lhesa image intensifier.

Sign in / Sign up

Export Citation Format

Share Document