18 research outputs found
Diffusion and bulk flow in phloem loading - a theoretical analysis of the polymer trap mechanism in plants
Plants create sugar in the mesophyll cells of their leaves by photosynthesis.
This sugar, mostly sucrose, has to be loaded via the bundle sheath into the
phloem vascular system (the sieve elements), where it is distributed to growing
parts of the plant. We analyze the feasibility of a particular loading
mechanism, active symplasmic loading, also called the polymer trap mechanism,
where sucrose is transformed into heavier sugars, such as raffinose and
stachyose, in the intermediary-type companion cells bordering the sieve
elements in the minor veins of the phloem. Keeping the heavier sugars from
diffusing back requires that the plasmodesmata connecting the bundle sheath
with the intermediary cell act as extremely precise filters, which are able to
distinguish between molecules that differ by less than 20% in size. In our
modeling, we take into account the coupled water and sugar movement across the
relevant interfaces, without explicitly considering the chemical reactions
transforming the sucrose into the heavier sugars. Based on the available data
for plasmodesmata geometry, sugar concentrations and flux rates, we conclude
that this mechanism can in principle function. We find that the water flow
through the plasmodesmata, which has not been quantified before, contributes
only 10-20% to the sucrose flux into the intermediary cells, while the main
part is transported by diffusion. On the other hand, the subsequent sugar
translocation into the sieve elements would very likely be carried
predominantly by bulk water flow through the plasmodesmata. Thus, in contrast
to apoplasmic loaders, all the necessary water for phloem translocation would
be supplied in this way with no need for additional water uptake across the
plasma membranes of the phloem.Comment: 29 pages with 5 figure
Hydrodynamics of small marine organisms: A mechanistic exploration of traits and trade-offs for flagellates and filter feeders
Energy harvesting through gas dynamics in the free molecular flow regime between structured surfaces at different temperatures
For a gas confined between surfaces held at different temperatures the
velocity distribution shows a significant deviation from the Maxwell
distribution when the mean free path of the molecules is comparable to or
larger than the channel dimensions. If one of the surfaces is suitably
structured, this non-equilibrium distribution can be exploited for momentum
transfer in tangential direction between the two surfaces. This opens up the
possibility to extract work from the system which operates as a heat engine.
Since both surfaces are held at constant temperatures, the mode of momentum
transfer is different from thermal creep flow that has gained more attention so
far. This situation is studied in the limit of free-molecular flow for the case
that an unstructured surface is allowed to move tangentially with respect to a
structured surface. Parameter studies are conducted, and configurations with
maximum thermodynamic efficiency are identified. Overall, it is shown that
significant efficiencies can be obtained by tangential momentum transfer
between structured surfaces
Swimming and feeding of mixotrophic biflagellates
Many unicellular flagellates are mixotrophic and access resources through both photosynthesis and prey capture. Their fitness depends on those processes as well as on swimming and predator avoidance. How does the flagellar arrangement and beat pattern of the flagellate affect swimming speed, predation risk due to flow-sensing predators, and prey capture? Here, we describe measured flows around two species of mixotrophic, biflagellated haptophytes with qualitatively different flagellar arrangements and beat patterns. We model the near cell flows using two symmetrically arranged point forces with variable position next to a no-slip sphere. Utilizing the observations and the model we find that puller force arrangements favour feeding, whereas equatorial force arrangements favour fast and quiet swimming. We determine the capture rates of both passive and motile prey, and we show that the flow facilitates transport of captured prey along the haptonema structure. We argue that prey capture alone cannot fulfil the energy needs of the observed species, and that the mixotrophic life strategy is essential for survival