Page 27 - 11-Bio-14 Transport

14. Transport eLearn.Punjab

Fig. 14.13 The Pressure-flow theory (1) A photosynthesizing leaf manufactures sucrose (red dots), which (2) is actively transported (red
arrow) into a nearby companion cell. The sucrose diffuses to sieve-tube element through plasmodesmata, raising the concentration
of sucrose. (3) Water (blue dots leaves nearby xylem and moves into the “leaf end” of the sieve tube by osmosis (blue arrow), raising
the hydrostatic pressure. (4) The same sieve tube connects to a developing fruit (sink); sucrose enters the companion cells by diffusion
through plasmodesmata. It is then actively transported out of the companion cells and into the fruit cells. (5) Water moves out of
the sieve tube by osmosis, lowering the hydrostatic pressure within the tube. (6) High pressure in the leaf end of the phloem and low
pressure in the fruit end cause water, together with any dissolved solutes, to flow in bulk from leaf (source) to fruit. (Black arrow).

27 V: 1.1