4.1.1  Osmotic engines for plant function

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Plants move, and when viewed at different organisational levels, such movement ranges from cellular (opening and closing of stomata) to whole organs (opening and closing of a flower). Growth can also be considered a directed movement, enabling roots to obtain nutrients (Chapter 3) and leaves to absorb light (Section 1.2). Growth is possible through co-ordination of solute and water transport at a membrane level, sustaining resource supply for biosynthesis and generating turgor pressure. Time scales of cell and organ movement can be staggeringly fast (thousandths of a second) or so slow that they can only be visualised over days with time-lapse photography.

One vital outcome of solute movement is a hydrostatic pressure difference between the inside and outside of plant cells, termed turgor pressure (P; Section 4.3). This substantial pressure (commonly 0.5–0.8 atmospheres) is exerted through a 5–10 nm thick plasma membrane appressed against a cell wall. Structural integrity of both plasma membranes and cell walls are vital to withstand P but these matrices are also modified during changes in cell shape or volume. For example, cell walls relax, stretch and rigidify, and new membranes are synthesised as cells grow. Cell movements might occur independently of those of their neighbours, such as the fine, turgor-driven move-ments of guard cells in a stomatal complex, or cooperatively, such as roots lifting a hefty concrete overburden (Section 16.1).

Ultimately, osmosis generates the hydrostatic gradients to sustain these processes. Osmotic fluxes in plants are based on proteins which catalyse solute and water transport; these proteins are embedded in membranes like components on an electronic circuit board. In this section we explore the mem-brane circuit board, the function of components and the physical principles by which they are governed.