3.1.3(d) - Water movement through plants

The Whole-Plant Water Gradient

The safest way to think about water movement is to follow water potential. Water moves from a region of higher water potential to a region of lower water potential. In plant cells and tissues, "higher" often means less negative, and "lower" often means more negative.

At the start of the pathway, soil water usually has a higher water potential than the contents of root hair cells. Water therefore enters root hair cells by osmosis, because it crosses a partially permeable cell surface membrane. From there, water continues across the root and into the xylem because each next region has a lower water potential.

At the end of the pathway, water leaves the leaf. Evaporation from wet mesophyll cell walls and diffusion of water vapour through stomata help keep the water potential of the leaf air spaces and surrounding air low. This maintains the gradient from root to leaf.

For example, water moves from soil at −0.10 MPa towards root cells at −0.30 MPa. Dissolved salt can lower soil water potential enough to reduce or reverse this gradient, so wet soil need not supply water effectively.

Apoplast And Symplast Pathways

Once water has entered the root, it can move across the cortex by two OCR-required pathways: the apoplast pathway and the symplast pathway. These are routes through tissue, not two different kinds of water.

That means apoplast movement stays outside cell membranes until the route is forced inward.

In the apoplast pathway, water does not cross cell surface membranes as it moves through the cell walls. This is a low-resistance route across much of the cortex. In the symplast pathway, water has crossed a cell surface membrane and then moves through cytoplasm from one cell to the next through plasmodesmata. After initial membrane entry, movement through plasmodesmata connects neighbouring cytoplasm without repeatedly crossing cell surface membranes. The driving force remains a water-potential gradient; osmosis specifically describes water crossing a partially permeable membrane.

[DIAGRAM: root_pathways: Lesson 068: Water movement through plants - diagram 01; asset_slug: 068_m03_1_3_water_movement_through_plants__diagram_01; recommended_method: drawn_biology; description: Monochrome root cortex pathway diagram showing soil water entering a root hair, apoplast movement through cell walls, symplast movement through cytoplasm and plasmodesmata, endodermis forcing water into living cells, and entry to xylem.]

A useful root detail is the endodermis. The Casparian strip, a waterproof band containing suberin in endodermal cell walls, blocks continued apoplast flow, so water that has been travelling in cell walls must enter living cells before reaching the xylem. Crossing an endodermal cell membrane allows selective control of mineral-ion entry before water reaches the xylem.

Now identify the route from its defining feature.