From the Soil to the Sky - How Water Moves

We’ve talked about water potential on the level of a single cell, and that would probably suffice if all we cared about were algae.  But land plants are much more complex, and interesting.  David Attenborough explains the problem in this video.

So how do you get water from the soil up to the leaf surface where you will use the water for photosynthesis and to transport necessary nutrients?   There really are only two ways to do this—you either push or pull.

It turns out that plants do both, but that one is much stronger than the other.  We’ll examine root pressure first and then transpirational pull.

To understand root pressure, you have to know something about transport across cell membranes.  Here’s a good video:

Of course, if you want to understand how a root works, you need to know something about its’ anatomy.

  

Look at the diagram first and see what you can deduce.

Things you should notice:

1.     Plant cells are connected by holes in their cell walls through which the plasma membranes are continuous.  These holes are called plasmodesmata.  You will notice that, once water passes a plasma membrane, it can travel unobstructed through the cells into the interior of the plant.
2.     Water does not have to pass a membrane at the root at a root hair.  It can enter at any point before the central ring.  This central ring of cells is called the endodermis.  They secrete a hydrophobic substance (a wax) that prevents water from moving between the cells of the endodermis.
3.     This makes the root cells one huge, continuous folded membrane in contact with the external environment (extremely high surface area to volume ratio).

Here’s a picture of a real root

The Y-shaped things are root hairs.  They are covered with fungi.  These fungi are called mycorrhizae and are mutualistic symbionts (90% of plant species have mycorrhizal partners) .  They help the plant with water and nutrient uptake in exchange for sugars.

Compare the diameter of a root hair (which is a single plant cell) with the diameter of one of the cottony fungal strands.  What does this do to the surface area:volume ratio?

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You should now realize that the root in combination with a mycorrhizal fungus is basically one huge membrane.  Now imagine that you put H+ pumps all along this membrane.  Along with these H+ pumps, you put coporters that allow the H+ ions back in, but bring along with them some nutrients like NO₃^2-, PO₄^3-.  What now happens to the osmotic conditions inside the endodermis?  Is it hypertonic or hypotonic to the soil?  Is the water potential higher or lower?

The osmotic conditions generated are enough to cause guttation on plants in wet soil in the morning. 

Root pressure is only enough to pump water up about 7 meters under the best conditions. 

The tallest tree in the world is 112 meters tall  (a football field including both end zones (122 yards).  Transpirational pull makes up the difference.  That will be the topic of my next post.