How Plants Control their "Breathing"

We've talked about how water moves through a plant and introduced the idea that gas exchange happens through the stomata.  Most people look at plants and see non-moving things; but plants actually have many moving parts.  One of these is the stomata.  Stomata are holes in the plant leaf that are surrounded by guard cells.  Plants regulate the size of this opening by osmotically swelling or shrinking the guard cells.

What gases pass through stomata?

• Carbon dioxide (in during the day, out during night)
• Oxygen (out during the day, in during the night)
• Water (out at all times)

Plants typically close their stomata at night--not all the way closed of course since they still need to get rid of carbon dioxide from respiration.  The receptor turns out to respond to blue wavelengths of light.  When you shine blue light onto the plant, the receptor triggers a signal cascade that results in the guard cells becoming turgid and the stomata open.

The mechanism for opening relies on a proton pump setting up a concentration gradient of H+ across the cell membrane.

 It uses this proton gradient to drive a K+ into the cell.  This lowers water potential in the guard cell causing water to enter.  The cells swell, opening the stomata.  Closing them seems to be a result of simply releasing the K+ by facilitated diffusion.  The exact signaling pathways that control stomata are an area of active research.  In any case, here is a fun demonstration of how stomata open and close.

From the Soil to the Sky Part II - Transpirational Pull

In the previous post, we saw that root pressure is only enough to account for water rising 7 meters.  In a tree that is 112 m tall, we still have 105 meters to go.  Using the football field analogy, we aren't even out of our own end zone using root pressure.  Transpirational pull accounts for the rest.

Water flows downhill.  It flows from where there is a lot of water to where there isn't a lot of water.  We have seen that water will flow into a cell as long as the solute concentration of the cell is higher than the external environment.  We call this cell hypertonic to its' hypotonic environment.  We say that the water potential of the cell is lower (or more negative) than the environment.  To put it in terms of water potential, water flows from high to low water potential.

Since we are talking about pulling water, we need to start where the water will be pulled from.

Leaves have openings in them called stomata which are created by two cells called guard cells (shown above).  Plants can adjust these openings so that they are wide open, partly open or even tightly closed (more on the mechanism later).  Most stomata are on the undersides of leaves.  Gases including carbon dioxide, oxygen and water vapor are exchanged through the stomata.

Leaf SEM

Leaf cross section (light microscope)

In the cross-section of a leaf above, you can see that the stomatal opening allows gases into the spongy mesophyll, which is an air-filled cavity in the leaf surrounding plant cells.  Each of these cells is exposed to the air, and is also connected to other cells which are connected to veins.  Veins contain xylem cells (pictured below) which carry water.

Wood is xylem.  Water travels in a continuous fashion from the spongy mesophyll all the way down the plant stem in the xylem.  Each water molecule hydrogen bonds to the ones around (and below) them--this is cohesion.  When one molecule evaporates into the air space in the leaf, other molecules are pulled up to take its place.  Because they are all connected, each water molecule pulls on the one below it, much like in the game barrel of monkeys.

Cohesion of water is like a barrel of monkeys.

Water flows into the air from the plant because there is less water there--the water potential is lower.  Depending on the humidity, air may be between -10 and -100 MPa.  As we move into the stomatal cavity, humidity goes up and water potential also becomes closer to zero (higher).  As long as the water potential is progressively higher as we go down the tree to the root, water will continue to flow from the roots up the stem to the leaf and out the stomata.

Remember that life is in a dynamic state of disequilibrium.  As the soil drys out, the plant will do things to conserve water and to maintain its internal water potential lower than the soil.  At some point this will be impossible and the plant.

What factors do you suppose affect the rate of transpiration (the loss of water from a leaf surface)?  How could you measure transpiration?  How would you design an experiment to test the effect of one of these factors?

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.

Atomsmith Demonstration

Atomsmith is a really cool program, but doesn't seem to work for everyone. A very computer-savvy student could probably contact the authors and help to de-bug it for multiple platforms. It really is a program that should be used in every science class.

Since many of my students have had problems with it, and my non-student readers don't have access to the program, I recorded a screencast of some of the important parts of the summer assignment I wrote. Here it is:

Overview of the Research Project with Due Dates