Plants ❮ 173
across selectively permeable membranes. It flows from a region with a high water concen-
tration to a region with a low water concentration. There is a higher mineral concentration
inside the vascular cylinder, which drives water into the xylem contained in this cylinder by
a force known as root pressure.
Capillary Action
Capillary actionis the force of adhesion between water and a passageway that pulls water
up along the sides. Along with osmosis, this mechanism is a minor contributor to the move-
ment of water up the xylem due to the counteracting force of gravity.
Cohesion-Tension Theory and Transpiration
This process is the major mover of water in the xylem. Transpiration creates a negative pres-
sure in the leaves and xylem tissue due to the evaporative loss of water. Water molecules dis-
play molecular attraction (cohesion) for other water molecules, in effect creating a single united
water molecule that runs the length of the plant. Imagine that you tie a bunch of soda cans to
a rope. If you are standing in a tree, and pull up on the cans at the top of the rope, the cans at
the bottom will follow—not really because they are loyal to the other cans, but because they
are connected to them, they are bonded. This is similar to the movement of water through the
xylem. When water evaporates off the surface of the leaf, the water is pulled up through the
xylem toward the leaves—transpiration is the force pulling water through the plant.
The Changing of the Guard: Regulating Stomata Activity
The stomata are structures vital to the daily workings of a plant. When closed, photosyn-
thesis is halted because water and carbon dioxide are inaccessible. When open, mesophyll
cells have access to water and carbon dioxide. But with every reward, there is always a risk.
When the stomata are open, the plant could dry out as a result of excessive transpiration.
This process of opening and closing the stomata must therefore be very carefully controlled.
Guard cells are the ones for the job. They surround and tightly regulate the actions of the
stomata. When water flows into neighboring guard cells (leading to an increase in turgor
pressure), a structural change occurs that causes the opening of the stomata. When the
water flows out of the guard cells (a decrease in turgor pressure), the stomata will close. It
is by this mechanism that guard cells control the opening and closing of the stomata.
“Move Over, Sugar”: Carbohydrate Transport Through Phloem
The transport of carbohydrates through the phloem is called translocation.After their
production, carbohydrates, the all-important product of photosynthesis, are dumped into
the phloem (the sugar superhighway) near the site of their creation, to be distributed
throughout the plant. The movement of the sugar into the phloem creates a driving force
because it establishes a concentration gradient. This gradient leads to the passive diffusion
of water into the phloem, causing an increase in the pressure of these cells. This pressure
drives the movement of sugars and water through the phloem. As the sugars arrive at various
destination sites, the sugar is consumed by plant cells, causing a reversal in the driving force
for water that pushes water out of the phloem. As water exits the phloem, the increased
pressure disappears and all is good once again.
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