Going out guide Quality seed sites Price Busters
Going out guide Latest Product info Price Busters
Advice Home
Growers Guide
some cheezy jokes.
Seed Sites
Go to the Shop
Transpirational pull
 YOU ARE HERE : Advice Home / Growers Guide / HOW A PLANT WORKS / Transpirational pull 

Generation of transpirational pull
Water diffuses from the xylem into the spaces inside the spongy mesophyll--> stomates-->atmosphere.
Water coating the surface of the mesophyll spaces forms crescent-shaped menisci as it enters the air spaces.
As water molecules are pulled from between the cells, the menisci become more curved, and water tension increases (due to the cohesiveness of the water molecules).
Meniscus tension is inversely proportional to the radius of its curved surface: as the meniscus becomes more concave, its radius decreases and the tension increases.
Tension thus generated is a NEGATIVE PRESSURE: it essentially "pulls" water from areas of greater hydrostatic pressure (i.e., fluid-filled areas such as xylem vessels and interstitial areas filled with water) into the areas of lower hydrostatic pressure (stomates).
This is the basis of "transpirational pull".

Bubbles sometimes can form in the water column in the xylem (this is more common in vessel elements than in tracheids--why?).

CAVITATION: rupture of the water column

EMBOLISM: filling of a vessel or tracheid with air.

Shoot tension is capable of lifting water up to 500 feet from the ground, which is more than enough to account for the tallest land trees' (redwoods) ability to lift water all the way to their tips. (350 feet).


Of lesser importance in moving water through xylem is ROOT PRESSURE.
at night, stomates are usually closed and transpiration stops.
endodermis-ringed root stele cannot "leak" ions, so water potential decreases as water from the soil osmoses into the stele.
this effectively "pushes" water up the stem and into the leaves.
Some herbaceous plants have special openings on the leaf margins called hydathodes. These allow root pressure water to escape, forming lovely little "beads" of "dew" overnight, and preventing cell rupture due to too much water pressure. This process is known as GUTTATION, and its results are generally observable only in the early morning, when humidity is very high.


Recall what you know about proton pumps in the cell:

Positively charged hydrogen ions are "pushed" out of the cell, a process requiring energy input.
The relatively higher proton concentration outside the plasma membrane creates a relatively negative charge inside the cell with respect to the outside.
The membrane potential can be used to do work, such as bringing positively charged ions into the cell following the gradient.
Negatively charged ions can also be brought into the cell via COTRANSPORT: some proteins carry not only hydrogen ions, but also can carry a specific negative ion (such as nitrite or nitrate) into the cell, using the membrane potential energy to do the work.

This is how SUGARS are loaded into cells.


Phloem sap is a thick solution containing up to 30% sugars (sucrose), amino acids, hormones etc.
In contrast, xylem sap is relatively thin and watery, as it contains mostly dissolved inorganics


Plants need to mobilize stored carbohydrates in order to perform cellular work:
1. convert starches/stored carbs into simple sugars
2. load simple sugar (usually sucrose) into phloem

3. transport sugar to wherever it needs to go

A SOURCE is any location where sugar is either being produced or has been stored. It is an area of higher sugar concentration than one to which it is being compared.

A SINK is any location where sugar is being used or actively stored (i.e., turning from sugar into starch)

How do we get the sugars from the source to the sink?

1. load sugar into sieve elements at the source and into the phloem via either of several pathways:

a. APOPLASTIC pathway: water travels along the outside of the cell walls (apoplast is the nonliving continuum formed by cell walls touching each other, creating a matrix)
b. SYMPLASTIC pathway: water travels from protoplast to protoplast via plasmodesmata (symplast is the cytoplasmic continuum formed by plasmodesmata)

c. TONOPLASTIC (transcellular) pathway: water travels from cell to cell by passing from vacuole to vacuole (tonoplast is the vacuole membrane)

The driving force causing the water movement is WATER POTENTIAL.

Transfer cells (recall these specialized types of parenchyma cells) facilitate the movement of water/solutes from apoplast to symplast and vice versa.
Because sugar accumulating in phloem transport cells may concentrate sugar 2-3 times what it is in regular mesophyll, ATP is needed to load the sugar to run the proton pumps. Transfer cells are very metabolically active!
Sucrose is cotransported into sieve tubes by transport proteins (see Figures in Chapter 32).
Phloem sap can move at a rate of 1m/hour, which is too fast for simple diffusion. It's moved via bulk flow: differences in pressure at opposite ends of a conduit cause movement with the potential gradient.
As sugar concentration rises in certain areas of the phloem (sink), water potential drops. This causes water from areas of higher water potential to flow into the sieve tube elements.
Result: water flows under pressure, somewhat like water through a hose!
Potential gradient goes from source to sink.
Once the sugars are unloaded, the water can diffuse into the xylem and be carried throughout the plant.

An ESSENTIAL NUTRIENT is one required by an organism for normal growth and development, but which it cannot manufacture on its own.
Animals, for example, have many essential organic nutrients (fatty acids, amino acids, vitamins, etc.) that they cannot manufacture themselves, and so must ingest them as other organisms containing those finished products.

Essential nutrients vary among species. Because PLANTS manufacture all the organic nutrients they need, they have no essential organic nutrients. However, plants do require very specific INORGANIC nutrients in order to grow, develop and thrive.

MACRONUTRIENTS These are elements (usually taken up in the form of compounds) required by plants in relatively large quantities. They are often major components of the plant's body. In plants, six of the main inorganic nutrients required are the six main components of organic molecules:

Three additional macronutrients needed by plants are:

MICRONUTRIENTS These are the elements (often taken up as compounds) needed in relatively small quantities.

The main function of these micronutrients is to serve as coenzymes in various enzymatic pathways.
Some of the functions of the various macro- and micronutrients are listed in your text:

A shortage of any of these nutrients will often have characteristic symptoms in the plant, with older plant parts showing the effects sooner than younger parts (which act as nutrient sinks, and draw more incoming material to themselves than the older parts do).

Print this page
About Us Contact Us