Back to 9 2 assessment statements
9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpirational pull, cohesion, adhesion, and evaporation. (3)
Transpiration stream is the upward flow of water from the roots to the leaves.
Higher rates of transpiration lead to a faster transpiration stream and higher rates of water uptake.
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evaporation
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- When water becomes heated by sunlight in spongy mesophyll and the water vapor goes out through the stomata which is called transpiration. The water needs to be replaced with water from xylem vessels.
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| transpirational pull |
- occurs when water loss cause negative (low) pressure in the xylem vessels due to evaporation (see "Water and Mineral Transport for description of evidence of the "pull" strength)
- generates upward movement of water through xylem
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cohesion
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- cause water molecules to form hydrogen bonds between one another
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| adhesion |
- occurs by the attraction of the water molecules and the cellulose in the walls of xylem vessels.
- This counteracts gravity
- The water molecules of adhesion and cohesion are bonded as the illustration below.
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structure of xylem vessels
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- No plasma membranes present- water can move in and out freely
- Helical (ring-shape) thickening of the cellulose wall build resistance for inward pressures
- Pores in the outer cellulose cell wall conduct water out of the xylem vessel and into cell walls of adjacent leaf cells.
See xylem SEM below;
from <http://science_boy.blogspot.com/2005_04_01_archive.html>
Xylem diagram:
from <http://www.clt.astate.edu/mhuss/toppage6.htm>
Xylem:
image from ...
Xylem and Phloem contrast;
from <http://kvhs.nbed.nb.ca/gallant/biology/xylem__phloem.jpg>
Colored micrograph of xylem and phloem:
image from <http://plantphys.info/plant_physiology/basiccytology1.shtml>
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From <http://www.bio.miami.edu/dana/226/226F09_10.html>
From: http://www.phschool.com/science/biology_place/labbench/lab9/images/xylem.gif
Symplast and Apoplast water pathways (details!)
Water enters plant roots via two primary pathways: symplast and apoplast.
From <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html>
Further reading and resources
Possible Assessment Questions:
- Outline advantages and disadvantages of carrying water by transpiration stream (2)
- List at least three structures of xylem vessels that contribute to transpiration stream (1)
- Explain the relationship between the rates of transpiration and the rates of water uptake involving the process of transpiration stream (3)
- Suggestions for Assessment Questions: (by Eunji)
- Compare and contrast adhesion and cohesion (2)
- Define transpirational pull (1)
Comments (2)
HL-Sunny said
at 5:06 pm on Apr 21, 2010
Mr. Reimer, I am not sure how to explain the pressure of air that we talked about in class in answering this question...
HL-Eun Ji said
at 1:36 am on Apr 21, 2010
Added suggestions for assessment questions
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