Water
In this chapter you will learn about molecular movement
through a comparison between balls in motion in a room and molecular activity.
There will also be a discussion of diffusion, osmosis, turgor, plasmolysis,
imbibition, and active transport. You will learn of the entry of
water into the plant, the movement of water through the plant, the evaporation
of water into leaf air spaces, and transpiration. The pressure-flow model of
photosynthate movement is presented as well as the topic of cohesion and tension
in the water column. The chapter concludes with a discussion of mineral
requirements for growth.
At the end of this chapter the successful student will be able to
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- Movement of molecules from a region of higher concentration to a region of lower concentration.
- Move along a concentration gradient.
- Move until equilibrium reached
- Osmosis is diffusion of water through a differentially permeable membrane from a region where the water is more concentrated to a region where it is less concentrated.
Water enters a cell by osmosis until the osmotic potential is balanced by the resistance to expansion of the cell wall.
- Water Potential of a plant is essentially its osmotic potential and pressure potential combined.
- Water flows from the xylem to the leaves, evaporates within the leaf air spaces, and transpires through the stomata into the atmosphere.
-
If you are interested in the way fluid flows, you might find
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm
interesting. If you want to learn more about diffusion, you might find
http://biology.arizona.edu/sciconn/lessons/mccandless/reading.html
interesting.
- Loss of water through osmosis is accompanied by shrinkage of protoplasm
away from the cell wall.
- Colloidal material and large molecules usually develop electrical charges
when they are wet, and thus attract water molecules.
- Plants absorb and retain solutes against a diffusion, or electrical,
gradient through the expenditure of energy.
Involves proton pump
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- Note on the left the moisture on a clear container placed over a plant.
- Note to the right the process from water entering via the root to its
exiting via the leaves.
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- Cohesion Tension Theory:
- When the negatively charged end of one water molecule comes close to the
positively charged end of another water molecule, weak hydrogen bonds hold
the molecules together.
- Water molecules adhering to capillary walls, and each other, create a
certain amount of tension.
- When water transpires, the cells involved develop a lower water
potential than the adjacent cells.
- Creates tension on water columns, drawing water from one molecule to
another, throughout the entire span of xylem cells
- Regulation of Transpiration
- Changes in turgor pressure occur when osmosis and active transport between
the guard cells and other epidermal cells cause shifts in solute
concentrations.
- When photosynthesis is not occurring in the guard cells, potassium ions
leave, and the stomata close.
- An increase in potassium ions causes a lowering of the water potential and
osmosis leading to turgid guard cells
Click here to see the cohesion-tension model or
xylem transport.Return to top
- Stomata of most plants are open during the day and closed at night.
- Stomata of many desert plants open only at night.
- Conserves water, but makes carbon dioxide inaccessible during the day.
- Humidity plays an inverse role in transpiration rates.
- High humidity reduces transpiration, while low humidity accelerates it.
- If a cool night follows a warm, humid day, water droplets may be produced
through hydathodes at the tips of veins of some plants (Guttation).
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- Transport of Organic Solutes in Solution
- One of most important functions of water in the plant involves the
translocation of food substances in solution by the phloem.
- Most of our knowledge on this subject came from studying aphids
feeding on phloem
- Pressure-Flow Hypothesis
- Organic solutes flow from a source where water enters by osmosis.
- Organic solutes are moved along concentration gradients between
sources and sinks
Click here to see the pressure-flow model of
phloem transport
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- Essential elements:
- Macronutrients are used by plants in greater amounts.
- Nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur.
- 99% of these needs are made by N, K, Ca, P
- The remaining elements, Micronutrients, are needed by the plants in very small amounts.
- present in "trace amounts"
- Note the deficiency symptoms on leaves p. 162 fig: 9.19
- when any of the essential elements are deficient in the soil, the plant
exhibits deficiency symptoms which disappear after the problem is corrected
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Last modified:
October 08, 2004 by
Cynthia Herbrandson 
© Copyright 1999, Kellogg Community College.
All rights reserved.
