1. Water movement in plants
1. water moves from soil to air
1. running down the pressure gradient
2. no energy input required
2. Cell-to-cell movement of water
1. depends upon solute potential and pressure potential
1. potential = ability to do work
1. solute potential is another name for osmotic potential
1. water moves from high solute potential (low solute concentration) to low solute potential (high solute concentration)
2. pressure potential is the pressure exerted upon the cell contents by cell membrane and wall
1. when water moves into the cell, the membrane presses upon the cel wall
2. this pressure is counteracted by the pressure of the wall upon the membrane
1. wall pressure counteracts osmotic flow
3. Water potential is the tendancy of water to move
1. water potentials of cells are usually negative
1. solute potential of cells are usually very negative
1. solute potential measured relative to pure water (=0)
2. pressure potential of cells is usually slightly positive
1. increased pressure potential = less negative water potential
2. water moves from high water potential (less negative) to low (more negative)
1. soil has a high water potential relative to plant roots
2. air has very low water potential relative to plant tissue
3. Water potential differences soil > plant > air establishes a water potential gradient
1. plants gain water from the soil & loose water to the air
4. Cohesion - tension theory of water movement
1. Water evaporates from leaves (transpiration)
1. steep water potential gradient from dry air and moist leaf space
2. surface tension at air-water interface pulls water from soil
1. hydrogen bonds among water molecules
2. removal of one water molecule "pulls" upon others in column
3. entire column of water is under tension
1. measurable with pressure bomb - see expts in text
5. Translocation is the movement of sugars
1. sugars move from source to sink
1. multiple sources, sinks
1. source: where sugar enters phloem
2. sink: where sugar leaves phloem
2. temporal changes in sources, sinks
2. Two cell types in phloem play different roles
1. sieve-tube elements conduct sap
1. alive, but lack nuclei, connected by open pores
2. companion cells provide metabolic support
1. nuclei and multiple organelles particularly mitochondria
2. loading sugar done by companion cells: costs energy
3. order of events in sugar loading and movement
1. sugar loaded by companion cells against gradient into sieve tube members
2. at source, phloem has high sugar concentration (low water potential)
3. adjacent xylem cells have high water potential
1. water moves from xylem into phloem at source
4. tugor pressure in sieve tube members at source increases
5. sugars in solution move along the pressure gradient
4. order of events in sugar unloading
1. cells in sink remove sugar from sap against gradient
2. water potential in sieve-tube elements increases (becomes less negative) as sugar is removed.
1. eventually, water potential in sieve-tube elements is less negative than in nearby xylem
3. water moves from sieve-tube element to xylem at sink
6. summary: a continuous loop of water flow driven by water potential gradients between xylem and phloem and unidirectional movement of sucrose molecures driven by pressure potential gradient in phloem (see figure)

1.  Why is the solute (or osmotic) potential of the plant cell negative?

2.  In what manner is osmosis involved in xylem flow?  In what manner is osmosis involved in phloem flow?

3.  Juvenile cicadas feed on xylem fluid in the roots, and take 13 - 17 years to reach maturity.  Aphids feed on phloem fluid in the stems, and take days or weeks to reach maturity.  Suggest a model that would explain these differences between these two heteropteran insects.

4.  Water movement up a xylem is a function of (mark all correct answers):

(A) positive pressure potential in turgid leaf cells
(B) negative (suction) pressure potential further up the plant
(C) negative osmotic potential in roots
(D) negative water potential in leaves due to photosynthesis and the evaporation of water
(E) cohesion of water molecules to other water molecules
(F) adhesion of water molecules to the cell walls in the xylem

5.  Which of the following statements regarding the transport of sugars and nutrients is currently regarded as correct?   (mark all correct answers)

(A) phloem tissue transports sugars in one direction
(B) sugars are loaded into the sieve cells only through the companion cells
(C) loading sugars into the sieve cells requires active transport
(D) unloading sugars from the sieve cells requires active transport
(E) the osmotic gradient produced by loading and unloading sugars causes movement of water in the phloem
(F) the flow of liquid in the phloem can be altered by altering the source and sink points for the sugars
 

6. Why is water stress such a major factor in the evolution of plant life?   (mark all correct answers)

(A) gas exchange for photosynthesis is very inefficient and a lot of water is lost
(B) transport of minerals in the xylem requires the evaporative loss of water
(C) the availability of water is unpredictable in all terrestrial environments
(D) water stress causes the stomata to close, and plants do not grow when the stomata are closed
(E) transport of sugars in the phloem requires the availability of relatively large quantities of water
(F) the male gametes cannot swim to the female egg without water

7.  The vessel element and tracheid cells of the xylem tissue are dead and contain no cells;  the sieve-tube cells of the phloem are alive and contain a cell (albeit greatly reduced, with few organelles).  In light of the currently supported models, why is the loss of the protoplast important in xylem flow but not in phloem flow?
 

8.  Be sure you understand why water movement in plants is entirely passive but sugar movement requires energy input.
 

See also:  Content review #2, 3, 4, 5, 6;  Concept review #1, 2, 4;  Applying ideas #1, 2, 3, 5

Answers