Chapter 7
Lecture Outline
Water in Plants
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Outline

Molecular Movement

Water and Its Movement Through the Plant

Regulation of Transpiration

Transport of Food Substances (Organic
Solutes) in Solution

Mineral Requirements for Growth
Molecular Movement

Diffusion
• Movement of molecules from a region of higher
concentration to a region of lower concentration
–
Molecules move along
concentration gradient.
–
Moving from lower to higher
concentration is against
concentration gradient.
–
State of equilibrium Molecules distributed
throughout available space
–
Rate of diffusion depends on
pressure, temperature and
density of medium.
Molecular Movement

Solvent - Liquid in which substances
dissolve

Semipermeable membranes - Membranes in
which different substances diffuse at
different rates
• All plant cell membranes

Osmosis - Diffusion of water through a
semipermeable membrane from a region
where water is more concentrated to a
region where it is less concentrated
Molecular Movement

Osmosis
Osmotic pressure - Pressure required to
prevent osmosis
• Osmotic potential balanced by resistance of cell wall.
–
Pressure potential (Turgor
Pressure) - Pressure that
develops against walls as
result of water entering cell
–
Turgid cell - Firm cell due to
water gained by osmosis
• Water potential of cell = osmotic
pressure + pressure potential
–
Turgid cell
Water moves from cell with higher water potential to
cell with lower water potential.
Molecular Movement

Osmosis
Osmosis is primary way water enters plants
from environment.
• Pathway of water
through plant:
–
–
–
Enters from soil into cell
walls and intercellular
spaces of root hairs and
roots
Crosses differentially
permeable membrane and
cytoplasm of endodermis,
then into xylem
Flows through xylem to
leaves and diffuses out
through stomata
Molecular Movement

Plasmolysis - Loss of water through osmosis
• Accompanied by shrinkage of protoplasm away
from the cell wall
Normal cells versus plasmolyzed cells
Molecular Movement

Imbibition
• Large molecules, such as
cellulose and starch,
develop electrical charges
when wet, and thus attract
water molecules.
• Water molecules adhere
to large molecules.
• Results in swelling of
tissues
• Imbibition is first step in
germination of seed.
Seeds before and after imbibition
Molecular Movement

Active Transport - Process used to absorb
and retain solutes against a diffusion, or
electrical, gradient by expenditure of energy
• Involves proton pump - Enzyme complex in
plasma membrane energized by ATP molecules
–
Transport proteins - Facilitate transfer of solutes to
outside and to inside of cell
Water and Its Movement Through the Plant

Transpiration - Water vapor loss from
internal leaf atmosphere
• More than 90% of the water entering a plant is
transpired.

Water needed for:
• Cell activities
• Cell turgor
• Evaporation for cooling
–
If more water is lost then taken
in, stomata close.
Water and Its Movement Through the Plant

The Cohesion-Tension Theory - Transpiration
generates tension to pull water columns
through plants from roots to leaves.
• Water columns created when water molecules
adhere to tracheids and vessels of xylem and
cohere to each other.
Water and Its Movement Through the Plant

The Cohesion-Tension Theory
• When water evaporates from mesophyll cells,
they develop a lower water potential than
adjacent cells.
• Water moves into mesophyll cells from adjacent
cells with higher water potential.
• Process is continued until veins are reached.
• Creates tension on water columns, drawing water
all the way through entire span of xylem cells
• Water continues to enter root by osmosis.
Regulation of Transpiration

Stomatal apparatus regulates transpiration
and gas exchange.
• Stomatal apparatus = 2 guard cells + stoma
(opening).
• Transpiration rates influenced by humidity, light,
temperature, and carbon dioxide concentration.
Regulation of Transpiration

When photosynthesis
occurs, stomata open.
• Guard cells expend
energy to acquire
potassium ions from
adjacent epidermal cells.
• Causes lower water
potential in guard cells
• Water enters guard cells
via osmosis.
• Guard cells become
turgid and stomata opens.
Regulation of Transpiration

When
photosynthesis
does not occur,
stomata close.
• Potassium ions
leave guard cells.
• Thus, water leaves.
• Guard cells become
less turgid and
stomata close.
Regulation of Transpiration

Stomata of most plants are open during day
and closed at night.

Water conservation in some plants:
• Stomata open only at night - Desert plants
–
Conserves water, but makes carbon dioxide inaccessible
during day.
o Thus, undergo CAM photosynthesis
« Carbon dioxide converted to organic acids and
stored in vacuoles at night.
« Organic acids converted to carbon dioxide during
day.
• Stomata recessed below surface of leaf or in
chambers.
–
Desert plants, pines
Regulation of Transpiration

Guttation - Loss of liquid water
• If cool night follows warm, humid day, water
droplets are produced through hydathodes at tips
of veins.
• In absence of transpiration at night, pressure in
xylem elements forces water out of hydathodes.
Guttation in
barley plants
Transport of Organic Solutes in Solution

Important function of water is translocation
of food substances in solution by phloem.

Pressure-Flow Hypothesis - Organic solutes
flow from source, where water enters by
osmosis, to sinks, where food is utilized and
water exits.
• Organic solutes move along concentration
gradients between sources and sinks.
Transport of Organic Solutes in Solution

Specifics of Pressure-Flow Hypothesis:
• Phloem loading - Sugar enters by active transport
into sieve tubes.
• Water potential of sieve tubes decreases and water
enters by osmosis.
• Turgor pressure develops and drives fluid through
sieve tubes toward sinks.
• Food substances actively removed at sink and water
exits sieve tubes, lowering pressure in sieve tubes.
• Mass flow occurs from higher pressure at source to
lower pressure at sink.
• Water diffuses back into xylem.
Mineral Requirements for Growth

Essential Elements - Essential as building
blocks for compounds synthesized by plants
Mineral Requirements for Growth

Macronutrients - Used by plants in greater
amounts
• Nitrogen, potassium, calcium, phosphorus,
magnesium and sulfur

Micronutrients - Needed by the plants in very
small amounts
• Iron, sodium, chlorine, copper, manganese,
cobalt, zinc, molybdenum and boron

When any required element is deficient in
soil, plants exhibit characteristic symptoms.
Review

Molecular Movement

Water and Its Movement Through the Plant

Regulation of Transpiration

Transport of Food Substances (Organic
Solutes) in Solution

Mineral Requirements for Growth