Transcript 9.2 Plant Transport - Twanow

9.2 Plant Transport
Learning Targets: Explain the process of
mineral ion absorption from the soil into
roots. Explain how water is carried by the
transpiration stream. State that guard cells
can regulate transpiration. Outline the role of
phloem in active translocation of sugars.
Absorption by Roots:
 Root hairs and mycorrhizae (symbiotic
associations of roots and fungus) increase
surface area for absorption
 Water and minerals travel from soil 
epidermis  cortex  endodermis 
vascular cylinder
 The endodermis protects the vascular
cylinder, and force materials to be
“screened” by traveling through a cell
membrane
Plant Transport Routes:


Symplastic – solutes move through
plasmodesmata (cytoplasm channels) between
cells
Apoplastic – solutes move through cell walls and
extracellular space
Root hairs and mycorrhizae
Mineral Ion Transport
 Mineral ions are needed by plants and
enter through three main processes:
 Carried through soil by the mass flow of
water, and diffuse into root cells
 Absorbed into symbiotic fungi
(mycorrhizae) and passed to root cells

Active transport into root cells (requires
ATP)


If the concentration of a mineral is higher
inside plant cells than in the soil
If ions cannot cross the cell membrane
Support
 To allow terrestrial plants to grow tall,
strong support is needed:
 Cell walls made of thickened cellulose
 Xylem vessels reinforced with lignin
 Turgor pressure - Supportive pressure of
the water inside cells
Xylem Transport
 xylem sap (water and dissolved minerals)
rises against gravity
 the “push” comes from root pressure:
since there is a higher concentration of
minerals in the vascular cylinder, water
moves in through osmosis
 the “pull” comes from transpiration,
cohesion, and adhesion (due to Hbonding)
Xylem Transport
 water evaporates and diffuses out of stomata
 the surface tension of the water film in the air
spaces pulls water out of the xylem
 this pull is transmitted down the column of water in
a xylem vessel
 adhesion to cell walls (hydrophilic cellulose) helps
resist gravity
Transpiration – the loss of water vapor
from leaves and other aerial parts of the
plant.
How is the flow of
water through
xylem vessels
solar powered?
Evaporation of
water from air
spaces in leaves
is driven by the
sun
Control of Transpiration
 guard cells control the size of stomata
 stomata let CO2 diffuse in, and O2 and H2O
diffuse out
 generally, stomata are open during the daytime,
and closed at night
Control of Transpiration
 opening: guard cells accumulate K+  water
enters by osmosis  guard cells swell  stoma
opens
 closing: guard cells lose K+  water leaves by
osmosis  guard cells collapse  stoma closes

K+ transport is stimulated by:
 Light
 CO2 depletion
 Internal clock (circadian rhythm)
 Abscisic acid – a plant hormone
produced by roots in dry conditions
(drought)
Xerophytes – plants adapted for water scarce
conditions
What are some plant adaptations that could limit
water loss?
 Small leaves, deep roots, waxy cuticles, fewer
stomata
 C4 photosynthesis – CO2 is stored as a 4 carbon
sugar and the Calvin cycle occurs in bundle
sheath cells
 CAM photosynthesis – stomata open at night
and CO2 is stored for use during the day when
stomata are closed
Phloem Sap Transport
 phloem sap (water and organic products
of photosynthesis) flows from sugar
sources to sugar sinks
What are some possible examples of sugar
sources and sinks?
 Sources – mature leaves, roots (early in
growing season)
 Sinks – growing roots, shoots, flowers or
fruits
Phloem Sap Transport
 sugar enters/leaves phloem through
diffusion or active transport
 the “push” comes from water pressure
 high solute concentrations at sugar
sources lead to water pressure from
osmosis
 low solute concentration at sugar sinks
leads to water loss by osmosis and
reduced pressure