Uptake and Transport of
Water and Ions in Plants

Uptake of Water INTO
& ACROSS Roots

  • STEPS
  • water enters root hairs by osmosis
    from soil
  • passes across root cortex
  • into xylem tissue
  • up xylem vessels in stem to leaf

Why these
steps occur?

root hairs well adapted for
absorbing water and mineral ions

  • thin walls - short diffusion pathway
  • shape - large SA/V ratio

water passes down wp gradient from H in soil
to low in root hair by osmosis

  • water in soil very weak solution mineral
    salts = higher water potential
  • root hair vacuole strong solution of sugars
    & other dissolved substances = low wp

water passes across cortex + enters
xylem tissue in centre of root

  • wp in xylem lower than in root hairs

Apoplast
Pathway

water passes through
cell walls of cortex cells

  • cell walls made of cellulose fibrils
  • parallel arrangement of microfibrils
    allow water easily between layers
  • as water seeps through, cohesive forces
    pull more water along apoplast

most water moves via this
route as limited resistance

Symplast
Pathway

water passes through
cytoplasm of cortex cells

  • gradient in wp across cortex
  • as route hair cells takes in water,
    water p less negative + is higher
    than adjacent cell
  • water moves via osmosis down
    wp gradient via plasmodesmata
  • Most water moves via apoplast way until it
    becomes blocked...
  • cell wall impregnated with waxy
    material (suberin) - CASPARIAN STRIP
  • suberin waterproof so CS stops
    water passing through

only way water can pass across endodermis
is move into protoplast + join water in is via
symplast pathway through cytoplasm

  • under metabolic control (impurities removed
    at this point as cannot pass through c.membrane
  • endodermal cells pump ions into xylem vessels by active transport
  • xylem vessels lowers wp so by osmosis H2O moves INTO xylem
  • creates root pressure force that helps move water up the plant
  • cohesive forces pull water from endodermis to xy
  • once enters stele - bulk water returns apoplast
  • water out of pits of xylem into cells

Uptake of Ions INTO
& ACROSS Roots

similar to
uptake of water

main uptake through root hairs
via facilitated diffusion/active transport

  • depends on concentration gradient

Transport of Water UP
the Root + Stem in Xylem

Negative Pressure
(tension)

created as water evaporates
out of stomata of leaf

responsible of pulling water
up the column as mass flow

in order to work, there must be
a continuous unbroken column water
through xylem - TRANSPIRATION STREAM

Cohesion

when molecules of a substance
attracted to each other

magnitude of this force depends on:

  • mass of the particles
  • distance apart

HYDROGEN bonding increases
cohesive forces

effect of large cohesion =
water molecules pulled inwards
towards each other allowing to be
sucked up xylem in continuous column

happens as water at top
of column evaporates out of leaf

Adhesion

weakest

large adhesive force between WALL of
XYLEM + WATER within

when molecules attracted to ones of a different type - CAPILLARITY

magnitude of this force depends on:

  • width of diameter
    (smaller diameter, higher it rises)

COHESION-TENSION
THEORY

  • water lost from leaves during transpiration
  • moves from saturated air inside leaf to drier air outside by evaporation

TRANSPIRATION STREAM
As water molecules removed
from xylem, more water molecules
move up to replace them

D: mechanism of water moving through
plant, involving tension + cohesive forces

transpiration is pulling water column
in xylem upwards and gravity pull downwards
so water column stretched + under tension

EVIDENCE

1.Diameter of Tress reducing during daylight

  • during day - transpiration rate highest
  • water evaporation fast so fast pull into plant
  • tension much greater
  • pull walls of xylem inwards
  • reducing diameter of tree
    (most obvi woody plants as more xylem)

2.Air gaps in water column of xylem

  • means water below cant be pulled up
  • cut flowers kept out of water for long time
  • air enters xylem column
  • placing flowers in water not make a difference
  • as water column already broken
  • flowers will wilt

Movement of Water
ACROSS Leaf

midrib divides into number of
veins that distribute across
the leaf

water enters leaf via
vascular bundle found
in midrib

water passes from veins
to surrounding cells for:

  • photosynthesis
  • providing turgor
  • lost in transpiration

Transpiration

  1. water evaporates from walls of spongey mesophyll into air spaces - becomes saturated
  2. water diffuses from region high wp (air spaces)
    to low wp (outside air)
  3. as water evaporates out, more drawn in to replace it
  4. sets up water potential gradient
  5. water passes from xylem to spongey mesophyll via APOPLAST or SYMPLAST