guide for science

1. PLANTWATERRELATIONS

3. Net flow in
whole plants

6. WATER IN PLANT LIFE
• Water makes up most of the mass of plant
cells.
• Water constitutes almost 80-95% of the mass
of growing plant tissues.
• Vegetables like lettuce contain about 95%
water.
• Sap wood 30-35% water, and
• Seeds 10-15% water.

7. • Water makes the medium for movement of
molecules within and between cells.
• Water also forms the environment in which almost
all the reactions occur in plants.
• Therefore plants continually absorb and loose water.
• On a warm, dry, sunny day a leaf will exchange upto
100% of its water in a single hour.
• This loss of water from plants is called as
transpiration.

9. Some Key Concepts
• Diffusion: movement of molecules from
high to low concentration.
• Osmosis: diffusion across a semi-
permeable membrane.
• Mass or bulk flow: movement of fluid due
to pressure or gravity differences.

11. Water Movement
Water movement is considered to be almost entirely passive
with water flow following a water potential gradient.
Water potential is the difference in free energy of water
in soils, cells or atmosphere and that of pure water.
Water potential of pure water is used for comparison and is
given a value of zero. When there are differences in water
potential, water will ALWAYS move passively from higher
to lower water potential.
Equation for water potential (must account for the factors
that influence the diffusion of water and other substances):
Yw = Yp + Ys
where Yw = water potential; Yp = pressure potential; Ys =
solute or osmotic potential

12. 1. Solute (or osmotic) potential (Ys).
This is the contribution due to dissolved solutes.
Solutes always decrease the free energy of water,
thus there contribution is always negative.
2. Pressure (or Pressure Potential)(Yp)
Due to the pressure build up in cells thanks to
the wall. It is usually positive, although may be
negative (tension) as in the xylem.

13. • Solute
• Effect of dissolved solutes on water potential
• Solutes reduce free energy of water
• Pressure
• It is the hydrostatic pressure of the solution.
• Positive pressure raises the water potential while
negative pressure decreases it.
• Positive pressure – turgor and negative pressure -
tension
• Gravity
• Is generally negligible.
• Overcome by transpirational pull.

14. Ψw = Ψs + Ψp + Ψg
• Where,
• Ψw = water potential
• Ψs = solute potential
• Ψp = pressure potential
• Ψg = gravitational potential

15. • Cell growth, photosynthesis, and crop
productivity are all strongly influenced by
water potential and its components.
• Like the body temperature of humans, water
potential is a good overall indicator of plant
health.
• Water always moves from a higher water
potential to lower water potential.

16. • Solute
• Effect of dissolved solutes on water potential
• Solutes reduce free energy of water
• Pressure
• It is the hydrostatic pressure of the solution.
• Positive pressure raises the water potential while
negative pressure decreases it.
• Positive pressure – turgor and negative pressure -
tension
• Gravity
• Is generally negligible.
• Overcome by transpirational pull.

19. The Colloidal System
• Protoplasm composed of substances in its
colloidal state.
• Enzymes are active only in their colloidal
state.
• In colloidal stage 100 or 1000 of molecules
lumped together.
• Dispersed particle found in the range of 1-
100 micro meter diameter.

20. EXAMPLE 1: let’s suppose we
drop a plant cell into pure water
Water can move by
osmosis across the
cell wall and cell
membrane but most
solutes cannot Pure water
Living
Plant Cell

21. EXAMPLE 2: Putting a plant cell
into salty water
Water can move by
osmosis across the
cell wall and cell
membrane but most
solutes cannot salty water
Plant Cell

22. Transport Structures in Vascular
Plants

23. Transport Structures in Vascular
Plants
• Root
– support
– Transport：
Epidermis
Cortex
Endodermis
Stele
（incl. xylem
and phloem)

24. Structural adaptation of root hair cells
Feature Function
Long extension increase surface area : free contact with soil water and ions
Thin/ Fine/ Slender penetrates soil particles for soil water contact
Large vacuole give osmotic control
Thin/ Unthickened cell wall for absorption of water and mineral salts
Absence of cuticle for absorption of water and mineral salts
Abundance of mitochondria for absorption of mineral salts by active transport

25. Structure/Function: Xylem
• Three types of xylem cells
– Tracheids
• elongate with tapered ends where
cells are connected into long tubes
• pits allow communication
between tracheids
– Vessel Elements
• highly specialized for transport
• many open-ended
• form vessels
– Rays
• lateral transport

26. Vessel Elements

27. Pulling Xylem Sap: The Transpiration-
Cohesion-Tension Mechanism
• Water is pulled upward by negative
pressure in the xylem
Transpirational Pull

28. Cohesion and Adhesion in the
Ascent of Xylem Sap
• 1 A negative pressure ( tension ) is created on the surface of the film of
water coating mesophyll cells when evaporation occurs from its surface.
(This occurs during transpiration).
• 2 This tension pulls on the interconnected chain of water molecules
within the xylem which extends from the leaf through the stem to the tips
of the root.
• 3 The water molecules in the chain are held together by hydrogen bonds
which exist between neighbouring water molecules. ( cohesion)
• 4 The chain of molecules is prevented from being pulled down because
each water molecule in the chain is attracted to the walls of the xylem by
hydropyllic attraction between water and the cellulose in the cell walls.
(Adhesion)
• 5 Hence the water column which is held together by cohesion and
prevented from lowering by adhesion is pulled up by the tension
generated from above by transpiration.
• 6 Solutes which are dissolved in the water also are pulled up within the
xylem.

29. • Ascent of xylem sap
Xylem sap
Outside air Y = –100.0 MPa
Leaf Y (air spaces)= –7.0 MPa
Leaf Y (cell walls)= –1.0 MPa
Trunk xylem Y= – 0.8 MPa
Water
potential
gradient
Root xylem Y= – 0.6 MPa
Soil Y= – 0.3 MPa
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration
Xylem
cells
Adhesion
Cell wall
Cohesion, by hydrogen
bonding
Water molecule
Root hair
Soil particle
Water
Cohesion
and adhesion
in the xylem
Water uptake
from soil

30. Root Pressure
When a plant is carefully severed close to the base of the
stem, sap oozes from the stump. The fluid comes out under
pressure which is called root pressure.
Root pressure is created by the osmotic pressure of xylem
sap which is, in turn, created by dissolved minerals and
sugars that have been actively transported into the apoplast
of the stele.
Although root pressure may play a significant role
in water transport in certain species or at certain
times, most plants meet their needs by
transpiration-pull.

31. The Pathway of Water in a plant root:
1) Apoplastic
2) Symplastic 3) Vacuolar

32. Soil water enters the root at the root hairs -
extensions of epidermal cells. It appears that water
then travels in both:
•in the nonliving parts of the root - called the
apoplast - that is, in the spaces between the cells
and in the cells walls themselves. This water has
not crossed a plasma membrane.
•the cytoplasm of root cells - called the symplast
- that is, it crosses the plasma membrane and then
passes from cell to cell through plasmodesmata.

33. However, the inner boundary of the cortex, the
endodermis, is impervious to water because of a
band of suberized matrix called the casparian
strip. Therefore, to enter the stele, apoplastic
water must enter the symplasm of the endodermal
cells. From here it can pass by plasmodesmata
into the cells of the stele.
Once inside the stele, water is again free to move
between cells as well as through them. In young
roots, water enters directly into the xylem vessels
and/or tracheids. These are nonliving conduits so
are part of the apoplast.

34. Mechanism of Water Absorption
Passive Absorption
Passive absorption is by osmosis. Passive absorption takes
place along the concentration gradient - when the
concentration of cell sap is higher than that of soil water.
Water is absorbed when transpiration rate is high or soil is
dry. Due to high transpiration rate, water deficit is created in
transpiring cells. Rapid transpiration removes water and
reduces turgor pressure in living cells of root.
The suction force thus developed is transmitted to root
xylem. It pulls water from surrounding root cells to make up
water deficit.

35. uptake of Minerals
Minerals enter the root by active transport (pumping
against the concentration gradient using ATP) into
the symplast of epidermal cells and move toward and
into the stele through the plasmodesmata connecting
the cells.
They return to the apoplast from the cells of the pericycle
through specialized transmembrane channels.
Once in the xylem, water with the minerals that have
been deposited in it move up in the vessels and tracheids.
At any level, the water can leave the xylem and pass
laterally to supply the needs of other tissues.

36. Cells types Features Adaptation
Xylem vessel
one elongated cell on top of another
forming vertical column
 hollow tubes result and facilitate mass/free flow of water
end wall and cell content lost
cell dead
cell wall lignified and strengthened
 to prevent collapse/pressure change due to -ve tension
develop during active transpiration
 to give mechanical support
pits
 allow inter-communication between cells
 vessels for free passage of water
 prevent air lock in transpiration stream
hex/octagonal shape  close packing for strength
Tracheid
elongated hollow cell, imperforated
with pit-closing membrane present in
the region of pits
 water flow from cell to cell through pits
cell wall lignified and strengthened  to give mechanical support
Fibre
long cell with cell wall thickened and
lignified
 to give mechanical support
Parenchyma cell cell wall may or may not be lignified  as storage cell.
Structural adaptation of xylem for transport

37. Vessel Elements

38. Transpiration is thought to
occur because the plant
requires CO2 from the
surrounding air for
photosynthesis. For most
plants, stomata open in the
morning after the plant has
been exposed to the sun
long enough to use up much
of the CO2 inside the leaf,
and stomata close at night.
The loss of water is
incidental to the need for
uptake of CO2.

39. Woody stems and mature roots are
sheathed in layers of dead cork cells
impregnated with suberin - a waxy,
waterproof (and airproof) substance.
So cork is as impervious to oxygen
and carbon dioxide as it is to water.
However, the cork of both mature
roots and woody stems is perforated
by nonsuberized pores called
lenticels. These enable oxygen to
reach the intercellular spaces of the
interior tissues and carbon dioxide to
be released to the atmosphere.
Lenticular &
cuticular transpiration

40. Leaf surfaces are dotted with millions of
stomata such as this one. This stoma is
lined by two guard cells that control its
aperture. Because control requires
movement, and movement requires
energy, these cells contain numerous
mitochondria and chloroplasts. Thus
they are the only cells in the
epidermis that are green.
Guard Cells

41. The opening or closing of the stomata is a
result of the changes in the turgor pressure of
the guard cells. The inner wall of each guard
cell is thick and elastic. When turgor
develops within the two guard cells flanking
each stoma, the thin outer walls bulge out
and force the inner walls into a crescent
shape. This opens the stoma.
When the guard cells lose turgor, the elastic
inner walls regain their original shape and
the stoma closes.

42. (OS) The stomata will open when cells are turgid. The reason for the
changes in turgor pressure is the movement of water from subsidiary
cells into the guard cells. This is a passive movement caused by
changes in water potential created by a K+ pump (active transport).

43. 43
Transpiration
• Transpiration is the loss of water from a plant by evaporation
• Water can only evaporate from the plant if the water potential is lower in the
air surrounding the plant
• Most transpiration occurs via the leaves
• Most of this transpiration is via the stomata.

44. Structure of STOMATA

46. How does transpiration occur?
• Due to endosmosis of water turgor pressure of
guard cells increase.
• Outer wall stretches and bulges out.
• Inner wall gets pulled apart.
• Stomata open.

47. Stomatal movement – how?
• Movement of stomata can be explained by active
potassium ion transport mechanism.
• It was proposed by FUJINO and later explained
by LEVITT.
• According to this theory opening of stomata is an
active process whereas closing is a passive
process.

48. Distribution and Number of Stomata
More stomata are found on the ventral leaf
surface, away from sunlight and wind. Some
plants have no stomata at all on the dorsal leaf
surface.

49. Transpiration does serve three useful
functions:
•It provides the force for lifting the
water up the stems.
•It cools the leaves.
•It helps absorb soil water.

50. Factors Affecting the Rate of Transpiration
1. Air Temperature
Leaves receiving direct sunlight
absorb about 80% of the radiant
energy falling on them. Part of this
is changed to heat, raising the
internal temperature of the leaf.
Part of this heat is used in
vaporizing water and a small part is
used in photosynthesis.

51. Increased vapour = Increased diffusion gradient = Increased
transpiration
Environmental factors
2. Relative humidity of the atmosphere
The drier the air around the plant, the
greater the transpiration rate.
The intercellular spaces and the cells of a
turgid leaf are usually almost saturated with
water. In this case a "steep" diffusion
gradient would exist between the leaf and
the atmosphere. The rate of transpiration
would therefore be high.

52. 3. Light Intensity
Illumination stimulates the opening of the
stomata and increases transpiration.
4. Wind/Air Movement
Removes the mantle of moist air
surrounding the leaf. If dry air is
blowing across the leaf the diffusion
gradient will be steep and rapid
transpiration results.