Chapter 36 – Plants & Transpiration
• The success of plants depends on their
ability to gather and conserve resources
from their environment
• The transport of materials is central to
the integrated functioning of the whole
plant
• Plants also depend on nutrient cycles to
obtain materials they need for life
processes
Evolution Connection :Underground
Plants
• Stone plants (Lithops) are adapted to life in the
desert
– Two succulent leaf tips are exposed above
ground; the rest of the plant lives below ground
Adaptations for acquiring resources were
key steps in the evolution of vascular
plants
• The algal ancestors of land plants absorbed water,
minerals, and CO2 directly from the surrounding
water
• Early nonvascular land plants lived in shallow
water and had aerial shoots
• Natural selection favored taller plants with flat
appendages, multicellular branching roots, and
efficient transport
Figure 36.2-1
H2O
H2O
and
minerals
Figure 36.2-2
CO2
O2
H2O
O2
H2O
and
minerals
CO2
Figure 36.2-3
CO2
H2O
O2
Light
Sugar
O2
H2O
and
minerals
CO2
Plant Adaptations Cont’
• The evolution of xylem and
phloem in land plants made
possible the long-distance
transport of water, minerals,
and products of
photosynthesis
• Xylem transports water and
minerals from roots to shoots
• Phloem transports
photosynthetic products from
sources to sinks
• Adaptations in each species represent
compromises between enhancing photosynthesis
and minimizing water loss
Shoots and Light Capture
• Stems serve as pathways for water and nutrients
and as supporting structures for leaves
• There is generally a positive correlation between
water availability and leaf size
Obtaining of Water and Minerals
• Soil is a resource used by the root system
• Root growth can adjust to local conditions
– For example, roots branch more in a pocket of
high nitrate than low nitrate
• Roots are less competitive with other roots from
the same plant than with roots from different plants
• Roots and soil fungi form mutualistic associations
called mycorrhizae
• Mutualisms with fungi helped plants colonize land
• Mycorrhizal fungi increase the surface area for
absorbing water and minerals
• Nitrogen fixation by bacteria
Transport of Water
• Plants must balance water uptake and loss
• Osmosis determines the net uptake or water loss
by a cell and is affected by solute concentration
and pressure
Transport of Water
• Water potential is a measurement that combines
the effects of solute concentration and pressure
• Water potential determines the direction of
movement of water
• Water potential is abbreviated as Ψ and measured
in a unit of pressure called the megapascal (MPa)
• Ψ = 0 MPa for pure water at sea level and at room
temperature
What Affects Water Potential?
• Both pressure and solute concentration affect
water potential
• This is expressed by the water potential equation:
Ψ  ΨS  ΨP
• The solute potential (ΨS) of a solution is directly
proportional to its molarity
• Solute potential is also called osmotic potential
Water Potential
• Pressure potential (ΨP) is the physical pressure
on a solution
• Turgor pressure is the pressure exerted by the
plasma membrane against the cell wall, and the
cell wall against the protoplast
• Consider a U-shaped tube where the two arms are
separated by a membrane permeable only to
water
• Water moves in the direction from higher water
potential to lower water potential
Figure 36.8a
Solutes have a negative effect on  by binding water
molecules.
Pure water at equilibrium
Adding solutes to the right
arm makes  lower there,
resulting in net movement of
water to the right arm:
Pure
water
H 2O
Membrane
H 2O
Solutes
Figure 36.8b
Positive pressure has a positive effect on  by pushing water.
Applying positive pressure to
the right arm makes  higher
there, resulting in net movement
of water to the left arm:
Positive
pressure
Pure water at equilibrium
H 2O
H 2O
Figure 36.8c
Solutes and positive pressure have opposing effects on water
movement.
In this example, the effect of
adding solutes is offset by
positive pressure, resulting in
no net movement of water:
Positive
pressure
Pure water at equilibrium
Solutes
H2O
H 2O
Water Movement Across Plant Cell
Membranes
• Water potential affects uptake
and loss of water by plant cells
• If a flaccid cell is placed in an
environment with a higher solute
concentration, the cell will lose
water and undergo plasmolysis
Figure 36.9a



Plasmolyzed
cell at osmotic
equilibrium with
its surroundings
P  0
S  0.9
  0.9 MPa
(a) Initial conditions: cellular   environmental 



0.4 M sucrose solution:
P
0
S 0.9
0.9 MPa

Initial flaccid cell:
P
0
S 0.7
0.7 MPa

Figure 36.9b
Initial flaccid cell:
P  0
S  0.7
  0.7 MPa
Pure water:
P  0
S  0
  0 MPa
Turgid cell
at osmotic
equilibrium with
its surroundings
P  0.7
S  0.7
  0 MPa
(b) Initial conditions: cellular   environmental 
• If a flaccid cell is placed in
a solution with a lower
solute concentration, the
cell will gain water and
become turgid
• Turgor loss in plants
causes wilting, which can
be reversed when the
plant is watered
• Transpiration drives the
transport of water and
minerals from roots to
shoots
The rate of transpiration is regulated by
stomata
• Leaves generally have broad surface areas and
high surface-to-volume ratios
• About 95% of the water a plant loses escapes
through stomata
• Each stoma is flanked by a pair of guard cells,
which control the diameter of the stoma by
changing shape
• Stomatal density is
under genetic and
environmental control
Figure 36.15a
Guard cells turgid/
Stoma open
Radially oriented
cellulose microfibrils
Guard cells flaccid/
Stoma closed
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening
and closing (surface view)
• This results primarily from the reversible uptake
and loss of potassium ions (K) by the guard cells
Figure 36.15b
Guard cells turgid/
Stoma open
H 2O
K
Guard cells flaccid/
Stoma closed
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H2O
H 2O
H 2O
(b) Role of potassium in stomatal opening and closing
Stimuli for Stomatal Opening and Closing
• Generally, stomata open during the day and close
at night to minimize water loss
• Stomatal opening at dawn is triggered by
– Light
– CO2 depletion
– An internal “clock” in guard cells
• All eukaryotic organisms have internal clocks;
circadian rhythms are 24-hour cycles
Nitrogen cycle and plants
• Plants need Nitrogen for
– Making proteins
– Cell replication: Making copies of the DNA
• Nitrogen is abundant in the air but is not available
to plants
• Nitrogen Fixation: conversion of N2 in the
atmosphere to ammonia (NH3) and nitrate (NO3-)
– Abiotic Fixation – lightning & radiation
– Biotic fixation – Soil bacteria & symbiotic relationships
with plants
Nitrogen cycle and plants
• Nitrification: Ammonia is oxidized into nitrite (NO2)
and nitrate (NO3-)
• Assimilation: Nitrates are most easily absorbed by
plants and therefore added to the food chain
• Ammonification: Decomposers break down
nitrogenous waste and convert it to NH3 for
absorbtion
• Denitrification – nitrates converted back to N2 and
lost to atmopshere
• Humans affect this cycle by adding excess fertilizer
& also by soil depletion
• Excess NH4+ can acidify soils and aquatic
ecosystems