Transport in
Angiosperms
Topic 9.2
Transpiration 9.2.5-9.2.10
 The loss of water vapor from leaves occurs
through stomata.
 Stomata are surrounded by 2 guard cells which
open and close
 As water exits the leaves, it is replaced by
water entering via the root
 Energy from the sun drives this process:
transpiration-adhesion-cohesion-tension
theory.
The process of
transpiration
 Heat is produced when light strikes a leaf.
 Water in the spongy mesophyll tissue
enters the vapour phase.
 Water evaporates through the stomatal pore
down a humidity gradient.
 The evaporation of water draws (pulls) more
water by mass flow into the spongy
mesophyll space.
 Water molecules are held together cohesion
due to hydrogen bonds between water
molecules.
 In turn this draws water from the end
of the xylem by the same cohesion.
 Water is therefore drawn up the stem
by cohesion between water molecules
and adhesion to the xylem vessel
walls.
 This transpiration 'pull or tension'
extends all the way down the xylem to
the root
The xylem
 2 major cell types
 Tracheids – dead at maturity. Have tapered
ends that connect to other tracheids via pits
 Vessel elements – dead at maturity with
lignified (woody) cell walls. These lie end to
end like straws. These are most efficient at
moving water.
 Ancient plants had only tracheids
 Most modern plants have only vessel
elements
The CO2 dilemma!
 Plants can only acquire carbon dioxide
when their stomata are open.
Remember CO2 is necessary for carbon
fixation in photosynthesis.
 Guard cells regulate the opening and
closing of the stomata
Guard cells
 The walls of the cells are of uneven
thickness (inner wall thicker than outer)
 When the cells take in water the outer part
bulges which opens the stomata. When they
lose water they sag and collapse over the
stomatal opening.
 Potassium ions are actively pumped into the
guard cells causing water to enter via osmosis,
thus opening the stomata.
 K ion pumps are stimulated by light from the
blue part of the spectrum.
 As the ions diffuse out so does water,
collapsing the guard cells.
 The hormone abscisic acid causes K ions
to diffuse from the guard cells rapidly.
This hormone is made by the roots in
times of drought
To study
 Table page 250 – how environmental
factors affect transpiration
 Table page 253 – details of the
transpiration-cohesion-tension theory
Xerophytes adaptations








Plants adapted to dry (arid) climates
Small thick leaves
Stomata in pits
Waxy cuticle
Fewer stomata
Hairs on leaves
Loss of leaves in dry months
Water storage in stems
Alternate photosynthetic
processes
 Both processes developed to conserve water
 CAM – crassulacean acid metabolism. Carbon
dioxide is fixed at night and incorporated into
organic acids. It is released during the day for
photosynthesis
 C4 photosynthesis – Carbon dioxide is
incorporated into a 4-C compound then moved
to the interior of the leaf. More Carbon dioxide
can be fixed than in a C3 plant
Roots 9.2.1 – 9.2.3
 Recall that root epidermal cells have
extensions called root hairs to increase
the surface area for water and mineral
absorption.
 As a root pushes through the soil it is
protected by a root cap.
•Zone of cell division –
closest to tip, cells
undifferentiated, mitosis is
occurring
•Zone of elongation – cells
are enlarging, G1 of cell
cycle
•Zone of maturation – cells
are differentiating and
beginning to function.
This is where root hairs
begin to be noticed.
Water movement
 Water must travel through the root to the
vascular cylinder which is in the center of
the root.
 The vascular cylinder is surrounded by
endodermis and pericycle
 Pericycle consists of cells that can
produce lateral roots
 Endodermis is a cylinder 1 cell thick that
forms a selective barrier which regulates
passage of substances from soil into VC
 Each endodermal cell has a barrier called
the Casperian strip. This strip is made of
suberin, a waxy, impermeable layer
 The impermeable suberin ensures that all
water and minerals entering the plant
must cross a semipermeable membrane.
Passage of water and
minerals into root
 Root hairs absorb soil solution (water and
minerals)
 The soil solution can travel 2 routes
 Apoplastic route
 Symplastic route
Apoplastic route
 Water and minerals travel between cell
walls through the cortex region
 Some solution enters cells some does
not
 When the solution reaches the
endodermis, the Casperian strip forces
the solution into a cell so that it has to
pass through a cell membrane
Symplastic route
 Soil solution passes through cells (and
their membranes) on their way to the
stele (vascular cylinder)
 Once past the endodermis, water and
minerals enter the xylem vessels to be
transported throughout the plant.
 Is the solution (now called xylem sap)
pushed or pulled through a plant?
 Push due to root pressure
 Pull by tension generated by transpiration
Movement of ions through
plants
 Minerals dissolved in water may enter by
diffusion if concentration inside the root is
lower than concentration outside root
 Fungal hyphae called mycorrhiza
increase the root surface area for water
and mineral absorption (mutualism)
 If mineral concentration inside the plant is
higher than in the soil, active transport is
required. This is also necessary if the ion
cannot cross the phospholipid bilayer.
 Active transport requires a transport
protein
 K + ions move through proteins called
potassium channels
Proton pump
 ATP provides energy to pump H+ ions out
of cell
 This makes the inside of the cell more
negative than the outside
 The hydrogen ion gradient causes a
voltage difference called membrane
potential. (-120 mv)
 Membrane potential is a form of potential
energy that can be used to absorb
mineral ions.
 Hydrogen ions may displace cations
attached to the soil, freeing them so they
may enter root
 Hydrogen ions may combine with anions
and drag the anion into the root via
cotransport
9.2.4 Plant support
 Cellulose cell walls – walls may thicken
for additional support
 Lignified cell - A complex polymer, the
chief noncarbohydrate constituent of
wood, that binds to cellulose fibers and
hardens and strengthens the cell.
 Collenchyma, vascular bundles,
sclerenchyma, provide flexible support
Turgor pressure
 How do osmosis, the plant cell vacuole,
and the plant cell wall work together to
provide turgor pressure?
 What happens if turgor pressure is lost?
9.2.11 Movement of
Sugars AKA Translocation
 Sugars and organic molecules are
transported through sieve tube members
and their companion cells
 Place where molecules originate is called
the source. Could be a leaf or a storage
organ
 Place where molecules are going is
called the sink. Could be where growth is
occurring or a fruit or tuber
 Companion cells load sucrose (soluble
and not metabolically active) into sieve
tubes.
 Water follows via osmosis causing
positive pressure in sieve tube. This
makes phloem sap flow.
 Companion cells unload sucrose at sink
(requires ATP as it is against
concentration gradient.
 Sugars may be converted to starch in
sink
 Water used in translocation is recycled
by xylem











1. Source produces organic molecules
2. Glucose from photosynthesis produced
3.Glucose converted to sucrose for transport
4. Companion cell actively loads the sucrose
5. Water follows from xylem by osmosis
6. Sap volume and pressure increased to give Mass
flow
7. Unload the organic molecules by the companion cell
8. Sucrose stored as the insoluble and unreactive
starch
9. Water that is released is picked up by the xylem
10. water recycles as part of transpiration to re supply
the sucrose loading
Source:
http://www.click4biology.info/c4b/9/plant9.2.htm#11