Download Grade 11 College Biology – Unit 4

Grade 11 University Biology – Unit 5 Plants
Transport in Plants
Section 13.4 Pages 560-566
TRANSPIRATION
 Process in which water evaporates from the inside of the leaf to the outside through the stomata


Every day, a growing plant takes in 10-20X more water than an animal of comparable size, BUT
almost 90% pf the water is lost by TRANSPIRATION, the evaporation of water from the leaves and
stems.
The RATE of transpiration is affected by the relative humidity in the air (i.e., plants transpire more
rapidly on hot, dry days).

Water loss occurs through the STOMATA in the EPIDERMIS. At night, the stomata are closed, so
water loss is reduced. At the same time, there is internal water loss as water evaporates from the wet
cells walls of the MESOPHYLL into the INTERCELLULAR SPACES. The process raises the internal
relative humidity of the leaf to almost 100 percent. When the sun rises, the stomata open to admit
carbon dioxide. Since the relative humidity of the external air is usually lower than 100 percent, water
vapour diffuses from inside the leaf to the outside through the stomata openings.

The water needs to be replaced in the mesophyll cells. Most of the new water comes from the
XYLEM. The water movement is fast and called BULK FLOW.
ABSORPTION BY ROOTS
 Root cells contain a higher concentration of dissolved nutrients than the surrounding soil. Thus,
water MOVES INTO the roots via osmosis.

ROOT HAIRS absorb most of the water. The water in the cell walls of the roots epidermis is usually
HYPOTONIC, a situation in which there is a greater concentration of water outside the cell wall. In
this situation, water moves passively via osmosis into the roots.

When the water reaches the cell walls of the ENDOERMIS, it reaches specialized cell wall region
called the CASPARIAN BAND which is impregnated with a waxy layer similar to the cuticle. Water
cannot pass through the Casparian band (i.e., it is impervious to water). The movement is controlled
by active transport of salt dissolved in water. That is, the cells expend energy to take in salts against
a CONCENTRATION GRADIENT. Salts are secreted from the cells on the inner side of the cell, and
as a result, water moves out. Both water and salt move by OSMOSIS, the diffusion of water across a
membrane in living system – osmotic pressure – the pressure created by water molecules as they
attempt to reach equilibrium across a membrane. By this process, salts and water move through the
cell wall into the xylem.
ROOT PRESSURE
 When transpiration in the leaves is slow at night, salt is still secreted in the endodermis of the root.
This creates an area of low water pressure in the xylem. More water moves into the xylem by
osmosis. As water moves into the xylem, a positive pressure called ROOT PRESSURE is created.
The osmotic pressure pushes the water up the xylem. Root pressure is the mechanism by which
positive pressure in the roots moves water upward in a plant. NOTE: While root pressure might
account for some of the upward movement of water in small herbaceous plants, it is insufficient to
overcome the force of gravity and move water up tall plants.
COHESION THEORY
 Cohesive forces between water molecules are very strong.

Water evaporates molecule by molecule from the cell walls of the leaf mesophyll. Because of
cohesiveness, a continuous chain of water molecules in pulled along. As water molecules cling to
each other as they move up the stem and into the leaves, they pull the molecules up as they transpire
out of the stomata. This creates a negative
pressure, and the negative pressure exerts
tension on the water confined in the xylem.
As water molecules are stuck together by
cohesion, the entire column of water in the
xylem adheres to the sides of the xylem. It is
said that the water in under tension as the
column moves up the xylem. At the same
time, the XYLEM TUBE narrows because of
the tension. Water moves up from the roots
to replace water lost from the leaves.

Cohesiveness is shown by the RATE OF
TRANSPIRATION. Transpiration begins in
the morning with the opening of the stomata
and reaches its peak by mid-afternoon. In
contrast, water uptake in the root does not
reach its peak until late-afternoon and
continues throughout the night.

Cohesion-Tension Model is a model of water
transport that explains how water is moved
from the roots to the leaves of a plant.
 Transpiration – The evaporation
of water molecules from the
shoot system becoming a force
that moves water and dissolved
minerals upward in the plant stem
 Cohesion – The force of attraction between water molecules
 Adhesion – The force that causes water molecules to stick together
FACTORS IMPACTING TRANSPIRATION
Temperature
 Transpiration increases when it is sunny and hot outside. This is because the holes on the
leaves that the water escapes from open up wider in heat. This allows more water to be released
in less time. In cold weather, the holes on the leaves close up tighter and transpiration rates
decrease.
Humidity
 At high humidity, the less transpiration will take place. High humidity means that there is a lot of
water in the air. When this happens, there is not as much room in the air for new water vapour.
The dryer the air is, the more plants will transpire because there is a lot of space left for new
water vapour.
Wind
 Transpiration occurs at higher levels when there is wind. Wind blows the air out from underneath
the leaves and keeps the air fresh. Without much wind, the same air stays underneath the leaves
where the water comes out of the holes. This old air gets filled up with water vapour and does
not absorb new vapour.
Soil
 Certain soils can affect the rate of transpiration. If the soil is not rich in moisture, the plant will dry
out and lose leaves. Thus, there is less opportunity for transpiration to take place.
Nutrient Transport in Phloem
Photosynthesis occurs in the green parts of the plant, mainly the leaves. The product of photosynthesis
is glucose. It is converted to sucrose for transport through the phloem.
TRANSLOCATION is the transport of sucrose and other organic molecule through the phloem.
PRESSURE-FLOW MODEL
Translocation is from a source to a sink.
The source is where the sucrose enters the sieve
tubes. The sink is any region where the plants are
stored.
Pressure-Flow is the combination of osmosis and
pressure dynamics that pushes materials from a
source to a sink.
For an experimental model of this model see
Figure 13.23 on Page 564. A change in the
concentration on sucrose also causes a movement
of water in the same direction. Internal pressure
increases at the source and pushes the sucroserich solution toward the sink. In the phloem, it is a
POSTIVE PRESSURE causing the flow.
HOMEWORK
 Page 566, Questions 1, 3, 4, 7, 11, 12, 15
Experiments to demonstrate transpiration in plants
Experiment #1
Materials
 Potted plant
 Empty pot
 Cellophane
 Vaseline
 Glass plate
 Bell Jar
Experiment Setup
Methods
 Take a potted plant and cover the pot and base of stem with cellophane or rubber.
 Place the potted plant on a “vaselined-glass plate”
 Invert a bell-jar over the pot and plant and leave them outside the laboratory.
 Set up a control experiment with no plant.
 Observe after one hour
 After an hour, drops of colourless liquid are seen inside the bell-jar with the plant. To show that
these drops are water, touch them with anhydrous copper sulphate (white) and its colour changes
to blue. No drops of water should be found in the control experiment where there was no plant.
Inference
The presence of stomata makes the leaves suitable for exchange of gases which goes on during
photosynthesis and respiration, and this makes loss of water inevitable. Stomata are tiny pores in the
epidermis of leaves, each pore being surrounded by a pair of special cells called guard-cells which control
the size of the stomata pores. The behaviour of the stomata and the rate of transpiration are determined
by environmental factors. Transpiration exerts a force against that of gravity, and this is known as
transpiration pull, enables water to be absorbed.
Experiment #2
Materials
 1 healthy geranium plant
 2 glasses that are the same size and kind
 Water
 Vaseline
 Shirt cardboard box
 Scissors
 Sharpened pencil
 Ruler
 Window area
Methods
 Use your ruler and pencil to measure and mark a
rectangle from the cardboard shirt box that is four by
six inches. Cut out the rectangle.
 With the sharp tip of the pencil, poke a little hole in the
middle of the cardboard rectangle.
 Break off a healthy leaf and stem from the geranium
plant.
 The leaf stem is called a petiole. Put the petiole of the
leaf in the hole in the cardboard rectangle.
 Fill one of the glasses three-quarters full with water.
 Cut the cardboard on top of the glass with the water in
it so the stem is down into the water and the leaf is on
top of the cardboard- not in the water.
 Take a little bit of vaseline and put it around the hole.
This is to keep evaporated water from the glass
seeping up into the top glass.
 Put the second glass upside down over the leaf, resting against the other glass' mouth.
 Put the glasses on a ledge or table top near a window where there is a good source of sunlight.
 Observe after what happens after 3 or 4 hours. Do you see little drops of water on the inside of
the top glass? Where is the water coming from? If you plugged the hole around the stem, the
water from the bottom glass should not be getting up into the top glass. This is transpiration, the
process whereby the leaves on green plants give off excess water. Look at the bottom of the
geranimum's leaves. You will see little dots which are called stomates. The stomates give off
water