Mr. West
AP Biology
1. Definition
• Transpiration is the evaporation of water
from the aerial parts of plants.
Of all the water plant absorbs, over 95-99%
is transpired to the air as water vapor.
4. From where water is transpired?
• Aerial parts of whole young plant
• Lenticels (lenticular transpiration) 0.1%
• Cutin (cuticular transpiration) 3%~10%
Stomatum (stomatal transpiration) ~ 90%
What is most likely leaving through the
stomata of the leaf picture here?
• Water (H2O)
What is this process called?
 Stomatal Transpiration
Stomatal transpiration
water loss
Site of
Guard cells
Openings allow gases
and water to move in
and out of leaf
Open and close
the stomata
Transport in plants
• H2O & minerals
– transport in xylem
– transpiration
• evaporation, adhesion & cohesion
• negative pressure
• Sugars
– transport in phloem
– bulk flow
• Calvin cycle in leaves loads sucrose into phloem
• positive pressure
• Gas exchange
– photosynthesis
• CO2 in; O2 out
• stomates
– respiration
• O2 in; CO2 out
• roots exchange gases within air spaces in soil
Why does
kill a plant?
Water & mineral absorption
• Water absorption from soil
– osmosis
– aquaporins
• Mineral absorption
– active transport
– proton pumps
• active transport of
root hair
proton pumps
Control of transpiration
• Balancing stomate function
– always a compromise between
photosynthesis & transpiration
• leaf may transpire more than its weight in water in
a day…this loss must be balanced with plant’s
need for CO2 for photosynthesis
Importance of transpiration
Guard Cells
What process
Guard Cells
using CO2 and H2O
releasing O2 as a waste
H2O product?
• Photosynthesis
Stoma Open
What is the plant using this
to make?
• Carbohydrates-glucose
If the plant needs water for
photosynthesis, why is
water coming out of the
Function of Stomata
• These stomata (leaf
openings) naturally allow
water to evaporate out.
Why would the plant close
stomata with guard cells?
• Prevent excess water loss
(conserve water)
So what is the point of having
• Allows gas exchange for
Guard Cells
Stoma Closed
Function of Guard Cells
How do the
guard cells react
to the availability
of water?
Dry – guard
cells CLOSE
lots of H2O –
guard cells
4. Guard cells:
• cells that open and
close the stoma
5. Stomata: openings in
leaf’s surface; when
Allows CO2 in & O2 out
of leaf
Guard Cells Stomata
5. Characteristics of guard cells
Guard cell properties and their
relationship with stomatal control
• Thickness of CW varies in the ventral and dorsal
part of the guard cells.
• Contains chloroplast and can perform light
reaction. (not dark reaction for the lack of key
• Structurally isolated from epidermal cells for the
lack of plasmodesmata (water and ions transmit
only through cellular pathway, thus helps to build
up water gradient)
• Little volume, little amount of water absorption or
loss controls stomtal aperture.
6. Factors influencing stomatal
Water content
Plant hormone
(1). Light
• Stomata of most plant open in the day and
close at night, while CAM plants are just
the opposite.
• Stomata opening are sensitive to red light
and blue light, and blue light is more
effective, it stimulates opening by a bluelight receptor: zeaxanthin.
(2) Temperature
• Stomatal aperture increase with Temp,
within 20- 30℃ (the optimal).
(3). CO2
• Low CO2 conc. promotes stomatal opening,
while high CO2 conc. inhibits stomatal
opening through its acidification of the
guard cell thus inhibits PM
(4) Water content
• Stomta open when the leaf contain
enough water. When there is a water
shortage, they close.
(6) Plant hormones
• CTK promotes opening
• ABA inhibits
Factors that influence transpiration
Transpiration from the leaf depends on
two major factors:
1. Difference in water vapor gradient
2. Diffusional resistance
The driving force
of transpiration is
the “vapor
gradient.” This is
the difference in
vapor pressure
between the
internal spaces in
the leaf and the
around the leaf
Diffusional resistance comprises
stomatal resistance and boundary
layer resistance
Transpiration rate=Driving force/resistance
water vapor inside the leaf - water vapor of the air
stomatal resistance + boundary layer resistance
Environmental factors that affect
the rate of transpiration
1. Light
Plants transpire more rapidly in the light
than in the dark. This is largely because
light stimulates the opening of the
stomata , Light also speeds up
transpiration by warming the leaf .
2. Temperature
Plants transpire more rapidly at higher
temperatures because water evaporates
more rapidly as the temperature rises.
3. Humidity
When the surrounding air is dry, diffusion
of water out of the leaf goes on more
• 4. Wind
When a breeze is present, the humid air is
carried away and replaced by drier air.
• 5. Soil water
A plant cannot continue to transpire rapidly if its
water loss is not made up by replacement from
the soil. When absorption of water by the roots
fails to keep up with the rate of transpiration,
loss of turgor occurs, and the stomata close.
This immediately reduces the rate of
transpiration. If the loss of turgor extends to the
rest of the leaf and stem, the plant wilts.
If you were an aquatic plant
where would your stomata be?
Fringed Water-lily
Stomata are found only on the upper epidermis because
the lower epidermis is submerged in water. If the stomata
were to be on the underside, they wouldn't be able to
perform their function (i.e to allow water to evaporate and
thus contribute to transpiration).
Reviewing Concepts:
Water Transport
• Movement of water and minerals
in a plant involves entry into
roots, xylem, and leaves.
• 3 processes:
1. Osmosis
2. Capillary Action (Adhesion)
3. Cohesion-Tension Theory
Water Transport
1. Osmosis - Water entering root cells creates
a positive pressure called root pressure.
a) Root pressure (primarily at night)
tends to push xylem sap upward in
b) Guttation is appearance of drops of
water along the edge of leaves, it is
result of root pressure.
 Root pressure is not a sufficient
mechanism for water to rise to the tops
of trees
Water Transport
2. Capillary Action – is the rise of
liquids in narrow tubes.
a) Adhesion – Molecular attraction
between UNLIKE substances.
 Capillary Action is also not a
sufficient mechanism for water to
rise to the tops of trees
Water Transport
3. Cohesion-Tension Theory
a) Transpiration – evaporation of
water from plants
b) Cohesion – water molecules
attracted to other water molecules.
(polarity & hydrogen bonds)
c) Bulk Flow – water movement from
roots to leaves as water molecules
evaporate from the leaf surface.
Opening and Closing of Stomates
• Each stomate has two guard cells with a
pore between them.
– Stomates OPEN - when guard cells take
up water = increase in turgor pressure
– Stomates CLOSE - when guard cells lose
water = decreases in turgor pressure .
• Guard cells are attached to each other at
their ends; inner walls are thicker than outer
• As they take up water, they buckle out,
thereby creating an opening between cells.

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