1. Exchange and Transport
Lesson 13
Learning objectives:
By the end of the lesson you
should understand…
•why transpiration is a
consequence of gaseous
exchange
•how the leaves of some
xerophytes are adapted to reduce
water loss by transpiration

2. Stomata

4. How do stomata open and close?
• Stomatal closing: Potassium ions move out of the vacuole and
out of the cells. Water then moves out of the vacuoles,
following potassium ions. The guard cells shrink in size and
the stoma closes.
• Stomatal opening: Potassium ions move into the vacuoles.
Water moves into the vacuoles, following potassium ions. The
guard cells expand and the stoma opens.

5. Cross section of stoma

6. Xerophytes
• Xerophytes live in deserts or environments where water is scarce
and evaporation is rapid, or in windy habitats where evaporation
can also be rapid. Their typical features are :
• deep roots to reach water far underground
• shallow spreading roots to collect occasional rainfall
• leaves reduced to spines with minimum surface area for
transpiration
• reduced number of stomata to reduce transpiration rate
• rolled leaves, leaf hairs and stomata sunk in pits to trap moist air,
increasing humidity and slowing diffusion of water vapour from
the stomata
• waxy leaf cuticle which is impermeable to water
• stomata opening at night and closed at midday when evaporation
rate would be highest
• storage of water in succulent tissues

8. Activity 16
• Comparing the transpiration rate in two
different plants.

9. Xerophytes activity
• http://www.kscience.co.uk/as/module3/exerc
ises/xerophytes.htm

12. When water enters the roots, hydrogen bonds link each water molecule to the next so
the molecules of water are pulled up the thin xylem vessels like beads on a string. The
water moves up the plant, enters the leaves, moves into air spaces in the leaf, and
then evaporates (transpires) through the stomata (singular, stoma).

13. There are hundreds of stomata in the epidermis of a leaf. Most are located in
the lower epidermis. This reduces water loss because the lower surface
receives less solar radiation than the upper surface. Each stoma allows the
carbon dioxide necessary for photosynthesis to enter, while water evaporates
through each one in transpiration.

14. A potometer is a device that measures the rate at which a plant draws up
water. Since the plant draws up water as it loses it by transpiration, you are
able to measure the rate of transpiration. The basic elements of a potometer
are:
• A plant cutting • A calibrated pipette to measure water loss • A length of
clear plastic tubing • An air-tight seal between the plant and the water-filled
tubing

15. Calculating area of leaf
• The rate of transpiration is measured as the amount of water lost/ square
meter/ minute. Because water evaporates through the many stomata on
the leaf surface, the rate of transpiration is directly related to the surface
area. To arrive at the rate of transpiration, therefore, you must calculate
the leaf surface area of each plant: Because most stomata are found in
the lower epidermis, you will determine that surface area.
• • Lay the leaves to be measured on a 1-cm grid and trace their outlines. •
Count the number of square centimeters. Estimate the area of the partial
squares. (Here's a simple method for this estimate: Count a partial square
if it is at least half covered by the leaf; do not count partial squares that
are less than half covered.) • Do not include the area of the stem (petiole)
in your calculations.
• http://www.phschool.com/science/biology_place/labbench/lab9/quiz1.ht
ml