UNIT 12: GAS EXCHANGE IN PLANTS
UNIT 12: GAS EXCHANGE IN PLANTS
Key Unit Competence
Describe structures of gaseous exchange organs in plants
Learning objectives
By the end of this unit I should be able to:
– Describe the structure of the stoma.
– Explain how stomata, lenticels and breathing roots are adapted to their
function.
– Explain the theories of opening and closure of stomata stating limitations of
each.
– Relate the differences between the structures of aquatic and terrestrial leaves
to a habitat.
– Draw and label a diagram of stoma as observed under alight microscope.
– Compare gaseous exchange structures of aquatic and terrestrial plants
– Relate the structure and function of aquatic and terrestrial plants
– Defend the relationship between structure and function in aquatic andterrestrial plants
Introductory activitySuggest the different parts of a plant that are used in gaseous exchange
12.1. Structure of stoma
Activity 12.1
Requirements
Light microscope, glass slide, cover slip, Camelina zebrine leaves, razor blade,forceps, Pasteur dropper and iodine solution.
Procedure
– Identify Commelina zebrina or commelina tradescantia plant nearby the
school. You can also use any other monocotyledonous plant with succulent
leaves.
– Remove a leaf from a plant. Then peel off gently the lower epidermis. It must
be thin enough to allow light to pass through
– Smear the epidermis on a slide containing one drop of dilute iodine solution.
– Put on a cover slip and then observe under the lower and medium
magnification.
– Repeat the observation in morning hours and in the afternoon hours.
Questions
1. Why should the sample used in the preparation must be transparent?
2. Draw and label structures observed under light microscope
Stomata (stoma in singular) are microscopic pores in epidermis of the leaves and
stems of terrestrial plants. They function in gas exchange between plant and the
atmosphere and in transpiration.
Each stoma is bordered by two saucer shaped cells called guard cells, which are
specialized epidermis cells whose movements control the size of the aperture
(pore). Unlike other epidermis cells, guard cells have kidney shape and have many
chloroplasts. Their inner cell wall is thick and less elastic while the outer cell wall is
thin and more elastic. Guard cells shrink when the plant has too little water. This
closes the stomata. When the plant has enough water, the guard cells swell up
again. This opens the stomata. In this way, the guard cells enable gaseous exchange.
Oxygen in the atmosphere diffuses through the stomata into the air spaces between
the cells of the spongy mesophyll tissue while carbon dioxide diffuses through thestomata out to the atmosphere.
Self – assessment 12.1Analyze the diagram below and answer to the following questions
1. What title fits better to this diagram:a. an open stoma2. The part labelled C is:
b. a closed stoma
c. of a guard cell and neighbouring cells
d. of a stoma and neighbouring cellsa. Vacuole3. The part which better represents the neighbouring cell is:
b. Thick inner cell wall
c. Chloroplast
d. Thin outer cell walla. Part A4. If the guard cells become more turgid, what is more likely to happen?
b. Part B
c. Part C
d. Part Da. The cells A will swell5. Which of the following statements is false about that diagram?
b. The pore will increase its diameter
c. The number of structures C will decrease
d. The structure B will stretch in warda. The stoma is closed12.2. Theories used to explain the mechanism of opening and
b. The inner cell wall of guard is thicker than the outer cell wall
c. There are many chloroplasts in neighbouring cellsd. The guard cells have many chloroplasts.
closure of stomata
Activity12.2The diagram below shows a closed and an open stoma.
Carefully analyse the diagrams above and brainstorm your observation.
Illustrate how stomata open and close
Many theories have been proposed regarding opening and closing of stomata. The
four important theories of stomatal movement are the following:– Theory of photosynthesis in guard cellsThe combined outcome of the four theories shows that in general stomata open
– Theory of starch sugar inter-conversion
– Theory of glycolate metabolism and– Theory of active potassium pump.
during the day (light) and close during the night (dark). But how does this happen?
In light, guard cells are stimulated. They absorb K+ ions from the neighbouring cells.
K+ ions make the guard cells more permeable to CO2
. As the guard cells perform photosynthesis, the concentration of CO2 falls and the pH rises. Elaborated starch
therefore splits into malate. The high concentration of malate and the rise of pH in
guard cells develop a decrease in water potential. Hence, the guard cells withdraw
water from the neighbouring cells and extend backward leaving an open pore inbetween whereby water is lost by evaporation.
During the night, there is no light to stimulate neither the absorption of K+ ions nor the
photosynthesis. Guard cells undergo cell respiration using photosynthetic products
as source of energy (carbohydrates: malate, glucose). Therefore, the concentration
of malate decreases making the guard cells hypotonic than neighbouring cells. As
guard-cells lose their water content, they shrive and the pore in between closes.
Stomatal transpiration ceases.
192
During the night, there is no light to stimulate neither the absorption of K+ ions nor the
photosynthesis. Guard cells undergo cell respiration using photosynthetic products as source of
energy (carbohydrates: malate, glucose). Therefore, the concentration of malate decreases making
the guard cells hypotonic than neighbouring cells. As guard-cells lose their water content, they shriveand the pore in between closes. Stomatal transpiration ceases.
Plant physiologists are certain that stomatal aperture varies as a result of changes in
\
the turgidity of the guard cells. But they are less certain about how these changes
are brought about, though the following observations have been made:– Most stomata open during the day and close at nightPlants can therefore vary the stomatal aperture. This allows a compromise between
– Some stomata show a circadian (daily) rhythm of opening and closing even
when kept in constant conditions
– Stomata generally close when a plant suffers water stress, for example, when
transpiration exceeds water absorption
– The stomata of some desert plants close during the day and open at night toreduce transpiration
the need to conserve water and the need to exchange gases for photosynthesis.
The compensation point is the point when the rate of photosynthesis is equal to the
rate of respiration. This means that the CO2 released from respiration is equivalent to
that which is taken up during photosynthesis. The compensation point is reached as
light intensity increases. If the light intensity is increased beyond the compensation
point, the rate of photosynthesis increases proportionally until the point of light
saturation is reached, beyond which the rate of photosynthesis is no longer affectedby light intensity.
For a plant under water stress, its need to conserve water is greater than its need to
obtain carbon dioxide for photosynthesis. Under these conditions a plant secretes
abscisic acid (ABA). This is a chemical messenger which causes stomata to close. It is
thought that ABA triggers a metabolic pump which actively secretes potassium ionsout of guard cells, causing the cells to lose water and become flaccid.
Self-assessment 12.2
1. According to the ionic theory of opening and closing stoma, what is the role
of potassium ions in the guard cell?
2. What would happen to guard cells if the concentration of malate doubled?
3. What is meant by compensation point?
12.3. Structural adaptations and function of stomata, lenticels
and breathing roots.
Activity 12.3Observe the adaptations of these plants for gas exchange.
1. How is each of these plants adapted for gas exchange?
2. Read through the notes that follow and describe any two adaptations for
gas exchange
The exchange of atmospheric gases is essential to photosynthesis and cell respiration. In plants, the
gas exchange takes place through stomata, breathing roots, lenticels and cuticles. Most stomata are
on the lower epidermis of the leaves on plants. Unlike other plant epidermal cells, the guard cells
contain chlorophyll to carry out photosynthesis. This allows the cells to expand or contract to open
or close the stomata.
Guard cells swells, through the process of osmosis, to allow opening of the stomata for CO2 to enter
and excess O2 and H2O to leave, and they shrink in order to force the stomata shut either partially or
completely to prevent dehydration. The number of stomata on the epidermal surface depends on the
ecology of plants. Usually, plants on wet climate have fast growth and a high concentration of
stomata. Plants on dry weather have lower rates of photosynthesis, lower growth and lower
concentrations of stomata.
Self-assessment 12.2
1) According to the ionic theory of opening and closing stoma, what is the role of
potassium ions in the guard cell?
2) What would happen to guard cells if the concentration of malate doubled?
3) What is meant by compensation point?
Activity 12.3
Observe the adaptations of these plants for gas exchange.
Figure 12: (a) Mangrove (b) Breathing roots (c) Pneumatophores
1) How is each of these plants adapted for gas exchange?
2) Read through the notes that follow and describe any two adaptations for gas exchange
(a) Mangrove (b) Breathing roots (c) Pneumatophores
1. How is each of these plants adapted for gas exchange?
2. Read through the notes that follow and describe any two adaptations for
gas exchange
The exchange of atmospheric gases is essential to photosynthesis and cell
respiration. In plants, the gas exchange takes place through stomata, breathing
roots, lenticels and cuticles. Most stomata are on the lower epidermis of the leaves
on plants. Unlike other plant epidermal cells, the guard cells contain chlorophyll to
carry out photosynthesis. This allows the cells to expand or contract to open or close
the stomata.
Guard cells swells, through the process of osmosis, to allow opening of the stomata
for CO2 to enter and excess O2 and H2O to leave, and they shrink in order to force the
stomata shut either partially or completely to prevent dehydration. The number of
stomata on the epidermal surface depends on the ecology of plants. Usually, plants
on wet climate have fast growth and a high concentration of stomata. Plants on dry
weather have lower rates of photosynthesis, lower growth and lower concentrationsof stomata.
Xerophytic plants or xerophytes are plants that inhabit arid regions (desert). They
have the following adaptations:– Stomata sunken in grooves and reduced in numberEpidermis infolded to reduce the surface area
– Ability to fix CO2 at night, so the stomata are closed during the day.– Leaves reduced to scales or thorns to reduce the surface area for transpiration
Hydrophytes or water plants are plants that grow submerged or partially submerged
in water. To thrive in this environment, hydrophytes have the following features
– developed stomata on large upper surface of their leaves (rather than
underside) making gas exchange more efficient.
– large air space to facilitate evaporation from the mesophyll.
– little or no lignified supporting tissues on the submerged parts.
– poorly developed transport tissue,
– stems and leaves have little or no lower cuticle but large continuous air spaces,
forming reservoir of oxygen and CO2
which also provides buoyancy to theplant tissues when submerged.
A halophyte is a plant that grows in water of high salinity and they come into contact
with saline water through its roots or by salt spray, such as in saline semi-deserts,
mangrove swamps, and marshes. Halophytes are adapted in the flowing ways:– Store water in succulent tissues which have high concentration of salt. They
can thus take up water from the sea water by osmosis.
– Extensive air spaces throughout the stem and roots making air available to all
cells, and giving buoyancy to the stem and leaves at highest tides.
– They develop breathing roots called pneumatophores which grow upwardand protrude out of the ground. e.g. mangrove tress.
Self-assessment 12.3What features are common to plants living in desert and saline soils?
End of unit assessment 12
Section A: Objective questions
1. You are provided with the diagram below. Analyze it and then choose the
correct answer. Transpiration in the leaf depends on the transport of potassium
ions into:
a. Into O
b. Into P
c. From M to L
d. From M to Qe. From P to L.
2. The theory that says than during the light time, potassium pumps open and
this brings about diffusion of CO2 from the atmosphere to the guard cells for
photosynthesis is called:a. Theory of photosynthesis in guard cells3. What is the main difference between the guard cells and the other epidermal
b. Theory of starch sugar inter-conversion
c. Theory of glycolate metabolismd. Theory of active Potassium Pump.
cells?a. Guard cells have chloroplast while the remaining epidermal cells have no4. Water lily is:
chloroplast
b. Guard cells have oval shape while other cells have cubic shape
c. Guard cells are beneath the spongy mesophyll
d. Guard cells are covered by a transparent cuticlea. Xerophytes5. Mangroves are plants adapted to estuaries or marine region with high salinity.
b. Halophyte
c. Hydrophyte
d. Helophyte
What statement does not describe the adaptations of mangroves?a. The presence of lenticels that help in gas exchange and evaporationSection B
b. Presence of large number of stomata on the upper side of the leaves
c. The presence of pneumatophores which are breathing rootsd. Presence of succulent tissues that have high concentration of salt
6. Explain how gaseous exchange occurs in the leaf.
7. How does gaseous exchange occur in woody stems?
8. Describe how roots get oxygen.
9. a. Draw a labelled diagram of a stoma
b. Draw arrows on the diagram to show how gaseous exchange occurs.
10.The drawing shows a 24-hour cycle for the opening and closing of stomata fromthe same plant.
a. Explain how this cycle of opening and closing of stomata is advantageous
to the plant.
b. The diagram shows the potassium (K+) concentrations in the cells around
open and closed stomata in commelina. The concentrations are in arbitraryunits.
i. Explain how the movement of K+ ions accounts for the opening of
stomata.ii. Explain how K+ ions are moved against a concentration gradient.