Biology

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 and terrestrial plants

Introductory activity
Suggest 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, Commelina 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 the stomata out to the atmosphere.

Self – assessment 12.1
Analyze the diagram below and answer to the following questions

1. What title fits better to this diagram:
a. an open stoma
b. a closed stoma
c. of a guard cell and neighbouring cells
d. of a stoma and neighbouring cells
2. The part labelled C is:
a. Vacuole
b. Thick inner cell wall
c. Chloroplast
d. Thin outer cell wall
3. The part which better represents the neighbouring cell is:
a. Part A
b. Part B
c. Part C
d. Part D
4. If the guard cells become more turgid, what is more likely to happen?
a. The cells A will swell
b. The pore will increase its diameter
c. The number of structures C will decrease
d. The structure B will stretch in ward
5. Which of the following statements is false about that diagram?
a. The stoma is closed
b. The inner cell wall of guard is thicker than the outer cell wall
c. There are many chloroplasts in neighbouring cells
d. The guard cells have many chloroplasts.

12.2 Theories used to explain the mechanism of opening and closure of stomata

Activity10.2
The 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 cells
–– Theory of starch sugar inter-conversion
–– Theory of glycolate metabolism and
–– Theory of active potassium pump.
The combined outcome of the four theories shows that in general stomata open
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 in between 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.

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 night
–– 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 to reduce transpiration
Plants can therefore vary the stomatal aperture. This allows a compromise between 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 affected
by 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 ions out 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 12.3
Observe 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.

Xerophytic plants or xerophytes are plants that inhabit arid regions (desert). They have the following adaptations:
–– Stomata sunken in grooves and reduced in number
–– Ability to fix CO₂ at night, so the stomata are closed during the day.

–– Epidermis infolded to reduce the surface area
–– 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 airspace 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 large continuous air spaces,
forming reservoir of oxygen and CO₂ which also provides buoyancy to the plant tissues when submerged.

A halophyte is a plant that grows in water of high salinity and they comeinto 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 upward and protrude out of the ground. e.g. mangrove tress.

Self-assessment 12.3
What 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 chose 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 Q
e. 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 cells
b. Theory of starch sugar inter-conversion
c. Theory of glycolate metabolism
d. Theory of active Potassium Pump.
3. What is the main difference between the guard cells and the other epidermal cells?
a. Guard cells have chloroplast while the remaining epidermal cells have no 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 cuticle
4. Water lily is:
a. Xerophytes
b. Halophyte
c. Hydrophyte
d. Heleophyte
5. Mangroves are plants adapted to estuaries or marine region with high salinity.
What statement does not describe the adaptations of mangroves?
a. The presence of lenticels that help in gas exchange and evaporation
b. Presence of large number of stomata on the upper side of the leaves
c. The presence of pneumatophores which are breathing roots
d. Presence of succulent tissues that have high concentration of salt

Section B
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 from
the 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 arbitrary units.

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.