Stomatal Movement Practice Questions
20 free Stomatal Movement practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
The opening and closing of stomata is primarily due to a change in the:
- A Turgidity of the guard cells
- B Temperature of the guard cells
- C Number of chloroplasts in guard cells
- D Thickness of the outer wall
Correct answer: Turgidity of the guard cells
When guard cells draw in water and become turgid, the stomatal aperture opens. When they lose water and become flaccid, the aperture closes.
Which of the following describes the structural feature of guard cell walls that facilitates stomatal opening?
- A Uniform thickness of all walls
- B Thin inner wall and thick outer wall
- C Lignified inner and outer walls
- D Thick inner wall and thin outer wall
Correct answer: Thick inner wall and thin outer wall
The inner wall of the guard cell (towards the stomatal aperture) is thick and elastic, while the outer wall is thin. This asymmetry causes the cells to bulge outward when turgid, pulling the pore open.
Cellulose microfibrils in the guard cells are oriented in which direction to assist in stomatal opening?
- A Longitudinally
- B Radially
- C Tangentially
- D Randomly
Correct answer: Radially
Cellulose microfibrils are oriented radially rather than longitudinally. This arrangement makes it easier for the guard cell to expand in length rather than width, helping the pore to open.
According to the most widely accepted theory, the primary ion responsible for changing the osmotic potential of guard cells is:
- A Sodium (Na+)
- B Potassium (K+)
- C Calcium (Ca2+)
- D Magnesium (Mg2+)
Correct answer: Potassium (K+)
The accumulation of K+ ions in the guard cells lowers their water potential, leading to water entry by osmosis. This increase in turgor pressure results in the opening of the stomata.
Which plant hormone is known as the 'stress hormone' because it triggers stomatal closure during water scarcity?
- A Auxin (Indole-3-acetic acid)
- B Gibberellin
- C Abscisic acid (ABA)
- D Cytokinin
Correct answer: Abscisic acid (ABA)
Abscisic acid (ABA) is produced under water-stress conditions. It signals the guard cells to release solutes, leading to a loss of turgidity and rapid closure of the stomata to conserve water.
Stomata typically open during the day and close at night. This pattern is reversed in which group of plants?
- A Mesophytes
- B Hydrophytes
- C CAM plants
- D C3 grasses
Correct answer: CAM plants
Crassulacean Acid Metabolism (CAM) plants, like cacti, exhibit scotoactive stomata. They open stomata at night to fix CO2 and close them during the day to minimize transpiration in arid environments.
What happens to the stomatal pore when guard cells become flaccid?
- A It opens wider
- B It remains unchanged
- C It fully closes
- D It becomes plugged with starch
Correct answer: It fully closes
Loss of turgor (flaccidity) causes the elastic inner walls of the guard cells to regain their original shape. This movement brings the inner walls closer together, effectively closing the pore.
In dicot leaves, guard cells are typically shaped like:
- A Dumbbell-shaped
- B Bean-shaped
- C Spherical
- D Rectangular
Correct answer: Bean-shaped
In most dicotyledons, guard cells are kidney-shaped or bean-shaped. In contrast, many monocots like grasses have dumbbell-shaped guard cells.
The influx of Potassium ions into guard cells is usually balanced by the entry of which anion or the production of which organic acid?
- A Chloride (Cl-) or Malate
- B Sulphate or Citrate
- C Phosphate (PO4) or Oxalate
- D Nitrate or Succinate
Correct answer: Chloride (Cl-) or Malate
To maintain electroneutrality, K+ influx is balanced by Cl- ions or the organic anion malate, which is produced from the breakdown of starch.
Which environmental factor directly causes stomata to close even during the day if it reaches excessively high levels?
- A Light intensity
- B Internal CO2 concentration
- C Relative humidity
- D Atmospheric oxygen concentration
Correct answer: Internal CO2 concentration
High internal concentrations of CO2 in the leaf sub-stomatal cavity act as a signal for the stomata to close. Conversely, low CO2 levels generally promote opening.
The subsidiary cells are specialized epidermal cells that:
- A Perform photosynthesis
- B Surround and support the guard cells
- C Are typically found only in root tissues
- D Secrete waxy cuticle
Correct answer: Surround and support the guard cells
Subsidiary cells (or accessory cells) are located adjacent to the guard cells. They assist in the ion exchange and physical movement of the guard cells during stomatal regulation.
Transpiration occurs primarily through the stomata. What percentage of total water loss in a plant is typically stomatal?
- A 1-5%
- B 10-20%
- C 50-70%
- D 90-95%
Correct answer: 90-95%
While some water is lost through the cuticle or lenticels, approximately 90-95% of transpiration occurs through the stomatal pores when they are open.
Blue light is known to be highly effective in:
- A Closing stomata
- B Promoting stomatal opening
- C Inhibiting photosynthesis
- D Decreasing guard cell turgidity
Correct answer: Promoting stomatal opening
Blue light triggers a specific photoreceptor in guard cells that activates a proton pump. This pump creates a gradient that facilitates the uptake of K+ ions, leading to stomatal opening.
During the 'Starch-Sugar Hypothesis' of stomatal movement, an increase in pH leads to:
- A Conversion of sugar to starch
- B Immediate death of the guard cell
- C Thickening of the outer wall
- D Conversion of starch to sugar
Correct answer: Conversion of starch to sugar
An increase in pH (more alkaline) occurs during the day due to CO2 consumption by photosynthesis. This pH shift activates enzymes that convert insoluble starch into soluble sugars, increasing osmotic pressure.
Which statement regarding stomatal distribution is true for most dorsiventral (dicot) leaves?
- A Stomata are only on the upper surface
- B Stomata are equally distributed on both surfaces
- C Stomata are absent on the lower surface
- D Stomata are more numerous on the lower surface
Correct answer: Stomata are more numerous on the lower surface
Dicot leaves are usually hypostomatic, meaning they have more stomata on the lower (abaxial) epidermis to reduce water loss from direct sunlight.
Guard cells differ from other epidermal cells because they contain:
- A Nuclei
- B Vacuoles
- C Chloroplasts
- D Mitochondria
Correct answer: Chloroplasts
Unlike typical epidermal cells, guard cells contain chloroplasts. While their photosynthetic rate is relatively low, they play a vital role in sensing light and producing solutes.
The immediate cause of water movement out of guard cells, leading to closure, is:
- A Increase in solute concentration
- B Active pumping of water into the cell
- C Decrease in temperature
- D Decrease in solute concentration
Correct answer: Decrease in solute concentration
When solutes (like K+ and malate) leave the guard cells, the water potential inside the cell increases. Water then moves out of the cells (exosmosis), causing them to become flaccid.
What is the collective term for the stomatal pore, guard cells, and surrounding subsidiary cells?
- A Stomatal apparatus
- B Stomatal pit
- C Lenticel exchange system
- D Hydathode complex
Correct answer: Stomatal apparatus
The stomatal apparatus (or stomatal complex) includes the aperture (pore), the pair of specialized guard cells, and any neighboring subsidiary cells.
In grasses, the guard cells are described as being:
- A Kidney-shaped
- B Heart-shaped
- C Dumbbell-shaped
- D Elliptical-shaped
Correct answer: Dumbbell-shaped
The guard cells in the Poaceae family (grasses) are dumbbell-shaped. They have bulbous ends and a narrow middle section with thickened walls.
The rate of transpiration is highest when the air is:
- A Dry and moving (windy)
- B Humid and still
- C Cold and humid
- D Saturated with water vapor
Correct answer: Dry and moving (windy)
Dry air increases the water potential gradient between the leaf and the atmosphere, while wind removes the saturated boundary layer of air near the leaf surface, both accelerating transpiration.