Transpiration Practice Questions
20 free Transpiration 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.
Which of the following is the primary purpose of transpiration in plants?
- A To transport food from the leaves down to roots
- B To create a pull for water absorption and transport
- C To decrease the overall rate of photosynthesis
- D To raise the temperature of the leaf surface slightly
Correct answer: To create a pull for water absorption and transport
Transpiration creates a 'transpiration pull' which is the main driving force for the ascent of sap. This process facilitates the absorption and movement of water and minerals from the roots to the leaves.
The immediate cause of the opening or closing of the stomata is a change in the:
- A Turgidity of the guard cells
- B Number of chloroplasts in guard cells
- C Thickness of the outer walls of guard cells
- D Orientation of xylem vessels
Correct answer: Turgidity of the guard cells
Stomatal movement is driven by turgor changes in guard cells. When water flows into guard cells, they become turgid and the stomatal pore opens; when they lose water, they become flaccid and the pore closes.
The inner wall of each guard cell (towards the stomatal aperture) is:
- A Thin and elastic
- B Thick and elastic
- C Thick and rigid
- D Thin and rigid
Correct answer: Thick and elastic
In guard cells, the wall towards the pore is thick and elastic, while the outer wall is thin. This differential thickening allows the cells to bulge outwards when turgid, pulling the inner walls apart to open the pore.
Which orientation of cellulose microfibrils in the cell walls of guard cells facilitates stomatal opening?
- A Longitudinal orientation
- B Radial orientation
- C Tangential orientation
- D Helical spiral
Correct answer: Radial orientation
Cellulose microfibrils are oriented radially rather than longitudinally in guard cell walls. This radial arrangement makes it easier for guard cells to expand in length rather than width, helping the stomatal pore to open.
Which of the following is NOT an external factor affecting the rate of transpiration?
- A Light and temperature
- B Humidity and wind speed
- C Stomatal number and position
- D Atmospheric moisture content
Correct answer: Stomatal number and position
Stomatal number and distribution are plant (internal) factors, not external environmental factors. External factors include light, temperature, humidity, and wind speed.
The transpiration driven ascent of xylem sap depends mainly on which physical property of water?
- A Cohesion alone
- B Adhesion alone
- C Surface tension alone
- D Cohesion, adhesion, and surface tension together
Correct answer: Cohesion, adhesion, and surface tension together
Cohesion (mutual attraction between water molecules), adhesion (attraction to polar xylem surfaces), and surface tension all work together to maintain a continuous water column and provide the high tensile strength needed for the ascent of sap.
How much of the water reaching the leaves is typically used in photosynthesis and plant growth?
- A Less than 1 percent
- B About 5 percent
- C Nearly 50 percent
- D More than 90 percent
Correct answer: Less than 1 percent
Plants absorb massive amounts of water, but less than 1 percent is actually used for growth and photosynthesis. The remaining 99 percent is lost to the atmosphere through transpiration.
Transpiration helps cool a leaf's surface mainly through which mechanism?
- A Evaporative cooling by latent heat loss
- B Radiation of heat to the night sky
- C Conduction of heat into the soil
- D It does not cool the leaf but heats it up
Correct answer: Evaporative cooling by latent heat loss
As water evaporates from leaf surfaces during transpiration, it absorbs latent heat of vaporisation, thereby cooling the leaf -- a process called evaporative cooling.
In a dorsiventral (dicotyledonous) leaf, stomata are usually:
- A Present only on the upper epidermis
- B More numerous on the lower epidermis
- C Equally distributed on both surfaces
- D Absent on the lower epidermis
Correct answer: More numerous on the lower epidermis
Dicot leaves are dorsiventral and typically have more stomata on the lower surface to reduce excessive water loss. In isobilateral (monocot) leaves, they are usually about equal on both surfaces.
Which process describes the 'Cohesion-Tension-Transpiration Pull' model?
- A Active pumping of water by xylem vessels
- B Pushing water upward from roots via positive pressure
- C Pulling water through xylem using evaporation energy
- D Movement of water driven by gravitational force alone
Correct answer: Pulling water through xylem using evaporation energy
The cohesion-tension-transpiration pull model proposes that water is pulled through the plant. Evaporation from leaves creates a negative pressure (tension) that is transmitted down to the roots through a cohesive, unbroken column of water in the xylem.
Which of the following describes the relationship between humidity and transpiration rate?
- A Transpiration increases as humidity increases
- B Transpiration decreases as humidity increases
- C Humidity has no effect on transpiration
- D Transpiration only occurs at 100% humidity
Correct answer: Transpiration decreases as humidity increases
Transpiration is driven by the vapor pressure gradient between the leaf interior and the outside air. High humidity reduces this gradient, thereby decreasing the rate of transpiration.
The evolution of the C4 photosynthetic system is probably a strategy to:
- A Increase water loss to cool the plant down
- B Fix more carbon while losing less water overall
- C Enable plants to survive in waterlogged conditions
- D Eliminate the need for stomatal openings entirely
Correct answer: Fix more carbon while losing less water overall
C4 plants are roughly twice as efficient as C3 plants at fixing carbon for the same amount of water lost. This is an adaptation that maximizes photosynthetic yield while minimizing water loss in hot, water-stressed environments.
Water molecules remain attached to each other in the xylem column due to:
- A Adhesion to xylem walls
- B Cohesion between molecules
- C Guttation pressure release
- D Root pressure buildup
Correct answer: Cohesion between molecules
Cohesion is the mutual attraction between water molecules due to hydrogen bonding. This strong intermolecular attraction allows the plant to maintain a continuous, unbroken column of water from root to leaf through the xylem.
The 'tensile strength' of water in xylem refers to its ability to:
- A Dissolve minerals
- B Resist a pulling force
- C Evaporate at low temperatures
- D Filter through cell walls
Correct answer: Resist a pulling force
High tensile strength is the ability to resist a pulling force without the column breaking. Because of cohesion and adhesion, the water column in the xylem can withstand the immense tension created by transpiration.
In isobilateral leaves, stomata are:
- A They become turgid and the pore opens wider
- B The inner walls spring back to their resting shape
- C The cellulose microfibrils snap under tension
- D The cell wall instantly increases in thickness
Correct answer: The inner walls spring back to their resting shape
When guard cells lose turgor and become flaccid, the elastic inner walls return to their original shape, causing the stomatal pore to close. This typically occurs at night or under water stress.
What happens to the guard cells when they lose water?
- A They become turgid and the stoma opens
- B The elastic inner walls regain their original shape
- C The microfibrils snap
- D The cell wall thickens instantly
Correct answer: The elastic inner walls regain their original shape
When water stress occurs or at night, guard cells lose turgor and become flaccid. The elastic inner walls return to their original shape, causing the stomatal pore to close.
Transpiration is often called a 'necessary evil' because:
- A It kills the plant if it happens too rapidly
- B It is needed for sap ascent but wastes most absorbed water
- C It only occurs in weed species, not crop plants
- D It blocks the plant from absorbing carbon dioxide
Correct answer: It is needed for sap ascent but wastes most absorbed water
Transpiration is a 'necessary evil' because it leads to massive water loss — over 99% of absorbed water is lost. However, it is essential for the ascent of sap, mineral distribution, and thermoregulation of the plant.
The ability of water to rise in thin tubes is called:
- A Cohesion
- B Surface tension
- C Capillarity
- D Adhesion
Correct answer: Capillarity
Capillarity is the ability of a liquid to flow in narrow spaces without the assistance of external forces. In plants, the small diameter of tracheids and vessels aids this upward movement.
During transpiration, water is lost from the leaf through:
- A Lenticels only
- B Stomatal pores
- C Hydathodes only
- D Xylem parenchyma cells
Correct answer: Stomatal pores
The vast majority (over 90%) of transpiration occurs through the stomatal pores in the leaves. Small amounts are also lost through the cuticle (cuticular transpiration) and lenticels (lenticular transpiration).
High wind speed generally ________ the rate of transpiration initially by ________ the boundary layer.
- A Decreases; thickening
- B Increases; removing
- C Decreases; cooling
- D Increases; saturating
Correct answer: Increases; removing
Wind speed removes the saturated air (boundary layer) from the leaf surface. This maintains a steep vapor pressure gradient, which increases the rate of transpiration.