Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Transpiration is the loss of water vapor from the leaves of a plant, primarily through the stomata. It is a consequence of gas exchange as stomata must open to allow to enter for photosynthesis.
The Cohesion-Tension Theory explains how water moves upwards through the xylem. Water molecules are polar and form hydrogen bonds, leading to cohesion (water sticking to water). Adhesion occurs when molecules stick to the lignin or cellulose in the xylem walls.
Water potential () is a measure of the tendency of water to move from one area to another. Water always moves from a region of higher water potential (less negative) to a region of lower water potential (more negative).
The transpiration stream is maintained by the evaporation of from the mesophyll cell surfaces into the air spaces, followed by diffusion out of the stomata, which lowers the hydrostatic pressure at the top of the xylem.
Environmental factors affecting the rate of transpiration include Light Intensity (increases stomatal opening), Temperature (increases kinetic energy of molecules), Humidity (decreases the concentration gradient), and Wind Speed (removes the boundary layer of saturated air).
📐Formulae
💡Examples
Problem 1:
In a potometer experiment, the air bubble moved a distance of mm in minutes. Calculate the rate of transpiration in mm/min.
Solution:
Explanation:
The potometer measures the rate of water uptake. Under most conditions, water uptake is almost equal to the rate of transpiration, so the movement of the bubble over time gives an estimate of the transpiration rate.
Problem 2:
Compare the water potential of the soil ( MPa) and the root hair cell ( MPa). Determine the direction of water movement.
Solution:
Water moves from the soil into the root hair cell.
Explanation:
Water moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential. Since , the moves down the gradient into the plant.