Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Xylem: Tissue responsible for the transport of water () and dissolved mineral ions from the roots to the leaves in a unidirectional flow (upwards).
Phloem: Tissue that transports sucrose () and amino acids from 'sources' (e.g., leaves) to 'sinks' (e.g., roots or flowers) in a process called translocation.
Root Hair Cells: Specialized cells with long extensions that increase the surface area for the absorption of water via osmosis and mineral ions via active transport.
Transpiration: The loss of water vapor from the aerial parts of a plant (mostly through the stomata in leaves) caused by evaporation at the surface of mesophyll cells.
The Transpiration Stream: The continuous column of water moving upwards through the xylem, maintained by cohesive forces between molecules and adhesive forces between water and the xylem walls.
Factors affecting Transpiration: 1. Temperature (increases kinetic energy), 2. Humidity (decreases concentration gradient), 3. Wind speed (removes water vapor), 4. Light intensity (opens stomata).
Vascular Bundles: The arrangement of xylem and phloem; in the stem, xylem is located towards the center, while phloem is located towards the outside.
📐Formulae
(Movement of water from high water potential to low water potential)
💡Examples
Problem 1:
In a potometer experiment, the air bubble moved a distance of mm in minutes. Calculate the rate of transpiration.
Solution:
Explanation:
To find the rate of transpiration using a potometer, we divide the distance the bubble travels by the time it took to move that distance.
Problem 2:
Explain why increasing the humidity around a leaf decreases the rate of transpiration.
Solution:
High humidity increases the concentration of molecules in the air outside the leaf, which reduces the concentration gradient between the internal air spaces and the atmosphere.
Explanation:
Transpiration relies on the diffusion of water vapor. According to Fick's Law, diffusion is faster when there is a steeper concentration gradient. High humidity makes the external air more like the internal air, slowing down the loss of .
Problem 3:
Describe the structural adaptation of Xylem vessels for their function.
Solution:
Xylem vessels are made of dead cells joined end-to-end with no cross-walls (forming a continuous tube) and are reinforced with a waterproof substance called lignin.
Explanation:
The lack of cytoplasm and organelles provides an empty lumen for flow, while lignin prevents the tubes from collapsing under the negative pressure of the transpiration pull.