What is the difference between the direction of flow in the xylem versus the phloem?

Flow in the xylem is strictly unidirectional, moving water and minerals only from the roots to the leaves. In contrast, flow in the phloem (translocation) is bidirectional, moving sugars from sources to sinks throughout the plant.

How do turgid guard cells differ from flaccid guard cells in their effect on stomata?

Turgid guard cells are swollen with water and curve outward, which opens the stomatal pore for gas exchange. Flaccid guard cells have lost water, causing them to go limp and close the pore to prevent further water loss.

What is the difference between the state of cells in xylem and phloem tissues?

Xylem vessels are composed of dead cells that have lost their internal structures to form hollow tubes for water transport. Phloem tubes consist of living cells that require metabolic energy to transport organic solutes.

Why is it a mistake to assume that high humidity increases the rate of transpiration?

High humidity actually decreases the transpiration rate because it reduces the concentration gradient of water vapor between the inside of the leaf and the outside air, making diffusion much slower.

What error occurs if a plant shoot is not cut underwater when setting up a potometer?

If cut in the air, an air bubble can enter the xylem vessels, creating an air lock. This breaks the cohesive column of water (the transpiration stream), preventing water from being pulled up the stem.

Why is it incorrect to say a potometer measures the exact amount of water transpired?

A potometer measures water uptake by the shoot. While most of this water is transpired, a small percentage is retained by the plant for photosynthesis and to maintain cell turgidity.

Define the term 'Transpiration Stream'.

The transpiration stream is the continuous movement of water through the xylem from the roots to the leaves, driven by the evaporation of water from the leaf surfaces.

What is lignin and what is its role in the xylem?

Lignin is a tough, waterproof polymer deposited in the cell walls of xylem vessels. It provides structural support to keep the vessels open under high pressure and prevents them from collapsing.

A potometer is a laboratory apparatus used to estimate the rate of transpiration by measuring the distance an air bubble travels in a capillary tube as a plant shoot takes up water.

Why does increasing light intensity usually increase the rate of transpiration?

Increased light intensity stimulates the guard cells to open the stomata wider to allow more $\text{CO}_2$ in for photosynthesis. This larger opening provides a greater pathway for water vapor to diffuse out of the leaf.

Transpiration | AQA GCSE Science

Combined Science Trilogy / Biology

Organisation

Transpiration

Summary

Transpiration is the physiological process by which plants lose water vapor through their leaves, primarily via the stomata. This process creates a negative pressure gradient known as the transpiration stream, which pulls water and dissolved minerals from the roots through the xylem to the rest of the plant, facilitating nutrient transport and cooling.

1. Definition & Core Concepts

Transpiration is defined as the loss of water vapor from plant leaves through evaporation at the surfaces of the mesophyll cells, followed by the diffusion of that vapor through the stomata into the atmosphere.
The process is driven by the energy from sunlight, which causes water to evaporate from the moist cell walls within the leaf's air spaces.
This loss of water creates a transpiration stream, a continuous column of water moving upwards from the roots to the leaves to replace the lost moisture.

Diagram showing the transpiration stream moving water from roots through the stem to the leaves where it evaporates.

2. Xylem Adaptations

Xylem vessels are specialized tissues adapted for the efficient transport of water; they consist of dead cells joined end-to-end to form long, hollow tubes.
The absence of cytoplasm and organelles within these cells ensures there is no internal resistance to the flow of the water column.
The cell walls are thickened with lignin, a waterproof and exceptionally strong polymer that prevents the vessels from collapsing under the intense tension created by transpiration.
Movement in the xylem is strictly unidirectional, occurring only from the roots toward the leaves.

3. Environmental Factors Affecting Rate

Temperature: Higher temperatures increase the kinetic energy of water molecules, leading to faster evaporation from mesophyll surfaces and a higher rate of transpiration.
Humidity: High humidity reduces the concentration gradient of water vapor between the leaf interior and the outside air, which significantly slows down the rate of diffusion.
Air Movement (Wind): Wind removes the accumulated water vapor from the leaf surface, maintaining a steep concentration gradient and thus increasing the transpiration rate.
Light Intensity: Increased light causes the stomata to open wider to allow for gas exchange during photosynthesis, which simultaneously allows more water vapor to escape.

4. Stomatal Regulation

5. Key Distinctions

6. Exam Strategy & Tips