Database results:
examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Transpiration Process
Plants are constantly losing water to the atmosphere through a process called transpiration. This might sound like a bad thing, but it's actually essential for plant survival! Transpiration helps plants move water and minerals from roots to leaves, cools plants down on hot days and maintains cell structure.
Key Definitions:
Transpiration is a bit like plant sweating, but with a purpose! Let's break down how it works step by step:
1. Water enters the plant through root hair cells by osmosis
2. Water moves across the root cortex and into the xylem vessels
3. Water travels up the stem through xylem vessels
4. Water reaches the leaves and moves into the mesophyll cells
5. Water evaporates from cell surfaces into air spaces inside the leaf
6. Water vapour diffuses out through the stomata into the atmosphere
Water potential gradient: Water moves from high water potential (roots) to low water potential (air)
Cohesion: Water molecules stick together, creating a continuous water column
Adhesion: Water molecules stick to the walls of xylem vessels
Surface tension: Creates a pulling force as water evaporates from leaves
Root pressure: Helps push water into the xylem at the roots
Stomata are tiny pores found mainly on the underside of leaves. Each stoma (singular of stomata) is surrounded by two guard cells that control whether the pore is open or closed. This control is crucial for balancing water loss with the need for gas exchange.
Opening: When guard cells take up water, they become turgid (swollen) and curve away from each other, creating an opening.
Closing: When guard cells lose water, they become flaccid (floppy) and move together, closing the pore.
This opening and closing is controlled by changes in the water content of guard cells, which is influenced by light, temperature, humidity and carbon dioxide levels.
Morning: Stomata open as light triggers photosynthesis
Midday: Stomata may partially close if water loss is too high
Evening: Stomata close as light levels decrease
Night: Stomata remain closed to conserve water
This rhythm helps plants balance water conservation with the need for carbon dioxide for photosynthesis.
Several environmental factors can speed up or slow down transpiration. Understanding these helps explain why plants lose water faster on some days than others.
Effect: Higher light intensity increases transpiration
Why: Light causes stomata to open for photosynthesis, allowing more water vapour to escape
Example: Plants transpire more on sunny days than cloudy days
Effect: Higher temperatures increase transpiration
Why: Heat increases the rate of evaporation and water molecules move faster
Example: Plants need more watering during hot summer days
Effect: Higher wind speed increases transpiration
Why: Wind removes water vapour around the leaf, maintaining a steeper diffusion gradient
Example: Plants in windy locations often have adaptations to reduce water loss
Effect: Higher humidity decreases transpiration
Why: Moist air reduces the water potential gradient between leaf and atmosphere
Example: Plants in rainforests often lack water-saving adaptations
Effect: Larger leaves increase transpiration
Why: More surface area means more stomata and greater water loss
Example: Desert plants often have small leaves or spines
Effect: Higher CO₂ levels decrease transpiration
Why: High CO₂ causes partial stomatal closure
Example: Plants in urban areas with high CO₂ may transpire less
Scientists measure transpiration rates using a simple piece of equipment called a potometer. This helps us understand how different factors affect water loss in plants.
A potometer measures the rate of water uptake by a leafy shoot, which approximates the rate of transpiration. Here's how it works:
Using a potometer, you can investigate how different conditions affect transpiration by changing one factor (like light or temperature) while keeping others constant.
Cacti and other desert plants (xerophytes) have amazing adaptations to reduce transpiration in their harsh, dry environments:
The saguaro cactus of Arizona can survive for years without rainfall by using these adaptations. During rare desert rains, its extensive shallow root system quickly absorbs water, which it stores in its expandable stem tissue.
Transpiration isn't just about water loss - it serves several vital functions for plants:
While transpiration is necessary, it can also create problems:
Transpiration represents a crucial balancing act for plants. They need open stomata to take in carbon dioxide for photosynthesis, but this inevitably leads to water loss. Different plants have evolved different strategies to manage this trade-off based on their environments.
Understanding transpiration helps us appreciate how plants survive in diverse conditions and how they might respond to environmental changes. It also informs agricultural practices, helping farmers develop efficient irrigation strategies and breed crops better suited to different climates.
A large oak tree can transpire up to 400 litres of water per day - that's enough to fill about two bathtubs! This shows the significant role plants play in the water cycle and local climate regulation.
Log in to track your progress and mark lessons as complete!
Login NowDon't have an account? Sign up here.