Transport in Plants
2. Transpiration
Learning outcomes
- I can define transpiration as the loss of water vapor from plant leaves.
- I can describe how water exits a plant through stomata.
- I can explain the role of transpiration in water transport.
- I can describe the transpiration stream.
- I can explain the benefits and costs of transpiration for plants.
Transpiration
Transpiration is the loss of water vapor from the aerial parts of a plant, especially the leaves.
Most transpiration occurs through tiny pores called stomata.
Water is absorbed by the roots, transported upward through the xylem, moves into leaf tissues, and eventually evaporates into the air.
The overall pathway is:
soil → roots → xylem → leaves → stomata → atmosphere
Where Does Transpiration Occur?
Transpiration happens mainly in the leaves.
Leaves contain:
- xylem vessels
- mesophyll cells
- air spaces
- stomata
Water reaches the leaf through xylem.
It then moves into mesophyll cells and evaporates from their moist surfaces.
The water vapour enters the air spaces inside the leaf and diffuses out through stomata.
What Are Stomata?
Stomata are tiny pores found mainly in the epidermis of leaves.
Each stoma is surrounded by two guard cells.
Guard cells control whether the pore is:
- open
- partly open
- closed
Stomata allow gases to move between the leaf and the atmosphere.
They allow:
- carbon dioxide to enter
- oxygen to leave
- water vapor to leave
How Water Leaves the Leaf
Water arrives in the leaf through the xylem.
It then moves into the cells of the mesophyll.
The walls of mesophyll cells are moist.
Water evaporates from these surfaces.
The water vapor collects in the air spaces inside the leaf.
It then diffuses through the stomata into the atmosphere.
The sequence is:
xylem → mesophyll cells → evaporation → leaf air spaces → stomata → atmosphere
Evaporation and Diffusion
Two processes are important during transpiration.
Evaporation
Liquid water changes into water vapor from the moist surfaces of mesophyll cells.
Diffusion
Water vapor then moves from an area of higher water vapour concentration inside the leaf to an area of lower water vapour concentration outside.
Therefore:
evaporation occurs first, followed by diffusion out through the stomata
Why Does Water Vapor Move Out?
The air spaces inside a healthy leaf are usually very humid.
The outside air is often less humid.
This creates a concentration gradient.
Water vapor moves down this gradient by diffusion.
Therefore:
high water vapour concentration inside leaf → lower water vapor concentration outside leaf
The steeper this gradient, the faster water vapour can leave.
The Transpiration Stream
The transpiration stream is the continuous movement of water from the roots through the xylem to the leaves.
The pathway is:
soil → root hair cells → root xylem → stem xylem → leaf xylem → mesophyll → atmosphere
This is not a separate process from transpiration.
Transpiration helps drive the stream.
How Transpiration Pulls Water Upward
When water evaporates from leaves, water is removed from the top of the xylem system.
This creates tension in the water column.
Because water molecules attract one another, water further down the xylem is pulled upward.
This force is called transpiration pull.
A simplified sequence is:
water evaporates from leaf → tension develops → water is pulled upward through xylem
Cohesion of Water
Water molecules are attracted to one another.
This attraction is called cohesion.
Cohesion helps keep the water inside xylem as a continuous column.
When water at the top moves upward, it pulls neighbouring water molecules with it.
Without cohesion, the transpiration stream would be much less effective.
Adhesion
Water molecules can also stick to the walls of xylem vessels.
This is called adhesion.
Adhesion helps:
- keep water in contact with xylem walls
- support the water column
- assist movement through narrow vessels
Cohesion and adhesion both contribute to water transport.
Why Transpiration Is Important
Transpiration has several benefits.
It helps:
- transport water
- transport mineral ions
- supply water for photosynthesis
- maintain turgor
- cool the plant
Although the plant loses water, this loss helps drive other essential processes.
Transpiration and Mineral Transport
Roots absorb mineral ions from the soil.
These ions dissolve in water.
As water moves upward through xylem, the mineral ions move with it.
Important minerals include:
- nitrate
- magnesium
- phosphate
- potassium
Therefore:
transpiration stream → movement of water and dissolved minerals
Transpiration and Photosynthesis
Water is one of the raw materials required for photosynthesis.
A simplified equation is:
carbon dioxide + water → glucose + oxygen
Light energy and chlorophyll are also required.
Transpiration helps maintain the movement of water toward the leaves, where photosynthesis occurs.
Transpiration and Turgor
Water helps keep plant cells turgid.
A turgid cell has a water-filled vacuole that pushes the cell contents against the cell wall.
This pressure helps:
- support leaves
- support soft stems
- maintain plant shape
If excessive transpiration causes too much water loss, cells may become flaccid.
The plant may wilt.
Transpiration and Cooling
Evaporation requires energy.
When water evaporates from leaf surfaces, heat energy is removed.
This can cool the leaf.
Therefore:
evaporation of water → heat loss → cooling
This is similar to the cooling effect of sweating in humans.
The Costs of Transpiration
Transpiration is useful, but it also has costs.
The main cost is water loss.
If a plant loses water faster than it absorbs water:
- cells lose turgor
- stomata may close
- leaves may wilt
- photosynthesis may decrease
- prolonged water loss may damage tissues
- severe dehydration may kill the plant
Plants therefore need to balance gas exchange with water conservation.
The Stomatal Trade-Off
Plants need carbon dioxide for photosynthesis.
Carbon dioxide enters mainly through open stomata.
However, when stomata are open, water vapor can escape.
This creates a trade-off:
open stomata → more carbon dioxide enters, but more water is lost
closed stomata → less water is lost, but less carbon dioxide enters
This balance is extremely important for plant survival.
Guard Cells and Water Loss
Guard cells change shape as their water content changes.
When guard cells become turgid, the stomatal pore usually opens.
When guard cells lose water, the pore closes.
Closing stomata can reduce water loss during dry conditions.
However, closing stomata also reduces carbon dioxide uptake.
This may reduce photosynthesis.
Factors Affecting Transpiration
The rate of transpiration is affected by environmental conditions.
Important factors include:
- temperature
- wind speed
- humidity
- light intensity
- water availability
Temperature
Higher temperature generally increases transpiration.
Warmer conditions increase the kinetic energy of water molecules.
Evaporation from mesophyll surfaces increases.
Therefore:
higher temperature → greater evaporation → faster transpiration
Humidity
Humidity is the amount of water vapor in the air.
When humidity is high, the outside air already contains a lot of water vapor.
The concentration gradient between the inside and outside of the leaf becomes smaller.
Therefore:
high humidity → slower transpiration
When the air is dry:
low humidity → steeper gradient → faster transpiration
Wind Speed
Still air allows a humid layer to build up around a leaf.
Wind removes this humid air.
This maintains a larger concentration gradient.
Therefore:
greater wind speed → usually faster transpiration
Very strong drying conditions may eventually cause stomata to close, which can reduce further water loss.
Light Intensity
Stomata often open in the light because carbon dioxide is needed for photosynthesis.
More open stomata usually allow more water vapor to escape.
Therefore:
greater light intensity → usually greater transpiration
At night, many plants partially or fully close their stomata, reducing water loss.
Water Availability
If soil contains plenty of water, roots can replace much of the water lost through transpiration.
If the soil becomes dry, water uptake becomes more difficult.
Plants may respond by:
- closing stomata
- slowing growth
- reducing photosynthesis
- wilting
Measuring Transpiration
A potometer can be used to estimate the rate of water uptake by a plant shoot.
Since most water taken up is lost through transpiration, water uptake can be used as an approximation of transpiration rate.
A typical potometer contains:
- a leafy shoot
- water-filled tubing
- a capillary tube
- an air bubble
As the shoot takes up water, the bubble moves.
The faster the bubble moves, the greater the rate of water uptake.
Limitations of a Potometer
A potometer does not directly measure water vapor leaving the leaf.
It measures water uptake.
Most water taken up is eventually lost through transpiration, but some water is also used for:
- photosynthesis
- growth
- maintaining cells
- other metabolic processes
Therefore, potometers provide an estimate rather than a perfect direct measurement.
Plant Adaptations That Reduce Transpiration
Plants living in dry environments often have adaptations that reduce water loss.
These may include:
- thick waxy cuticles
- fewer stomata
- sunken stomata
- small leaves
- rolled leaves
- leaf hairs
- stomata that close during the hottest part of the day
Plants with adaptations for dry environments are often called xerophytes.
Thick Waxy Cuticle
The cuticle is a waterproof layer covering the leaf surface.
A thicker cuticle reduces evaporation directly through the epidermis.
This helps conserve water.
Sunken Stomata
Some plants have stomata positioned in pits below the leaf surface.
These pits trap humid air.
The humid air reduces the water vapor concentration gradient.
Therefore:
sunken stomata → humid pocket → reduced transpiration
Small or Reduced Leaves
Small leaves have less surface area.
Less surface area means less area from which water can evaporate.
Some desert plants have leaves reduced to spines.
Their stems may carry out much of the photosynthesis instead.
Rolled Leaves
Some grasses roll their leaves during dry conditions.
This traps moist air inside the rolled leaf.
It also reduces exposure to moving air.
Both effects reduce transpiration.
Benefits vs Costs of Transpiration
| Benefits | Costs |
|---|---|
| Drives water transport | Causes water loss |
| Helps transport mineral ions | Can cause wilting |
| Supplies leaves with water | May reduce growth during drought |
| Helps maintain mineral movement. | May cause stomata to close |
| Can cool leaves | Closed stomata reduce photosynthesis |
| Supports the transpiration stream. | Severe water loss can damage or kill the plant |
Transpiration is therefore useful but must be carefully regulated.
Worked Example: Dry Windy Day
A plant is placed outside on a hot, dry, windy day.
Predict the effect on transpiration.
High temperature increases evaporation.
Low humidity creates a steep water vapor concentration gradient.
Wind removes humid air from around the leaf.
Therefore, the rate of transpiration is likely to increase.
If water loss becomes too high, stomata may close.
Worked Example: High Humidity
A plant is placed in very humid air.
What happens to transpiration?
The concentration of water vapor outside the leaf is high.
The difference between the inside and outside of the leaf becomes smaller.
Diffusion of water vapor slows.
Therefore:
transpiration decreases
Worked Example: Stomata Close
What happens when stomata close?
Water vapor loss decreases.
Therefore, transpiration decreases.
However, carbon dioxide entry also decreases.
As a result, photosynthesis may slow.
This shows the cost-benefit balance involved in stomatal control.
Worked Example: Wilting
A plant loses water faster than its roots can absorb it.
Plant cells lose water.
Their vacuoles become smaller.
Turgor pressure decreases.
Cells become flaccid.
Leaves and stems droop.
The plant wilts.
Worked Example: Transpiration Stream
Describe how water moves from the soil to the atmosphere.
A complete pathway is:
soil → root hair cells → root tissues → xylem → stem → leaf xylem → mesophyll → leaf air spaces → stomata → atmosphere
This movement links root absorption, xylem transport, evaporation, and diffusion.
Common Misconceptions
Transpiration means water moving through the xylem.
Not exactly. Transpiration specifically refers to the loss of water vapor from the plant. This loss helps drive water movement through xylem.
Plants lose water only through their roots.
Roots mainly absorb water. Most transpiration occurs from leaves.
Stomata are used only to release water vapor.
Stomata also allow carbon dioxide to enter and oxygen to leave.
All water absorbed by a plant is lost by transpiration.
Most is, but some water is used in photosynthesis, growth, and other processes.
Transpiration is always harmful.
Transpiration has important benefits, including water and mineral transport and cooling.
Closing stomata solves all water-loss problems.
Closing stomata conserves water, but it also restricts carbon dioxide uptake and may reduce photosynthesis.
Did You Know?
A large tree can move enormous amounts of water from its roots to its leaves over time without using a pump like an animal heart.
The driving force begins at the leaves. As water evaporates, tension develops in the xylem and helps pull a continuous column of water upward from the roots.
In this sense, much of the plant's water transport system is powered indirectly by solar energy, because sunlight promotes evaporation from leaves.
Key Terms
Transpiration – Loss of water vapor from the aerial parts of a plant, mainly through stomata.
Stoma – A small pore in the leaf surface used for gas exchange.
Stomata – Plural of stoma.
Guard cells – Specialized cells that control the opening and closing of stomata.
Evaporation – Change of liquid water into water vapor.
Diffusion – Net movement of particles from higher to lower concentration.
Transpiration stream – Continuous movement of water from roots through xylem to leaves.
Transpiration pull – The pulling force created as water evaporates from leaves.
Cohesion – Attraction between water molecules.
Adhesion – Attraction between water and another surface.
Humidity – Amount of water vapor in the air.
Turgor – Pressure produced when water-filled plant cells press against their cell walls.
Potometer – Apparatus used to estimate plant water uptake.
Xerophyte – A plant adapted to dry conditions.
Key Takeaways
- Transpiration is the loss of water vapor from plant leaves.
- Most transpiration occurs through stomata.
- Water evaporates from moist mesophyll cell surfaces.
- Water vapor diffuses through leaf air spaces and exits through stomata.
- Guard cells control stomatal opening.
- Transpiration helps create the transpiration pull.
- This pull helps move water upward through the xylem.
- The continuous movement of water from roots to leaves is called the transpiration stream.
- Cohesion helps maintain a continuous column of water.
- Transpiration helps transport dissolved mineral ions.
- Water transported to leaves is important for photosynthesis and turgor.
- Evaporation can help cool the plant.
- The major cost of transpiration is water loss.
- Excessive water loss can cause wilting and dehydration.
- Open stomata allow carbon dioxide uptake but also increase water loss.
- Plants must balance photosynthesis with water conservation.
- Temperature, humidity, wind, light, and water availability affect transpiration.
- Plants in dry environments often have adaptations that reduce water loss.