Transpiration Revision Notes

Transpiration is the loss of water vapor from the leaves of plants.

Water absorbed by roots moves up through the plant and evaporates from leaf surfaces.

This evaporation mainly occurs through tiny openings called stomata.

• Water vapor leaves leaves during transpiration
• Occurs mainly through stomata
• Water moves from roots to leaves before loss

Stomata are small pores on the leaf surface that open and close to regulate gas exchange and water loss.

They allow water vapor to escape while controlling water loss to prevent dehydration.

Stomata are the main sites where transpiration occurs.

• Stomata regulate water vapor loss
• Open and close to balance gas exchange
• Located mainly on leaf undersides

Transpiration creates a pull that draws water upward from the roots through the xylem.

It helps transport minerals dissolved in water throughout the plant.

Evaporation of water cools the plant, preventing overheating.

• Drives upward water movement
• Supports mineral transport
• Cools the plant by evaporation

Inside the leaf, mesophyll cells contain water that evaporates from their surfaces into the surrounding air spaces.

This evaporation changes water from liquid to vapor inside the leaf.

• Water evaporates from mesophyll cell surfaces
• Liquid water changes to vapor inside leaf
• Evaporation is the first step in transpiration

Air spaces between mesophyll cells allow water vapor to accumulate and move freely inside the leaf.

These spaces connect to the outside air through stomata, facilitating vapor movement outwards.

• Air spaces collect water vapor
• Provide pathway to stomata
• Connect internal leaf to external environment

Water vapor diffuses from the air spaces inside the leaf to the outside air through stomata.

Diffusion occurs because water vapor concentration is higher inside the leaf than outside.

Stomata are pores on the leaf surface that open and close to regulate this gas exchange.

• Water vapor diffuses out through stomata
• Diffusion driven by concentration gradient
• Stomata regulate water vapor loss

As temperature rises, water molecules inside leaves gain kinetic energy, causing them to evaporate more quickly from the moist surfaces of mesophyll cells. This speeds up the rate of transpiration.

Higher temperatures increase the evaporation rate, leading to greater water loss through stomata.

Plants in warmer environments typically transpire faster due to this increased evaporation.

• Higher temperature increases kinetic energy of water molecules.
• Faster evaporation from leaf surfaces raises transpiration rate.
• Temperature directly affects water loss speed.

Wind removes the layer of humid air that surrounds the leaf surface, which otherwise slows down water vapor diffusion.

When wind speed increases, this humid air is blown away, maintaining a steep concentration gradient for water vapor to diffuse out of the leaf.

Therefore, stronger wind speeds increase the rate of transpiration by allowing more water vapor to escape.

• Wind removes moist air near stomata.
• Maintains concentration gradient for water vapor diffusion.
• Higher wind speed leads to increased transpiration rate.

Using a potometer, water uptake by a plant shoot can be measured as an estimate of transpiration rate.

By varying temperature and wind speed in controlled experiments, their effects on transpiration can be observed.

Data shows that both increasing temperature and wind speed raise water uptake, indicating higher transpiration rates.

• Potometer measures water uptake as transpiration indicator.
• Controlled changes in temperature and wind speed show their effects.
• Data confirms increased transpiration with higher temperature and wind speed.

Mesophyll cells inside leaves are separated by many air spaces, creating a large internal surface area.

Water evaporates from the moist surfaces of these cells into the air spaces, increasing water vapor inside the leaf.

This large surface area allows more water to evaporate, enhancing transpiration potential.

• Air spaces between mesophyll cells increase internal surface area.
• Water evaporates into these air spaces from cell surfaces.
• More surface area means more water vapor inside the leaf.

Stomata are pores on the leaf surface that allow water vapor to exit from the internal air spaces to the outside air.

The size and number of stomata determine how much water vapor can leave the leaf.

More or larger stomata increase water loss; fewer or smaller stomata reduce it.

Stomata can open or close to regulate water loss and gas exchange.

• Stomata control water vapor exit from leaves.
• Number and size of stomata affect transpiration rate.
• Stomatal regulation helps balance water loss and gas exchange.

Water evaporates into the large internal air spaces and then diffuses out through stomata.

Both the internal surface area and stomatal characteristics work together to control the rate of water vapor loss.

This structure-function relationship is key to plant water regulation.

• Evaporation occurs inside leaf air spaces.
• Water vapor exits through stomata pores.
• Leaf anatomy controls transpiration efficiency.

Water evaporates from the leaf surfaces during transpiration, creating a negative pressure (tension) inside the leaf xylem vessels.

This negative pressure pulls water upward from the roots through the xylem vessels.

The pulling force generated by transpiration is called transpiration pull.

• Water evaporation creates tension in xylem.
• Negative pressure pulls water upward.
• Transpiration pull drives water ascent.

Water molecules stick together due to cohesive forces caused by hydrogen bonding.

This cohesion forms a continuous column of water molecules inside the xylem vessels.

Cohesion prevents the water column from breaking as it is pulled upward.

• Cohesion holds water molecules together.
• Continuous water column forms in xylem.
• Prevents breaks in water flow during ascent.

Water moves from roots, through xylem vessels, to leaves where it evaporates.

Transpiration pull and cohesion work together to maintain continuous water flow.

This process is essential for delivering water and minerals to all parts of the plant.

• Water pathway: roots → xylem → leaves.
• Transpiration pull and cohesion enable upward movement.
• Essential for plant hydration and nutrient transport.

Higher temperatures increase the kinetic energy of water molecules, causing faster evaporation from leaf surfaces.

This leads to an increased rate of transpiration as more water vapor forms inside the leaf.

Plants lose water more rapidly in warmer conditions.

• Temperature raises water molecule energy.
• Faster evaporation increases transpiration.
• Warmer air accelerates water loss.

Wind removes the humid air layer around leaves, preventing saturation near stomata.

This maintains a steep concentration gradient for water vapor to diffuse out of the leaf.

Higher wind speeds therefore increase transpiration rate by enhancing diffusion.

• Wind blows away moist air near leaves.
• Maintains concentration gradient for diffusion.
• Increases transpiration rate with stronger wind.

High humidity means the air contains more water vapor, reducing the difference in water vapor concentration between inside and outside the leaf.

This smaller concentration gradient slows down the diffusion of water vapor from the leaf.

Therefore, high humidity decreases the rate of transpiration.

• High humidity saturates air with water vapor.
• Reduces concentration gradient for diffusion.
• Slows transpiration rate.

Wilting happens when water loss from leaves through transpiration is greater than water absorbed by roots.

This imbalance causes plant cells to lose water and turgor pressure.

Loss of turgor pressure makes cells flaccid, causing leaves and stems to droop.

• Water loss exceeds water uptake.
• Cells lose water and turgor pressure.
• Plant tissues become limp and droop.

Turgor pressure is the pressure of water inside cells pushing against cell walls, keeping cells firm.

When cells lose water, turgor pressure decreases, making cells soft and unable to support the plant structure.

Maintaining turgor is essential for plant rigidity and upright growth.

• Turgor pressure keeps cells firm.
• Loss of turgor causes wilting.
• Water inside cells maintains plant shape.

Wilting reduces the surface area exposed to sunlight and air, lowering water loss through transpiration.

This helps the plant conserve water during drought or hot, dry conditions.

If water becomes available, plants can regain turgor and recover from wilting.

• Wilting reduces exposed surface area.
• Helps conserve water under stress.
• Plants can recover when watered.