B2 - Transpiration

What is transpiration?

The loss of water vapour from leaves by evaporation of water from mesophyll cells and diffusion through stomata.

What is the transpiration stream?

The continuous flow of water from roots to leaves through xylem, ending in evaporation from leaf surfaces.

How does water move to replace that lost via transpiration?

Water moves from xylem into leaf cells and is drawn up from roots through xylem vessels.

Why is transpiration important?

It brings water for photosynthesis, transports dissolved mineral ions, and cools the leaf through evaporation.

How do dissolved mineral ions (e.g. magnesium) get to leaves?

They travel with water in the transpiration stream through xylem tissue.

How does evaporation cause water to leave leaves?

Water evaporates from mesophyll cells, then diffuses through air spaces and exits via stomata.

Which factors affect the rate of transpiration?

Light intensity, temperature, humidity, and wind.

How does increasing temperature affect transpiration?

It increases the rate because evaporation is faster at higher temperatures.

How does humidity affect transpiration?

Low humidity increases transpiration by creating a steep diffusion gradient; high humidity reduces it.

How does air movement (wind) affect transpiration?

Wind removes water vapour near leaf surfaces, maintaining a steep gradient and increasing transpiration.

How does light intensity affect transpiration?

High light intensity causes stomata to open, increasing water loss and transpiration rate.

What is a stomata?

A pore in the leaf epidermis that allows gas exchange and water vapour to escape.

What are guard cells?

Specialized cells that flank stomata and control their opening and closing.

How do guard cells control stomata opening?

They become turgid (swollen with water) and change shape, opening the stomata.

Under which conditions do guard cells open stomata?

In the presence of light and sufficient water, allowing CO₂ in for photosynthesis.

When do guard cells close stomata?

In hot, dry conditions or when water is scarce to reduce water loss.

Why is stomatal closure useful?

It prevents excessive water loss by reducing transpiration.

Where are most stomata located on a leaf and why?

On the underside of the leaf to reduce direct exposure to sunlight and minimize water loss.

How are guard cells adapted for their function?

They have unevenly thickened cell walls and change shape to open or close stomata.

What happens to stomata at night?

They close because photosynthesis doesn't occur, so CO₂ isn't needed, conserving water.

What is the relationship between photosynthesis and transpiration?

Stomata open for CO₂ during photosynthesis, which also allows water vapour to escape (transpiration).

Why does light intensity increase transpiration?

Because it causes stomata to open, which increases water loss.

What is the limiting factor when humidity is high?

A reduced diffusion gradient slows down water loss and lowers transpiration rate.

What is the combined effect of temperature and light on transpiration?

Both increase evaporation and stomatal opening, greatly increasing transpiration.

How might a plant adapt to dry climates to reduce transpiration?

Fewer stomata, waxy cuticle, sunken stomata, small or needle-like leaves, or stomata that only open at night.

What is one method used to measure transpiration rate?

Using a potometer to measure the rate of water uptake.

Why is water uptake a good estimate for transpiration?

Because nearly all water taken up by roots is eventually lost by transpiration.