Transpiration

Transpiration is the evaporation of water from the surface of the plant.

UPTAKE OF WATER IN ROOTS (1)

root hair cells have a fine extension that extends out into the soil, this provides a large surface area for absorption of water and mineral ions

Root hair cells have thin walls which provides a short diffusion distance

(these are adaptations of root hair cells)

MOVEMENT OF WATER FROM SOIL→ ROOT HAIR CELL→ XYLEM VESSEL (2)

Each root hair is a single specialised cell of the root epidermis. Soil water has a higher water potential so water moves into the root hair cell via osmosis. This water dilutes the contents of the root hair cell and increases it's water potential so water moves from the root hair cell into the root cortex by osmosis, to keep a high water potential gradient, this allows water to be taken up by the xylem in the centre of the root

LOSS OF WATER BY THE LEAVES (3)

Water vapour diffuses out through the stomatal pores of the leaves. Loss of water from the surrounding mesophyll cells sets up a water potential gradient which draws water from the surrounding mesophyll cells

The Xylem vessels supply the leaf mesophyll tissues with water

Transpiration causes water to be pulled up by the xylem in the stem and roots in a continuous column called the transpiration stream

UPTAKE OF MINERAL IONS

Mineral ions are taken up by active transport, unless the concentration of the ion is higher in the soil water (fertiliser added) then the mineral ion can enter by diffusion

Once inside the root the mineral ions pass across the root cortex mainly by diffusion and enter the xylem vessels. They are then carried around the plant in the transpiration stream

Transpiration Stream Functions

  • supplies water for leaf cells to carry out photosynthesis

  • carries mineral ions dissolved in water

  • provides water to keep plant cells turgid

  • allows evaporation from leaf cell surface which cools the leaf

..

Stomata are mainly found on the underside of leaves as they are not exposed to direct sunlight, this will produce a high rate of diffusion. Also less air movement which reduces water loss

Turgid/Open = less flexible + thicker, H20 enters by osmosis causing vacuoles to expand

Flaccid/Closed = vacuoles shrunk, H20 lost by osmosis

FACTORS AFFECTING RATE OF TRANSPIRATION

  • Light intensity - rate of transpiration increases in light because of the opening of stomata

  • Temperature - increase in temperature increases the rate of evaporated water. It provides particles with more heat energy causing particles to move faster

  • Humidity - when it’s humid, the air contains a higher concentration of water vapour, this reduces diffusion gradient between the leaf and external air. rate of transpiration decreases in humid air and speeds up in dry air

  • Wind speed - rate of transpiration increases with faster air movement. It maintains a steep diffusion gradient as it removes water vapour from the leaf

POTOMETER

A potometer can be used to test factors of transpiration. For example a fan or hair dryer on a plant shows effect of moving air (wind speed), putting the plant under a bright light or in the dark sees the effects of light intensity.

a simple piece of apparatus which measures the rate of water loss as transpiration or the rate of water uptake

assumption: Rate of water uptake = Rate of water loss

Method

Shoot is cut at a slant under water to prevent air bubble blocking xylem

Shoot is placed in potometer under water and as potometer is removed from the water

Petroleum jelly is placed at the junction of the shoot to prevent leakages

Air bubble is introduced, leaves are dried, and plant is left to acclimatise

Mark position of air bubble at start of scale and leave for 30-60 mins