Grade 12 Xylem Transport

Transpiration is the inevitable consequence of gas exchange in the leaf
Plants transport water from the roots to the leaves to replace losses from transpiration
The cohesive property of water and the structure of the xylem vessels allow transport under tension
The adhesive property of water and evaporation generate tension forces in leaf cell walls
Active uptake of mineral ions in the roots causes absorption of water by osmosis

Transpiration is the loss of water vapour from the stems and leaves of plants

Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)

Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)

As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis
Hence, transpiration is an inevitable consequence of gas exchange in the leaf

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls

Xylem Structure

Pathway

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