Stomata and Transpiration

Root Hair CellsLook at the following pictures. What adaptations do roots have to allow for absorption of water?

Root Hair Cells

Transpiration

Water MovementMass flow of water in xylem vessels –

pressure lowered as water leaves vessels. Water moves up from the roots where the pressure is higher.

Diffusion of water through cells from xylem.Water vapour diffuses through leaf air spaces.If the concentration of water vapour in the

leaf is higher than outside, water vapour will diffuse out of the leaf through the stomata.

Capillary action (water molecules attracted to polar molecules in xylem walls (adhesion)).

Cohesion – tension theory.

Cohesion-Tension TheoryWater molecules have dipoles which cause an

attraction between them.Water is ‘pulled’ up the xylem vessels by

transpiration. When this happens, the pull is transmitted all the way down the water column, pulling all of the water molecules up the vessel.

For this to work, the xylem vessel must be a continuous column of water i.e. contain no bubbles.

Cross Section of A Leaf

Opening/ Closing StomataStomata can be closed to prevent water loss.Guard cells curve apart when turgid.When flaccid, the edges of the cells lie close

together.However, this drastically slows transpiration

and means that no carbon dioxide can enter the leaf (= no photosynthesis).

Stomata Diagrams

Animation link

Factors Affecting Water LossDiffusion rate is affected by:

Surface area (the higher the surface area, the greater the rate of diffusion).

Difference in concentration (the greater the concentration gradient, the higher the rate of diffusion).

Length of the diffusion path (the greater the length, the lower the rate of diffusion).

http://www.sciencemag.org/site/feature/misc/webfeat/vis2005/show/transpiration.swf

Preventing Excess Water LossCuticle (waxy layer) on leaf which is

impermeable to water.Most stomata found on underside of leaf as it

is cooler in dicotyledons.Thick leaves = reduced water loss.Spines/ hairs increasing boundary layer

(undisturbed layer of air).Stomata closed at certain times of the day.Stomata may be sunken and found in pits.

Xerophytes – Marram Grass• Found on sand dunes.• When dry, leaves roll up, so stomata open to an enclosed space.• Water vapour accumulates in this space = reduced diffusion gradiant.• Spines increase width of boundary layer.

Xerophytes Cont.Some xerophytes may have large numbers of

stomata.Xerophytes cells may have extra support to

prevent cells collapsing when they dry out.Extensive root system.Leaves may have evolved to become spines,

with water being stored in the stem e.g. cacti.