Plant Biology Leaves and transport in the xylem

Plant Biology

Leaves and transport in the xylem • • Transpiration is the consequence of gas exchange in the leaf The transport of water from the roots to the leaves to replace water lost Cohesive and adhesive forces enable transpiration Active uptake of mineral ions causes absorption of water by osmosis

Gas exchange and water loss • • Water loss from a leaf is the consequence of gas exchange. Carbon dioxide is a taken in for photosynthesis and oxygen released (some oxygen will be used in respiration) Gas exchange and water loss happens through the stomata Water los through the stomata is called transpiration

Stomata and guard cells • Guard cells control the opening of the stomata. • The swell with water to open • When do you think guard cells swell?

Leaf anatomy

Transpiration - start 11 mins

Modelling water transport – capillary action Demonstrates the adhesive and cohesive properties of water

Tension Forces - - Water evaporates from surface of leaf → adhesion/cohesion causes water to be drawn from nearest available source to replace the water lost xylem Force of adhesion between water and cell walls in the leaf = strong enough to suck water out of xylem. Lowers pressure in xylem drawing water up the xylem from below – cohesion. Passive xylem Water

Exam Q

Active Uptake of Minerals…Water Follows Water is absorbed into root cells by osmosis… Root hair cells (providing a large SA) actively pump mineral ions into roots. Solute concentration inside the root cells is greater than in the soil. Water follows by osmosis. Water travels through the roots into the xylem taking the minerals along too. By capillary action Apoplastic pathway Water travels through cells walls (most) Symplastic pathway Water travels through plasmodesmata and the cytoplasm of cells

Exam Q Plant transport 7 mins

Losses from Transpiration (summary) - Water leaves through stomata by transpiration - Replaced by water from xylem - Water in xylem climbs stem through pull of transpiration plus forces of adhesion and cohesion - Water moves from soil into roots by osmosis due to active transport of minerals into roots - Water in root travels to xylem through cell walls (apoplastic) and cytoplasm (symplastic)