AP Biology Transport in Plants

AP Biology General Transport in plants  H 2 O & minerals  transport in xylem  transpiration  evaporation, adhesion & cohesion  negative pressure  Gas exchange  photosynthesis  CO 2 in; O 2 out  stomates  respiration  O 2 in; CO 2 out  roots exchange gases within air spaces in soil  Sugars  transport in phloem  bulk flow  Calvin cycle in leaves loads sucrose into phloem  positive pressure Why does over-watering kill a plant?

AP Biology Start with Water  Review of the structure of water

AP Biology Review of Water Potential   =  s +  p

AP Biology Water and minerals must be absorbed through the roots first before it can travel in xylem  Water: Root hairs increase surface area. Water is small enough to diffuse through membrane. Transport proteins called aquaporins also aid in this

AP Biology Mineral absorption Minerals cannot pass through the membrane because they are charged  Proton pumps  active transport of H + ions out of cell  chemiosmosis  H + gradient  creates membrane potential  difference in charge  drives cation uptake  creates gradient  cotransport of other solutes against their gradient

AP Biology Water flow through root  Porous cell wall  water can flow through cell wall route & not enter cells = apoplast  It can also flow through cells but then must pass through the cell membrane and plasmodesmata = symplast  However once water reaches endodermis (Casparain strip) it must enter symplastic route (i.e. go into to cells) before it can enter the xylem Casparian strip

AP Biology Controlling the route of water in root  Endodermis  cell layer surrounding vascular cylinder of root  lined with impermeable Casparian strip  forces fluid through selective cell membrane  filtered & forced into xylem cells Aaaah … Structure – Function yet again!

AP Biology Water is now in the xylem!!! How do we fight gravity and get to the leaves!  BULK FLOW! The long-distance transfer of fluids by pressure  Water is pulled through the xylem by negative pressure driven by transpiration (the loss of water at stomata) at the leaves

AP Biology Transpiration  Air outside the leaf is drier (has lower water potential) then the air inside the leaf. Therefore water vapor exits the lead via the stomata.  The vaporization of water creates negative pressure (tension)

AP Biology Ascent of xylem fluid Transpiration pull is generated by leaf as water exits through the stomata (negative tension is created). To replace the water that is lost water from the xylem is pulled into air spaces

AP Biology In SUM  The negative pressure potential created by transpiration lowers the water potential.  Therefore water moves up the xylem from an area of high water potential to an area of low water potential

AP Biology The structure of water and xylem also helps this STRUCTURE OF WATER  Cohesion: Water molecules help pull each other up  Adhesion: Attraction of xylem walls helps offset the force of gravity STRUCTURE OF XYLEM  Since xylem is composed of dead/empty cells water does not have to pass the cell membrane  Walls of xylem can adhere to water

AP Biology Chloroplasts Epidermal cell Nucleus Guard cell Thickened inner cell wall (rigid) Stoma openStoma closed H2OH2O water moves into guard cells H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O How can transpiration be stopped? K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ water moves out of guard cells  Uptake of K + ions by guard cells  proton pumps  water enters by osmosis  guard cells become turgid  Loss of K + ions by guard cells  water leaves by osmosis  guard cells become flaccid

AP Biology Control of transpiration  Balancing stomate function  always a compromise between photosynthesis & transpiration  leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO 2 for photosynthesis

AP Biology How is K+ pumped into guard cells?  Light stimulates guard cells to activate protein pumps which help with the cotransport of K+ into cells.  Lack of CO 2 opens stomata  Lack of water in environment  Abscisic acid “tells” stomata to close in response to lack of water RECALL CAM PLANTS!!!!!!!!!!!!

AP Biology Transport of sugars in phloem  Flow from source (where sugar is made) to sink (where sugar is needed)  Loading of sucrose into phloem  flow through cells via plasmodesmata  proton pumps  cotransport of sucrose into cells down proton gradient

AP Biology can flow 1m/hr Pressure flow in phloem  Mass flow hypothesis  “source to sink” flow  direction of transport in phloem is dependent on plant’s needs  phloem loading  active transport of sucrose into phloem  increased sucrose concentration decreases H 2 O potential  water flows in from xylem cells  increase in pressure due to increase in H 2 O causes flow which in turn pushes sugar through phloem

AP Biology Don’t get mad… Get answers!! Ask Questions!