Transport in Plants
Leaf anatomy (Fig.35.6c)
- cells in leaf that do photosynthesis are parenchyma
- arranged on top of leaf in regular array (palisade mesophyll)
- on bottom of leaf more spongy, fewer cells and more space (spongy mesophyll)
- holes on outside of leaf for air exchange (CO2 in, O2 out) are stomata
- cells that make stomata are guard cells, can open and close depending on needs of the plant
- stomata also loose water
- spongy mesophyll covered in layer of moisture
- every time stomata open, water evaporates
o example – maple tree 15 m tall, 177,000 leaves, surface area of 675 m2 (1.5 basketball courts) loses 220L water/hr!!!!
Water movement into and out of plant cells
- movement of water across membranes which are permeable to water but not solute (like biological membranes)
- what rules govern which direction water flows in?
- water moves towards more concentrated solution (osmosis)
o water’s tendency to move towards more concentrated solutions is solute potential ys
o more dissolved solutes, more negative the solute potential
o solute potential of pure water with no solutes is 0
- but plant cells have a cell wall, which limits how much water a cell can take up
o walls exert a pressure back on the cell membrane and cell walls are not very flexible (Fig. 38.3)
o this pressure is pressure potential (yp) (a positive number)
o turgor pressure is the pressure the cell membrane exerts on the cell wall
- water potential (yW)is sum of solute potential (negative number) and pressure potential (usually positive)
- water always moves across a membrane towards lower water potential
- water potential important for plant structure (Fig. 38.4)
o if cytoplasm less concentrated than surrounding fluid, cells will lose water and shrink
§ wilting is too little water in cells
o if cytoplasm more concentrated water will move in until pressure potential counteracts
- plants get water from the soil
- water needs to get to vascular tissue in root to be transported to rest of plant
o vascular tissue (xylem) is in the center of the root, so how does water get all that way in?
- two paths (Fig. 38.7)
o between cells, in the space called the apoplast
o through cells, through the space called the symplast
o water in a root may do both from dermal cells to endodermis
- endodermis
o single layer of cells
o sealed around edges by waterproof strip (Casparian strip)
- so for water and mineral to get to vascular tissue of root, must go through endodermis cells
o these act as filter or gatekeeper – nothing gets into vascular tissue except through them
o (Fig. 38.8)
- Once past endodermis the water and dissolved solutes enter the xylem tubes
Transport in xylem and phloem
- Xylem transports water and dissolved minerals up, phloem transports organic molecules up or down
- This is a simplified picture
o Xylem transports sugars from roots to shoot and leaves in the spring
o Phloem can have large amounts of dissolved minerals
- So transport is more complex than the first simple statement
Architecture of xylem
- tracheids are long cylindrical cells connected by lots of pits
o when cell does programmed cell death, wall stays and many form tube connected by pits (Fig. 38.12)
o all vascular plants have these
- vessel elements
o only in angiosperms (Fig. 38.13)
o when cells die the ends disintigrate, leaving a tube
Models for xylem
- how does water and dissolved minerals and ions move up a tree?
- No pump
- Is water pushed up from roots, or pulled up?
- Pushing - root pressure
o pressure exerted by roots to force water up
o more negative water potential (more solutes) in roots than soil
o water comes in
o must push up fluid in xylem
o there is some, this is shown by guttation, fluid forced out stomata in leaves (Fig.38.14)
o most plants this is minor and unable to push up more than a few meters
- cohesion – tension model (pull) (Fig. 38.16)
o plant leaves lose water through stomata – this is transpiration
§ As water evaporates the surface tension at air-water interface exerts a strong pull on water in xylem
o generates a tension (negative pressure potential) in the water
o pulls on fluid in xylem in the veins of the leaf (tension)
o pulls on fluid in xylem of whole plant, down to roots (tension)
o water molecules are hydrogen bonded to each other (cohesion)
§ also walls of xylem are hydrophilic and narrow, again pull water (adhesion)
o cohesiveness of water is great enough to pull whole column of water up the tree
o no ATP used, driven by differences in water potential
o tracheids and vessel elements have thick secondary walls to prevent collapse like a straw does if you suck too hard
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