Lab 1: Xylem and Phloem

Xylem

• Transports water

  • One-way flow, stiffened with lignin

• Contents: vessel elements, tracheids, fibers

Tracheids

• Long, slender cells with tapered ends that conduct water from roots to shoots

• Has pits where only primary cell walls are present

• Water moves from cell to cell via the pits with lowest resistance

Vessel Elements

• Short and wide

• Conduct water

• Have pits but also perforations at the ends of each cell

  • Perforations lack primary and secondary cell walls and allow for more efficient water transfer

  • Think a straw

Fibers

• Important elements of plan architecture

• Provide mechanical strength and support to the plant (in general and to phloem)

Phloem

• Transports sugar

  • Two-way flow

• Contents: sieve tubes and companion cells

Transpiration

Moves water from roots → stem → leaves → air

1. Water leaves the leaf through the stomata (which let water vapor escape into the air)

2. Water evaporates inside the leaf (the liquid water turns into vapor)

3. Creates a “pull” (as water leaves, it creates tension

4. Water gets pulled up the xylem (the plant is acting like a straw, pulling water upward)

5. Water is pulled from the roots to replace what is lost

6. Water comes from the soil (roots absorb the water)

Water Potential

The potential of water to move out of a cell (water wants to move from where there’s lots of water to where there is less)

• Moves from high water potential to low

• Lots of solutes (like sugar) = LOW water potential

• Less solutes = HIGH water potential

Translocation

Movement of sugar

• Transporting sugars from source to sink cells (active transport)

• Source cells = photosynthetic cells in leaves

• Sink cells = actively dividing cells that need energy in roots and stems

Translocation Details

• Phloem sap moves from areas of high water potential to low water potential

• High water potential in phloem is due to the high turgor pressure in the sieve-tube members near source cells

  • This pressure is created by pumps that actively load sucrose into companion cells against a concentration gradient

  • Higher concentration of sucrose = low water potential, so the surrounding water has high potential and wants to move into the cell

  • Won’t stop until the concentration gradients are equal, so the only option to avoid exploding when turgid is for the cell to move that sugar to the next cell

• At sinks, turgor pressure is much lower than at sources

  • As it sinks, sucrose is removed from phloem sap by growing tissues or storage cells