Mass Transport in Phloem

Question 1

Features of Phloem

Answer 1

• vessels that transports sucrose from the leaves to other cells
• living cells
• run close to xylem
• sieve pores for ease of flow
• thick walls to withstand pressure
• companion cells have many mitochondria to produce ATP for active transport
• sieve element has few organelles for ease of flow

Question 2

Mass Flow Hypothesis for transport of sugars through sieve tubes

Answer 2

• active mechanism/ ATP from source moves solute to sieve tube elects (phloem) by companion cells
• water potential lowers in phloem due to solubility of solute (sucrose)
• water moves via osmosis into phloem from the xylem down its water potential gradient
• high hydrostatic pressure created
• mass flow of water and sugars from high to low pressure (down pressure gradient) from source to sink
• sucrose unloaded at sink via active transport
• water follows by osmosis down its water potential gradient
• excess water reenters xylem
• 2 way flow (this can occur in both directions)

Question 3

Evidence for the Mass Flow Hypothesis (translocation) for organic substances

Answer 3

• there is a pressure-> puncturing a tree results in sap (organic substances) being forces out
• if phloem is removed then the organic substances cannot travel past the this point
• the organic substance can move up or down direction, not determined by gravity