Phloem

Question 1

Describe structure

Answer 1

Phloem sieve tubes

Sieve plates(walls between cells become perforated)

Filled with phloem sap and has no nucleus

Companion cells, which have many mitochondria for active transport

Question 2

Phloem loading - Symplast pathway

Answer 2

Sucrose moves down a concentration gradient from source cells through cytoplasm

Into the companion cells and then into sieve tubes

This is a largely passive process

Water moves into sieve tubes by osmosis causing hydrostatic pressure

This pushes sucrose along phloem

Question 3

Phloem loading - Apoplast pathway

Answer 3

Moves down concentration gradient through cell walls and intercellular spaces

Active transport to bring sucrose into companion cell

Leads to lower water potential in cell, results in osmosis which produces hydrostatic pressure

Sap moves towards sink

Diffusion gradient is maintained

Question 4

Evidence for translocation

Answer 4

Radioactive isotopes, Carbon-14 is labelled in glucose and can be seen in sucrose

Jet of steam used to kill bark. Movement through phloem stops

Aphids penetrate phloem with mouthparts. Phloem sap appears as droplet at end of body

Question 5

Why was mass flow hypothesis proposed

Answer 5

Phloem sap is transport medium

Hydrostatic pressure is force in system

Phloem sieve tubes is the system

Mass transport systems involve transport of materials, a system, a transport medium, and a pressure to cause movement

Question 6

Cons of passive mass flow hypothesis

Answer 6

Doesn’t account for active loading of companion cells in apoplastic pathway

Translocation is continuous and doesn’t have fixed concentrations

Question 7

Pressure flow hypothesis is better version of mass flow hypothesis

Answer 7

Sucrose loaded actively into sieve tube elements

Osmotic gradient causes hydrostatic pressure

Turgor increases in sieve tube walls

Mass flow of water takes place down turgor pressure gradient

Sucrose moves with water

Sucrose and water removed from sieve tube elements