9.2 Transport in the phloem of plants
1.
Explain how plants transport organic compounds from sources to sinks.
 Plants transport organic compounds from sources to sinks by a process called
translocation.
 Plants transport organic compounds from sources to sinks via a vascular tube system
called the phloem.
 Sources are the parts of the plant where organic compounds are produced by
photosynthesis, such as leaves.
 Sinks are the parts of the plant where organic compounds are consumed or stored, such
as roots, fruits, and seeds.
 The main organic compound transported by the phloem is sucrose, a disaccharide sugar
that is soluble but metabolically inert.
 Phloem is composed of sieve tube cells and companion cells. Loading of sucrose into
the companion cell is carried out by active transport.
 Hydrogen ions are first pumped outside companion cells into the source using ATP.
Hence, a proton gradient is established and sucrose along with hydrogen ions are
transported into the companion cells by a channel called symport.
 When sucrose enters companion cells, it creates concentration gradient between CC and
sieve tube cells, which in turn causes sucrose to enter the sieve tube cells by passive
diffusion.
 The presence of sucrose in ST cells decrease the water potential in the ST and water
from the neighbouring xylem enters the ST cell by osmosis.
 That water exerts a pressure called hydrostatic pressure in the ST due to the
incompressibility of water from hydrogen bonds between water molecules. This
pressure moves the soluble substances from a place of high hydrostatic pressure to a
place of LHP.
 Sucrose molecules diffuse into the sink by passive transport when the sucrose
concentration is lower in the sink as well as by active transport using ATP when the
sucrose concentration is higer in the sink than in the phloem.
 Lower sucrose concentration in the ST causes an increase in water potential and it
makes the water in the ST to move back into xylem by osmosis.
 This whole process is called translocation in which organic compounds are transported
from sources to sinks.
2.
Outline how incompressibility of water allows transport along hydrostatic pressure
gradients.
3.
Explain how active transport is used to load organic compounds into phloem sieve
tubes at the source.
4.
Explain how high concentrations of solutes in the phloem at the source lead to water
uptake by osmosis.
5.
Outline how hydrostatic pressure causes the contents of the phloem to flow toward
sinks.
6.
Analyse data from experiments measuring phloem transport rates, using aphid stylets
and radioactively labelled carbon dioxide.
7.
Explain the structure–function relationships of phloem sieve tubes.
8.
Identify xylem and phloem in microscope images of stem and root.