What is it?
Translocation is the movement of organic substances
made by the plant which are called assimilates.
These are things such as sugars and other chemicals
made by plant cells and are transported in the
Assimilates are transported in sieve elements, which
work closely together with companion cells to
The part of the plant that releases the assimilate into
the phloem is called a source, and the part of the
plant that removes the assimilate from the phloem is
called a sink.
Figure 1 shows a sieve tube. A sieve tube is
made up of elongated sieve elements
that are joined at the ends to form one
Sieve elements are cells that are found in
phloem tissue, they have non- thickened
cellulose walls and they are also very
narrow, often having a diameter between
10 and 15µm. A sieve element has a similar
composition to any other plant cell.
However, they do not have a nucleus,
ribosomes and a tonoplast.
A sieve elements walls are made of
cellulose so that substances can move
laterally as well as vertically. When the end
walls of 2 sieve elements meet, a sieve
plate is formed. This is made up of the
walls of both sieve elements and has
many large pores, allowing water and
solutes to pass through to the next sieve
Each sieve element has at least 1 companion cell
alongside it. Companion cells have a similar
structure of a normal plant cell as they have a
cellulose cell wall, a plasma membrane,
cytoplasm, a vacuole and a nucleus, but they
have a larger number of mitochondria and
ribosome as these cells are very metabolically
active. Also, the vacuole of a companion cell is
relatively small instead of large.
Many plasmodesmata pass through the walls
between sieve elements and companion cells,
providing a direct pathway between the
cytoplasm of the companion cell and the
cytoplasm of the sieve element.
How translocation occurs
Inside a sieve tube is phloem sap. This moves by mass
flow, so a pressure difference needs to be created in
order to achieve mass flow in the phloem, meaning that
the plant has to use its own energy.
This pressure difference is the outcome of active loading
of sucrose into sieve elements at the place from which
the sucrose is to be transported (normally a
photosynthesising leaf). This movement of sucrose
decreases the water potential in the sap inside the sieve
tube, which causes water to move into the sieve
element down a water potential gradient, by the
process of osmosis.
In the leaf (source), water moves into the sieve tube and
moves out of it in the root (sink) and a pressure
difference is created. This pressure difference causes the
liquid inside the tube to flow from the higher pressure
area to the lower one.
Loading sucrose into the phloem
Some sugars produced by photosynthesis are
converted into sucrose. This sucrose is transported
to other parts of the plant by the phloem. Sucrose
is a soluble substance, so it dissolves in water in the
mesophyll cells, and moves out of the cell and
across the leaf by either the apoplast or symplast
The sucrose is then transferred into the companion
cells by active transport and then into the sieve
tube, through the plasmodesmata.
Apoplast and Symplast pathway:
what's the difference?
Water soaks into wall and then seeps
from cell wall to cell wall, and through spaces
between the cells without ever entering a cell.
The water moves from cell to cell by
osmosis or through plasmodesmata and
actually enters the cells.
Unloading sucrose from the
occurs in any tissue that requires the
sucrose. The sucrose usually moves out of the
phloem and into the tissue by facilitated
diffusion and it is then converted into another
substance to maintain a concentration