Translocation

During photosynthesis, plants produce glucose from simple inorganic molecules - carbon dioxide and water - using light energy. Some of the glucose produced by photosynthesis is used for respiration. This releases energy for the seven life processes.

Translocation is the movement of sugar produced in photosynthesis to all other parts of the plant for respiration and the other processes described above. This occurs in phloem cells.

Phloem

The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as:

  • growing parts of the plant for immediate use
  • storage organs such as bulbs and tubers
  • developing seeds

Transport in the phloem therefore takes place both up and down the stem - in contrast to transport in the xylem, which is just upwards.

Transport of substances in the phloem is called translocation.

Phloem consists of living cells. The cells that make up the phloem are adapted to their function:

  • Sieve tubes - specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next.
  • Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s).
Diagram showing how the phloem moves food substances around the plant

Comparison of transport in the xylem and phloem

XylemPhloem
Type of transportPhysical processRequires energy
Substances transportedWater and mineralsProducts of photosynthesis; includes sugars and amino acids dissolved in water
Direction of transportUpwardsUpwards and downwards

The xylem and phloem are distributed differently in roots and stems. In the root, the xylem forms a central column. It forms a solid support. The phloem is towards the centre, outside the xylem.

In the stem, the transport tissues of the xylem and phloem are grouped into vascular bundles.

Cross section of a plant root and plant stem
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