Page 22 - 11-Bio-14 Transport

14. Transport eLearn.Punjab

or transport sugars and other organic material throughout the plant are called sieve elements.

In addition to sieve elements, phloem tissue also contains companion cells, parenchyma cells, and
in some cases fibres, sclereids and latex containing cells. However, only sieve tube cells are directly
involved in transport of organic solutes.

Sieve elements are characterised by ‘sieve areas’ portions of the cell wall where pores interconnect
the conducting cells. Some of the sieve areas of sieve tube members are generally formed in end
walls of sieve tube members where the individual cells are joined together to form a longitudinal
series called a sieve tube. Sieve plate pores of sieve tubes are essentially open channels, that allow
transport between cells (Fig. 14.10).

Fig. 14.10 (a) This diagram shows part of the root phloem consisting of sieve tube members stacked end to end. Adjoining end walls
have common pores. Each sieve tube member is associated with a companion cell (b) Sieve tube member showing the pores in its end
walls. Note the scarcity of cytoplasmic components in these sugar conducting cells.

Each sieve tube member is associated with one or more companion cells. Sieve tubes and companion
cells are in communication with each other by plasmodesmata. Companion cells supply ATP and
proteins to sieve tubes. The photosynthetic products from photosynthesizing cells, the mesophyll
and palisade layer of leaf, pass into sieve tubes, through the companion cell via plasmodesmata.

Patterns of Transport

Phloem transport does not occur exclusively in an upward or a downward direction and is not

defined with respect of gravity. Transport or translocation occurs from the areas of supply (sources)

to areas of metabolism or storage (sinks). 22

V: 1.1