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Summary BOT 2601 LEARNING UNIT 5
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 5STRUCTURE AND FUNCTION
https://www.youtube.com/watch?v=xGCnuXxbZGk
Textbook reference: chapter 7 pg. 259-293
VASCULAR TISSUES:
o The vegetative body of self-supporting land plants is adapted to terrestrial life. These adaptions include xylem
and phloem – vascular tissues, needed to survive life away from an aquatic habitat
PHLOEM STRUCTURE AND TRANSLOCATION OF ORGANIC SUBSTANCES:
o The functional units of phloem tissue are sieve tube elements and companion cells
o Sieve plates begin development at the same time as the nucleus begins to degenerate and cell autonomy
diminishes, resulting in wall perforations at the pore sites, and enlarged sieve plate pores
o Sugars are transported from the leaves to non-photosynthetic, metabolically active tissues. This involves loading
sugars into sieve elements at the sites of production, long-distance transport, and the removal or unloading of
the sugars. The terms collection phloem, transport phloem and release phloem can be used
o Phloem transport is aided by pressure flow
MINOR LEAF VEINS AND PHLOEM LOADING AND UNLOADING:
o Leaves are the primary sites of photosynthesis in higher plants
o The flattened leaf blade is highly specialised for the surface-dependant processes of gas exchange and
conduction of water
o Minor veins possess an encircling sheath of parenchymatous cells that also enclose the terminal vascular
elements. They collect photosynthetic assimilates from nearby mesophyll cells where they are synthesized and
depositing them into the transport sieve tubes
o The process by which the major products of photosynthesis are selectively and actively delivered to the sieve
elements is known as phloem loading
o Sugars are subsequently transported to the major veins and away from the leaf where they are delivered to the
sink tissues and then released in a process called phloem unloading
o Protophloem cells provide a one-way path for unloading sugars in target regions of undifferentiated tissues such
as apical meristems and immature leaves
o The role of metaphloem elements of minor veins is to collect sugars from the mature leaf mesophyll and provide
transport to the sites of release
o Secondary sieve elements are involved in long-distance transport but also participate in the unloading process
Models of phloem loading:
o In one model of phloem loading, photosynthetic assimilates move across the mesophyll and into the bundle
sheath cells and sieve elements by travelling through intervening protoplasts and interconnecting
plasmodesmata
o In some species, assimilates travel against a concentration gradient. Sugar entering the companion cell move
into the sieve tube element through the plasmodesmata between the 2 cells. The accumulation of high sugar
concentrations in the sieve elements of the leaf veinlets creates the concentration gradient of assimilates
between 2 regions along the length of the same sieve tube
o Photosynthate can also pass into the cell walls and intercellular space (the apoplast) and then be actively
transported across the cell membrane of the sieve tube element and companion cell complex
o The structure of the phloem of minor veins plays an important role in loading photosynthetic products in leaves.
The minor leaf bundles of dicotyledons have small sieve tube elements and large phloem parenchyma cells. The
parenchyma cells have a dense assemblage of organelles and are connected to the sieve tube elements by
plasmodesmata
o A symplasmic mode of phloem loading is structurally indicated by high numbers of branched plasmodesma links
between leaf mesophyll, bundle sheath cells and the companion cell and sieve elements. The passage of
photosynthates occurs through the bundle sheath cell into a companion cell, with a final transfer into the sieve
tube element.
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