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Lecture notes of 20 pages for the course Biology at Sixth year / 12th Grade (.)
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Transport in plantsOrganisation of the Plant Body
Vascular plant consists of :
1. Root system – anchors the plant to the soil, from which it absorbs water and
mineral salts
2. Shoot system – consists of stems and leaves
Function : photosynthesis
Xylem
2 main functions :
1. Conduction of water and mineral salts
2. Support
Xylem consist of 4 type of cells:
Tracheids
Vessel elements
Parenchyma
Sclerenchyma fibres
The water conducting tissues – vessels and tracheids
Vessels
Main conducting units of angiosperm xylem
Efficient at conducting water due to wide lumen
Consists of long, tubular structures formed by fusion of several cells end to end
in a row. Resulting vessel is a hollow tube which does not have any end walls so
there is no restriction to the flow of water and mineral salts up the plant
Lignification – important in resisting pressure in xylem
Also provides support to plant
Vessels have pits which link one vessel to another
Pits allow water to pass sideways between one xylem vessel and the next
Tracheids
Elongated cells with tapering ends
Consist of dead cells with empty lumens when mature
Conduct water but are less adapted than vessels – Unlike vessels tracheids do
not have open ends so that water ha to pass from cell to cell via pits
,Tracheids represent the original water conducting cells of vascular plants. More
evolved plant groups have less tracheids and more vessels. Tracheids function
efficiently since they are the only type of water conducting tissue in conifers, most of
which are large trees
Tracheids have tapering end walls which overlap with adjacent tracheids giving
mechanical support to the plant
Lignification in tracheids resembles that in vessels
Xylem parenchyma
Packing tissue that keeps the other xylem elements in place
Cells have thin cellulose wall and living contents as is typical in parenchyma
cells
Xylem fibres
Function : Support
They have no role in water transport
Cells have overlapping end walls
Have very thick cell walls
Narrow lumens
Provide xylem with mechanical strength
Phloem
Function : translocation of solutions of organic solutes like sucrose throughout the
plant
Consists of long tubular structures that consist of living cells with cytoplasm and no
mechanical function
5 cell types in phloem :
Sieve tube elements
Companion cells
Parenchyma
, Fibres
Sclereids
Sieve tubes and Companion cells
Sieve tubes – long tube like structures formed by fusion of cells called sieve tube
elements or sieve elements end to end
Sieve tube elements have cells walls made of cellulose and pectin
Their nuclei + other organelles degenerate and are lost as they mature
Cytoplasm becomes confined to a thin layer around the periphery of the cell
Although sieve elements lack nuclei + organelles they remain living due to the
presence of companion cells. Each sieve tube cells has one or more companion cells.
Companion cells have dense and active cytoplasm
Companion cells are connected to the sieve tube cells by plasmodesmata, and provide
the proteins, ATP and other nutrients needed to maintain the sieve tube elements
Region of cell wall between one cell and adjacent cell from a structure called sieve plate
Sieve plates are perforated making the end walls looks like sieves
Allows the flow of the solution from one sieve element to the next
Cytoplasmic strands pass through the pores of the sieve plates so forming continuous filaments
Transport in plants
The need for a Transport
Diffusion is an inadequate for transport within multicellular organisms since :
Their S:A:V is small
Distances that materials have to travel are large
Quantity of materials moving is large
Makes diffusion alone an inefficient means of distributing substances around the body.
For this reason, transport systems were developed in multicellular organisms
Both plants and animals have efficient transport systems which allow substances to
be transported around the body efficiently
Transport system in plants consist of :
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