Summary Transport in plants; Xylem and Phloem transport
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Module
Biology
Institution
Book
Cambridge International AS and A Level Biology Coursebook with CD-ROM
Compiled and summarised covering everything falling under Transport in plants unit with diagrams including Xylem, Phloem, Adaptations and Transpiration process
7.1 The transport needs of plants
Plants have to make their own organic food using photosynthesis; they can get minerals
and water from the soil
The main photosynthetic organs are the leaves; they require light
In order to obtain minerals and water, plants have extensive root systems underground
The plant body therefore tends to spread out above ground to get light and spreads out
below ground to get inorganic mineral ions and water
7.2 Vascular system: xylem and phloem
The substances transported such as mineral ions and organic compounds, dissolve in
water to form solutions
These solutions are moved through specialised tubes
The tubes form a system called the Vascular system
(Plants contain a vascular system but there also vascular systems in animals such as the blood
vascular system)
In plants the vascular system contains two tissues, xylem and phloem
Both contain specialised tubes for transporting fluids
The fluid is called Sap (xylem sap/phloem sap)
Xylem sap moves in tubes called Xylem vessels
Phloem sap moves in tubes called Phloem vessels
Xylem and phloem are together known as Vascular tissue
Xylem:
Carries mainly water and inorganic ions (mineral salts) from roots to the parts above
ground
Xylem sap can move in only one direction, from roots to the rest of the plant
Phloem:
Carries substances made by photosynthesis from the leaves to other areas of the plant
Can also carry substances from storage organs to other parts of the plant
At any one time, the fluid in the phloem can be moving in different directions in different
tubes of the phloem, up or down the plant
7.3 Structure of stems, roots and leaves and the distribution of xylem and phloem
> Xylem: a tissue containing tubes called vessels and other types of cell, responsible for support
and the transport of water and mineral salts through a plant
> Phloem: a tissue containing tubes called sieve tubes and other types of cells, responsible for
the transport through the plant of organic solutes (assimilates) such as sucrose
> Vascular tissue: a tissue in plants consisting mainly of xylem and phloem but also containing
sclerenchyma and parenchyma cells
Stems, roots and leaves are the main organs involved in transport in plants
In stems and leaves, the xylem and phloem are found in structures called Vascular
bundles (which also contain other types of cell)
In roots, the xylem and phloem are found at the centre
The epidermis is one cell thick and covers the outside of the plant
, Most of the cells outside the vascular tissue are Parenchyma cells, these have thin cell
walls and vary in size, some may contain nuclei
The outer region of stems and roots is known as the Cortex, it is mainly made of
parenchyma
There are zones of cells like parenchyma with thicker walls, collenchyma, for more
support
These tend to be found at the outside of stems just below the epidermis and in the
midrib of leaves
The Endodermis is one cell thick like the epidermis (‘ends’ means inside; the
endodermis can be considered as an internal epidermis)
Vascular bundles in stems have a cap of fibres, sclerenchyma, for extra strength in the
stem. Like xylem, their walls contain a strengthening material, Lignin
> Vascular bundle: a strand of vascular tissue running longitudinally in a plant; within the
bundle, the arrangement of tissues like xylem, phloem and sclerenchyma varies in different
plants and organs
> Parenchyma: a basic plant tissue typically used as packing tissue between more specialised
structures; it is metabolically active and plays an important role in the movement of water and
food products in the xylem and phloem, and may have a variety of functions (I.e food storage
and support)
> Sclerenchyma: a plant tissue consisting of thick-walled cells with a purely mechanical function
(strength and support); the cell walls have become impregnated with lignin and the mature cells
are dead with no visible contents; many sclerenchyma cells take the form of fibres
> Collenchyma: a modified form of parenchyma in which the corners of the cells have extra
cellulose thickening, providing extra support, in the midrib of leaves and the corners of square
stems (in 3D the tissue occurs in strand I.e celery petioles)
> Epidermis: the outer layer of cells covering the body of a plant or animal cell; in plants it is
usually one cell thick and may be covered with a cuticle which provides additional protection
against loss of water and disease
> Endodermis: the layer of cells surrounding the vascular tissue of plants; most clearly visible in
roots
> Lignin: a hard material made by plants and used to strengthen the cell walls of certain types of
cell, it is the main material in wood, particularly xylem vessel elements and sclerenchyma cells
Dicotyledons:
Flowering plants may be monocotyledons or dicotyledons
Monocotyledonous plants, such as grasses, typically have long, narrow leaves
Dicotyledonous plants typically have leaves with wide blades and narrow stalks
(petioles)
Their seeds contain an embryo with two cotyledons (seed leaves) in their seeds and the
adult plant typically has leaves with a blade (lamina) and stalk (petiole)
The mechanisms of transport through both types of plant are the same, but there are
differences in the distributions of xylem and phloem in their roots, stems and leaves
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