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Biology OCR A level Transport in plants Summary Notes
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Biology OCR A level Transport in plants Summary Notes
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B9- Transport in plants3.1.3 Transport in plants
(a) The need for transport systems in multicellular plants.
Need for plant transport systems:
• Metabolic demands: many parts of the plant do not photosynthesise but they still need oxygen
and glucose transported to them and the waste products of cell-metabolism removed.
• Size: some plants grow throughout their lives, many perennial plants, e.g. coastal redwood, are
very large, so they need very effective transport systems to move substances.
• Surface area: SA: V is difficult in plants as leaves are adapted to have a large SA: V for
exchange of gases but the size and complexity of multicellular plants means that when the
stems, trunks and roots are taken into account they still have a small SA: V, therefore, they can't
rely on diffusion alone to supply their cells with everything they need.
(b) (i) The structure and function of the vascular system in the roots, stems and leaves of
herbaceous dicotyledonous plants.
Key words:
Dicotyledonous plants- (dicots) make seeds that contain two cotyledons, organs that act as
food stores for the developing embryo plant and form the first leaves when the seed germinates.
Herbaceous dicots- with soft tissue and a relatively short life cycle, leaves and stems that die
down at the end of the growing season to the soil level.
Woody (arborescent) dicots- have hard, lignified tissues and a long life cycle.
Perennial plants- plants that live a long time and reproduce year after year.
Sclerenchyma fibre- can be found below the epidermis of monocotyledons, organised in two or
three layers of cells. Perivascular sclerenchyma fibres are found protecting vascular bundles.
Xylary sclerenchyma fibres are components of the primary xylem.
Plasmodesmata- microscopic channels through the cellulose cell walls linking the cytoplasm of
adjacent cells. Is a narrow thread of cytoplasm that passes through the cell walls of adjacent plant
cells and allows communication between them.
Tracheid- a type of water-conducting cell in the xylem which lacks perforations in the cell wall.
Sieve tube- a series of sieve tube elements placed end to end to form a continuous tube.
Companion cell- cells are linked to the sieve tube elements by many plasmodesmata. They
maintain their nucleus and all their organisms. They are very active cells and it is thought that they
function as a ‘life support system’ for the sieve tube cells, which have lost most of their normal
cell function.
Vessel element- one of the cell types found in xylem, the water-conducting tissue of plants.
Vessel elements are typically found in flowering plants but absent from most gymnosperms such
as conifers.
• Dicotyledonous plants have a vascular system.
In herbaceous dicots, it is made up of two
many types of transport vessels, the xylem and
the phloem. These transport tissues are
arranged together in vascular bundles in the
leaves, stems, and roots of herbaceous dicots.
Xylem:
• Largely non-living tissue.
• The flow of materials in the xylem is up from the
roots to the shoots and leaves.
• Is made up of several types of cells, most of which
are dead when they are functioning in the plant.
• Xylem vessels are the main structure and are long,
hollow structures made by several columns of cells
fusing together end to end.
• Two other tissues associated with xylem in
herbaceous dicots. Thick-walled xylem
parenchyma packs around the xylem vessels,
, storing food and containing tannin deposits. Tannin is a bitter, astringent-tasting chemical that protects
plant tissues from attack by herbivores.
• Xylem fibres are long cells with lignified secondary walls that provide extra mechanical strength but don’t
transport water.
• Lignin can be laid down in the walls of the xylem vessels in several different ways. It can form rings,
spirals or relatively solid tubes with lots of small unlignified areas called bordered pits. This is where water
leaves the xylem and moves into other cells of the plant.
Phloem:
• A living tissue that transports food in the form of organic solutes
around the plant from the leaves where they are made by
photosynthesis.
• It supplies the cells with the sugars and amino acids needed for
cellular respiration and for the synthesis of all other useful molecules.
The flow of materials in the phloem can go both up and down the
plant.
• Main transporting vessels of the phloem are the sieve tube elements.
They are made up of many cells joined end to end to form a long,
hollow structure.
• The phloem tubes are not lignified. In the areas between the cells,
the walls become perforated to form sieve plates, which let the
phloem contents flow through.
• As the large pores appear in these cell walls, the tonoplast (vacuole
membrane), the nucleus and some of the other organelles break down. The phloem becomes a tube filled
with phloem sap and the mature phloem cells have no nucleus.
• Linked to the sieve tube elements are companion cells, which form with them. These cells are linked to
the sieve tube elements by many plasmodesmata.
• They maintain their nucleus and all their organelles. The companion cells are very active cells and it is
thought that they function as a ‘life support system’ for the sieve tube cells, which have lost most of their
normal cell functions.
• Also contains supporting tissues including fibres and sclereids, cells with extremely thick cell walls.
(ii) The examination and drawing of stained sections of plant tissue to show the distribution of
xylem and phloem.(PAG)
(iii) The dissection of stems, both longitudinally and transversely, and their examination to
demonstrate the position and structure of xylem vessels. (PAG)
(c) (i) The process of transpiration and the environmental factors that affect transpiration rate
transpiration rates.
Key words:
Stomata- pores in the surface of a leaf or stem that may be opened and closed by guard cells
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