Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
Biology spec led revision
Chapter 9 - Transport in Plants
THE NEED FOR A TRANSPORT SYSTEM
1) Flowering plants take water and minerals in through their roots and make glucose in their
leaves form photosynthesis
2) Water and glucose are needed by ALL CELLS in the plants, and so need to be transported
around
Glucose is transported as SUCROSE
Transport Systems
Two transport systems in flowering plants:
1) Xylem - for transporting water and mineral ions
2) Phloem - for transporting nutrients such as photosynthates and amino-acids
Plants have a BRANCHING BODY SHAPE
• A large SA:V ratio, even in large plants
• Thin leaves, so they can maximise their SA:V ratio
• Root hairs on roots which greatly increases their SA:V ratio too
No system for CO2 or O2 transport, because:
• the large SA:V ratio
• chloroplasts present in cells to produce own O2 and CO2 usage
• Low metabolic rates of plant tissue
All leads to LOW DEMAND for oxygen for AEROBIC RESPIRATION
TRANSPORT SYSTEM STRUCTURE
Plants have a vascular system, but it comprises of two systems:
Xylem Vessels:
• Transport water and mineral ions from the roots, up the plant
Phloem Sieve Tubes:
• Transport absorbed substances, photosynthates and minerals up and down the plant
XYLEM
• Consists of several cell types
• But the cells primarily involved in transport are the xylem vessel elements
• The structure of the xylem vessels adapts them well for transporting water and dissolved ions
• Their secondary function of supporting and strengthening the plant is made possible the
presence of lignin in cell wall, which can be in rings, coils or strips.
• The end wall of the xylem can be perforated rather than completely missing
Structure Breakdown:
• The end wall is lost to allow free passage of water to the next cell
• The cell is dead and has no cytoplasm, allowing water to pass through the inner gap
• The cell wall is thickened by lignin which makes it rigid and able to give support
• Pits allow water to move transversely from cell to cell. Pits are areas where the cellulose cell wall
is not thickened by lignin and so water can diffuse through.
,PHLOEM
Unlike xylem, the phloem consist of LIVING CELLS
Phloem consist of several cells, but two that are concerned with transport are sieve tube
elements and companion cells
Sieve Tube Elements:
• Sieve tube cells form continuous tubes, and each has an associated companion cell.
• The sieve tube cells are alive, but have no nucleus, few organelle and few cytoplasm
• This means that they can dedicate more space to their role in transporting assimilates, but
they are not self-sufficient
• The companion cell
provides the sieve tube cell
with all required cell
activity products, which
are transferred to the sieve
tube cell via
plasmodesmata, which
aids transport of molecules.
Plasmodesmata are gaps in
the cell wall through which
connects the cytoplasm of
two cells, lined with plasma
membranes.
, ABSORBING WATER
Water Potential and Water Movement
• Movement of water between plant cells depends upon the water potential of two cells and the
same applied to water movement between cells and the atmosphere.
• Water always moves from an area of higher water potential to one of lower water potential.
• In transpiration, water usually moves out of the leaf because the water potential inside of the
leaf is higher than that outside the leaf.
• Water potential gradients involve the absorption of water in the roots.
• As water ENTERS via the roots, water LEAVES via the leaves, creating a water potential
gradient from the roots (highest water potential) up the stem, to the leaves (lowest water
potential).
• Water enters the roots from soil down a water potential gradient, assisted by the increase in SA
provided by the root hair cells
• It then needs to move across the root cortex and through the epidermis to reach the xylem in
which it can be transported up the stem.
• This movement of water goes down a water potential gradient, and can be achieved via several
pathways
Pathways of Water Movement
1) Apoplastic Pathway
• Goes through the CELL WALLS, the cell walls are readily permeable, so offering little
resistance. Most water travels this path.
Issues with apoplastic:
• No to select what goes into the plant and is kept out, everything dissolved in water is
carried with it.
Problem solved
• Via the Casparian strip in the walls of the endodermal cells.
2) Symplastic
• Goes through the CYTOPLASM, and from cell to cell via the plasmodesmata
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