Summary of 3.3: Transport in Plants
3.3.1
Why do plants need a transport system?
Small SA:V ratio (larger plants)
Demand for water & minerals is high – need transport system (xylem)
Demand for sugars is high – need transport system (phloem)
Distribution of vascular tissue (xylem and phloem):
Unlike in animals, there is no pump and respiratory gases aren’t carried.
Dicotyledonous plants (2 seed leaves) have xylem and phloem found together in
vascular bundles, along with collenchyma and sclerenchyma for support.
Xylem and phloem in the young root:
Vascular bundles found at the centre of young root. The xylem forms an X-shape
and phloem is in between its arms; this structure provides strength to withstand
forces that roots are exposed to.
Around the bundle is the endodermis which gets water into xylem. There is a layer
of meristem cells inside the endodermis called the pericycle.
Xylem and phloem in the stem:
Vascular bundles found on outer edge of stem.
Non-woody plants: bundles separate
Woody plants: bundles separate in young stems, but become a ring in older
stems. This means there is a ring of vascular tissue under the bark, providing
flexibility to withstand bending forces to which stems are exposed.
Xylem is found towards inside of bundle and phloem towards outside. In between
them is the cambium – a layer of meristem cells that divide to produce new xylem
and phloem.
Xylem and phloem in the leaf:
Vascular bundles form midrib and veins of a leaf
Dicotyledonous leaves have veins which get smaller as they spread away from
the midrib. Within each vein, xylem is on top of phloem.
Dissection of plant material to view distribution of vascular tissue:
Requires staining
Demonstrated in the leaf stalk of celery, but can also be carried out with busy
lizzie stems.
Thin sections are cut and viewed at low power. Allow the leafy stem to take up
water by transpiration. The stem can then be cut longitudinally/transversely and
viewed with a microscope.
3.3.2
What does xylem tissue consist of?
- vessels to carry water and minerals
- fibres to support plant
- living parenchyma cells which separate the vessels
Formation of xylem vessels:
As vessels develop, lignin impregnates the walls of the cells – this kills them but makes
them waterproof. This results in a hollow column of dead cells.
, Summary of 3.3: Transport in Plants
The lignin forms patterns: spiral, annular (rings) or reticulate (network of broken rings)
In places where lignification is not complete, there are pits in the cell wall (bordered pits).
The pits of 2 adjacent vessels allow water to leave xylem and pass into living parts of
plant.
Adaptations of xylem to its function (carrying water & minerals upwards):
- flow of water not impeded as it is hollow
- lignin thickening prevents walls from collapsing and keeps it open even when there is
low water supply
- lignin patterns allow xylem to stretch as plant grows
- tube is narrow so water column doesn’t break
Structure of phloem
- sieve tube elements: make up tubes in phloem tissue which carry sap up/down the
plant. Have no nucleus and little cytoplasm to allow mass flow. Separated by perforated
cross walls called sieve plates, which allow movement of sap from one element to the
next.
- companion cells: carry out metabolic processes to actively load sucrose into the sieve
tubes. Have a large nucleus, dense cytoplasm, many mitochondria.
Plasmodesmata: gaps in the cell wall containing cytoplasm which connects 2 cells.
Apoplast pathway: water moves by mass flow through spaces in cell walls and spaces
between the cells.
Symplast pathway: water enters through plasma membrane into cytoplasm. Then passes
through plasmodesmata from one cell to the next
Vacuolar pathway: water passes through plasmodesmata and vacuoles
Water uptake:
Refer to pressure potential in your answer.
Placing a plant cell in pure water will cause it to take up water by osmosis. This is
because the water potential of the cell is more negative than pure water (0kPa). Water
molecules move down the water potential gradient and eventually this exerts pressure
on the cell wall (pressure potential) which reduces the influx of water. The cell won’t
burst due to the cellulose cell wall; instead it will become turgid.
Water loss:
Refer to plasmolysis in your answer.
Placing a plant cell in salt solution will cause it to lose water by osmosis. This is because
the water potential of the cell is less negative than the salt solution. Water molecules
move down the water potential gradient and eventually plasmolysis will occur – the
plasma membrane loses contact with the cell wall. The tissue is now flaccid.
3.3.1
Why do plants need a transport system?
Small SA:V ratio (larger plants)
Demand for water & minerals is high – need transport system (xylem)
Demand for sugars is high – need transport system (phloem)
Distribution of vascular tissue (xylem and phloem):
Unlike in animals, there is no pump and respiratory gases aren’t carried.
Dicotyledonous plants (2 seed leaves) have xylem and phloem found together in
vascular bundles, along with collenchyma and sclerenchyma for support.
Xylem and phloem in the young root:
Vascular bundles found at the centre of young root. The xylem forms an X-shape
and phloem is in between its arms; this structure provides strength to withstand
forces that roots are exposed to.
Around the bundle is the endodermis which gets water into xylem. There is a layer
of meristem cells inside the endodermis called the pericycle.
Xylem and phloem in the stem:
Vascular bundles found on outer edge of stem.
Non-woody plants: bundles separate
Woody plants: bundles separate in young stems, but become a ring in older
stems. This means there is a ring of vascular tissue under the bark, providing
flexibility to withstand bending forces to which stems are exposed.
Xylem is found towards inside of bundle and phloem towards outside. In between
them is the cambium – a layer of meristem cells that divide to produce new xylem
and phloem.
Xylem and phloem in the leaf:
Vascular bundles form midrib and veins of a leaf
Dicotyledonous leaves have veins which get smaller as they spread away from
the midrib. Within each vein, xylem is on top of phloem.
Dissection of plant material to view distribution of vascular tissue:
Requires staining
Demonstrated in the leaf stalk of celery, but can also be carried out with busy
lizzie stems.
Thin sections are cut and viewed at low power. Allow the leafy stem to take up
water by transpiration. The stem can then be cut longitudinally/transversely and
viewed with a microscope.
3.3.2
What does xylem tissue consist of?
- vessels to carry water and minerals
- fibres to support plant
- living parenchyma cells which separate the vessels
Formation of xylem vessels:
As vessels develop, lignin impregnates the walls of the cells – this kills them but makes
them waterproof. This results in a hollow column of dead cells.
, Summary of 3.3: Transport in Plants
The lignin forms patterns: spiral, annular (rings) or reticulate (network of broken rings)
In places where lignification is not complete, there are pits in the cell wall (bordered pits).
The pits of 2 adjacent vessels allow water to leave xylem and pass into living parts of
plant.
Adaptations of xylem to its function (carrying water & minerals upwards):
- flow of water not impeded as it is hollow
- lignin thickening prevents walls from collapsing and keeps it open even when there is
low water supply
- lignin patterns allow xylem to stretch as plant grows
- tube is narrow so water column doesn’t break
Structure of phloem
- sieve tube elements: make up tubes in phloem tissue which carry sap up/down the
plant. Have no nucleus and little cytoplasm to allow mass flow. Separated by perforated
cross walls called sieve plates, which allow movement of sap from one element to the
next.
- companion cells: carry out metabolic processes to actively load sucrose into the sieve
tubes. Have a large nucleus, dense cytoplasm, many mitochondria.
Plasmodesmata: gaps in the cell wall containing cytoplasm which connects 2 cells.
Apoplast pathway: water moves by mass flow through spaces in cell walls and spaces
between the cells.
Symplast pathway: water enters through plasma membrane into cytoplasm. Then passes
through plasmodesmata from one cell to the next
Vacuolar pathway: water passes through plasmodesmata and vacuoles
Water uptake:
Refer to pressure potential in your answer.
Placing a plant cell in pure water will cause it to take up water by osmosis. This is
because the water potential of the cell is more negative than pure water (0kPa). Water
molecules move down the water potential gradient and eventually this exerts pressure
on the cell wall (pressure potential) which reduces the influx of water. The cell won’t
burst due to the cellulose cell wall; instead it will become turgid.
Water loss:
Refer to plasmolysis in your answer.
Placing a plant cell in salt solution will cause it to lose water by osmosis. This is because
the water potential of the cell is less negative than the salt solution. Water molecules
move down the water potential gradient and eventually plasmolysis will occur – the
plasma membrane loses contact with the cell wall. The tissue is now flaccid.
