4.4 - Transport Tissues in Plants Flashcards
What is xylem?
It is the main tissue that transports up water and mineral ions around a plant. This is called the ‘transpiration stream’.
How does the xylem become a dead tissue?
- Starts off as living tissue
- The first xylem formed is called ‘protoxylem’
- At this stage, it is capable of stretching and growing as it is not fully lignified
- Then, cellulose microfibrils in the wall of the xylem laid somewhat vertically in the stem
- This increases the strength of the tube and allows it to withstand great compression forces
- As the stem ages, the amounts of lignin increases this results in the cells to become impermeable to water and other substances
- The contents of the cells die
- This lignified tissue is called ‘metaxylem’
- The end walls between the cells break down so the xylem forms hollow tubes running from root to shoot
What are the five main types of cells in the xylem and explain their role?
- Tracheids - are single cells that are elongated and lignified, have tapering end walls, they are dead with empty lumens used as conducting tissue
- Vessel elements - used as conducting tissue
- Parenchyma cells - unspecialised plant cells with no thickening, found in vessels or tracheids as living tissue used as packing tissue
- Sclerenchyma cells - long, narrow and highly thickened cells with only a small lumen and are used for support
- Fibres - used for support
Name and explain the structural adaptation of xylem tissue for the water and mineral ion transport and for support
- Dead, empty xylem vessels create a wide lumen for unrestricted water and mineral ion flow
- End walls of vessels break down creates a long continuous tube for water transport
- Cells walls are impregnated with lignin which prevents vessels collapsing when contents are under tension
- Cell walls are lignified with rings, spirals and in a reticulate manner which allows vessels to be flexible preventing breakage as the stem bends in the wind
- Pits in the lignified walls allow lateral transport of water between adjacent vessels
What is the phloem and its function?
It is the main tissue transporting dissolved solutes and assimilates around the plant
It is a living tissue
Materials can be transported both up and down the stems
This is called ‘translocation’
What is the Sieve Tube Element of the phloem and their function?
Sieve tube elements
- are elongated living cells with very specialised structure
- As they develop they lose their nuclei and their cytoplasm is restricted to a thin layer with few organelles but they remain alive.
- Their end walls have many holes called ‘sieve pores’ which form a ‘sieve plate’.
- Running through their large lumen and sieve pores between each tube element strands of phloem protein.
- These sieve tubes are connected to their companion cells through cytoplasmic bridges called ‘plasmodesmata’
What are companion cells and their function?
- they run alongside the sieve tubes
- they are typical plant cells but with dense cytoplasm and many organelles
- the cytoplasm is connected to the sieve tube through plasmodesmata
- thought to carry out many cellular functions that enable the sieve tubes to stay alive and transport materials even though they have no nucleus and very little cytoplasm
Structural adaptations of phloem for sucrose/assimilate transport
- Sieve tubes have limited peripheral cytoplasm and organelles which creates space for transport of sucrose + assimilates through the cell
- End walls form sieve plates with sieve pores which form direct connections for transport from one sieve tube element to the next
- Companion cells and sieve tubes are connected by plasmodesmata which enables the sieve tubes to stay alive without nucleus and with very little cytoplasm
- Companion cells have dense cytoplasm and many organelles which are thought to be needed to support sieve tubes.
What are the main similarities between xylem and phloem?
- both are plant transport tissues
- both have a lot of water moving through them
- both found in vascular bundles in plant
What are the main differences between xylem and phloem?
- xylem transports water and mineral ions up the plant whereas phloem transports the products of photosynthesis (sucrose) up and down the plant
- xylem are dead tissues whereas phloem are living tissues
- xylem comprises hollow tubes running through the stem of the plant whereas the phloem is made from many sieve tube elements joined together to form tubes with a sieve plate in-between them.
- The direction of flow in the xylem is only upwards, whereas the direction of flow in the phloem is up and down
What is the vascular bundle?
phloem
- cambium
- xylem
Vascular Bundles Distribution
- In the roots, a vascular bundle is found in the centre and on the edges of the centre is the phloem tissue
-In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to epidermis) - In the leaves, vascular bundles form the midrib and veins and spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)
Why is it beneficial that the stele forms a central cylinder through the root structure?
It provides strong anchoring so that the plant is firmly rooted to the ground during harsh weather conditions
Why is it beneficial to have the vascular bundle around in the stem structure?
Having it around in the stem structure allows the plant to bend during harsh winds or weather conditions rather than snapping.
