3.3.2: Transport in tissues Flashcards
What is the role of xylem cells?
To transport water and mineral ions from the roots up to the leaves and other parts of the plant.
What does xylem tissue consist of?
- vessels to carry water and dissolved nutrients.
- fibres to help support the plant.
- living parenchyma cells which act as packing tissue to separate and support the vessels.
Describe how xylem cells form.
- Lignin impregnates walls of the cells making the walls waterproof. This kills the cells.
- The end walls and contents of the cell decay, leaving a long column of dead cells with no contents.
Why is the lignin important?
It strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessels open even when water may be in short supply.
What types of patterns may the lignin thickening form in the cell wall and why is this useful?
The lignin may form spiral, annular (rings), or reticulate (a network of broken rings).
This prevents the vessel from being to rigid and allows some flexibility of the stem or branch.
In some places, lignification is not complete, leaving gaps in the cell wall, what do these gaps form?
- These gaps form pits or bordered pits These bordered pits in two adjacent vessels are aligned to allow water to leave one vessel and pass into the next vessel.
- They also allow water to leave the xylem and pass into the living parts of the plant.
How are xylem vessels adapted to their function?
- Made of dead calls aligned end to end to form continuous columns.
- Tubes are narrow so that the water column does not break easily and capillary action can be effective.
- bordered pits and the lingnified walls allow water to move sideways from one vessel to another.
- Lignin deposition in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or the branch to bend.
The flow of water in xylem vessels is not impeded because…
- There are no cross walls.
- There are no cell contents, nucleus or cytoplasm.
- Lignin thickening prevents the wall from collapsing.
What is the function of phloem tissues?
To transport assimilates (mainly sucrose and amino acids) around the plant. The sucrose is dissolved in water to form a sap.
What does phloem tissue consist of?
Sieve tubes- made up of sieve tube elements- and companion cells.
What do sieve tubes not contain?
Why is this useful?
They contain no nucleus and very little cytoplasm. This leaves space for mass flow of sap to occur.
What is at the end of the sieve tube elements?
Perforated cross-walls called sieve plates.
What do these sieve plates allow?
Movement of sap from one element to the next.
What are the cells in between the sieve tubes called and describe their contents?
Companion cells. They each have a large nucleus and dense cytoplasm. Also they have numerous mitochondria.
What is the function of companion cells?
They carry out the metabolic processes needed to load assimilates into the sieve tubes. Their numerous mitochondria produce the ATP needed for active processes.
