Xylem and phloem Flashcards
What is the xylem made of?
A tissue made of several different cell types including xylem vessels, xylem parenchyma and xylem fibres
What are xylem vessels?
Long tubes that carry water and mineral ions
What is the name for a singular xylem vessel cell?
A xylem vessel element
What are xylem vessels made of?
Columns of dead cells fused end-to-end
What are xylem vessels like before they mature?
Elongated living cells
What happens as xylem vessels mature?
They end wall breaks down, cytoplasm disappears and the cellulose cell walls become thickened with lignin
What is the effect of lignin?
It causes cell contents to die and the end wall of each cell disappears
What is the function of lignin?
To strengthen and reinforce the walls of the xylem vessels so they don’t collapse under transpiration pull
What are the types of lignin thickening?
Spirals, rings or relatively unbroken tubes of lignin which has pits
What is the purpose of pits?
So water can pass sideways into other cells of the plant of to other xylem vessels if one becomes blocked
What are xylem parenchyma?
Thick walled cells
What is purpose of xylem parenchyma?
They pack around xylem vessels for support and store food
What do xylem parenchyma contain?
Tannin which is bitter tasting
What is the purpose of tannin?
To protect plants from herbivores
What are xylem fibres?
Long cells with lignified secondary walls
What is the purpose of xylem fibres?
Provide extra mechanical strength for support but don’t transport water
What are the types of cells in the phloem?
Sieve tube elements and companion cells
Where are sugars made by photosynthesis stored?
Roots, leaves, seeds and fruits
How are sugars transported?
As sucrose which are dissolved in water to form sap
What is the pressure in the phloem?
2000 kPa
What do sieve tube elements form?
A tube to transport sugars
What are sieve tube elements?
Living cells which are mainly cytoplasm which are lined up end to end to form tubes
What is the shape of sieve tubes?
They are thin walled and are 5/6 sided
What do sieve tube elements contain?
Perforated cross walls called sieve plates
What is the purpose of sieve plates?
They allow phloem contents to flow
What is the result of pores in size plates developing?
Tonoplast and organelles break down so nucleus dissapears
What are companion cells?
Small cells that lie between sieve tubes
What is the structure of companion cells?
Large nucleus, dense cytoplasm and numerous mitochondria
What is the function of companion cells?
Complete metabolic processes for both companion and sieve tube so very metabolically active living cells
What is an example of metabolic process completed by companion cells?
Produce ATP for loading sucrose into sieve tubes (active transport)
How are sieve tubes and companion cells linked together?
By plasmodesmata to exchange materials
What are other phloem tissues?
Sclereids and fibres
What are sclereids?
Cells with extremely think cell walls
What is the function of sclereids and fibres?
To support
