Xylem And Phloem Flashcards
Two main types of xylem tissue
Xylem vessel and xylem fibres
Xylem vessels
Xylem vessels start as a series of plant cells running up the stem from the roots to the leaves- at a certain point the carbohydrate lignin forms within the cell walls- lignin is impermeable- the living contents of the cells die and end walls between cells break down.
-regions of cell wall remain free of longing- pits- allow water and dissolved substances to pass between vessels.
What happens if a xylem vessel is blocked or damaged
Water can move through pits to different vessels- pits also allow water to move out of the xylem, like to cells in leaves
Lignin
Arranged in spirals or rings, continuous apart from pits
-supports structure of xylem vessel, water is pulled up xylem vessels, this causes pressure in vessels to fall slightly, lignin prevents collapse.
Xylem fibres
Formed from long narrow cells, very large amounts of lignin form in these cells- interior contents of cells die
-xylem fibres are not used to transport water- provide mechanical support for the plant.
What does the xylem also contain
Parenchyma cells- can act as store of starch- parenchyma cells can also contain tannins which are bitter compounds which deter herbivores from eating the plant.
What is an assimilate
-Glucose used to form other compounds like sugars and amino acids-assimilates
Job of phloem
Transport assimilates from leaves to other parts of the plant like roots or flowers- up or down
What’s the fluid moving in the phloem called
Phloem sap
Is phloem living or dead
Living
Two types of tissue in phloem
Sieve tube element cell- long line of cells arranged end to end, almost all organelles have been lost- interior mostly free to transport sap- end walls of cells have been modified to contain large pores- sieve plate
Companion cells- contain nucleus and lots of mitochondria, microscopic channels link the companion cells to the sieve tube element cells-plasmodesmata
What’s a sieve plate
Modified end wall of sieve tube element cell- allow phloem sap to move between cells
What are plasmodesmata
Microscopic channels which link the companion cells to the sieve tube element cells. Molecules such as ATP and proteins can move through the plasmodesmata into the sieve tube element cells
Role of the companion cell
To provide essential molecules to the sieve tube element cells
Do phloem tubes contain lignin in their cell walls
No
Two types of phloem tissue which provide support
- Fibres and sclereids- both have thickened cell walls
- fibres are long and narrow whereas sclereids have a variety of shapes.
Where do root hairs grow from
Cells in the epidermis of the root- water move in to the root hair by osmosis
How are root hair cells adapted
-densely packed root hairs massively increase the SA:V ratio of the root
-The surface of the root hair consists only of the cell wall and cell membrane, makes surface extremely thin increasing rate of osmosis.
-water potential inside the root hair cell is lower than in the soil- mineral ions in root.
How does water from root hair cells get to the xylem
Through the root cortex
What’s the symplast pathway in the cortex
-water moves from the cytoplasm of one cell to the cytoplasm of an adjacent cell, to do this water moves through the plasmodesmata linking the cells
-The symplast pathway is driven by the water potential gradient between the root hair cells and the xylem, water continually moves into the root hair cells by osmosis from the soil
-symplast pathway is relatively low-pathway for water in the cytoplasm is obstructed by the organelles
What’s the apoplast pathway in the cortex
-water moves within the cell walls and the spaces between the cells
-the cellulose cell walls have a relatively open structure allowing water to move easily between the cellulose fibres.
-apoplast pathway offers much less resistance to water flow than the symplast pathway
What’s cohesion
Water molecules are attracted to each other- water molecules can form hydrogen bonds to each other- as water moves into the xylem and is carried away, more water moves along the apoplast pathway due to cohesion.
What does water have to pass through before entering xylem
-Layer of cells called the endodermis- a band of waterproof material called Suberin runs around the cell wall-casparian strip
-because of casparian strip water can no longer move through the apoplast pathway, instead water passes through the cell membrane and into the cytoplasm- becoming part of the symplast pathway
-by forcing all water into cytoplasm, this allows the cell membrane to control which substances can enter the xylem
What’s the casparian strip
Where cells contain a band of waterproof material called Suberin which runs around the cell wall- in endodermis
Where is active transport used in the endodermis - root pressure
Cells in endodermis use active transport to pump mineral ions into the xylem- lowers water potential of xylem, triggering water to move into the xylem vessels by osmosis- root pressure
-active process requiring respiration
What’s the surface of a leaf covered with
Waterproof layer called the waxy cuticle- reduces water loss from the surface of the leaf by evaporation
What are the tiny pores on the lower surface of the leaf called
Stomata- open when the plant photosynthesises- CO2 in and O out.
What are the surface of the cells in the leaf covered with
A thin layer of water-evaporates from surface of cells- internal leaf spaces contain a high concentration of water vapour.
-generally the level of water vapour in external air is relatively low - water vapour travels out of leaf
-this evaporation of water followed by diffusion of water out of the leaf is transpiration.
-conc of water vapour in cells starts to decrease, causes water to move by osmosis from adjacent cells, lowers water potential of these cells causing water to move into them, at some point this reaches the xylem, with water passing out of the xylem to adjacent cells.
-so when transpiration is taking place water is continually being pulled out the xylem vessels
What is tension
-when transpiration is taking place water is continually being pulled out the xylem vessels- this pulling effect is tension.
What’s the transpiration stream
The movement of water from the roots, up the xylem and out of the leaf
What can water also form hydrogen bonds to- adhesion
Molecules in the xylem vessel walls like carbohydrates- adhesion.
Effect of cohesion and adhesion-capillary action
Water can move up very thin tubes against the force of gravity - water moves up xylem vessels by capillary action to replace water lost by the xylem vessels.
What is transpiration pull
water moves up xylem vessels by capillary action to replace water lost by the xylem vessels.
What’s the whole process of transpiration called
Cohesion-tension theory
Two pieces of evidence which support the cohesion tension theory
- If a plant stem is cut, then air is sucked into the xylem, suggesting xylem vessels are under tension. However the air prevents cohesion between the water molecules, so water movement stops.
-If we measure the diameter of a tree trunk we can see that this reduces when transpiration is at its maximum, this supports the idea that transpiration pull is generating a negative pressure or tension in the xylem.
