Transport in Plants Flashcards
What do the xylem and phloem transport? Which way do they transport these substances?
Xylem: water up the plant
Phloem: sugars and other assimilates up and down the plant
How are the xylem and phloem found in plants?
Together in vascular bundles, also contain other tissue types to give bundle strength and plant support
Where is the vascular bundle found in a root and what does it look like?
at centre of young root, large central core of xylem in X shape, phloem is found between the arms of the X
What is the sheath of cells around the vascular bundle called and what does it do?
endodermis, gets water into xylem vessels
What is inside the endodermis?
Layer of meristem cells (cells that can divide) called the pericycle
Where is the vascular bundle found in the stem?
Near the outer edge of the stem, non-woody plants the bundles are separate, in woody plants the bundles are separate in young stems but become continuous in older stems
How is the vascular bundle arranged under bark and how is this beneficial efficient?
complete ring of vascular tissue under bark, provides strength and flexibility to withstand being forces
What is between the xylem and phloem within the vascular bundle?
Layer of cambium: meristem cells which divide to produce new xylem and phloem
What do vascular bundles form in a leaf?
The midrib and veins
What are the 2 main groups of flowering plants?
dicotyledons and monocotyledons (based on number of first seed leaves - cotyledons - they have)
What is the arrangement of veins in a dicotyledon leaf?
Branching network of veins, which get smaller as they spread away from the midrib - the xylem is on top of the phloem in each vein
What cells make up phloem tissue?
sieve tube elements and companion cells
How are cells arranged in the xylem?
hollow columns of dead cells end to end, reinforced with lignin, living parenchyma cells
How do lignin thickenings arranged themselves in xylem vessel cell wall?
spiral, annular (rings), reticulate (network of broken rings)
What are the areas of the xylem vessel called where lignification is not complete? What is the purpose of these areas?
pits/bordered pits, allow water to leave one vessel and pass into an adjacent one, or pass into the living parts of the plant
What are the 4 ways that the xylem is adapted for its function?
made from dead cells to form continuous column, tubes narrow so water column doesn’t break and capillary action is effective, pits allow sideways movement, lignin allows xylem to stretch with growth and stem to bend
Why is the flow of water in the xylem not impeded?
no end walls or cell contents, no nucleus or cytoplasm, lignin thickening prevents the walls from collapsing
What are sieve tubes?
not true cells: little cytoplasm, no nucleus, lined end to end, cross walls at intervals with lots of pores so sap can flow (sieve plates) sieve tubes thin walls and 5/6 sided
What are companion cells?
in between sieve tubes, large nucleus, dense cytoplasm, numerous mitochondria, carry out metabolic processes needed by the sieves, cytoplasm of the 2 cells are linked by plasmodesmata
What are plasmodesmata?
gaps in cells walls allowing communication and flow of minerals between sieve tube and companion cells
What is water potential?
measure of the tendency of water molecules to diffuse from one place to another
What is the water potential of pure water?
0 kPa
What is it called when a plant cell is full of water? What does the water inside the cell do?
Cell is turgid, the water exerts pressure on the cell wall: pressure potential. As pressure builds up, the influx of water is reduced
What happens to a plant cell when it is placed in a solution with low water potential?
water is lost by osmosis, cell loses turgidity, cytoplasm and vacuole shrink: incipient plasmolysis
What is plasmolysis?
when water continues to leave the cell, the plasma membrane will lose contact with the wall
What is the water potential of the cell? Why is this?
Negative because cells contain dissolved salts and sugars
How does water move between cells?
When plant cells touch each other, water mols pass from the cell with less negative water potential to the cell with more negative
What are the 3 ways that water molecules can travel between cells?
apoplast, symplast, vacuolar pathways
What is the apoplast pathway?
cellulose cell walls have many water filled spaces between the cellulose mols, water can move through these spaces, doesn’t pass through plasma membranes so dissolved mineral ions and salts can be transported
What is the symplast pathway?
Water enters the cell cytoplasm through plasma membrane, then through plasmodesmata from one cell to next, plasmodesmata contain a thin strand of cytoplasm
What is the vacuolar pathway?
similar to the symplast pathway, but water isn’t confined to the cytoplasm of the cells, able to pass through vacuole as well
What is cohesion?
attraction of water mols for one another
What is adhesion?
the attraction of water mols to the walls of the xylem
What are root cells covered in?
The epidermis contains root hair cells to increase the SA of the root
What do the root hair cells do?
Absorb minerals from the soil by active transport, the minerals reduce the water potential of the cytoplasm
How is water taken up by the roots?
Across the plasma membrane by osmosis, as the mols move down the water potential gradient
What is the endodermis that surrounds the xylem made of? (movement of water across the roots)
layer of cells, starch sheath containing starch granules, sign that energy is being used. Special cells that have waterproof strip in some of their walls: Casparian strip
What does the Casparian strip do?
blocks the apoplast pathway between the cortex and the xylem, forcing water into the symplast pathway
Where do the endodermis cells move minerals? What does this do to water potential?
by active transport from the cortex to the xylem, decreasing the water potential in the xylem so water moves from the cortex through the endodermal cells to the xylem by osmosis
How is a water potential across the whole cortex created?
water potential in the cells outside the endodermis is lowered (from water into the xylem), water entering the root hair cells
How does water move from the root hair cells to the xylem overall?
water moves through symplast pathway from root hair cells across the cortex and into xylem, water also moves through the apoplast pathway across the cortex but then joins the symplast pathway to go through the endodermis
Why is the Casparian strip important?
ensures water and dissolved nitrate ions have to pass through the cytoplasm and cell membranes which have transporter proteins in to actively transport nitrates
What cannot happen once water has entered xylem?
water cannot pass back into the cortex as the apoplast pathway of the endodermal cells is blocked
What are the 3 processes that help water move up the stem?
root pressure, transpiration pull, capillary action
How does root pressure aid water movement in the stem?
water is driven into the xylem by osmosis, forcing water into the xylem and pushes the water up for a few metres
What is the transpiration pull?
Loss of water by evaporation in the leaves is replaced with water from the xylem, cohesion keeps water in a column so as mols are lost at the top, the whole column is pulled up: cohesion tension theory
What happens if the water column is broken in one xylem vessel?
The water column can still be maintained through another vessel via the pits
How does capillary action aid water movement up the stem?
Adhesion: water mols attracted to sides of xylem vessel, narrow vessels mean this attraction can pull water up
How does water leave the leaf?
most through stomata, tiny amount from waxy cuticle, water evaporates from cells lining the cavity below the guard cells
