Lecture: Xylem Flashcards
what meristem produces primary xylem and phloem?
procambium
what is the function of primary xylem?
conduction of water and dissolved minerals
what is the function of secondary xylem?
strength
what makes up the structure of xylem?
- parenchyma
- sclerenchyma
- conducting (tracheary) elements
the parenchyma in xylem is arranged in
rows or strands
what is an important feature of parenchyma of xylem? How does that happen?
can resume cell division. Induced by pathogen
describe the structure of parenchyma transfer cells in xylem
cell wall protrudes into cell, which increases area of cell membrane.
transfer cells of parenchyma are important to ___ through ____
short-distance transport through lateral veins
what is the structure of sclerenchyma in xylem?
fibres/sclerides
what is the function of sclerenchyma in xylem?
strength, especially around vulnerable cells, like veins
what are the conducting elements of xylem?
tracheids and vessels
what are tracheids and vessels and what three things do they have in common?
elongated dead cells
- no protoplasts
- secondary cell walls
- have pits
what makes tracheary elements distinctive?
- wall thickenings
2. perforations
what are wall thickenings in tracheary elements, and their function?
secondary cell wall gradually laid in different patterns, that allow for elongation
what are perforation is tracheary elements and their function?
holes for passage of water
how are tracheary elements formed?
secondary cell wall laid down gradually, the cell dies (apoptosis), the nucleus and primary cell wall go deteriorate, making large channel for water
which tracheary element is most primitive and found in all vascular plants?
tracheids
describe the structural elements of tracheids
pits and pit fields with plasmodesmata, the pit membrane (primary cell wall) still intact
the primary walls of tracheids are
pit membranes across pits
vessels are made up of smaller
vessel elements stacked
what is different about the walls of vessels?
end walls degenerate (no 1/2 cell wall)
what replaces end walls in vessels?
perforation plates
what is a simple perforation plate?
fully open ends
what is a scalariform perforation plate?
bars across opening
perforations occur only in which tracheary element?
vessels
why do angiosperms keep their tracheids if they have vessels?
capillary action, and can contain air bubbles formed in vessels and relocate them
which angiosperms have vessels?
both monocots and dicots
what is the structure of vessels?
wide with no end walls
what is an advantage of vessels?
more water flows with less friction
what is a disadvantage of vessels?
no filter for air bubbles, which stop flow
what is the structure of tracheids?
narrow with end walls with only small openings
what is an advantage of tracheids?
filters air bubbles, rerouting them and water
what is a disadvantage of tracheids?
slow water flow
what are the 4 cell types of xylem?
- parenchyma
- sclerenchyma
- tracheids
- vessels
what is the source of water for plants
soil
what is the sink of water for plants?
air
what creates turgor pressure?
osmotic flow
what prevents the cell from bursting when water flows in by osmosis?
cell wall is tough and pushes back. Equilibrium is reached by pushing out 1 water molecule each time a new one is brought in
what does the cell look like when it is at maximum turgor pressure?
fully inflated
how do root hairs increase water absorption?
increase surface area and do not have waxy cuticle
best water uptake happens when the soil is
dilute (after rain)
what happens to soil/plant after a dry spell or salt in snow?
soil concentration of minerals increases, osmotic flow decreases, water still lost to transpiration, so turgor pressure crashes and plant wilts
how do plants in salt marshes not wilt?
they reverve the osmotic gradient and actively pump in salts, which are secreted by glands. This pulls water into the plant too
what are the three ways water moves in from root hairs?
- apoplastic
- symplastic
- trans-cellular
how does water move through the apoplastic pathway?
through cell walls
how does water move through the symplastic pathway?
through protoplasms
how does water move through the trans-cellular pathway?
through vacoules
provide the process by which water is unloaded from a plant. What is it called?
Transpiration. Water in tracheary elements within Ps organ evaporates into stomatal chambers through stomates and out to the air
what are the two long-distance transport theories?
- root pressure theory
2. transpiration cohesion theory
what is the root pressure theory for long-distance transport?
water is pushed up by osmotic flow which creates turgor pressure and forces water to next cell
what evidence supports the root pressure theory?
sap oozes out of cut; hydrathodes in some species
what are some potential problems with the root pressure theory?
- only works in dilute soil
2. not powerful enough to push water to the top of the tree
what is the transpiration -cohesion theory/ tension cohesion theory?
passive movement of water, which cohered by H bonds and adhered to cell wall and then pulled up by transpiration
what three things are needed by the Ts-Cohesion theory?
- cohesion
- adhesion (capillary action)
- Ts to pull
what is a problem with Ts sucking in Ts-cohesion? What is the solution?
soft tubes could collapse, solution: secondary cell walls of tracheids and vessels
what action alone is enough to move water through plant?
cohesion/capillary action
how much water absorbed is lost to Ts rather than used for growth and metabolism?
99%
what are the benefits of Ts?
- evaporation cools leaf surface
2. creates faster upward flow, resulting in more absorption, therefore more water and nutrients
what are the added benefits of the strength of xylem?
allows plant to grow taller to have more space and access to sunlight
what is the direction of water in xylem? Can it reverse?
upwards. It cannot reverse because water must be lost through stomates, waxy cuticle of the endodermis in root prevents water leaving
why are tracheids and vessels reinforced with secondary walls. Why are they inside?
strength so Ts sucking does not make them collapse. If only the outside was strong, the inside could still collapse
In which tracheary element would water rise higher (even without Ts)?
tracheid because of capillary action (adhesion)
what plants make syrup?
woody deciduous plants in temperate climates
what do developing leaf buds need?
- PGS to break bud dormancy
- sugar to fuel mitosis
- water to enlarge new cells
what makes sap sweet?
parenchyma in roots convert starch to sugar, along with PGS, move into tracheary elements, making a sweet sap
what problem is faced when new leaves need to be formed?
need water to expand pre-formed leaves, and need Ts to do that, but Ts requires leaves, and there is noot enough root pressure alone
how do woody diciduous plants cope with a lack of Ts when they have no leaves?
create pressure in stem by undergoing a freeze-thaw cycle (warm days, cold nights)
how does the cold part of the freeze-thaw cycle increase stem pressure?
below 0C, gases in xylem dissolve, increasing pressure and bringing water in from adjacent cells. water willl freeze and the gas will compress
how does the warm part of the freeze-thaw cycle increase stem pressure?
above 0C water melts, the compressed gases expand, increasing the turgor pressure, pushing the water containing sugar and PGS
where would the sweetest syrup be in a tree?
closer to base of trunk, becasuse not yet made it to leaves
is maple syrup sweet in the fall?
no, because its not freeze thaw, and the leaf buds have not formed, or do not yet need the sweet resources
what is the function of parenchyma in xylem?
long-distance transport
what induces division in parenchyma of xylem?
pathogen
what is the function of wall-thickenings in tracheary elements?
allow for elongation
are tracheids found in monocots or dicots of angiosperms?
both, baby!
at maximum turgor pressure, the cell is fully __-
inflated
why does the cell not burst at max turgor pressure?
equilibrium (as one water molecule is brought in, another is pushed out)
