BIOL 240W Study Guide - Midterm Guide: Desiccation, Passive Transport, Casparian Strip
36- Resource acquisition and transport
apoplast: consists of everything external to plasma membrane (cell walls,
extracellular spaces, interior of vessel elements and tracheids)
symplast: consists of the cytosol of all the living cells in a plant, plasmodesmata-
connection between individual cells ,(living part of plant)
3 transport routes for water and solutes: apoplastic route (continuum through
cell walls), symplastic route (continuum through the cytosol, crossing one plasma
membrane entering the plant), transmembrane route ( across multiple cell walls
and membranes)
solute transport: selective permeability of plasma membrane for short distance
movement in/out of cell, active and passive transport
membrane potential: difference in voltage across cells plasma membrane due
to differential distribution of ions. Affects activity of excitable cells and
transmembrane movement of all charged substances- established through
pumping H+ by proton pumps
•neutral solutes like sucrose can be co-transported with protons, ions like nitrate
can also be co-transported with protons in roots
•ion channels: specific for certain ions, most channels gated, responding to
certain stimuli
short distance transport of water: across plasma membranes
•osmosis: diffusion of free water into/out of cell- affected by solute concen. and
pressure. water moves from area of higher to lower free water concentration
(lower to higher solute concentration). Water flows from regions of higher
potential to lower (potential= water's capacity to perform work).
•measured in unit of pressure called megapascal (psi=0 MPa for pure water at
sea level and room temp.)
•psi= solute potential(molarity) + pressure potential
•solute potential: concentration of dissolved material, decreases with addition of
solutes because solutes bind water molecules and reduce ability to do work.
solute potential for a 0.1M sugar solution= -.23 MPa
•Pressure potential: physical pressure on solution. Turgor pressure: pressure
exerted by the protoplast (living part of cell) against the cell wall.
hypotonic: a solution surrounding a cell that will cause cell to take up water
(turgid-normal)- equilibration of psi leads to positive pressure potential due to cell
wall
isotonic: a solution surrounding a cell that will cause no net movement of water
(flaccid)
hypertonic: a solution surrounding a cell that will cause cell to lose water
(plasmolyzed)
- plant cells function in hypotonic solution because of cell wall (healthy plant cells
stiff from turgor pressure
plasmolysis: shrinkage of cytoplasm and detachment from cell wall in
hypertonic solution- equilibration of psi in hypertonic solution leads to
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plasmolysis. Cellular potential>environmental
general idea: if a flacid cell is placed in a solution with a lower solute
concentraion (hypotonic) the cell will gain water and become turgid. If a flaccid
cell is placed in solution with higher solute conentraion (hypertonic) plant will lose
water and will wilt
bulk flow: what long distance transport depends on, through xylem and phloem.
Depends on pressure gradient, always occurs from higher pressure to lower
pressure. independent of solute concentration. Rapid and prolific
•efficient movement possible bc mature tracheids and vessel elements have no
cytoplsm (part of apoplast) and sieve-tube elements have few organelles
•driven entirely by water potential, psi (p)
•driven by transpiration and does not require energy from the plant- solar powered
•bulk flow v diffusion: bulk flow is much faster and driven by differences in
pressure potential in dead cells, not solute potential across membranes of living
cellslike in diffusion.
root pressure: active tranport of solutes into root steele lowers water potential,
water flows into stele, increasing pressure, forcing water up the xylem - minor
mechanism of bulk flow
transpiration: movement and evaporative loss of water from plant. Drives
transport of water and minerals from roots to shoots via xylem (xylem sap).
Cohension of water molecules transmits upward pull. Because outside of leaf is
often drier than inside, water vapor diffuses down its water potential gradient and
exits via stomata. This loss of water is the pulling force for upward movement.
Also influenced by adhesion of water molecules to xylem cell walls (cellulose).
Occurs within a thin film of water, cell walls of mesophyll act like thin capillary
network- cell walls made of microfibrils.
•look at diagram
•the "sucking force" of negative potential is due to cohension, adhesion to cells
walls of tracheids and vessel elements help fight againt gravity
water and mineral uptake by roots: epidermal cells (especially root hairs)
absorb. Water moves through apoplastic route then into xylem. Cohension
between water molecules due to H-bonds (adhesion prevents water from falling
back down)
casparian strip: waxy barrier that forces all water and solutes coming through
apoplastic route to pass through plasma membrane to get to
endodermis/vascular cylinder. Allows the plant to select what moves into plant
cavitation: pocket in chain of water vapor, failure of cohension, water molecules
can detour around bubble through pits in tracheids or by root pressure. caused
by drought stress or freezing
stomata: regulate amount of gas exchange and water loss through transpiration.
Guard cell turgor is controlled by regulation of K+ uptake. Generally, stomata
open during day and close at night- opening at dawn triggered by light, CO2
depletion, internal clock in guard cells.
- abscisic acid (ABA): produced in response to water deficiency, causes closure
of stomata
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circadian rhythms: all eukaryotic organisms have these internal clocks, 24-hour
cycles
xerophytes: plants adapted to arid climates. Some desert plants complete life
cycle during rainy season, some have fleshy stems that store water/reduce
transpiration rate
CAM plants: use specialized form of photosynthesis where stomatal gas
exchange occurs at night
translocation: how products of transpiration transported through phloem.
Movement always from source to sink but sources and sinks can switch (summer
v winter). Sugars from source tissues loaded into sieve tube elements by
symplastic diffusion and active loading from apoplast. 1) Phloem near source-
solute potential lowered, active transport of sugar against concentration gradient
by co-transport. 2) water rushes in, pressure potential increases. 3) phloem near
sink- sugars are withdrawn, increases solute potential. 4) water exits and
recycled in xylem
•sugar source: organ that is a net producer of sugar- mature leaves
•sugar sink: organ that is net consumer/storer , such as tuber or bulb
•active transport: dependent on energy from proton pump, cotransport sucrose
with H+
•phloem is an "information superhighway"- many plant viruses spread through
phloem. Functional mRNA can be transported long distances in phloem- signals
indicating infections are moved through phloem. For ex: RNA-interference
defense against viruses used phloem transport
38- life cycle of plants
sporophytes: diploid plant produces haploid spres via meiosis
gametophytes: spores divide by mitosis, form male and female haploid forms
that produce gametes
- fertilization results in diploid zygotes (divide by mitosis to form new
sporophytes)
flower structure and function: reproductive shoots of angiosperm sporophyte,
attach to receptacle (ovaries inside receptacles)
•carpel: stigma, style, ovary.
•stamens: anther (where pollen is produced) and filament.
•petals:
•sepals:
sexual reproduction: generates variation in offspring where conditions often
change, seeds facilitate dispersal and can remain dormant until conditions
improve, seed germination is precarious and energetically expensive
angiosperm life cycle:
•female gametophyte development: the embryo sac develops within the ovule.
Within the ovule, two integuments surround a megasporangium(2n). One cell in
the megasporangium undergoes meiosis, producing four haploid megaspores,
only one of which survives. The megospores divides without cytokinesis,
producing one large cell with 8 haploid nuclei- which is partitioned into a
multicellular female gametophyte, the embryo sac. The fates of nuclei
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