ch. 36 Flashcards
what does the stem do
- conduits for water and nutrients
- supports structures for leaves
what do shoot length and branching pattern affect?
light capture
what is there a tradeoff between?
growing tall and branching
- more energy invested into branching, less energy available for growth in height
what is there a positive correlation between?
water availability and leaf size
phyllotaxy
arrangement of leaves on the stem
- phyll - leaf
- tax - movement toward/away
phyllotaxy of most angiosperms
leaves arranged in spiral
angle between leaves
137.5 degrees, likely minimizes shading of lower leaves
what does leaf orientation affect
light absorption
low light conditions
horizontal leaves capture more sunlight
high light conditions
vertical leaves less damaged by sun and allow light to reach lower levels
why are stomatal pores necessary
diffusion of CO2 into the photosynthetic tissues of leaves
how is over 90% of water lost by plants
evaporation from stomatal pores
what do shoot adaptations represent compromises against
enhancing photosynthesis and minimizing water loss
what can root growth adjust to?
local conditions
- roots branch extensively into pockets with high nitrate and grow straight through pockets of low nitrate availability
roots and competition
roots from same plant less competitive than roots from dif plants
mycorrhizae
roots and hyphae of soil fungi form mutualistic associations
- mycorrhizal fungi increase surface area for absorbing water and minerals, especially phosphate
3 transport routes for water and solutes
- apoplastic
- symplastic
- transmembrane
apoplastic route
through cell walls and extracellular spaces
symplastic route
cross plasma membrane once and then travel through cytosol
- use plasmodesmata
transmembrane route
repeatedly cross plasma membranes as they pass from cell to cell
apo
away, furthest point
sym
with, in company
trans
cross
plastic
to grow or form, capable of being deformed without rupture
plasma membrane permeability controls what?
short-distance movement of substances
what types of transport occur in plants
both active and passive
establishment of membrane potential in plants
pumping H+ by proton pumps
potential =
voltage
establishment of membrane potential in animals
pumping Na+ by sodium=potassium pumps
different types of energy conversion
- H+ and membrane potential
- H+ and cotransport of neutral solutes
- H+ and cotransport of ions
- ion channels
osmosis
diffusion of water into or out of a cell that is affected by solute concentration and pressure
water potential =
solute potential + pressure potential
- determines direction of movement of water
- higher to lower
definition of potential
ability to do work
solute potential of a solution is inverse to what?
its molarity
what is solute potential also called?
osmotic potential
pressure potential
physical pressure on a solution
positive pressure potential
pushing
- solution being expelled from syringe
negative pressure potential
sucking
- solution withdrawn by syringe
turgor pressure
positive pressure exerted by plasma membrane against the cell wall and the cell wall against the protoplast
protoplast
living part of the cell, which includes plasma membrane
what does water potential affect?
uptake and loss of water by plant cells
what happens when a flaccid/limp cell is placed in an environment with a higher solute concentration
the cell will lose water through negative pressure and undergo plasmolysis
aquaporins
transport proteins in the cell membrane that facilitate the passage of water
what do the opening/closing of aquaporins affect the rate of
osmotic water movement across the membrane
bulk flow
movement of a fluid driven by a pressure gradient
what does efficient long-distance transport of fluid require
bulk flow
where do water and solute move together?
tracheids and vessel elements of xylem and sieve-tube elements of phloem
what do branching veins within leaves ensure
that all living cells are within a few cells of the vascular tissue
what is bulk flow enhanced by
structural adaptations of xylem and phloem cells
- mature tracheids/vessel elements have no cytoplasm
- sieve-tube elements have few organelles in cytoplasm
- perforation plates connect vessel elements
- porous sieve plates connect sieve-tube elements
where does most water and mineral absorption occur
near root tips
- where root hairs are located and epidermis is permeable to water
what accounts for much of the surface area of roots?
root hairs
what happens after soil solution enters the roots?
the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals
what does active transport enable in the roots
essential minerals to accumulate at much higher concentrations in roots compared to the surrounding soil
endodermis
innermost layer of cells in the root cortex
- surrounds vascular cylinder and is last checkpoint for selective passage of minerals from cortex into the vascular tissue
water can cross the cortex via what?
symplast or apoplast
Casparian strip
waxy strip in endodermal wall that blocks apopastic transfer of minerals from the cortex to the vascular cylinder
what must water/minerals cross to enter the vascular cylinder of roots
the plasma membrane of an endodermal cell
what do endodermal cells discharge?
water and minerals from their protoplasts into their own cell walls
what are involved in the movement from symplast to apoplast
diffusion and active transport
once in the apoplast after the endodermis, what do water and minerals do?
enter the tracheids and vessel elements
xylem sap
water and dissolved minerals transported from roots to leaves by bulk flow
what does the transport of xylem sap involve?
transpiration
transpiration
evaporation of water from a plant’s surface
how is transpired water replaced
as water travels up from the roots
what do root cells do at night?
continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
root pressure
a push of xylem sap as water flows in from the root cortex
what does root pressure sometimes result in?
guttation
guttation
exudation of water droplets on tips or edges of leaves
positive root pressure is relatively…
weak
- minor mechanism of xylem bulk flow
cohesion-tension hypothesis
transpiration and water cohesion pull water from shoots to roots
what is xylem sap normally under
negative pressure, or tension
transpirational pull
- water vapor in air spaces of leaf diffuses down water potential gradient and exits leaf via stomata
- as water evaporates, air-water interface retreats into the mesophyll cell walls
- surface tension of water at air-water interface creates a negative pressure potential
what lowers water potential?
negative pressure potential
how are water molecules pulled from more hydrated areas of the leaf
by the negative pressure potential created at the air-water interface
what transfers the pulling forces to the water in the xylem
the cohesion of water molecules
how are water molecules attracted to each other
cohesion
- water molecules exiting xylem tug on adjacent water molecules down the column
what does adhesion do in the xylem
offset the force of gravity
why do vessel elements and tracheids not collapse under negative pressure
thick secondary walls
what can cause a break in the chain of water
drought stress or freezing
- causes cavitation
cavitation
formation of a water vapor pocket
how can transport of xylem sap continue after cavitation
- move between adjacent xylem cells through pits
- move from xylem to phloem tissue and back again
- cavitation can be repaired
- new xylem is added by secondary growth
what is blue flow in xylem sap ascent driven by
water potential difference at opposite ends of xylem tissue
- driven by transpiration
what does bulk flow require energy from
not from plant, but is solar powered like photosynthesis
how does bulk flow differ from diffusion
- driven by differences in pressure potential, not solute potential
- occurs in hollow dead cells, not across membranes of living cells
- moves entire solution, not just water or solutes
- much faster
do leaves have higher or lower surface area compared to volume
higher
what happens when high surface areas of leaves increase the rate of photosynthesis
increase in water lost through stomata
what do guard cells do
open and close stomata to help balance water conservation with gas exchange for photosynthesis
what controls stomatal density
genetic and environmental control
what happens to guard cells when they are turgid
guard cells bow outward and pore between them opens
what happens to guard cells when they are flaccid
become less bowed and pore closes
what do changes in turgor pressure result from
reversible uptake and loss of potassium ions by guard cells
what are required to move K+ across the plasma membrane?
(under guard cell slides)
proton pumps that generate membrane potential
when do stomata generally open and close
open during day, close at night
what is stomatal opening triggered by?
- light
- CO2 depletion
- internal clock in guard cells
circadian rhythms
24-hour cycles - internal clocks of eukaryotic organisms
abscisic acid (ABA)
hormone produced in response to water deficiency and causes closure of stomata
what do sunny, warm, dry, and windy conditions cause?
evaporation and increased transpiration rates
true or false, some evaporative water loss occurs through the cuticle when stomata are closed
true
what happens if uptake/transport of water not sufficient to replace lost water
plant will wilt
evaporative cooling
- another result of transpiration
- can lower temperature of leaf
xerophytes
plants adapted to arid climates
xero
dry
when do some desert plants complete their life cycle
- during the rainy season
- others have fleshy stems that store water or leaf modifications that reduce rate of transpiration
crassulacean acid metabolism (CAM)
stomatal gas exchange occurs at night
translocation
products of photosynthesis are transported through phloem
elements for translocation in angiosperms
sieve-tube elements
phloem sap
aqueous solution that is high in sucrose
where does phloem sap travel
from sugar source to sugar sink
sugar source
organ that is net producer of sugar, such as mature leaves
sugar sink
organ that is net consumer or depository of sugar, such as roots, buds, and fruits
true or false: A storage organ can be a sugar sink in the summer and sugar source in the spring
true
where must sugar be loaded before being exported to sinks
sieve-tube elements
does sugar move by symplastic of apoplastic pathways
depends on species
what do companion cells do
enhance soul movement between apoplast and symplast
what kind of transport does phloem loading require in many plants
active transport
what enables the cells to accumulate sucrose
proton pumping and contransport of sucrose and H+
what occurs at the sink
sugar molecules diffuse from the phloem to sink tissues and are followed by water
how does phloem sap move
through a sieve tube by bulk flow driven by positive pressure called pressure flow
- flows from high pressure sources to low pressure sinks
self-thinning
dropping of sugar sinks such as flowers, seeds, or fruits
- occurs when there are more sugar sinks than the sources can support
symplast
continuum of cytosol linked by plasmodesmata
apoplast
water-filled cell walls and intercellular spaces
