Transcript Plant Transport

Transport in Plants
What is the tallest tree on the planet?
Sequoia sempervirens - The coastal redwood
(115m = 379 feet)
Seems like it would require a pump, like
you and I have, but a much larger one to
transport substances from roots to
leaves. Trees as we know do not have
any “pumps” of that nauture. So how do
they do it?
Maybe Plants Push Xylem Sap: Root Pressure
• Water flows in from the root cortex generating a positive pressure that forces
fluid up the xylem. This is upward push is called root pressure
• Root pressure sometimes results in guttation, (the exudation of
water droplets on tips of grass blades or the leaf margins of
some small, herbaceous dicots in the morning). More water
enters the leaves than is leaves it (transpired), and the excess is
forced out of the leaf.
Plant transport mechanisms solve a
fundamental biological problem:
• The need to acquire materials from the environment and distribute
them throughout the entire plant body
• Clear nail polish
• Leaves
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•
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•
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Activity1
Activity 2
Flaccid carrot and cucumber slices
Bowl
dH2O, bottled water, tap water
Salt
Precursor 1: Water chemistry and characteristics
• Polarity
*H-bonds (Strong or weak? Can you draw and Hbond between 2 or more water molecules?)
• Consequences include: Cohesion, Adhesion,
Surface Tension…etc (properties of water)
Activity 3: A mini-experiment/demonstration
• Indirect and relative measure of H-bond
strength (as well as cohesion andadhesion)
– Glass slides
– Plastic cups
– Water
– Pennies
– Masking Tape (Thumbs)
Precursor 2. Selective Permeability of
Membranes
• The selective permeability of a the plasma
membrane controls the movement of solutes
into and out of the cell AND the role of:
• Specific transport proteins are involved in
movement of solutes (and water too!)
• Passive Transport – Diffusion, Facilitated
Diffusion, Osmosis (Differences?)
• Active Transport (Features of?)
Proton Pumps
Proton pumps create a hydrogen ion gradient that is a form of potential energy
that can be harnessed to do work
They contribute to a voltage known as a membrane potential (Plant cytoplasm is
(-) compared to extracellular fluid)
Consequences include:
Fac diffusion of other cations
Cotransport: symport and antiport (secondary active transport)
EXTRACELLULAR FLUID
CYTOPLASM
ATP
–
+
–
–
+
+
H+
H+
H+
H+
H+
H+
–
–
+
+
H+
H+
Proton pump generates
membrane potential
and H+ gradient.
Membrane potential and cation uptake
• Plant cells use the proton gradient and membrane
potential to drive the transport of many different
solutes (e.g. cation (+) uptake: opposites attract)
CYTOPLASM
–
–
K+
K+
+
EXTRACELLULAR FLUID
+
+
–
Cations ( K+ , for
example) are driven
into the cell by the
membrane potential.
K+
K+
K+
K+
K+
–
+
–
+
(a) Membrane potential and cation uptake
Transport protein
Cotransport (symport)
• In cotransport a transport protein (known as a
symport) couples the passage of one solute to
the passage of another in the same direction
H+
–
+
–
+
–
+
H+
H+
H+
H+
H+
H+
H+
–
+
–
+
–
+
(b) Cotransport of anions
H+
H+
H+
H+
Cell accumulates
anions ( NO3–, for
example) by
coupling their
transport to the
inward diffusion
of H+ through a
cotransporter.
Cotransport (Antiport)
• Energy released as a molecule (e.g.H+) diffuses
back into the cell and powers the active
transport of a second molecule (ex. Ca++ or
Na+) out of the cell
•
Sucrose uptake
• The cotransport is also responsible for the uptake
of the sugar sucrose (a neutral solute) by plant cells
–
H+
H+
+
H+
H+
–
+
–
+
Plant cells can
also accumulate a
neutral solute,
such as sucrose
H+
H+
S
–+
H
H+
H+
–
–
+
+
H+
steep proton
gradient.
H+
S
–
(c) Cotransport of a neutral solute
+
( S ), by
cotransporting
H+ down the
H+
An important membrane protein side
note
Water Potential
• To survive plants must balance water uptake and loss
• Water potential is a measurement that combines the
effects of solute concentration and physical pressure
(due the presence of the plant cell wall) It is a
measurement of the FREE amount of water molecules
and the direction of movement of water (i.e. water’s
potential to do work).
• Water flows from regions of high water potential
(areas of more free water molecules) to regions of low
water potential (less free water molecules)
Ex. of water “doing work” on an
organismal level
Which has the greatest water
concentration?
• A or B
A or B
• Water potential is essentially not much different
Getting a little technical - The water potential
equation. Don’t freak out! Think Poseidon!
By convention, plant physiologists measure water potential in units
of pressure called megapascals (MPa). Note: bars is acceptable
For a baseline, the water potential for pure water at 1ATM is
expressed as having 0 Mpa or 0 bars
Breaking it down….
Cont’d
Consider this (U-tube Examples – AP Loves them)
• An artificial model
Cont’d: Addition of Solute example
Cont’d – Positive Pressure Example
Cont’d: A negative pressure example
Connection to plants:
AP will not be thrilled however if that was your
response to an “Explain what happens” prompt
• So what’s a better answer?
AP “Explain what happens” prompt
possible answers
• 1 star = The cell gains water
• 2 stars = Since water moves from high water potential to low
water potential, it will enter the cell.
• 3 stars = (include the data if provided) – Since the water
potential for the cell is -0.7 bars and the surrounding
environment has a water potential of 0 bars, water moves into
the cell.
• 4 stars = (include consequences ) Since the water potential for
the cell is -0.7 bars and the surrounding environment has a
water potential of 0 bars, water moves into the cell making it
turgid.
Cont’d – Produce 4 star answer for
scenario B(At home, not now )
• Note: The original cell has a starting water potential of -0.7 bars
Compare each situation with respect to the cytoplasm’s water potential
and the surrounding environment’s water potential
cell
env.
cell env.
cell env.
Water Pot:
Bonus info, free of charge: What could you say about each
situations:
cell env cell env cell env
Water concentration?
Solute Concentration?
Osmotic potential?
Collaborative Review/Study Break
• On mini-poster paper
– 1. Explain the role(s) of a gradient of protons in
moving substances across a plant cell’s plasma
membrane
– 2. How do symports and antiports differ? Give an
example of key substances each mechanism
transports.
– 3. What is “water potential” and discuss why it is
important with respect to plant cells
CHECK YOUR VEGETABLE AND YOUR
FRUIT!!
• Evaluate your slices
• Explain what has happened to them to a
classmate (or to a teacher)
Next Step: How do roots take in water and
minerals from the soil
• Water and mineral salts from the soil enter the
plant through the epidermis of roots and
ultimately flow to and through the shoot system
(xylem tissue) by bulk flow and active transport
respectively.
• Bulk flow – the group movement of molecules
in response to a difference in pressure between
two locations (see more later)
• Soil solutionRoot Hair EpidermisRoot
Cortex Root Xylem
Cont’d
• Root Hairs
– Much of the absorption of water and minerals occurs near
root tips, where the epidermis is permeable to water and
where root hairs are located
– Root hairs account for much of the surface area of roots
A mutulaistic symbiotic relationship.
and a surface area multiplier
Plant Cell Structure- more info for
understanding transport
• The vacuole is a large organelle that can occupy as much as 90%
of more of the protoplast’s volume
• The vacuolar membrane (the tonoplast)
– Regulates transport between the cytosol and the vacuole
Cell wall
Transport proteins in
the plasma membrane
regulate traffic of
molecules between
the cytosol and the
cell wall.
Cytosol
Vacuole
Plasmodesma
Plasma membrane
Transport proteins in
the vacuolar
membrane regulate
traffic of molecules
between the cytosol
and the vacuole.
Vacuolar membrane
(tonoplast)
(a) Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
Water travels to the root xylem by one of three pathways
Water and minerals can travel through a plant by one of three routes
1. Out of one cell, across a cell wall, and into another cell (transmembrane
route)
2. Via the symplast (symplastic route)
Key
3. Along the apoplast (apoplastic route)
Symplast
Apoplast
Transmembrane route
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
Symplast
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Symplastic route
Apoplastic route
Transport routes between cells. At the tissue level, there are three passages:
the transmembrane, symplastic, and apoplastic routes.
Lateral transport of minerals and water in roots
Casparian strip
Endodermal cell
Pathway along
apoplast
Pathway
through
symplast
1 Uptake of soil solution by the
hydrophilic walls of root hairs
provides access to the apoplast.
Water and minerals can then
soak into the cortex along
this matrix of walls.
Casparian strip
2 Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
1
Plasma
membrane
Apoplastic
route
Vessels
(xylem)
2
3 As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
Symplastic
route
Root
hair
4 Within the transverse and radial walls of each endodermal cell is the
Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals. Only minerals already in
the symplast or entering that pathway by crossing the plasma
membrane of an endodermal cell can detour around the Casparian
strip and pass into the vascular cylinder.
Epidermis
Cortex Endodermis Vascular cylinder
5 Endodermal cells and also parenchyma cells within the
vascular cylinder discharge water and minerals into their
walls (apoplast). The xylem vessels transport the water
and minerals upward into the shoot system.
The Endodermis
– Is the innermost layer of cells in the root cortex
– Surrounds the vascular cylinder and functions as the
last checkpoint for the selective passage of minerals
from the cortex into the vascular tissue
• Water can cross the cortex via the symplast or apoplast
• The waxy Casparian strip of the endodermal wall
blocks apoplastic transfer (but not symplastic) of water and
minerals from the cortex to the vascular cylinder
Ascent of Xylem Sap
•Plants lose an enormous
amount of water through
transpiration (the loss of
water vapor through the
stomata) and the transpired
water must be replaced by
water transported up from
the roots
•Xylem sap rises to heights of
more than 100 m in the
tallest plants
Pulling Xylem Sap
The Transpiration-Cohesion-Tension Theory
• Transpirational Pull
– Water transport begins as water evaporates from the
walls of the mesophyll cells inside the leaves and into
the intercellular spaces
– Driven by the
Cohesion and Adhesion in the Ascent of Xylem Sap
•
•
•
The transpirational pull on
xylem sap:
Solar Powered
Bulk Flow (pressure differences
created by water potential
differences)
Is transmitted all the way
from the leaves to the root
tips and even into the soil
solution
It is facilitated by the
cohesion and adhesion
properties of water
Narrow diameter of xylem
Cont’d
• Transpiration produces negative pressure (tension) in
the leaf which exerts a pulling force on water in the
xylem, pulling water into the leaf
• This water vapor escape through the stomata
• The Transpiration Dance
and
• Transpiration animations
• https://www.youtube.com/watch?v=U4rzLhz4HHk
Stomata and Transpiration Control
• Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas and high surface-tovolume ratios. Good and bad:
–  increase photosynthesis;
–  Increase water loss through stomata
20 µ
Check your nail-polished spinach
leaves
• Tear your leaf as to produce a “lip” of dried nail polish
• Peel off as large a section of the dried nail polish only
• Microscopic observation reveals imprint of the organization of
the leaf surface – specifically stomata (guard cell) arrangement
Stomata cont’d
• About 90% of the water a plant loses escapes through stomata (lenticel,
cuticle other 10%)
• Each stoma is flanked by guard cells which control the diameter of the stoma
by changing shape
Cells turgid/Stoma open Cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Guard Cells
Shape changes due to multiple factors including:
• Changes in turgor pressure that open and close
stomata result primarily from the reversible
uptake and loss of potassium ions (K+) by the
guard cells
• Creates water potential differences
Role of potassium in stomatal opening and closing.
The transport of K+ (potassium ions, symbolized
here as red dots) across the plasma membrane and
vacuolar membrane causes the turgor changes of
guard cells.
H2O
K+
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Xerophyte Adaptations That Reduce Transpiration
• Xerophytes are plants adapted to arid
climates
– They have various leaf modifications
that reduce the rate of transpiration
• The stomata of xerophytes
– Are concentrated on the lower leaf
surface
– Are often located in depressions that
shelter the pores from the dry wind
– Possess thicker waxy cuticles
– Sunken stomata
– Trichomes (“hair)
Cuticle Upper epidermal tissue
Lower epidermal Trichomes
tissue
(“hairs”)
Stomata
100 m
Stomata in
recessed
crypts of
Oleander plant
Second Major Plant Tranport Event Translocation of Phloem Sap
• Organic nutrients are translocated through the
phloem (translocation is the transport of organic nutrients in
the plant)
• Phloem sap
– Is an sucrose solution
– Travels from a sugar source to a sugar sink
– A sugar source is a plant organ that is a net producer
of sugar, such as mature leaves
– A sugar sink is an organ that is a net consumer or
storer of sugar, such as a tuber or bulb or a leaf too!
Translocation of phloem sap cont’d: The
pressure-flow hypothesis
Seasonal Changes in Translocation
•A storage organ such as a
tuber or bulb may be a sugar
sink in summer as it
stockpiles carbohydrates.
•After breaking dormancy in
the spring the storage organ
may become a source as its
stored starch is broken down
to sugar and carried away in
phloem to the growing buds
of the shoot system
Phloem loading
• Sugar from mesophyll leaf cells must be loaded into sieve-tube
members before being exported to sinks
• Depending upon the species, sugar moves by symplastic and
apoplastic pathways
In many plants phloem
loading requires active
transport.
Proton pumping and
cotransport of sucrose
and H+ enable the cells
to accumulate sucrose.
Answers to first study break sesssion
• 1. After an H+ gradient is established (by
pumping protons out of the cell) the resulting
inward flow of H+ down its concentration
gradient provides energy to actively transport
other substances into the cell
• 2. In symport, two substances move in the same
direction through a cell membrane; in antiport
two substances cross the cell membrane in
opposite directions
• Sample AP FR and Key