Download Chapter 36: Resource Acquisition and Transport in Vascular Plants

Chapter 36:
Resource Acquisition and Transport in Vascular Plants
An Overview of Transport Mechanisms in Plants
1. List three levels in which transport occurs in plants.
--Cell to cell, cell to environment, and organ to organ transport.
2. Compare the processes of passive and active transport. Distinguish between the two main
categories of transport proteins.
--Passive transport: the diffusion across the membrane, do not require energy
--Active transport: the pumping of solute across the membrane, require energy, move
against it electrochemical gradient.
- The two main categories of transport proteins are the transport protein that binds
selectively to a solute, change shape and release the solute across the membrane and the
transport protein that provides selective channels across the membrane that only allow
certain ion to enter.
3. Describe the role and importance of proton pumps in transport across plant membranes.
--The proton pump used the energy from ATP to pump the H+ out of the cell. The
outside of the cell has higher H+ concentration than the inside. Therefore, the inside of
the cell is negative to the outside of the cell. The H+ gradient across the membrane is a
form of potential (stored) energy, and the flow of H+ back into the cell can be harness to
do work.
4. Define cotransport and chemiosmosis.
--Cotransport: a transport protein that have the combination of passive transport and
active transport.
--Chemiosmosis: an energy coupling mechanism that used energy from ATP to drive
cellular work across the membrane.
5. Define osmosis and water potential. Explain how water potential is measured.
--Osmosis: the diffusion of water across the membrane
--Water potential: the combined effects of solute concentration and physical pressure are
incorporated into a quantity.
--The water potential is measured in the unit of megapascals (PMa), the water potential is
the combination of the solute potential and the pressure potential. The U-shape tube was
used to determine the water potential. The two sides of the U-shape tube were separated
by the membrane that is permeable to water but not to solutes. If the right arm of the tube
contains a 0.1 M solution (-0.23 MPa) solute potential) and the left side contains pure
water (0 MPa), and there is no physical pressure, the water potential will be equal to the
solute potential. -0.23 MPa on the right side will be equal to 0 MPa of the left side
because the water potential move from the region of higher water potential to the region
of lower water potential, the net movement of the water will from the left side to the right
side.
6. Explain how solutes and pressure affect water potential.
--The solute affects the water potential because when the solute binds to water molecule,
it reduces the number of free water molecules and lowering the capacity of the water to
move and do work. Adding solutes always lower the water potential because solute
potential of a solution is always negative.
--The pressure affects the water potential because when add the pressure to either side of
the U-shape tube, it will either reduce or increase the water potential. And beside in the
equation of water potential, the pressure and solute potential were important to water
potential. Pressure potential can be positive or negative.
7. Explain how the physical properties of plant cells are changed when the plant is place into
solutions that have higher, lower, or the same solute concentrations. Define flaccid,
plasmolyze, turgor pressure, and turgid.
--The plant cell will become flaccid and soon will undergo plasmolysis when placed in
the solutions that have lower solute concentration because through osmosis, the water
moves from high solute concentration to low solute concentration. The plant cells will
not change when placed in the same solute concentrations. But when the plant cells
placed in the solutions that have higher solute concentration, the cell will be turgid and
soon will burst.
--Flaccid: when the plant cell is limp, where there is no tendency for water to enter the
cell
--Plasmolyze: when the cell’s protoplast shrinks and pulls away from the cell wall.
--Turgor pressure: the force directed against a plant cell wall after the influx of water and
swelling of the cell due to osmosis
--Turgid: swollen or distended or the entry of water
8. Explain how aquaporins affect the rate of water transport across membranes.
--The selective channel, aquaporins can affect the rate of water transport across
membranes because the phosphorylation of the aquaporin proteins regulated the water
movement by increase the calcium ions or decrease the pH in the cytoplasm.
9. Describe the three major compartments in vacuolated plant cells, noting their
interrelationships.
--Cell wall: outside of the protoplast, ions can diffuse across a tissue through apoplast
(the continuum formed by cell wall) and the dead interiors of tracheids and vessels
--Cytosol: form a continuum, the middle compartment
--Vacuole: the most inner compartment
*The transport proteins in help to regulate traffic of molecules between the
compartments.
10. Describe the three routes available for lateral transport in plants.
--Apoplastic: water and solutes move along the continuum of cell walls and extracellular
spaces
--Symplastic: water and solutes move along the continuum of cytosol within a plant
tissue, only one crossing of plasma membrane is needed
--Transmembrane routes: water and solutes move out of one cell, across the cell wall, and
into the neighboring cell, which may pass them to the next cell in the same way, requires
repeated crossing of plasma membranes as water and solutes exit one cell and enter the
next.
11. Define bulk flow and describe the different types of forces that generate pressure.
--Bulk flow is the movement of a fluid driven by pressure in long distance transport.
--Xylem forces: pull the water movement upward through the cohesive water column
--Phloem forces: water follows by osmosis raising the pressure.
12. Relate the structure of sieve-tube cells, vessel cells, and tracheids to their functions in bulk
flow.
--The Sieve-tube cells belong to phloem forces, their cytoplasm is almost devoid of
internal organelles, their sugar concentration must maintain to simplify lower resistance.
--The vessel elements and tracheids are dead cells, have no cytoplasm and they are empty
for lower resistance.
Absorption of Water and Minerals by Roots
13. Explain how the structure of root hairs promotes their functions. Explain how mycorrhizae
facilitate the functions of roots.
--The root hairs absorb the soil solution and the solution flows into the hydrophilic walls
of the epidermal cells and passes freely along the cell walls and the extracellular spaces
into the root cortex. This provides a much greater membrane surface area for absorption
than the surface area of the epdidermis alone.
--The mycorrhizae increase the surface area for absorption and enable the older region of
the root to supply water and minerals to the plant.
14. Explain how the endodermis functions as a selective barrier between the root cortex and
vascular tissue.
--The endodermis acts as a final checkpoint for the selective passage of minerals from the
cortex into the vascular tissue. The apoplast is blocked by suberin in the Casparian strip
around each endodermal cell, the Casparian strip forces water and minerals to pass
through the symplast to enter stele, the endodermis ensures that no minerals can reach the
vascular tissue of the root without crossing a selectively permeable plasma membrane,
the endodermis also prevent the solutes that have accumulated in the xylem from leaking
back into the soil solution.
Transport of Xylem Sap
15. Describe the potential and limits of root pressure to move xylem sap. Define root pressure,
transpiration, and guttation.
--The potential of root pressure to move xylem sap: pumping the minerals ions into the
xylem of the stele even at night, the endodermis helps prevent the ions from leaking out,
and lower the water potential within the stele.
--The limits of root pressure to move xylem sap: sometime cause more water to enter the
leaves than is transpired, not all plants generated root pressure, root pressure cannot keep
pace with transpiration after sunrise.
--Root pressure: a push of xylem sap
--Transpiration: the loss of water vapor from leaves and other aerial parts of the plant
--guttation: the exudation of water droplets that can be seen in the morning on the tips or
edges of some plant leaves
16. Explain how transpirational pull moves xylem sap up from the root tips to the leaves.
--The more water evaporates from the cell wall, the more curvature of the air-water
interface increases and the pressure of the water become more negative. Water molecules
from the more hydrated parts of the leaf are then pulled toward this area to reduce
tension. The water molecule is cohesively bound to the next by hydrogen bonds.
Because water moves from areas of higher water potential to the areas of lower water
potential, the more negative pressure potential at the air-water interface cause water in
xylem cells to be pulled into mesophyll cells, which lose water to the air spaces, where it
diffuses out through stomata.
The Control of Transpiration
17. Describe the role of guard cells in photosynthesis-transpiration.
--The guard cell opens and closes the stomata, and they also help to balance the plant’s
requirement for photosynthesis.
18. Explain the advantages and disadvantages of the extensive inner surface area of a leaf.
--Advantages: increases surface area for absorption of CO2 and light during
photosynthesis, helps to release O2 as a by-product of photosynthesis, and increases the
rate of photosynthesis.
--Disadvantage: increases water loss, about 95% of water a plant losses through stomata.
19. Explain how the transpiration to photosynthesis ratio is calculated and what it indicates
about a plant.
--The transpiration to photosynthesis ratio is calculated under the same environmental
conditions to which the amount of water lost by a leaf depends largely on the number of
stomata and the average size of the pores. This indicates about the water loss of a leave
through the stomata and the efficiency of water usage.
20. Explain how transpiration changes the temperatures of leaves and why this is adaptive.
Explain how plants with low transpiration rates compensate for higher temperatures.
--The transpiration changes the temperatures of leaves by evaporate the cooling, which
lower a leaf’s temperature compared to the surrounding air. This cooling prevents the
leaf from reaching temperatures that could denature enzymes that involved in
photosynthesis and other metabolic processes. The plants with low transpiration adapted
to be stable at higher temperature.
21. Explain how and when stomata open and close.
--The stomata are open during the day and close at night. The stomata open when the K+
flow in across the plasma membrane of the guard cell that is coupled to the generation of
a membrane potential by proton pumps, the stomata open when the active transport of H+
out of the guard cell, and the stomata can open when the absorption of K+ causes the
water potential to become more negative within the guard cells, and the cells become
more turgid as water enter by osmosis. The stomata close when the K+ is flow out from
the guard cells to the neighboring cells, which leads to an osmosis loss of water. Three
cues that contribute to stomatal open:
-Light: the guard cells accumulate K+ and become turgid by the illumination of
blue-light receptors. The activation of the blue-light receptors stimulates the activity of
proton pumps in the plasma membrane of the guard cells to promote the absorption of
K+.
-CO2 depletion: as the CO2 concentrations decreases during the day, the stomata
progressively open if sufficient water is supplied to the leaf.
-Internal “clock” in guard cells: the memory of daily activity of opening and
closing of stomata; even for plants that kept in the dark place.
22. Explain how xerophytes reduce transpiration.
--The Xerophytes are plants that adapted to deserts and other regions with little moisture.
--Xerophytes reduce transpiration because some of the species avoid drying out by
completing their short life cycles during the brief rainy season, they have unusual
physiological or morphological adaptations that enable them to withstand the harsh desert
conditions, they also have highly reduced leaves that resist excessive water loss, and they
carry out photosynthesis mainly in their stems.
Translocation of Phloem Sap
23. Define and describe the process of translocation. Trace the path of phloem sap from the
primary sugar source to common sugar sinks.
--Translocation: the transportation of organic nutrients in the phloem of vascular plants.
--The process of translocation: the process that the phloem transports the organic
products of photosynthesis throughout the plant.
--The path of phloem sap: sugar must be transported into sieve-tube elements before
being exported to sugar sinks. It can either moves from mesophyll cells to sieve-tube
elements via the symplast, passing through plasmodesmata or it can moves by symplastic
and apoplastic pathways. Most of them then moves into the apoplast and is accumulated
by nearby sieve-tube elements; it can be either direct or through companion cells. Some
of the plants enhancing solute transfer between apoplast and symplast. Some requires
active transport when sucrose is more concentrated in sieve-tube elements and
companion cells than in mesophyll. Both proton pumping and cotransport enable sucrose
to move from mesophyll cells to sieve tube elements.
24. Describe the process of sugar loading and unloading.
--Sugar loading: the higher level of sugar at the source reduces the water potential and
causes water to flow into the tube.
--Sugar unloading: the removal of sugar at the sink increases the water potential and
causes water to flow out of the tube.
25. Define pressure flow. Explain the significance of this process in angiosperms.
--Pressure flow: the positive pressure that drives the movement of the sap along the sieve
tube. The building of pressure at the source end and reduction of that pressure at the sink end
cause water to flow from source to sink and carrying the sugar along. This also reduces that
water potential inside the sieve-tube members and causes the uptake of water.
Vocab:
- Transport proteins: proteins that embedded to the cell membrane, act as the gate to allow
ions to enter or leave the cell
- Selective channels: channels that let a certain ion to enter the cell
- Proton pump: a transport protein that used energy from ATP to pump the H+ out of the cell
- Cotransport: the transport protein that have a combination of the diffusion of one solute and
the active transport of the other.
- Chemiosmosis: an energy coupling mechanism that used energy from ATP to drive cellular
work across the membrane.
- Osmosis: the diffusion of water.
- Water potential: the physical property predicting the direction in which water sill flow,
governed by solute concentration and applied pressure.
- Megapascal (MPa): the measuring unit of water potential.
- Tension: the state of being stretch tight.
- Flaccid: when the plant cell is limp, where there is no tendency for water to enter the cell.
- Plasmolyze: when the cell’s protoplast shrinks and pulls away from the cell wall.
- Turgor pressure: the force directed against a plant cell wall after the influx of water and
swelling of the cell due to osmosis.
- Turgid: swollen or distended, when the cell has a greater solute concentration than its
surroundings.
- Aquaporin: a transport protein in the plasma membrane of a plant, animal, or
microorganism cell that specifically facilitates osmosis, the diffusion of water across the
membrane.
- Tonoplast: membrane that bounds the chief of vacuole of a plant cell.
- Symplast: the continuum of cytoplasm connected by plasmodesmata between cells
- Apoplast: the continuum of cell walls plus the extracellular spaces.
- Bulk flow: the movement of a fluid driven by pressure
- Mycorrhizae: a mutualistic association of plant roots and fungus.
- Endodermis: the innermost layer of the cortex in plant roots
- Casparian strip: a water impermeable ring of wax in the endodermal cells of plants that
blocks the passive flow of water and solutes into the stele by way of cell walls.
- Transpiration: the loss of water vapor from leaves and other aerial parts of the plant.
- Root pressure: the upward push of xylem sap in the vascular tissue of roots.
- Guttation: the exudation of water droplets, caused by root pressure in certain plants.
- Transpiration to photosynthesis ratio: surface to volume ratio.
- Circadian rhythm: a physiological cycle of about 24 hours that is present in all eukaryotic
organisms and that persists even in the absence of external cues.
- Xerophytes: a plant that adapted to deserts and other regions with little moisture.
- Translocation: the transport of organic nutrients in the phloem of vascular plants
- Sugar source: a plant that is a net producer of sugar, by photosynthesis or by breakdown of
starch.
- Sugar sink: an organ that is a net consumer or depository of sugar.
- Transfer cells: a companion cell with numerous ingrowths of its wall, which increase the
cell’s surface area and enhance the transfer of solutes between apoplast and symplast.