T RANSPORT IN V ASCULAR P LANTS C HAPTER 36 Nicole Moenck Per. 1.

TRANSPORT IN VASCULAR PLANTS

CHAPTER 36Nicole Moenck

Per. 1

36.1 PHYSICAL FORCES DRIVE THE TRANSPORT OF MATERIALS IN PLANTS OVER A RANGE OF DISTANCES

Selective Permeability of Membranes: Solutes diffuse down their gradients and that

diffusion across a membrane is called passive transport.

Active transport is the pumping of solutes across membranes against their electrochemical gradients.

It is “active” because the cell must expend energy in the form of ATP to transport the solute.

Most solutes cant pass across the lipid bilayer so they pass through transport proteins. The transport proteins bind to the solute and release it to the other side.


The Central Role of Proton Pumps: The most important active transport protein in the plasma

membrane of plant cells is the proton pump. The proton pump uses energy from ATP to pump hydrogen ions out of the cells. Results in a proton gradient with a higher H+ concentration outside of the cell then inside.

Because the proton pump moves positive charge out of the cell the pump also contributes to a voltage known as a membrane potential. Which is a separation of opposite charges across a membrane. In the form of potential energy uses to perform cellular work. (figure 36.3)

The role of proton pumps in transport is an application of chemiosmosis. The key feature of chemiosmosis is a transmembrane proton gradient, which links energy-releasing processes to energy-consuming processes in cells.


Effects of Differences in Water Potential:

To survive, plants must balance water uptake and loss. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane.

Water will move by osmosis from the solution with the lower solute concentration to the solution with the higher solute concentration. But physical pressure also effects osmosis.

The combined effect of solute concentration and physical pressure is called water potential.


Plant biologists measure water potential in units of pressure called megapascals.

Water potential expressed: Water potential = solute potential + pressure potential

Solute potential of a solution is proportional to the number of dissolved solute molecules. It affects the direction of osmosis.

Pressure potential is the physical pressure on a solution. The water in living cells is usually positive pressure.

The cells contents press against the plasma membrane producing tugor pressure.


When the external solution has the lower water potential, water will leave the cell by osmosis, the cells protoplast with phasmolyze (shrink) and pull away from the wall.

A walled cell that has a greater solute concentration inside than outside is turgid (healthy).

You can see the effect of tugor loss in wilting, he dropping of leaves and stems as a result of cells becoming flaccid.

Water typically crosses vacuolar and plasma membranes through transport proteins called aquaporins. These channels don’t affect the water potential or direction of water flow, but rather the rate at which water diffuses down the gradient.


Three Major Compartments of Vacuolated Plant Cells:

The cells, the cytosol, and the vacuole. The vacuolar membrane, or tonoplast,

regulats molecular traffic between the cytosol and the vacuolar contents, called cell sap.

Plasmodesmata connect the cytosolic compartments of neighboring cells forming a continuous pathway for transportation. The cytoplasmic continuum is called the symplast. The continuum of cells walls is called the apoplast.


Bulk Flow in Long-Distance Transport: Bulk flow is the movement of fluid drive by

pressure. In bulk flow, water and solutes move through the

tracheids and vessels of xylem and through the sieve tubes of the phloem.

In the phloem, the loading of sugar generates a high positive pressure at one end forcing sap to the opposite end.

In xylem, tension (negative pressure) drives transport. Transpiration, evaporation of water from the leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.

36.2 ROOTS ABSORB WATER AND MINERALS FROM THE SOIL

The Roles of Root Hairs, Mycorrhizae, and Cortical Cells:

Much of the absorption of water and mineral occurs near the root tips. Root hairs account for much of the surface area of roots.

Most plants form beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil.

Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hypae. It then transfers to the host plant. Supplies water and minerals to the plant.

36.2 ROOTS ABSORB WATER AND MINERALS FROM THE SOIL The Endodermis: A Selective Sentry: Water and minerals that pass from the soil into the

root cortex cannot be transported to the rest of the plant until they enter they xylem of the vascular cylinder.

The endodermis, the outermost layer of cells in the root cortex, surrounds the vascular cylinder and functions as a last checkpoint for the slective passage of minerals from the cortex into the vascular tissue.

Minerals cross into the vascular cylinder. These minerals had to be screened in order to the enter the symplast. Those minerals that reach the endodermis via the apoplast encounter a dead end. In the transverse and radical walls of each endodermal cell is the Casparian strip, a belt made of suberin, a waxy material imperivous to water and dissiolved materials. Making the only way past the barrier is to cross the plasma membrane and the enter the vacular cylinder through the symplast.

36.2 ROOTS ABSORB WATER AND MINERALS FROM THE SOIL The endodermis, with its Casparian strip, ensures

that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane.

It also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution.

The structure and its location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.

The water and mineral nutrients tracked from the soil to the root xylem can now be transported upward as xylem sap to the shoot system.

36.3 WATER AND MINERALS ASCEND FROM ROOTS TO SHOOTS THROUGH THE XYLEM

Factors Affecting the Ascent of Xylem Sap:

The sap flows upward from roots throughout the shoot system to veins that branch throughout each leafs. Leaves depends on water.

Plants loose a big amount of water by transpiration, the lose of water vapor from the leaves and other aerial parts of the plant.

Unless the transpired water is replaced, the plant will wilt and die.


When transpiration is low, root cells are still pumping minerals to the xylem of vascular cylinder. The endodermis helps prevent the minerals from leaking out. The resulting accumulation of minerals lowers the water potential. Water flows in from the root cortex, generating root pressure, an upward push of xylem sap.

The root pressure sometimes causes more water to enter the leaves than transpired, resulting in guttation, the exudation of water droplets (dew).

For the most part, xylem sap isn’t pushed from below by root pressure but pulled upward by the leaves.


To move material upward, we can apply positive pressure from below or negative pressure from above. We will see that transpiration provides the pull and the cohesion of water due to hydrogen bonding transmits the upward pull.

Stomata, the microscopic pres on the surface of a leaf, lead to a maze of internal airspaces that expose the mesophyll cells to the carbon dioxide they need for photosynthesis.

The hypothesis is that negative pressure that causes the water to move up through the xylem develops air-water interface in mesophyll cells.


The movement depends on adhesion of water to cellulose microfibrils and other hydrophilic components in plant cell walls.

The role of negative pressure fits with water potential. Since water move from where its potential is higher to where it is lower, the increasingly negative pressure at the air-water interface causes xylem cells to lose water to mesophyll cells, which lose water to the airspaces, where it diffuses out through the stomata.

The negative water potential of leaves provides the “pull” in transpirational pull.

36.3 WATER AND MINERALS ASCEND FROM ROOTS TO SHOOTS THROUGH THE XYLEM The transpirational pull on xylem sap is transmitted all the way

from the leaves to the root tips and into the soil solution. The cohesion of water due to hydrogen bonding makes it

possible to pull of column of sap from above without the water molecules separating.

Transpirational pull can extend down to the roots only by unbroken water molecules. Cavitation, the formation of water vapor pocket in a vessel.

Then processes like root pressure can help plants age and change each season.

In transport of water from roots to leaves through bulk flow, the movement of fluid is driven by water potential differences at opposite ends of a conduit. A conduit are vessels of chains of tracheids. The water potential difference is generated at the leaf end by transpirational pull, which lowers the water potential at the “upstream” end of the xylem.

36.4 STOMATA HELP REGULAR THE RATE OF TRANSPIRATION Leaves generally have large surface areas and high

surface area-to-volume ratios. The large surface area is a morphological adaptation that

enhances the absorption of light needed to drive photosynthesis.

The high surface area-to-volume ratio aids in the uptake of carbon dioxide during photosynthesis as well as in the release of oxygen produced as a by-product of photosynthesis.

Effects of Transpiration on Wilting and Leaf Temperature:

If transpiration pulls water upward they will not wilt. The rate of transpiration is the highest on sunny days.

Transpiration also results in evaporative cooling. It prevents leaf from reaching high temperature that could

stop photosynthesis.

36.4 STOMATA HELP REGULAR THE RATE OF TRANSPIRATION Stomata: Major Pathways for Water Loss: About 90% of the water a plant loses is from the

stomata. Guard cells control the stomata. Because of natural selection, desert plants have lower

stomatal densities than do marsh plants because of its surroundings.

When guard cells take in water by osmosis, they become turgid. The cell walls are uneven, therefore, causing an increase in the size of the pore.

The change in tugor pressure that open and close the stomata result from the reversible uptake and loss of potassium ions by the guard cells.

In general, stomata are open during the day and closed at nigh so that water isn’t lost when it is to dark for photosynthesis.

36.4 STOMATA HELP REGULAR THE RATE OF TRANSPIRATION Three reasons for stomatal opening at dawn.1 – Light stimulates guard cells to accumulate potassium

ions and become turgid.2 – Depletion of carbon dioxide within the air spaces of the

leaf, which occurs when photosynthesis begins in the mesophyll.

3 – An internal “clock” in the guard cells. Cycles that have intervals of 24 hours are called

circadian rhythms. Environmental stress can cause the stomata to close

during the day. Xerophyte Adaptation That Reduce Transpiration: Plants adapted to climates, called xerophytes, have

various leaf modification that reduce the rate of transpiration.

36.5 ORGANIC NUTRIENTS ARE TRANSLOCATED THROUGH THE PHLOEM Xylem sap flows from roots to leaves, in a direction

opposite to that necessary to transport sugars from the leaves to other parts of the plant. The phloem, a second vascular tissue, that transports the products of photosynthesis. The transport of organic nutrients is called translocation.

Movement from Sugar Sources to Sugar Sinks: Sieve tubes carry sugars from a sugar source to a

sugar sink. Sugar source is a plant organ that is a net producer of

sugar, by photosynthesis or a breakdown of starch. Sugar sink is an organ that is a net consumer or

storer of sugar. Example: Growing roots, buds, stems, and fruits.

36.5 ORGANIC NUTRIENTS ARE TRANSLOCATED THROUGH THE PHLOEM In some plants, transfer cells are needed to transfer

solutes between the apoplast and symplast. Phloem loading sometimes requires active transport

because the sucrose concentration is 2 to 3 times higher than in mesophyll. Phloem unloads sucrose at the sink end of a sieve tube.

Pressure Flow: The Mechanism of Translocation in Angiosperms:

In angiosperms, sap moves through the sieve tubes by bulk flow driven by positive pressure (Pressure flow).

Sugar transport on 3 levels: cellular level across the plasma membrane, short-distance (sucrose migration from mesophyll to phloem via symplast and apoplast, and long-distance (bulk flow in sieve tubes).

T RANSPORT IN V ASCULAR P LANTS C HAPTER 36 Nicole Moenck Per. 1.

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