Ch. 36 -- Resource Acquisition & Transport in Vascular Plants

Turgid

(adj.) swollen, bloated, filled to excess; overdecorated or excessive in language a walled cell w/ greater [solute] relative to outside is firm or _________. Causes wilting

Endodermal cell significance:

(selectively permeable p. membrane barrier for minerals, nutrients ) -facilitates endodermal transport of NEEDED minerals from the soil into the xylem -barrier to many unneeded/toxic substances -prevents solutes that have accumulated in xylem from leaking back into soil (backflow back into soil) -These cells, as well as living cells w/in the vasc cylinder, discharge minerals from their protoplasts into own cell walls -both diffusion and active transport facilitate the transfer of solutes from symplast to apoplast -water and minerals can now enter tracheids and vessel elements -where they are transported to shoot system by BULK FLOW

phloem

Living vascular tissue that carries sugar and organic substances throughout a plant

Root Architecture and Acquisition of Water and Minerals

Root growth can adjust to local conditions Roots are less competitive with other roots from the same plant than with roots from different plants Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate plants rapidly adjust their architecture and physiology to exploit patches of available nutrients in soil (often shift to fix nitrate efficiently) Soil contains resources mined by the root system For example, roots branch extensively into pockets of high nitrate availability and grow straight through pockets of low nitrate availability Mutualisms with fungi helped plants colonize land

Loading of Sucrose into Phloem

Sugar must be loaded into sieve-tube elements before being exported to sinks -Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways -Companion cells enhance solute movement between the apoplast and symplast In MANY species, requires Active Transport

sieve tube elements

found in phloem; stacked end to end; have holes so materials can get in and out of the phloem conduits for translocation

solute potential (ΨS) [Osmotic Potential]

directly proportional to its molarity. (solutes are mineral ions, sugars) Increase in [solute] has negative effect on water potential

How does water move against gravity in plants?

electrostatic H bonds Transpiration Cohesion Tension Mechanism

sugar sink

is an organ that is a net consumer or depository of sugar, such as roots, buds, and fruits; STORAGE organ can be a sugar sink in summer and sugar source in spring (starch broken down --> sugar) usually receive from nearest sources

Sugar source

is an organ that is a net producer of sugar, such as mature leaves

trees transport water and minerals to highest leaves by

losing water -- transpiration

companion cells

make up phloem vessels, along with sieve tube elements

active transport of sugar in plants why is active transport required?

required bc there is a higher [sucrose] in sieve tube elements and companion cells than in mesophyll. Proton pumping (H+) & H+/Sucrose-cotransport -allows sucrose to move from mesophyll cells to sieve tube elements or companion cells (phloem structures)

Sugars are transported from ________ to ________ via the __________

sources to sinks via the phloem water and minerals -- upward; unidirectional flow -- XYLEM sugar -- bidirectional flow (often downward) from leaves (sources) to sinks (plant organs; primarily in roots/buds/fruits -- PHLOEM translocation -- transfer of sugars

mycorrhizae

symbiotic/mutualistic relationships between fungal hyphae and plant roots ***CRITICAL STEP IN PLANTS COLONIZING LAND*** hyphae indirectly endow root systems of many plants with enormous SA (water/mineral/nutrient absorption) [especially phosphate] Mycorrhizal fungal filaments in the soil are truly extensions of root systems and are more effective in nutrient and water absorption than the roots themselves. More than 95 percent of terrestrial plant species form a symbiotic relationship with beneficial mycorrhizal fungi, and have evolved this symbiotic relationship over the past several hundred million years. These fungi predate the evolution of terrestrial plants, and it was the partnership with mycorrhizal fungi that allowed plants to begin to colonize dry land and create life on Earth as we know it. The mycorrhizal symbiotic relationship centers on the plant's ability to produce carbohydrates through photosynthesis and share some of these sugars with the fungus in return for otherwise unavailable water and nutrients that are sourced from the soil or growing media by the extensive network of mycelial hyphae produced by the fungus. It's a two-way relationship of sharing resources between two species, thus a classic symbiotic mutualism. The endomycorrhizal fungi rely on the plant, and the plant's performance and survival are enhanced by the fungus. In return for carbon, these fungi improve plant health and tolerance to environmental stress. This symbiosis is over 400 million years old

Because of __________ _______ water and minerals cannot pass the endodermis and enter vascular cylinder via apoplast. Why?

Casparian strip instead water and minerals that are passively moving through the apoplast must cross the SELECTIVELY PERMEABLE p. membrane of the endodermal cell before they can enter vascular cylinder

Leaf area index

Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows Leaf area indexes higher than .7 result in shading to the point that self-pruning occurs (not advantageous to be over 0.7) quantitative variable that measures how much light can be absorbed by a plant ratio of total upper leaf surface of plant /Surface Area of land its growing on Leaf orientation (horizontal and vertical leaves) -- vertical/angle allows leaves below it to capture light as well

Where is sucrose unloaded?

at sink end of a sieve tube [free sugar] in sink is always lower than sieve tube bc the unloaded sugar is consumed during growth/metabolism of cells of the sink, or converted into insoluble polymers (starch)----> [SUGAR] Gradient

photosynthesis-water-loss-compromise

broad surface of most leaves favors light capture; open stomatal pores allow diffusion of CO2 into photosynthetic tissues -Open stomata pores---> evaporation of water (transpiration) -Shoot adaptations represent compromises between: maximizing photosynthesis and minimizing water loss

2. capillary action Movement of Water Against Gravity

capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). Capillarity occurs due to three properties of water: Surface tension, which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water. Adhesion, which is molecular attraction between "unlike" molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls. Cohesion, which is molecular attraction between "like" molecules. In water, cohesion occurs due to hydrogen bonding between water molecules. (links an unbroken chain of water molecules from roots to entire organism)

3. transpiration-cohesion-tension mechanism

A transport mechanism that drives the upward movement of water in plants: transpiration exerts a pull that is relayed downward along a string of molecules held together by cohesion and helped upward by adhesion. The cohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension essentially combines the process of capillary action with transpiration, or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem. Transpiration (evaporation) occurs because stomata are open to allow gas exchange for photosynthesis. As transpiration occurs, it deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction). The tension created by transpiration "pulls" water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw. Cohesion (water sticking to each other) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward the stomata.

Water movement across plant cell membranes: Water potential affects uptake and loss of water by plant cells

If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis (H2O follows high [SOLUTE] // lower water potential) Plasmolysis occurs when the cell membrane shrinks and pulls away from the cell wall... Opposite = turgidity = wilting

Short-Distance Transport of Solutes Across Plasma Membranes

In plants, membrane potential is established through pumping H+ by proton pumps (selective) Plasma membrane permeability controls short-distance movement of substances Both active and passive transport occur in plants e.g. proton pump -- cytoplasm to ECF -- pH gradient and membrane potential is established -- pumps [H+] - Higher {H+] in ECF -SUCROSE & NITRATE (a) H+H+ and membrane potential. The plasma membranes of plant cells use ATP-dependent proton pumps to pump H+H+ out of the cell. These pumps contribute to the membrane potential and the establishment of a pH gradient across the membrane. These two forms of potential energy can drive the transport of solutes. (a) H+ and membrane potential. The plasma membranes of plant cells use ATP-dependent proton pumps to pump H+ out of the cell. These pumps contribute to the membrane potential and the establishment of a pH gradient across the membrane. These two forms of potential energy can Drive the transport of solutes. (b) H+ and cotransport of neutral solutes. Neutral solutes such as sugars can be loaded into plant cells by cotransport with H+ ions. H+/sucrose cotransporters, for example, play a key role in loading sugar into the phloem prior to sugar transport throughout the plant. (c) H+ and cotransport of ions. Cotransport mechanisms involving H+ also participate in regulating ion fluxes into and out of cells. for example, H+/NO3- cotransporters in the plasma membranes of root cells are important for the uptake of NO3- by plant roots. (d) Ion channels. Plant ion channels open and close in response to voltage, stretching of the membrane, and chemical factors. When open, ion channels allow specific ions to diffuse across membranes. For example, a K+ ion channel is involved in the release of K+ from guard cells when stomata close.

vascular cylinder

central region of a root that includes the vascular tissue-xylem and phloem

Bulk flow differs from diffusion:

-It is driven by differences in pressure potential, not solute potential -It occurs in hollow dead cells, not across the membranes of living cells -It moves the entire solution, not just water or solutes -It is much faster

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. minerals that reach endodermis via apoplast encounter dead end that blocks their passage into vascular cylinder; located in transverse and radial walls of each endodermal cell Made of suberin, a waxy/fatty material impervious to water and dissolve minerals

plasmodesmata

An open channel in the cell wall of plants through which strands of cytosol connect from adjacent cells

Transport Continuums: The Apoplast & Symplast

Apoplast -Everything external to the plasma membrane -Cell walls, extracellular spaces, and the interior of dead cells such as vessel elements and tracheids Symplast -The entire mass of the cytosol of all the living cells in a plant, as well as the plasmodesmata, cytoplasmic channels that interconnect.

Aquaporins: Facilitating Diffusion of Water

Aquaporins are transport proteins in the cell membrane that facilitate the passage of water Opening and closing of aquaporins affect the rate of osmotic water movement across the membrane -a diff in water potential determines direction of water movement across membranes -open and close -- affects osmotic flow rates permeability is decreased by increase in cytosolic [Ca2+], or decreases in pH permeability is INCREASED by decrease in cytosolic [Ca2+], or increase in pH

Long-Distance Transport: The Role of Bulk Flow

Efficient long-distance transport of fluid requires bulk flow, the movement of a fluid driven by a pressure gradient -- always flows from HIGH pressure to LOW Water and solutes move together through: -tracheids and vessel elements of xylem -sieve-tube elements of phloem (diffusion is too slow for long distance) INDEPENDENT OF SOLUTE CONCENTRATIONS The branching veins in leaves ensure that all cells are within a few cells of the vascular tissue -Bulk flow is enhanced by the structural adaptations of xylem and phloem cells -Mature tracheids and vessel elements have no cytoplasm -sieve-tube elements have few organelles in their cytoplasm Perforation plates connect vessel elements, and porous sieve plates connect sieve-tube elements

Transpiration

Evaporation of water from the leaves of a plant gives of purified water to be evaporated... via Stomata [Stomata are pores in the leaf that allow gas exchange where water vapor leaves the plant and carbon dioxide enters. Special cells called guard cells control each pore's opening or closing. When stomata are open, transpiration rates increase; when they are closed, transpiration rates decrease.]

Transport of Water and Minerals into Xylem

MUST enter first root hairs, then: -xylem of vascular cylinder OR -Stele (vasc tiss of stem or root) to get to the rest of plant Endodermis -- the last checkpoint for selective passage of minerals from cortex into vascular cylinder (innermost layer of cells in root cortex) Minerals already in symplast when they reach endodermis continue through the plasmodesmata of endoderm cells then pass into vascular cylinder (had to already cross p. membrane to enter symplast in epidermis or cortex) last segment in soil-to-xylem pathway: passage of water and minerals into tracheids and vessel elements of xylem

Resource Acquisition and Transport in a Vascular Plant

Movement of water is UNIDIRECTIONAL (up against gravity) Flow of sugar is BIDIRECTIONAL (leaves to roots, roots to stem/leaves) [Roots primary storage of sugar] Stomata -- CO2 brought in, O2 is released Sugars are made in leaves via photosynth.

Bulk Flow by Positive Pressure in a Sieve Tube Mechanisms of Translocation in Angiosperms

Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow Phloem sap flows from sources (where pressure is high), to sinks (where pressure is low)

wilting

The drooping of leaves and stems as a result of plant cells becoming flaccid due to cells losing water

mesophyll

The ground tissue of a leaf, sandwiched between the upper and lower epidermis and specialized for photosynthesis.

protoplast

The living part of a plant cell, which also includes the plasma membrane.

Ascent of Xylem Sap (against gravity)

The movement of xylem sap against gravity is maintained by the Transpiration-cohesion-tension mechanism

water potential

The physical property predicting the direction in which water will flow, governed by solute concentration and applied/physical pressure. (Water flows from regions of higher water potential to regions of lower water potential) Abbreviated by the Greek letter Ψ Units of pressure=megapascal

turgor pressure

The pressure inside of a cell as a cell pushes itself against the cell wall. i.e. is the positive pressure exerted by the plasma membrane against the cell wall and the cell wall against the plasma membrane e.g. -2MPa (tension) -- often water in hollow nonliving xylem cells (tracheids/vessel elements) of a plant

Sugars are transported from sources to sinks via the phloem

The products of photosynthesis are transported through phloem by the process of translocation A sugar source is an organ that is a net producer of sugar, such as mature leaves A sugar sink is an organ that is a net consumer or depository of sugar, such as roots, buds, and fruits In angiosperms, sieve-tube elements are the conduits for translocation Phloem sap (sugar source ---> sugar sink) organ can be a sugar sink in summer and sugar source in spring

Transpirational Pull

The suction force caused by transpiration that is the main factor causing water movement up the xylem. Negative pressure/water potential w/in leaves pulls from soil up stem to veins of leaves transmitted through entire organism xylem sap will diffuse into neighboring cells once it reaches veins of leavesd

Shoot Architecture and Light Capture

There is a trade-off between growing tall and branching; the more energy invested into branching, the less energy available for growth in height Phyllotaxy Stems serve as conduits for water and nutrients and as supporting structures for leaves Shoot length and branching pattern affect light capture There is generally a positive correlation between water availability and leaf size

transpiration-cohesion-tension mechanism & Movement of Water Against Gravity

These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees: 1. Root pressure pushes water up 2. Capillary action draws water up within the xylem 3. Cohesion-tension pulls water up the xylem

Apoplast and Symplast: Transport Continuums

Three transport routes for water and solutes are: -The apoplastic route, through cell walls and extracellular spaces -The symplastic route, where water and solutes cross a plasma membrane once and then travel through the cytosol -The transmembrane route, where water and solutes repeatedly cross plasma membranes as they pass from cell to cell not mutually exclusive

Solutes and Pressure Affect Water Potential

Turgor pressure

Different mechanisms transport substances over short or long distances

Two major transport pathways through plants: Apoplast -Everything external to the plasma membrane -Cell walls, extracellular spaces, and the interior of dead cells such as vessel elements and tracheids Symplast -The cytosol of all the living cells in a plant, as well as the plasmodesmata

The process of xylem sap transport in plants involves the loss of an astonishing amount of _______ via _______________

Water via Transpiration

cohesion-tension mechanism

a mechanism for the transport of water in xylem; water is pulled up the xylem tubes, powered by the force of evaporation of water from the leaves (producing tension) and held together by hydrogen bonds between nearby water molecules (cohesion).

phloem sap

a mixture of sugar, nutrients, and water that flows through phloem vessels (SIEVE TUBE ELEMENTS) in a plant is an aqueous solution that is high in sucrose; also aa, Hormones, minerals

Uptake of H2O and Minerals

absorb H2O at the roots (unidirectionally) -- up stem to leaves Transpiration

sieve tubes

an element of phloem tissue consisting of a longitudinal row of thin-walled elongated cells with perforations in their connecting walls through which food materials pass. allow flow of sap

tracheids and vessel elements

parts of xylem, in the wood you would see large vein like thing that transport water these are vessels, the smaller more common holes are called Tracheids

pressure potential (ΨP)

physical pressure on a solution positive or negative relative to atm pressure

sieve plates

porous end walls that allow fluid to flow between cells along the sieve tube

1. Root pressure Movement of Water Against Gravity

relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake of water in the roots increases Ψp in the root xylem, driving water up. -In extreme circumstances, root pressure results in guttation, or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree.

[SUGAR] Gradient

sugar molecules diffuse from phloem into sink tissues and water follows by osmosis

companion cells.

the active cells found next to sieve tube elements that supply the phloem vessels with all of their metabolic needs may enhance solute transfer between apoplast and symplast

Phyllotaxy

the arrangement of leaves on a stem, is specific to each species important for light capture Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral

translocation

the movement of carbohydrate through phloem from a source to a sink (somewhere that uses carbs). transports products of photosynthesis targets are often root tips carried out by Phloem

self pruning

the shedding of lower shaded leaves, occurs when they respire more than photosynthesize

xylem sap

the solution of water and minerals that accumulates in the root xylem and flows through xylem vessels in a plant transported by tracheids and vessel elements

most of absorption of water and minerals occurs @ ?

the tips of root cells (epidermal cells) permeable to water many differentiated into root hairs

Water transport in Plants

trees move water by losing water -- Transpiration Water evaporation from plants water diffuses from inside the leaf to outside air via pores transported from roots to veins -- Xylem Transpiration Cohesion Tension Mechanism

Bulk Flow via Xylem

water & minerals from soil enter plant through epidermis of roots -----> cross root cortex ---------> pass into vascular cylinder ----> xylem sap (water and min solution) rapid water and minerals in xylem is transported long distances via _________ _____ in xylem -primarily driven by transpiration - is driven by a water potential difference at opposite ends of xylem tissue -does not require energy from the plant; like photosynthesis, it is solar powered (transpiration)

Root hairs absorb

water and minerals from soil solution (water and mineral ions are not bound tightly to soil particles ) soil solution is is drawn into the hydrophilic walls of epidermal cells and passes freely along the cell walls and extracellular spaces into the root cortex -this flow enhances exposure of the cells of the cortex to the soil solution -- provides a larger membrane SA for absorption (than that of epidermis alone) -ACTIVE transport of minerals occurs (since soil usually has low mineral concentration) e.g. K+ can accumulate large concentrations this way