transport in plants notes

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Transport in Plants NotesAP Biology Mrs. Laux

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3 levels of transport occur in plants:1. Uptake of water and solutes by individual cells

-for photosynthesis and respiration-ex: absorption of H2O /minerals by root hairs

2. Short distance cell-to-cell transport at level of tissues and organs

3. Long distance transport of sap by xylem and phloem at wholeplant level

A. Transport at the cellular level-plasma membraneselective permeability

1. Passive transport-when solute travels down a concentration gradient-no energy-transport proteins sometimes used

-aid/speed up carrying of materials across

membrane

2. Active transport-against concentration gradient-energy requiring process-proton pumpimportant in plants




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-like in respiration and photosynthesis in makingof ATP-here, opposite, proton pump hydrolyzes ATP and

uses energy to drive H+ ions out of the cell-produces a proton gradient and electrochemicalgradient outside of cell-can accomplish two things:a. entry of + ions, such as K+, when concentrationis greater outside the cell than in

-because inside is more (-) with all H+spumped out, K+ can easily enter the cellbecause of charge with electrochemicalgradient

b. entry of (-) ions, such as nitrate ion-NO3-

-energy stored by H+ ions on outside of cell(water behind dam) can be used totransport solutes against their concentration gradient-inside cells is (-), NO3

- moving into cellrequires energy-energy comes from H+ gradient

-energy is used in respiration tomake ATP

c. entry of a neutral solute-sucrose




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Water Potential and Osmosis-osmosisnet uptake or loss of water by the cell; depends on whichcomponent, the cell or extracellular fluid has higher water potential

-water potential, Ψ, psifree energy of water that is a consequenceof solute concentration and applied pressure-water moves high Ψ low Ψ -unitMPa (megapascals) ~10 atm-pure H2O-open container 0 MPa

-add solutes, lower Ψ to (-)-increase pressure, increase Ψ -decrease pressure, decrease Ψ (-)

-bulk flowmovement of H2O due to pressure differencesΨ = pressure potential and solute concentration potential

Ψ = Ψp –Ψs0.1 M solution has a solute potential of 0.23 MPa

-in open container Ψp = 0-therefore, Ψ = Ψp – Ψs

= 0 – 0.23= -0.23

-plants will gain or lose water to intercellular fluids depending ontheir water potential-if flaccid (limp) cell is placed in a hypertonic solution, cell willplasmolyze

-protoplast will pull away from cell wall-if flaccid cell is placed in hypotonic solution, cell will swell andturgor pressure develops




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-when pressure on cell wall is equal to osmotic pressure,equilibrium is reached-no more net movement of H2O

Tonoplast-membrane surrounding large central vacuole in plant cells-important in regulating intracellular conditions

-contains integral proteins-control movement of solutes between cytosoland vacuole-has membrane potential

-proton pumps maintain higher pH in cytosol because pump H+s intothe vacuole-this proton gradient helps transport solutes into vacuole for storage

B. Short Distance transport at the level of tissues and organs-can happen via:

1. across plasma membrane and through cell walls2. symplast routesolutes move from cell to cell through cellsvia plasmodesmata

-doesn’t go through membrane but through the thinstreams of cytoplasm

-ex: endodermis3. the apoplast routewater and solute move past outside of cell walls

-between cells-never enters cells




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C. Long distance travel through whole plant-via vascular tissues

-passive transporttoo slow-instead travel via:

-bulk flow through xylem and phloem-transpiration- pulls sap up tree from roots in xylem-hydrostatic pressure develops at one end of sieve tubes inphloem-forces sap along

Absorption of water and minerals by roots-absorbed via the following pathway:

soilepidermisroot cortexxylem




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1. soilepidermis:-most absorption occurs near root tips where epidermis is permeableto water -root hairs increase surface area

2. epidermis to root cortex:

-lateral transport is usually a combination of apoplast and symplastroutesa. apoplast route

-water and minerals flow through hydrophilic cell walls andpass freely

b. symplast route-makes mineral absorption possible-active transport into epidermal and cortex cells allowssufficient supply of minerals to pass through

-diffusion will not allow enough into cells-concentration toolow-ex: transport proteins of tonoplast and plasma membraneactively pump K+ into root cells and Na+ out

3. root cortexxylem




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-only minerals using symplast route move directly into vascular tissues-minerals and water passing through apoplasts are blocked by theCasparian strip-water and minerals enter stele through the cells of the endodermis

-Casparian strip only allows certain ions to pass into stele-endodermal cells discharge contents that it is carrying via symplast,into the apoplast of xylem

-tracheids and vessels are part of apoplast-as water and minerals enter apoplast (via diffusion and activetransport) they are free to enter tracheids and vessels

Transpiration of sap through xylem

-sap=water + minerals-water transported up from roots must replace that lost bytranspiration (evaporation of water from leaves)-also provides nutrients (minerals) to shoot system

-How water (sap) climbs xylem1. pushing xylem saproot pressure




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-as water enters the stele, it enters via osmosis

-brings with it minerals/ions from the soil-as water moves up the column, the concentration decreasesin roots, even though mineral concentration remains high, thiscauses water flow because of the higher concentration of solutes inside the stele-during the day, this forces water upward (root pressure) to anextent and water evaporates through air spaces viatranspiration-at night, transpiration is decreased, stomata are closed, androot pressure continues

-this causes water/sap droplets to escape throughleaves-called guttation-this can be seen as small droplets of sap on grass andleaves in the morning

-Root pressure, although it does have an effect on the movement of sapthrough the xylem, it is not the leading mechanism driving the ascent of xylem sap

a. cannot keep up with rate of transpiration-water evaporates more quicklyb. can only force water up a few meters

2. Pulling xylem sapcohesion-tension theory




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-deals with two aspects:-transpiration pulls sap upward-cohesion of water transmits upward pull along entire length of thexylem

a. transpirational pull-depends on negative pressure-gaseous water in air spaces diffuses into drier atmospherethrough stomata

-lost water vapor is replaced by evaporation from mesophyllsbordering the air spaces-as water evaporates, this causes a negative pressure, tensionin xylem (syringe)

-this tension causes water to be pulled from xylemthrough mesophyll, towards surface film on cellsbordering the stomata

-bulk flow of water through xylem cells occurs as moleculesevaporate from leaf; therefore, as one water moleculeevaporates from leaf, it pulls all the other water molecules upalong behind it

-this is because of:b. cohesion and adhesion of water -allows for transpirational pull to occur -cohesion between water molecules due to H-bonding allowsone cell to pull “chain” of cells behind it-adhesion of water (via H-bonding) to hydrophilic cell walls of xylem cells also helps pull against gravity




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-small diameter of tracheids and vessels is important toadhesion-results in capillary action

-movement of liquids in narrow tubes-capillary action makes small contribution unless

coupled with transpirational pull-upward pull of sap causes tension in xylem, decreases Ψ, andallows passive flow soilstele-flowing water via capillary action forms a meniscus in xylem

Water vapor diffuses from the moist air spaces of the leaf to the drier air outside via stomata. Evaporation from the water film coating the mesophyll cells maintains the high humidity of the air spaces. This loss of water causes the water film to form menisci that

become more and more concave as the rate of transpiration increases. A meniscus has a tension that is inversely proportional to the radius of the curved water surface. Thus, as

the water film recedes and its menisci become more concave, the tension of the water film increase. Tension is a negative pressure- a force that pulls water from locations where

hydrostatic pressure is greater. The tension of water lining the air spaces of the leaf is the physical basis of transpirational pull, which draws water out of xylem.

-actively flowing water in xylem tissues never formsmeniscus because there is continuously flowing water

If water vapor or another gas is allowed to enter xylem tissue-cavitationformation of a water vapor pocket in xylem




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-vessels cannot function again unless refilled by root pressure-this can only happen in small plants, root pressure can onlyraise water a few meters

-pits between xylem vessels allow for detours around cavitated area-secondary growth adds new xylem vessels each year

-ascent of xylem sap is solar powered-sun causes evaporation; therefore, negative pressure-bulk flow in xylem depends only on this pressure-while osmosis in roots and leaves are due to small gradients inwater potential caused by both solute and pressure gradients

Control of the Stomata-opening and closing of the stomata influence gas exchange, transpiration,ascent of sap, and photosynthesis-when stomata are open, CO2 can enter: therefore, photosynthesis canoccur

-H2O can escape-good, because water and nutrients can be carried up xylem toleaf cells

-also evaporative cooling-allows cooling-bad, because too much water can escape and causedesiccation

-When stomata are closed-no water loss, therefore, escape wilting (too much evaporation, notenough delivery-No CO2 enters; therefore, no photosynthesis can occur

-also as concentration of CO2 decreases in air spaces and

concentration of O2 increases (product of photosynthesis) inair spaces, run risk of photorespiration occurring-each stoma is surrounded by 2 guard cells

-cell walls are not uniform, the cell wall bordering stoma is thicker than the rest of cell wall-composed of cellulose microfibrils arranged radially, from outsideto stoma side

-when turgid (full of water) cells “buckle” due to microfibrilsand stomata open-when flaccid (water leaving) guard cells sag and openingsclose




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-Guard cells control the following:

1. stomata close when temps are high-reduces water loss-shuts down photosynthesis

2. stomata open when CO2 concentrations are low inside the leaf -allows photosynthesis

3. stomata close at night and open during the day-because in leaf, CO2 low during the day-being used for photosynthesis, at night, no photosynthesis- CO2

concentration remains high (because of respiration)

How guard cells control stomata:-guard cells open when turgid (filled with water)-closed when no water (flaccid)-this uptake and loss of water is controlled mainly by the uptake and loss

of K+ ions by guard cells-when K+ diffuses into cell and then vacuole by plasma membraneand then the tonoplast, this decrease water potential (Ψ) in guardcells

-water flows into guard cell-highΨ to low Ψ -most of water is stored in vacuole; therefore, tonoplast alsoplays a major role-this increase in positive charge is balanced by:

-uptake of Cl- ions-uptake of other negative ions, especially organic acids

-loss of H+, from organic substances/acids-closing of guard cells results when K+ exits the cell andcreates osmotic loss of water

Guard cells open and close because of internal and external cues-stomata open at dawn in response to:

1. light induces guard cells to take up K+ by-beginning photosynthesis, making ATP for H+ pumps




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2. decrease of CO2 in air spaces due to onset of photosynthesis3. internal clock will cause guard cells to open even if kept indark (circadian rhythm~24 hours)

-guard cells may close stomata if:

1. water deficiency results in flaccid guard cells2. a hormone is produced due to water deficiency3. high temps increase CO2 in air spaces due to increasedrespiration, closing guard cells

Evolutionary aspects that reduce transpiration:1. Plants have structural adaptations to hot, dry climates2. CAM photosynthesis

Transport of sugarstranslocationtransport of products of photosynthesis by phloemto the rest of the plant-in angiosperms, sieve tube members are specialized cells thatfunction in translocation-pores in sieve plates allow phloem sap to move freely along sievetube-phloem sap=sucrose, minerals, amino acids, and hormones

-Considered “source-to-sink” transport-sourceorgan where sugar is produced (usually leaves) or wherestarch is broken down (leaves, stems, roots, etc.)-sinkorgan that consumes or stores sugar

-ex: growing parts of plants, fruits, non-green parts of plants,stems, trunks, etc.

-sugar flows sourcesink-can depend on season

-ex: tuber sink when storing in summer, source inspring

-minerals are also transported to sinks




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-sink is usually supplied by nearest source-direction of phloem can change, depending on location of sourceand sink-whereas, xylem, only up plant

Phloem Loading and Unloading-sugar from source must be loaded into sieve member before being

translocated to a sink-process follows:

1. Sugars enter sieve tube members-soluble carbohydrates, sucrose and fructose, move fromsight of production to sieve tube members by active transport-this develops a higher concentration of solutes in the sievetube than in the sink

2. Water enters sieve tube members- Ψ is higher outside sieve tube than in sieve tube; therefore,water diffuses down concentration gradient and into sievetube member

3. Pressure in source sieve tube member move water and sugars tosieve tube member at sink through sieve tubes

-move via bulk flow (because of pressure)4. Pressure is reduced at the sink as sugars are removed for use bynearby cells

-by active and passive transport-water diffused out of sieve tube members, relieving pressure

transport in plants notes

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