Transport in Plants Notes AP Biology Mrs. Laux 3 levels of transport occur in plants: 1. Uptake of water and solutes by individual cells

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1 3 levels of transport occur in plants: 1. Uptake of water and solutes by individual cells -for photosynthesis and respiration -ex: absorption of H 2 O /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 whole plant level A. Transport at the cellular level -plasma membrane selective 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 pump important in plants 1

2 -like in respiration and photosynthesis in making of ATP -here, opposite, proton pump hydrolyzes ATP and uses energy to drive H+ ions out of the cell -produces a proton gradient and electrochemical gradient outside of cell -can accomplish two things: a. entry of + ions, such as K+, when concentration is greater outside the cell than in -because inside is more (-) with all H + s pumped out, K+ can easily enter the cell because of charge with electrochemical gradient b. entry of (-) ions, such as nitrate ion-no 3 - -energy stored by H+ ions on outside of cell (water behind dam) can be used to transport solutes against their concentration gradient -inside cells is (-), NO 3 - moving into cell requires energy -energy comes from H+ gradient -energy is used in respiration to make ATP c. entry of a neutral solute -sucrose 2

3 Water Potential and Osmosis -osmosis net uptake or loss of water by the cell; depends on which component, the cell or extracellular fluid has higher water potential -water potential, Ψ, psi free energy of water that is a consequence of solute concentration and applied pressure -water moves high Ψ low Ψ -unit MPa (megapascals) ~10 atm -pure H 2 O-open container 0 MPa -add solutes, lower Ψ to (-) -increase pressure, increase Ψ -decrease pressure, decrease Ψ (-) -bulk flow movement of H 2 O due to pressure differences Ψ = pressure potential and solute concentration potential Ψ = Ψp Ψs 0.1 M solution has a solute potential of 0.23 MPa -in open container Ψp = 0 -therefore, Ψ = Ψp Ψs = = plants will gain or lose water to intercellular fluids depending on their water potential -if flaccid (limp) cell is placed in a hypertonic solution, cell will plasmolyze -protoplast will pull away from cell wall -if flaccid cell is placed in hypotonic solution, cell will swell and turgor pressure develops 3

4 -when pressure on cell wall is equal to osmotic pressure, equilibrium is reached -no more net movement of H 2 O Tonoplast -membrane surrounding large central vacuole in plant cells -important in regulating intracellular conditions -contains integral proteins-control movement of solutes between cytosol and vacuole -has membrane potential -proton pumps maintain higher ph in cytosol because pump H + s into the 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 walls 2. symplast route solutes move from cell to cell through cells via plasmodesmata -doesn t go through membrane but through the thin streams of cytoplasm -ex: endodermis 3. the apoplast route water and solute move past outside of cell walls -between cells -never enters cells 4

5 C. Long distance travel through whole plant -via vascular tissues -passive transport too 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 in phloem-forces sap along Absorption of water and minerals by roots -absorbed via the following pathway: soil epidermis root cortex xylem 5

6 1. soil epidermis: -most absorption occurs near root tips where epidermis is permeable to water -root hairs increase surface area 2. epidermis to root cortex: -lateral transport is usually a combination of apoplast and symplast routes a. apoplast route -water and minerals flow through hydrophilic cell walls and pass freely b. symplast route -makes mineral absorption possible -active transport into epidermal and cortex cells allows sufficient supply of minerals to pass through -diffusion will not allow enough into cells-concentration too low -ex: transport proteins of tonoplast and plasma membrane actively pump K+ into root cells and Na+ out 3. root cortex xylem 6

7 -only minerals using symplast route move directly into vascular tissues -minerals and water passing through apoplasts are blocked by the Casparian 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 active transport) 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 by transpiration (evaporation of water from leaves) -also provides nutrients (minerals) to shoot system -How water (sap) climbs xylem 1. pushing xylem sap root pressure 7

8 -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 decreases in roots, even though mineral concentration remains high, this causes water flow because of the higher concentration of solutes inside the stele -during the day, this forces water upward (root pressure) to an extent and water evaporates through air spaces via transpiration -at night, transpiration is decreased, stomata are closed, and root pressure continues -this causes water/sap droplets to escape through leaves -called guttation -this can be seen as small droplets of sap on grass and leaves in the morning -Root pressure, although it does have an effect on the movement of sap through 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 quickly b. can only force water up a few meters 2. Pulling xylem sap cohesion-tension theory 8

9 -deals with two aspects: -transpiration pulls sap upward -cohesion of water transmits upward pull along entire length of the xylem a. transpirational pull -depends on negative pressure -gaseous water in air spaces diffuses into drier atmosphere through stomata -lost water vapor is replaced by evaporation from mesophylls bordering the air spaces -as water evaporates, this causes a negative pressure, tension in xylem (syringe) -this tension causes water to be pulled from xylem through mesophyll, towards surface film on cells bordering the stomata -bulk flow of water through xylem cells occurs as molecules evaporate from leaf; therefore, as one water molecule evaporates from leaf, it pulls all the other water molecules up along 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 allows one 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 9

10 -small diameter of tracheids and vessels is important to adhesion -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, and allows passive flow soil stele -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 forms meniscus because there is continuously flowing water If water vapor or another gas is allowed to enter xylem tissue -cavitation formation of a water vapor pocket in xylem 10

11 -vessels cannot function again unless refilled by root pressure -this can only happen in small plants, root pressure can only raise 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 in water 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, CO 2 can enter: therefore, photosynthesis can occur -H 2 O can escape -good, because water and nutrients can be carried up xylem to leaf cells -also evaporative cooling-allows cooling -bad, because too much water can escape and cause desiccation -When stomata are closed -no water loss, therefore, escape wilting (too much evaporation, not enough delivery -No CO 2 enters; therefore, no photosynthesis can occur -also as concentration of CO 2 decreases in air spaces and concentration of O 2 increases (product of photosynthesis) in air 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 outside to stoma side -when turgid (full of water) cells buckle due to microfibrils and stomata open -when flaccid (water leaving) guard cells sag and openings close 11

12 -Guard cells control the following: 1. stomata close when temps are high -reduces water loss -shuts down photosynthesis 2. stomata open when CO 2 concentrations are low inside the leaf -allows photosynthesis 3. stomata close at night and open during the day -because in leaf, CO 2 low during the day-being used for photosynthesis, at night, no photosynthesis- CO 2 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 membrane and then the tonoplast, this decrease water potential (Ψ) in guard cells -water flows into guard cell -high Ψ to low Ψ -most of water is stored in vacuole; therefore, tonoplast also plays 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 and creates 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 12

13 2. decrease of CO 2 in air spaces due to onset of photosynthesis 3. internal clock will cause guard cells to open even if kept in dark (circadian rhythm~24 hours) -guard cells may close stomata if: 1. water deficiency results in flaccid guard cells 2. a hormone is produced due to water deficiency 3. high temps increase CO 2 in air spaces due to increased respiration, closing guard cells Evolutionary aspects that reduce transpiration: 1. Plants have structural adaptations to hot, dry climates 2. CAM photosynthesis Transport of sugars translocation transport of products of photosynthesis by phloem to the rest of the plant -in angiosperms, sieve tube members are specialized cells that function in translocation -pores in sieve plates allow phloem sap to move freely along sieve tube -phloem sap=sucrose, minerals, amino acids, and hormones -Considered source-to-sink transport -source organ where sugar is produced (usually leaves) or where starch is broken down (leaves, stems, roots, etc.) -sink organ that consumes or stores sugar -ex: growing parts of plants, fruits, non-green parts of plants, stems, trunks, etc. -sugar flows source sink -can depend on season -ex: tuber sink when storing in summer, source in spring -minerals are also transported to sinks 13

14 -sink is usually supplied by nearest source -direction of phloem can change, depending on location of source and 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 from sight of production to sieve tube members by active transport -this develops a higher concentration of solutes in the sieve tube 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 sieve tube member 3. Pressure in source sieve tube member move water and sugars to sieve 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 by nearby cells -by active and passive transport -water diffused out of sieve tube members, relieving pressure 14