LECTURE 4: SHORTDISTANCE TRANSPORT OF NUTRIENTS http://vitae-scientia.tumblr.com/ COMPETENCY After completing this Lecture and mastering the lecture materials, the competency expected to develop includes the ability To explain the short distance routes of nutrient transport in plants To explain the process of membrane transport for plant nutrients 1
LECTURE FLOW Transport Routes Symplast pathway Apoplast pathway Transmembrane Pore size WFS & DFS CEC of Roots Membrane transport Membrane Characteristics Downhill and Uphill transport Proton pumps TRANSPORT PATHWAY 1. TRANSPORT ROUTES a. Three routes are available for lateral transport, the movement of water and solutes from one location to another within plant tissues and organs. a. Symplast route b. Apoplast route c. Transmembrane route The Symplast route is the route via the symplast that requires only one crossing of a plasma membrane. After entering one cell, solutes and water move from cell to cell via plasmodesmata. 2
b. The apoplast route is the route along the apoplast, the extracellular pathway consisting of cell wall and extracellular spaces. c. Water and solutes can move from one location to another within a root or other organ through the continuum of cell walls before ever entering a cell. The transmembrane route is the route where substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by same mechanism. This transmembrane route requires repeated crossings of plasma membranes. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3
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2. Cell Wall Pores Primary cell wall consist of a network of cellulose, hemicellulose (including pectins), and glycoprotein. The middle lamella is a pectin layer which cements the cell walls of two adjoining cells together 5
Cellulose is made of repeating molecules of glucose attached end to end in a β(1-4) linkage. These long thin cellulose molecules are united into a "Microfibril". These microfibrils are arranged in a very regular, ordered arrangement and because of this they exhibit almost "crystalline" properties. These crystalline regions of the microfibrils are known as micelles. The microfibrils wind together to form fine threads which may coil around one another like a cable. Each "cable" is called a "Macrofibril". The network contains pores, the so-called interfibrillar and intermicellar spaces which differ in size. A maximum diameter of 3.5-3.8 nm has been calculated for root hair cells of radish, and maximum values for plant cell walls are in the range of 5.0 nm. 6
The dimensions of hydrated ions such as K+ and Ca2+ are small compared with the diameter of these pores, thus the pores themselves should not restrict the movement of these ions within the free space The volume of roots available for the passive transport (free space) is about 10% of total space in young roots Materials Rhizodermal cell wall Cortical cell wall Pores in cell wall Sucrose Hydrated ions K+ Ca2+ Diameter (nm) 500-3000 100-200 <5 1.0 0.66 0.82 In the free space of the roots, the carboxylic groups (RCOO-) act as cation exchangers, and cations can accumulate in a nonmetabolic step in the free space, where anions are repelled Plant species differ considerably in their cation exchange capacity (CEC), that is the number of cation exchange sites in their cell walls CEC (meq/100 g As a rule, the CEC of Plant species dry weight) dicotyledonous species Wheat 23 is much higher than Maize 29 that of Bean 54 monocotyledonous Tomato 62 species CEC = Cation-Exchange Capacity 7
In the free space of the roots, the carboxylic groups (RCOO-) act as cation exchangers, and cations can accumulate in a nonmetabolic step in the free space, where anions are repelled Because of the negative charges in the cell walls (the apoplast), the terms were introduced Apparent Free Space (AFS) AFS = WFS+DFS WFS = Water Free Space DFS = Donnan Free Space R-COO- - Micropore 3. + ++ ++ ++ ++ ++ Macropore + + + + + + - ++ + Anion - + + +- DFS Indiffusible anions + - + + ++ Cation WFS Root Zone Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located. Root hairs are extensions of epidermal cells that account for much of the surface area of roots. The soil solution flows into the hydrophilic walls of epidermal cells and passes freely along the apoplast into the root cortex, exposing all the parenchyma cells to soil solution and increasing membrane surface area. 8
PLASMA MEMBRANE 1. Chemical Composition Membranes are typically composed of two classes of compounds: protein and lipid. Carbohydrate comprise only a minor fraction of membrane Its framework consists of a double layer of phospholipids. The major types of proteins are tightly coiled, rod-shaped, fibrous proteins, and the more compact, globular-shaped integral protein and pheripheral proteins Lipid utama dari Membran: phospholipid, glucolipid & sulfolipid Lipid utama lain : Sterol khususnya cholesterol pada khewan, dan bsistosterol pada tanaman 9
2. Permeability Because the cell membrane is mostly lipid, it only allows lipid-soluble substances (e.g. oxygen, carbon dioxide, and steroids) to go through. Water-soluble substances (e.g glucose, amino acids, ions, and water) need the help of proteins transporters Permeability (cm.s-1) Therefore, the main sites of selectivity in the uptake of cations and anions as well as solutes in general are located in the plasma membrane of individual cells 10
3. This can be seen that most of the Ca2+ (45Ca) taken up within 30 min (influx) is still readily exchangeable (efflux) and is almost certainly located in the AFS In contrast, only a minor fraction o0f the K+ (42K) is readily exchangeable within the 30-min period, most of the K+ having already been transported across the membranes into the cytoplasm and vacuoles ( inner space ) Passive and Active Transport Downhill & Uphill transport from Knox, Ladiges & Evans 11
4. Proton pumps Proton pumps play a central role in transport across plant membranes The most important active transporter in the plasma membrane of plant cells is the proton pump. 1. 2. It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell. This creates a proton gradient because the H+ concentration is higher outside the cell than inside. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 12
Tranport kation dan anion lintas membran plasma ke cytoplasm dapat digerakkan oleh H+-ATPase dengan berbagai cara. Countertransport (antiport) : transport kation dengan fluks H+ (1:1) dengan arah yang berlawanan Cotransport (symport) : transport anion dan H+ dengan arah yang sama H+-ATPase membran plasma : pengeluaran H+ dari cytoplasm yang distimulasi oleh kation monovalen; tidak sensitif terhadap anion H+-ATPase tonoplast : transport H+ ke vakuola ; sensitif pada anion (distimulai Cl- & dihambat NO3-) relatif tidak sensitif pada kation 13
TRANSMEMBRANE PROTEINS Nomenclature of transport proteins. Schematic representation of primary active transport mechanisms, such as ABC transporters (e.g., glutathione conjugate pump), metal transporters (e.g., Ca 2+-ATPase) and H+-ATPases, secondary active transport mechanisms, such as the K +/H+ symporter or the Na+/H+ antiporter, and passive transport mechanisms, such as the NH4 + carrier and the K+ channel. Figure adapted from White (2003). 14
3. It also creates a membrane potential or voltage because the proton pump moves positive charges (H+) outside the cell, making the inside of the cell negative in charge relative to the outside. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work. These are often used to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of potassium ions (K+) by root cells. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 15
The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3- or sucrose. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The role of protons pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying principle in cellular energetics. In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energyconsuming processes. The ATP synthases that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis function somewhat like proton pumps. However, proton pumps normally run in reverse, using ATP energy to pump H+ against its gradient. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 16
Examplen of Proton pump activity and apoplastic ph. Apoplastic ph of substomatal cavities of Vicia faba as influenced by FC and cyanide, added to the chamber harbouring the cut leaf petiole. Upper curve: acidification of the apoplast following the addition of 5 μm fusicoccin (FC). Middle curve: effect of 1 mm NaCN and subsequent light off and light on. Lower curve: alkalinization of the apoplast by 10 mm NaCN and effect of light off and light on both in the presence of NaCN. 5. Representative of at least three equivalent kinetics, each. Membran Potential If only one type of ion penetrates the membrane, the Nernst equation can be used to calculate the membrane potential with a fair degree of accuracy. The equation is based upon the idea that at equilibrium the concentration gradient forces acting on ions will be exactly balanced by opposite electrical forces. Em C 2.3RT log 2 zf C1 Em = membrane potential R = gas constant (1.9787 cal.mol-1,0k-1) z = ion charge T = 0K F = Faraday constant (23.06 cal.mv-1.mol-1) C2 = internal concentration C1 = external concentration 17
A simple calculation of the Nernst potential E can be made as a function of H+ concentration in two compartments as follows: E = (RT/zF ) ln (C0/Ci) where R is the gas constant, T is the absolute temperature, z is the charge of the ion, and F is the Faraday constant. C0 and Ci represent the ion concentrations outside and inside the cell respectively (considered here as two compartments). Since H+ is the monovalent cation, the value of z would be 1 (z = 1). The numerical values of the constants R, F, and T at 30 C (303 K) can be substituted, and after converting from natural logarithm to log10 (x2.303), we obtain: E = 60 log (C0/Ci). Suppose the H+ concentration across the membrane is 10 5 M (ph 5.0) and 10 7 M (ph 7.0) as shown in Figure 2, then, E = 60 log(10 5 M/10 7 M) =log (100), and therefore, the membrane potential inside the cell is 120 mv. For z = 1 or 2 at 250C, Em Em C C 2.3 * 1.987 273 25 log 2 59.06 log 2 1 * 23.06 C1 C1 C C 2.3 * 1.987 273 25 log 2 29.53 log 2 2 * 23.06 C1 C1 18
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