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LU2: BIOCHEMISTRY, CELL STRUCTURE & FUNCTION: TRANSPORT SYSTEM & GASEOUS EXCHANGE in Plant Mohamad Fhaizal bin Mohamad Bukhori [email protected] 012-3942055 Pejabat Akademik...
LU2: BIOCHEMISTRY, CELL STRUCTURE & FUNCTION: TRANSPORT SYSTEM & GASEOUS EXCHANGE in Plant Mohamad Fhaizal bin Mohamad Bukhori [email protected] 012-3942055 Pejabat Akademik 2 LEARNING OBJECTIVES 1. XYLEM and transpiration. ▪ Describe the basic principle of water and minerals uptake by roots. ▪ Describe the basic concept of root pressure and theory of capillarity: adhesion-cohesion tension. ▪ Explain the basic mechanism of transport based on water potential. 2. PHLOEM and translocation. ▪ Describe the basic concept of mass flow and pressure flow hypothesis. ▪ Explain the basic mechanism of active transport and pressure flow model. PLANT TRANSPORT SYSTEM Occurs in 3 LEVELS 1. The uptake and loss of water and solutes from cell. 2. Transport of water and substances from cell to cell. 3. Long distance transport with sap in xylem and phloem along the whole plant. Xylem Transports sap (water) from roots to Transports sap (water and leaves. sugar) from source to sink. Complex principal water conducting tissue in vascular plants. It is also involved in conducting dissolved minerals, in food storage, and in supporting the plant body. Source ▪ Location/region of where photosynthesis occurs and organic solutes are synthesized. ▪ E.g., Green leaves of plants. Organic solutes - sucrose and amino acids loaded into sieve tubes of phloem. Sink ▪ Growing shoot, root regions, developing flowers, fruits, storage organs such as tubers, bulbs etc. ▪ E.g., Sucrose is unloaded from sieve tube. 1. XYLEM AND TRANSPIRATION ▪ Plants imbibe and transpire water more than animals do, as they have no re-circulation system. About 99% of all water entering the roots, leave the leaves via the stomates (transpiration) without ever taking part in metabolism. ▪ A single plant can transpire ±60 L of water in one growing season. ▪ Water movement is due to differences in potential between soil, root, stem, leaf and atmosphere. ▪ Under normal condition, the water potential in soil is higher than in root cell cytosol, resulting in water flowing to follow the potential gradient. This is known as bulk flow, and it is the primary force driving water through xylem. 1. XYLEM AND TRANSPIRATION ▪ The aqueous solution of dissolved minerals in the xylem is known as xylem sap. ▪ Bulk flow is much faster than diffusion or osmosis, reaching the rate of 15-45 moles/hour, depending on environmental conditions and the size of the xylem lumen. ▪ Xylem raises water up to 350’ above the ground in some of the largest trees on earth. ▪ But, is the water being pushed or pulled along the xylem for transpiration? Absorption and Transport of Water and Minerals in Plants ▪ Mineral's ions are needed for the plant’s metabolism. ▪ Solutes tend to diffuse down concentration gradients. ▪ Root hairs increase the surface area for absorption. ▪ Surface cell membranes and tonoplast of root cells have transmembrane proteins. ▪ Transmembrane proteins functioned as channels, carriers or pumps to move solutes into cells and from cell to cell. Passive Transport ▪ Osmosis: Diffusion across a membrane. ▪ Generally slow, unless solutes travel through transport proteins (or selective channels: gated-environmental stimuli open or close) in the membrane. Active Transport ▪ Require energy to move solutes up to concentration or charge gradient. ▪ E.g., Proton pump create membrane potentials; potential energy can be used to perform cellular work. The membrane potential provides the energy to uptake some minerals, e.g., K+ ions The membrane potential provides the energy for co-transport of ions up their concentration gradients, as H+ ions move down theirs. The membrane potential provides the energy for co-transport of some neutral molecules (e.g., sugar) up their concentration gradients, as H+ ions move down theirs. Root Pressure : Pushing Xylem Sap ▪ Water enters the root because the water potential of the root tissues is almost always lower than of the soil, with its high dissolved mineral content. ▪ Water entering the stele may travel via one of the 3 routes defined: Apoplast pathway (passive diffusion: important route) ▪ Water moves across spaces between cellulose fibres in the cell wall. Symplast pathway (active transport) ▪ Water moves through cytoplasm of the cells. ▪ Cytoplasmic strands in the plasmodesmata allow water to move between cytoplasm from one cell to adjacent cell down a water potential gradient. Vacuolar/Transmembrane pathway ▪ Water moves through vacuoles, cytoplasm & cell wall. At night; ▪ stomates are usually closed. ▪ endodermis prevents leakage of ions out of the stele into other tissues. ▪ resulting decrease in water potential in the stele (due to the accumulation of ions). ▪ resulting flow of water from the cortex into the stele (root pressure: xylem sap being pushed up the xylem because of incoming water from the root). ▪ root pressure, exerted from below, is positive pressure potential, since potential increases as one moves up the stem. ▪ root pressure is sufficient to lift water no more than a few feet above the ground. Guttation ▪ In a small plant, root pressure could result in potentially harmful water pressure buildup at night, when stomates are closed. ▪ Many herbaceous plants have special openings on the leaf margins called hydathodes. ▪ These allow root pressure water to escape (forming lovely little beads of dew overnight) and preventing cell rupture due to too much water pressure. ▪ This process is known as guttation, and its results are generally observable only in the early morning, when humidity is relatively high. Water Transport in Plant Roots ▪ The characteristics of root hairs: i. Thin cell walls. ii. Large surface area over volume ratio for absorption of water and minerals. ▪ Water [high water potential in Soil > Root hair] → Root hair → Cortical cell → Cell to cell. ▪ Water is drawn by osmosis process; i. Root cells actively pump ions (use ATP) into cells. ii. High concentration of ions creates a greater osmotic pressure in plant than surrounding soil water; water moves into cells by osmosis. ▪ 3 main routes for movement of water across root: i. Apoplast pathway ii. Symplast pathway iii. Vacuolar pathway Shoot Tension : Pulling Xylem Sap 1. Water movement in plants not only from below, via +ve pressure, but also by pulling from above via -ve pressure potential: potential decreases as one (water) moves up the stem. 2. This occurs via transpiration. 3. The air spaces inside spongy mesophyll are quite humid, as they are constantly in contact with moist cell walls and vascular tissue filled with xylem sap. 4. On typical, non-rainy days, the water potential of the atmosphere is far lower (more -ve) than that of the spaces inside the mesophyll. 5. This means that water will want to travel out of the stomates to the area of relatively low water potential. 6. As one moves down the plant, water potential increases. Here's a hypothetical array of water potentials in a soil and plant system: Theory of Transpiration Water evaporate from cell walls of palisade and spongy mesophyll cells into sub-stomatal cavity; ▪ Lower the water cavity. ▪ Drawn water from neighboring mesophyll cells that has high water potential. ▪ Until water is drawn from xylem vessels in the leaf (via apoplast / symplast / vacuolar) Cohesion: Water molecules form continuous water column in the xylem vessels ≈ by hydrogen bond that formed between the water molecules. Adhesion: Attractive forces between water molecules and the hydrophilic xylem walls ≈ prevent water column from moving down. Lower Ψ at the top is the tension that pulls water up from the bottom Transpiration creates a water pressure gradient 4. Endodermal cells actively secrete mineral into xylem 3. Water potential decrease 2. Water from root cells drawn into xylem and produce root pressure 1. Root pressure – a +ve hydrostatic pressure to push the water up the stem i. Evaporation at the surface of the leaf keeps the water column moving. ii. This is the strongest force involved in transpiration. Cohesion and adhesion cause water to crawl up narrow tubes. The narrower the tube the higher the same mass of water can climb. i. Cohesion between water molecules creates a water chain effect. ii. As molecules removed from the column by evaporation in the leaf, more are drawn up. i. Pressure differences created by transpiration draws water out of the roots and up the stems. ii. This creates lower water pressure in the roots, which draws in more water. Transpiration ▪ Loss of water as water vapour from plant to atmosphere (99%). ▪ More water is loss through stomata, little by cuticle layer, lenticels (woody stems). ▪ 1% photosynthesis or other metabolic process, to maintain turgidity. ▪ Rate regulated by two guard cells surrounding each stoma (or stomate). ▪ Guard cells open when water moves into cells by osmosis (i.e., cells are turgid). Turgor results from active uptake of Potassium (K+) ions, stimulated by light; and subsequent influx of water by osmosis. Factors Effecting Rates of Transpiration Importance of Transpiration ▪ Maintain water potential gradient that moves water and dissolved minerals from roots to aerial part of plants. ▪ Evaporation of water from leaves absorbs latent heat of vaporization, cooling leaves during hot and dry weather. External Internal 1. Temperature 1. Leaf Surface ▪ Increase kinetic energy, increase movement water ▪ Increase leaf surface, increase transpiration. molecules, thus, diffuse through stomata faster. ▪ Less stomata, thin needle-shaped and rolled leaves, reduce ▪ High temperature, low humidity, increase rate of diffusion of surface area exposed to air ≈ decrease evaporation of water vapour from leaf. water from leaves. 2. Light 2. Location of Stomata ▪ Day: High light intensity, stimulates opening of stomata ≈ ▪ Dicotyledonous plants have stomata on the lower leaf increase transpiration. surface ≈ reduce transpiration. ▪ Night: Low light intensity, stimulates stomata to close ≈ reduce transpiration. 3. Humidity 3. Density of Stomata Pores per unit of leaf and size of stomatal pore. ▪ Low humidity (High vapour pressure). ▪ Increase gradient of water vapour saturation between substomatal cavity and atmosphere ≈ Increase transpiration. 4. Air Movement ▪ Carries away water vapour outside stomata. ▪ Thus, creates steep concentration gradient and water vapour from substomatal cavity diffuses rapidly to outside. ▪ High dry, windy condition, increase transpiration. ▪ Low air movement, water vapours accumulate around stomata, reduce concentration gradient, reduces water loss. 5. Water Supply ▪ Rate of transpiration higher than rate water absorption from soil ≈ stomata close and reduce water loss by transport. 4. Water from top of xylem vessel reduces 3. Reduce hydrostatic pressure 2. Water column under tension and is pulled from roots to leaves 1. Movement of water up the xylem vessel is by mass flow. Thus, what are the characteristic of xylem vessel in facilitating water transport in plants? Characteristic of Xylem Vessel 1. Hollow, tubular cells. 2. Composed of 5 cell types: tracheids, vessels, parenchyma, sclereids (short sclerenchyma cells), and fibers. ▪ The tracheids (small diameter) and vessel (large diameter) elements form the bulk of the tissue. They are heavily strengthened and are the conducting cells of the xylem. ▪ Parenchyma cells are involved in storage, while fibers, and sclereids provide support. 3. Dead at maturity: no protoplasm, forming a hard skeleton that serves only to support the plant. 4. Lignified cell walls: to give strength (withstand the tension and prevent from collapsing) and makes xylem waterproof; vessels does not collapse under tension and water does not steep out. * Pits: present in the lignified walls, for water to move out laterally to neighbouring cells. 5. Long continuous tube. 6. Narrow and cappilarity: increases adhesion between water molecules and walls of xylem vessels. 2. PHLOEM AND TRANSLOCATION ▪ Movement of organic solutes, e.g., Sucrose, amino acids, organic acids, K, Cl, PO, Mg from leaves (source, site produced or stored) to sieve tubes to be carried to other parts of plant (sink, site used). ▪ Unlike movement of xylem sap, movement of phloem sap requires energy expenditure on the part of the plant. How solutes moved around the phloem? Pressure Flow Model EXPLANATION 1 ▪ Explained by consideration of osmosis, applied to solutions of two sugar solutions across a semi-permeable membrane. i. Carbohydrates actively transported into phloem at source. ii. High concentration of carbohydrates causes greater osmotic pressure in phloem; water moves in from adjacent xylem by osmosis. iii. Water influx creates (turgor) pressure inside phloem; pushing water and dissolved carbohydrates through phloem. iv. At sink, carbohydrates actively removed from phloem; reducing osmotic pressure in phloem. v. water leaves phloem and reenters xylem, maintaining an osmotic pressure gradient between sources and sinks. Characteristic of Phloem Vessel 1. Sieve Tubes ▪ Long cylindrical structure consisting narrow living, elongated sieve elements joined. ▪ End walls meet to form sieve plate and plasmodesmata enlarge to form sieve tube. When mature, ▪ Nucleus, Golgi apparatus and ribosome degenerates. ▪ A thin layer of cytoplasm and some small mitochondrion found lining inside of the thin cellulose cell wall. This presents less barrier to flow of sap through sieve element. Sieve tubes ruptured, ▪ Protein strands form plugs to block the pores. ▪ Callose deposits are then formed across the sieve plates to prevent loss of phloem sap. 2. Companion Cells ▪ Associated with sieve tubes. ▪ Smaller, shorter. ▪ Contains large nucleus, dense cytoplasm with numerous mitochondrial ribosomes and endoplasmic reticulum. ▪ Cells metabolically very active, connected to sieve tube elements by plasmodesmata.
