Transport of Water and Nutrients in Vascular Plants: Xylem and Osmosis - Prof. Arthur E. S, Study notes of Biology

Download Transport of Water and Nutrients in Vascular Plants: Xylem and Osmosis - Prof. Arthur E. S and more Study notes Biology in PDF only on Docsity! Chapter 36 TRANSPORT IN VASCULAR PLANTS Plants absorb water and minerals through their roots and transport them to the leaves and stems for metabolic use, e. g. photosynthesis. Xylem transport water and minerals from roots to shoots. Phloem transport sugars from where they are produced or stored to where they are needed for growth and metabolism. PHYSICAL FORCES DRIVE THE TRANSPORT OF MATERIALS IN PLANTS. A variety of physical processes are involved in the transport of materials in plants. The transport in plants occurs at three scales: 1. Within cells, e. g root hairs. 2. Short-distance transport from cell to cell. 3. Long-distance transport within xylem and phloem. SELECTIVE PERMEABILITY OF MEMBRANES Transport at the cellular level depends on the selective permeability of membranes. Biological membranes are usually permeable to small molecules and lipid-soluble substance.  Water, gases (O2, N2, CO2, CO), small polar molecules (glycerol), larger non-polar molecules (hydrophobic substances like hydrocarbons). Biological membranes are impermeable to and use proteins to transport the following types of molecules,  Ions, amino acids and sugars, e. g. glucose. Transport methods (review chapter 7): 1. Simple diffusion: Along concentration gradient. 2. Facilitated diffusion: protein channels, along concentration gradient. 3. Carrier-mediated transport (active transport): pumps require ATP. 4. Cotransport 5. Endocytosis 6. Exocytosis Proton pumps Proton pumps play an important role in transport across the membrane. The energy stored in a proton gradient is used to transport solutes across the membrane. In chemiosmosis, the proton gradient is used to synthesize ATP. Because the proton pump moves positive charge (H+) out of the cell, the pump contributes to a voltage known as a membrane potential, a separation of opposite charges across a membrane. The proton gradient across the membrane creates membrane potential that can be harnessed to perform cellular work. Plants use the energy stored in the proton gradient and membrane potential to drive the transport of many different solutes, e. g. chemiosmosis, cotransport. Osmosis Osmosis is the passive transport of water across a membrane. Water will move across a cell membrane in the hypotonic (low solute concentration) → hypertonic direction (high solute concentration).  Remember that high solute concentration means relatively low water concentration In plants, the presence of the cell wall that limits the expansion of the cell adds another factor that affects osmosis. The combine effect of solute concentration and pressure makes what is called the water potential, represented by the letter psi, ψ. Water moves across the membrane from the region of high water potential to that of low water potential. Water potential is measured in megapascals, MPa.  1 MPa = 10 atmospheres or 14.5 pounds/inch2. Examples:  Your lungs exert less than 0.1 MPa.  A car tire is usually inflated to about 0.2 MPa.  Water pressure in home plumbing is about 0.25 MPa.  Plant cells exist at about 1 MPa. Water potential for pure water in an open container is standardized a 0 MPa. Adding solutes lowers the water potential because the water molecules surrounding the solute have less freedom of movement due to intermolecular attractions. Any solution at atmospheric pressure has a negative water potential.  There is an inverse relationship of ψ to solute concentration (osmotic potential). Plant cells have a cell wall, which adds another factor affecting osmosis: physical pressure. Increasing the pressure on water increases the ψ.  There is direct relationship of ψ to physical pressure. Cells in the root pump ions into the root stele. The endodermis prevents these ions from leaking back into the cortex. The water potential in the stele is lowered and water flows in from the root cortex generating a positive pressure that forces fluid up the xylem. This pressure is called root pressure. Root pressure pushes water from the root up the stem.  Not strong enough to push the up tall plants.  It is very low or non-existent during the summer months.  Movement of water is greatest in the summer months when root pressure is the lowest. Guttation is the release of water droplets through small openings on leaves of plants. Guttation is the result of root pressure. Root pressure can force the water up a few meters only. It is not the main mechanism that brings water to the top of the plant but it contributes. Many plants do not generate root pressure at all. 2. Transpiration-Cohesion-Tension Theory Also known as the Transpiration-Cohesion Theory. Transpirational pull Water is constantly being lost through the stomata. This water is replaced with water vapor from the mesophyll cells. 1. On most days, the air is drier than the air in the mesophyll air spaces; the outside air has a lower water potential than the air inside the leaf. 2. Water vapor in the air spaces diffuses down its water potential and out of the leaf through the stomata. 3. As water evaporates, it is replaced by water found in the water film that surrounds the mesophyll cells. Water adheres to the hydrophilic areas of the cellulose microfibrils of the cell wall. 4. As water evaporates, a meniscus is formed in the interphase of the water and air in the air spaces of the mesophyll. This curvature increases the surface tension and the rate of transpiration. 5. Cohesive forces also operate on the surface of the water film. 6. These two forces, adhesion and cohesion, create the meniscus that has a negative force. 7. This negative pressure draws water out of the xylem through the mesophyll, and toward the cells and surface film bordering the air spaces near stomata. See figure 36.14 on page 774.  There is a gradient in water potential from the atmosphere down to the soil.  The atmosphere has very negative water potential.  Leaves have higher water potential than the atmosphere and lose water to it.  Stems have higher water potential than the leaves; the roots higher than the stem; and the soil higher than the roots. The gradient creates a pull of the column of water in the xylem due to the hydrogen bonds that exist between the water molecules (cohesion). Adhesion of the water molecules to the xylem walls maintains an unbroken column of water. The walls of the vessels and tracheids are hydrophilic and increase the adhesion of water molecules. The transpirational pull is transmitted from the leaves to the root tips and even into the soil solution. The plant does not spend any of its energy in bringing the water up to the top. Solar energy drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by maintaining a high humidity in the air spaces with a leaf. CONTROL OF TRANSPIRATION Leaves have a high surface area-to-volume ratio. This ratio facilitates the uptake of CO2 needed in photosynthesis and the release of the waste product, O2. Photosynthesis consumes CO2 and produces O2. Both gases diffuse in and out of the leaf respectively through the stomata. The spongy mesophyll of the leaf increases the surface area exposed to CO2 but also increases the surface area of evaporation. The internal surface area of the leaf may be 10 to 30 times greater than the external surface area. Stomata: major pathway of water loss. A plant loses 95% of the water through the open stomata. The waxy cuticle covering most of the leaf surface prevents evaporation. The stoma is the opening located between two kidney or dumbbell-shaped guard cells.  Dumbbell shape in monocots.  Kidney shape in dicots. The guard cells are suspended over an air chamber by subsidiary epidermal cells. Guard cells control the diameter of the stoma by changing shape. When the guard cells become turgid, the stoma opens. When flaccid, the stoma closes. Potassium ion mechanism. The changes in turgor pressure in the guard cells are the result of the reversible uptake and loss of K+.  Light triggers an influx of K+ into the guard cells.  It occurs through active transport; ATP required. A proton pump is probably involved and K+ move through channels driven by a membrane potential.  Osmotic pressure decreases and water moves into the guard cells.  The increase turgidity of the cells causes a change in shape and the stoma opens.  Most of the K+ are stored in the central vacuole. The tonoplast plays a role here.  Regulation of aquaporins may also be involved by varying the permeability of the membranes to water. Opening of the stomata is most pronounced in blue light, and to a lesser extent in red light.  Light  proton pump moves H+  K+ transported into the cell  water diffuses into out of the guard cell through specific K channels the guard cells  guard cells change shape and open the stoma. The stoma may close by a reversal of the process when light decreases. Loss of turgidity closes the stoma. Stimuli to open and close the stomata: 1. Stomata are open or closed according to the physiological needs of the plant.  Photosynthesis: depletion of CO2. A low concentration of CO2 in the leaf induces stomata to open even in the dark.  Transpiration: loss of water causes loss of turgor throughout the plant.  The hormone abscisic acid is produced in response to water deficiency and causes the guard cells to close.  Increase in temperature increases cellular respiration and CO2 production. In mesophytes, the stomata are usually open during the day and closed at night.  CAM plants do the reverse. 2. Light triggers the intake of K+ by the guard cells.  There are blue-light receptors in the membrane of the guard cells that trigger ATP- powered proton pumps, which in turn promotes the uptake of K+ ions. 3. An internal clock causes the stomata to open and close periodically.  Cycles of 24 hours are called circadian rhythms. Xerophytes have leaf adaptations that reduce the rate of transpiration.