Transport in plants

• Higher will increase rate of transpiration as increased water vapour gradient between inside of leaf and outside air.
• Transport systems in multicellular plants are necessary to meet metabolic demands and ensure efficient transport of substances up and down the plant.
• Aphids can demonstrate translocation of solutes in phloem, with pressure in phloem forcing sap out through stylet, and pressure and flow rate is lower closer to sink than to near source.
• The flow of sugars in phloem is 10000 times faster than it would be diffusion alone, suggesting an active process is driving mass flow.
• The concentration of sucrose in phloem sap is higher near source than sink.
• The size of a plant determines the need for transport systems as large plants need to transport substances effectively from the roots to the leaves.
• Small specific activity to volume ratio (SA:V) in plants means that diffusion alone can supply cells with necessary substances.
• The vascular system in roots, stems and leaves of herbaceous dicotyledonous plants is made up of xylem and phloem transport vessels arranged together in vascular bundles.
• Xylem is a non-living tissue that transports water and mineral ions up plants and provides support.
• Xylem is made up of several types of cells, most of which are dead when functioning in the plant.
• Xylem forms long, hollow structures made by several columns of cells fusing together end to end.
• Xylem parenchyma packs around xylem vessels, stores food and contains tannin deposits.
• Xylem has lignified secondary walls that provide extra mechanical strength but do not transport water.
• Phloem is a living tissue that transports food in the form of organic solutes up and down plants from leaves where it’s made by photosynthesis.
• Phloem supplies cells with sugars and amino acids needed for cellular respiration and synthesis of other useful molecules.
• The main transporting vessel in phloem are sieve tube elements, many cells joined end to end to form long hollow structure.
• In areas between cells, walls of sieve tube elements become perforated to form sieve plates, allowing phloem contents to flow through.
• Companion cells form with sieve tube elements, linked to them by many plasmodesmata, supporting sieve tube cells and containing supporting tissues such as fibres and sclereids.
• Transpiration is a consequence of gas exchange where stomata are opened by guard cells and CO2 and O2 are exchanged along with water vapour inside leaf by diffusion.
• Stomata open and close to control the amount of water lost by plant, opening during the day with photosynthesis and closing at night with respiration.
• Transpiration is a turgor driven process, when turgor is low, asymmetric config of guard cell walls closes pore.
• When environmental conditions are favourable, guard cells pump in solutes by active transport, increasing turgor.
• Cellulose hoops prevent water swelling, guard cells extend length, when water is scarce hormonal signals from roots trigger turgor loss from guard cells, closing pore, conserving water.
• Factors affecting transpiration include light, humidity, temperature.
• Light is required for photosynthesis and so stomata open for gas exchange.
• In the dark, most stomata are closed, reducing gas exchange.
• Humidity affects the rate of transpiration as a higher humidity will lower the rate of transpiration as a reduced water vapour gradient between inside of leaf and outside air.
• Some poisons affect mitochondria and the production of ATP, suggesting a role for active transport in root pressure.
• Water moves through the cell walls and intercellular spaces of the xylem, filling spaces between the loose, open network of fibres in the cellulose cell wall.
• As water molecules move into the xylem, more are pulled through the apoplast behind them, creating tension in the xylem and giving the plant a continuous flow of water through an open structure with little to no resistance.
• If levels of oxygen or respiratory substances fall, root pressure does too.
• The rate of water uptake is calculated by measuring the distance moved by an air bubble and the time it takes.
• Mineral ions are transported into root hair cells by active transport while water is transported via osmosis.
• Water must pass through a selectively permeable cell surface membrane to enter the symplast pathway, preventing the entry of toxic solutes from reaching living tissues.
• The solute conc in the cytoplasm of endodermal cells is relatively dilute compared to cells in the xylem, increasing the rate of water moving into the xylem by osmosis from the endodermis through the symplast pathway.
• Potometer measures the rate of water uptake, which is similar to the rate of transpiration as 99% of water usually is lost from transpiration.
• Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem and move up the plant.
• Increase in KE of H2O molecules leads to increased rate of evaporation from spongy mesophyll cells into air spaces of leaf.
• All joints should be sealed with Vaseline to prevent air bubbles and the plant should be cut under water to prevent air entering the xylem and no water on the leaves.
• Water moves through the continuous cytoplasm of living plant cells that are connected through plasmodesmata by osmosis.