Plant Transport

• Water moves through the xylem vessels by cohesion tension theory.
• Xylem vessels have thick walls made up of lignin, which provides strength and support.
• The water molecules are attracted to one another, forming chains or columns called capillary action.
• Transpiration is the process whereby plants lose water vapor through their stomata.
• As water evaporates from leaves, it creates negative pressure (suction) that pulls more water up into the plant.
• Sieve plates are the perforated ends of sieve tube elements
• The symplastic route is where water diffuses via osmosis across plasmodesmata between cytoplasms down water potential gradients from root hair cell to xylem or between leaf cells
• The apoplastic route is where water diffuses via osmosis through gaps in the cell walls of plants down a water potential gradient from root hair cells to the xylem
• To measure the rate of transpiration, use equation pi x r squared x length travelled and divide by time taken
• Set up a potometer to measure the rate of transpiration underwater to keep a closed system
• Evidence for cohesion tension theory:

  When broken the xylem will not leak

  When broken the xylem cannot transport water through the transpiration stream

  Tree trunks are thinner during the day due to evaporation of water through stomata
• Sucrose enters sink companion cells via active transport using ATP
• Sucrose is diffuses via facilitated diffusion from source cells into companion cells
• H+ ions are actively transported into the cell wall spaces of companion cells using ATP, causing sucrose molecules to diffuse into the sieve tube elements as H+ ions diffuse via facilitated diffusion via cotransport
• Sucrose molecules don’t move entirely by mass transport as they don’t move to where they are needed - they are evenly distributed in a plant
• Cohesion tension theory is when water molecules undergo cohesion between themselves and undergo adhesion between the water and the xylem walls, causing water to be pulled up the stem. Negative pressure as water evaporates from stomata creates tension
• The phloem has smaller circles in the vascular bundle surrounded by parenchyma
• The apoplastic route is faster than the symplastic route
• During translocation, solutes can travel both up and down a plant but not in the same sieve tube
• Mass flow theory states that, as sucrose diffuses into the sieve tube elements, the water potential becomes more negative causing water to diffuse by osmosis into the phloem creating high hydrostatic pressure
• As sucrose diffuses into sink cells, their high sucrose concentration lowers their water potential causing water to diffuse via osmosis into sink cells and back into the xylem, lowering the hydrostatic pressure
• Translocation moves sucrose down a hydrostatic gradient
• Sink cells include the tips of roots and stems due to their high rate of respiration and storage of glucose as starch
• The phloem can be identified using tracers, by giving a plant radioactive carbon and allowing it to assimilate this carbon into sucrose, thin slices of the plant can be X rayed to see the phloem
• Dicots have regularly arranged vascular bundles separated by cambium
• Downward phloem flow will stop at night due to no photosynthesis so no sucrose being produced in source cells
• Using sucrose is more efficient as a transport sugar as it is a disaccharide so stores more energy than glucose and it is non reducing so won’t react
• Evidence for mass flow theory: sap will leak from the phloem when cut indicating it is under pressure, concentration of sucrose in source cells is higher, downward phloem flow stops at night, sucrose increasing in the leaf leads to sucrose increase in the phloem after some time, metabolic poisons inhibit translocation
• Evidence against mass flow theory: sieve tube elements seem to prevent mass flow, not all solutes move at the same speeds, sucrose is delivered evenly rather than where most needed
• The apoplastic route is blocked by the casparian strip