transport in plants

• Xylem is the specialized tissue responsible for transporting water and minerals from the roots to the rest of the plant.
• the air surrounding the leaves is dry and therefore water potential is lower than the leaf air spaces therefore water leaves the stomata by transpiration along a water potential gradient.
• the shoot was cut and out into the potometer under water so there will be no air bubbles in the xylem vessels to block the water.
• repeated measures were done to increase reliability
• the rate of transpiration is higher during the daylight/afternoon as stomata are open so water can evaporate. Since the rate of transpiration is high, the rate of flow of water in xylem is high since water is being sucked and negative pressure is present.
• Turgid pressure - POSITIVE while Tension pressure - NEGATIVE (as there is a pull/suction)
• xylem cells are under tension as water is being lost - negative pressure.
• • The cell wall of xylem consists of lignin, which is very adhesive and acts as an effective water-attracting surface to help keep the water column in place and prevent collapse
• Transpiration-cohesion-tension mechanism:

  • Transpiration causes a negative pressure (tension) that generates a pulling force on xylem sap (water)
  • Cohesion between water molecules pulls the water column up the xylem vessel due to the presence of H-bonds and the high tensile strength of the water column
  • Adhesion is present as the water column sticks to the hydrophilic cellulose cell walls of the xylem vessels, allowing the water column to flow up the xylem
• Cavitation - is when air bubbles enter the xylem and xylem vessels are temporarily unavailable.
• Xylem vessels are dead upon maturity - so the water flow is not disturbed since it has no protoplasm.

  Xylem have a narrow lumen which increases adhesive forces which are responsible for capillarity (water flow up a xylem vessel)
• Xylem vessels are dead upon maturity - so the water flow is not disturbed since it has no protoplasm.

  Xylem have a narrow lumen which increases adhesive forces which are responsible for capillarity (water flow up a xylem vessel)
• Transpiration is essential as - it generates a transpiration pull which drives the absorption of water from the roots and upward movement of water and minerals to the leaves - in large trees, transpiration is the only way for the minerals and water to reach such heights and reach the leaves.
• Transpiration is essential as it may regulate the temperature of the plant by an evaporating cooling effect which protects the plant from heat injury.
• Waxy cuticle reduces water loss but it is impermeable to the co2.
• stomata - allow gas exchange to occur for respiration and photosynthesis. Regulate the rate of water loss + transpiration.
• Stomata close during the night to limit water loss (prevent water loss)
• Guard cells are specialised epidermal cells and they guard the opening of the stomata
• Plasmodesmata - connect the cytoplasm of adjacent cells together
• casparian strip - found in the cell walls of the endodermis (innermost layer of the root cortex) and blocks the apoplastic pathway.
• Once the water and minerals reach the casparian strip, they must pass through the cell membrane into the cytoplasm -symplastic poathway
• symplastic pathway - water travels through connected cytoplasm of the neighboring cells via plasmodesmata.
• transmembrane pathway - transport accross tonoplast and plasma membrane
• apoplastic, symplastic and transmembrane = water and minerals move through the root cortex via these pathways along a water potential gradient towards the xylem
• root xylem sap has a low water potential as tension -negative pressure and low solute potential
• the transpiration stream is the flow of water from the roots to the leaves which are the transpiring surfaces
• water potential is lowest in the air and highest in the soil
• Guard cells are found in pairs surrounding the chloroplast and guard cells may be surrounded by subsidiary cells which are structurally different which serve as ion and water reserves
• guard cells are sausage or kidney shaped in dicots and non grass monocots
• dummbell shaped in grasses
• cellulose microfibrils prevent the cells from getting wider. they hold the sides of the cells as a way that they can not get wider when they swell.
• The guard cells have thick inner walls, to therefore they will stretch less, so they will curve, that is why the guard cells are shaped like sausages
• to open the stoma - Guard cells uptake potassium ions, and water follows by osmosis from epidermal/subsidiary cells which increase pressure potential, and the cells become turgid as water enters the guard cells.
• The pore opens as guard cells become turgid with water by osmosis from subsidiary cells. The pore closes as guard cells lose water and become flaccid
• transpiration is the loss of water vapour through evaporation from plant surfaces such as leaves
• To close stoma - guard cell lose water by osmosis (from high water potential to low water potential - transpiration) and pressure potential decreases, the cells will become flaccid and collapse against one another and the stomata pore closes
• Guard cells are joined end to end
• during the day - stomata open as light is available, allowing co2 to enter the leaf so photosynthesis could occur
• during the night light is not present so photosynthesis can not occur, so the stomata close preventing water loss (to conserve h2o)
• During the day, chloroplasts in guard cells carry out photosynthesis and they produce ATP, as the sugar produced in photosynthesis is broken down by the mitochondrion into ATP. ATP acts as a fuel for active transport processes.,