Transport in plants

transpiration stream

1. water evaporates from cell walls inside leaf cell, and water vapour diffuses out through the stomata

   • cell walls are made of cellulose and are hydrophilic

2. more water is drawn from nearest xylem vessels to replace water lost by evaporation

   • happens due to adhesion of water to cellulose, and cohesion between water molecules

     • this is capillary action

3. tensions (negative pressure) are generated in xylem vessels when water is drawn out,

   • due to cohesion between water molecules, tension generated in leaf is transmitted down continuous columns of water in xylem until the roots

     • this is transpiration pull, allows water to be moved upwards through the plant against force of gravity

adaptations of xylem vessels

• lack of end walls and cell contents

  • creates long, hollow continuous tubes, allowing unimpeded flow of xylem sap

  • mature xylem vessels are non-living

• pits for entry and exit of water

  • gaps in the thickening of the cell wall through which water can enter and exit

• walls thickening and lignin

  • walls of xylem thickened and lignified

  • prevents vessels from collapsing when pressure is low during transpiration

dicotyledonous stem structure

dicotyledonous root structure

root pressure in xylem

• when a plant is not transpiring (e.g due to high humidity), but needs xylem sap to flow upwards, root pressure mechanism is used

  • minerals ions are actively transported into the xylem cells, lowering their water potential

    • this makes xylem sap hypertonic, so water moves in by osmosis

    • this causes positive pressure potential inside the xylem and pushes the sap upwards, against force of gravity

translocation

• phloem tissues are used to translocate carbon compounds from sources to sinks

  • sources: parts of plant where photosynthesis occurs generating carbon compounds

  • sinks: part of the plant which need a supply of carbon compounds

adaptations of phloem

• companion cells have many pump proteins in their plasma membrane to pump sucrose into their cytoplasm, and many mitochondria to make the ATP needed for active transport

• plasmodesmata allow the sucrose-rich sap to flow from companion cell into the sieve tube

process of translocation in phloem sieve tubes

• organic compounds (usually sucrose) produced at the source are loaded into sieve tube by active transport by companion cells, using pump proteins in membrane

  • increases solute conc, (lowers water potential), meaning water moves in by osmosis

  • this increases the hydrostatic pressure inside the sieve tube

• pressure gradient from the source to the sink causes flow

  • sieve plates have pores through which phloem sap can flow