ISA 1 exam paper revision

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ISA REVISION

Movement of water through plants

  • plants cant absorb water over their general body surface due to waxy waterproof layer
  • So their exchange surface is the root hair cell
  • Root hair cells have to replace the water lost by transpirtation

Root hair cells:

 They are effcient because they provide a large surface area ( long extensions and occur in thousands on each of the branches of a root)

They also have thin surface layer ( materials can move across easily.

How water moves from soil and in to the root hair cell...

  • The water potential in the soil is higher than that of the root hair cell,
  •  so water moves by osmosis from soil solution and into the root hair cell ( down the W.P gradient)
  • water continues its journey by one of two ways: apoplastic pathway , symplastic pathway

Apoplastic Pathway

  • water is drawn into endodermal cells and due to water being cohesive pulls more water with it
  • This creates tension that draws water along the cell walls of cells of the root cortex
  • The mesh like structure of the cellulose cell walls of the cells has waterfilled spaces and so there is little or no resistance in pulling the water along the cell walls.

Symplastic Pathway

  • Takes place across the cytoplasm of the cells of the cortex as a result of osmosis
  • water passes the cell walls through openings called plasodesmata
  • Each plasmodesma is filled with a thin strand of cytoplasm, so there is a continuous collumn of cytoplasm extending from the RHC to xylem.

How water moves along the passage.

1) water entering by osmosis increases the water potential of the root hair cell

2) root hair cell now has a highter W.P than that of 1st cell in cortex

3) water, therefore, moves in to the 1st cell in the cortex down the W.P gradient

4) 1st cell now has a higher W.P than neighbouring cell 

5) So water moves into neighbouring cell by osmosis along the W.P gradient

6) ( this process repeats to 3rd cell along the cortex)

7) At the same time as this the WP in the 1st cell lowers its W.P causing more water to enter root hair cell

8) SO a W.P gradient is set up across all cells of the cortex to carry water along the cytoplasm from root hair cell to endodermis.

Passage of water in to Xylem

When the water reaches the endodermis, by the apoplastic pathway, the casparian strip ( water proof band) in endodermal cells prevents it progressing further along the cell wall. It is then forced in to the living protoplast of the cell. So it joins the symplastic pathway.

  • Active transport of salts is the most likely mechanism by which water gets in to the xylem. 
  • Endodermal cells actively transport salts into xylem. 
  • Requires energy so only happens with in living tissue, takes place along carrier proteins in the cell
  • Water has to enter the cytoplasm

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