In this topic of IBDP Biology, we will learn about the second part of the passage of water through a plant.

Apoplast pathway

• Movement of water along cell walls and through spaces between cells.
• From root hair cells, through the cortex cells through to the endodermis.
• When it comes to IBDP Biology,  Pathway of least resistance.

Symplast pathway

• When it comes to IBDP Biology,  Cytoplasm of root hair cells is at a lower water potential than soil water.
• Water enters by osmosis.

• There is a water potential gradient from the soil, through the cortex, to the xylem in the centre.
• Water moves from cell to cell through plasmodesmata (pores in adjacent cell walls).
• Then through the cell surface membranes into cytoplasm. 
• This provides more resistance, so less water molecules pass this way.

Endodermis:

• A ring of cells close to the xylem.
• Has a Casparian strip in the cell wall.
• When it comes to IBDP Biology,  This is a waxy layer that is impermeable to water.
• Water must enter the cytoplasm at this point.
• This enables the plant to control entry of mineral ions.
• They are actively pumped into the cell.
• This reduces water potential.
• This increases water uptake.

Xylem:

• Water enters xylem through pits.
• As the water moves up the xylem it lowers the water potential in the roots, maintaining the gradient.

• When it comes to IBDP Biology, Transpiration = the loss of water from the plant by evaporation
  • Mesophyll cells in a leaf have moist cells walls.
  • Some water evaporates into the air spaces.
  • If stomata are open, and the water vapour concentration is lower outside than inside, the water will diffuse out.
• Transpiration stream = the movement of water up the plant as a consequence of evaporation from the leaves

• Delivers water to all parts of the plant – although most of the water actually leaves the plant.
• When it comes to IBDP Biology,  Distributes mineral ions throughout plant.
• Evaporation of water from leaves cools the plant.
• Lack of water means that more water may be lost than can be replaced, cells lose water and become flaccid – this may lead to wilting and death.

That's the end of this topic.

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