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In this topic of IBDP Biology, we will learn about the second part of the passage of water through a plant.
Pathways of absorbed water
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.
Transpiration
- 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
Role of transpiration
- 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.
