3.3.7 Transpiration in Plants Flashcards
What are plant roots responsible for?
Plant roots are responsible for the uptake of water and mineral ions from the soil
Root hair cells are adapted to maximise water and ion uptake
How do water and minerals enter the root?
minerals are taken up from the soil by either active transport or diffusion, depending on soil mineral concentrations
mineral ions lower the water potential of the root hair cells, and water enters the cells from the soil by osmosis
Once water enters the root hair cell what must happen?
Once water has entered the root hair cells it must be transported across the root cortex to the xylem
What are the two pathways that water can take across the cortex?
The apoplast pathway
The symplast pathway
What is the apoplast pathway?
Water moving on the apoplast, or apoplastic, pathway travels by diffusion within the cell walls and intercellular spaces of plant tissue
Note that this is not osmosis because the water does not cross any cell membranes
Water is drawn across the root via the apoplast pathway due to cohesive forces between water molecules:
Water moves upwards in the xylem due to transpiration
Cohesion between water molecules means that more water is drawn along the apoplast pathway within the root to replace the water that has moved upwards
What is the symplast pathway?
Water moving on the symplast, or symplastic, pathway travels via cell cytoplasm and vacuoles
Water enters the symplast pathway and moves between cells, and into cell vacuoles, by osmosis
Water can also move from cell to cell by diffusion via the plasmodesmata
Water is drawn across the root via the symplast pathway as follows:
water moves into root hair cells from the soil by osmosis, increasing the water potential of the root hair cell
water moves down its water potential gradient into neighbouring root cells, increasing their water potential
water continues to move across the root from high to low water potential
When water reaches the centre of the root what must it cross to enter the xylem?
The endodermis
What are the cells of the endodermis surrounded by?
The cells of the endodermis are surrounded by a waxy band known as the Casparian strip, which forms an impassable barrier to water
The waxy material is known as suberin
What does the casparian strip do?
The Casparian strip blocks the cell walls of the endodermis cells, preventing water from entering the xylem via the apoplast pathway and instead forcing it into the symplast pathway
it is thought that this may help the plant control which mineral ions reach the xylem
How is water drawn upwards in the xylem due to transpiration?
water evaporates from the surface of cells in the leaves, lowering the water potential of leaf cells
water is drawn out of the xylem and into leaf cells by osmosis down its water potential gradient
more water molecules are drawn upwards in the xylem in a continuous column due to forces of cohesion between water molecules
attractive forces of adhesion between water molecules and the sides of the xylem aid this process
What is the upward movement of water in the xylem known as?
The transpiration stream
How does water move through the leaves of plants due to transpiration?
Water vapour diffuses out of leaf air spaces and into the surrounding environment down a water vapour potential gradient
The loss of water vapour from the air spaces creates a water potential gradient between leaf mesophyll cells and the leaf air spaces, so more water moves from the leaf mesophyll cells into the air spaces
Water first moves from the cell cytoplasm to the cell surface, before evaporating into the air space
Losing water lowers the water potential of the leaf mesophyll cells, so water moves into the cells by osmosis from neighbouring cells and the xylem
Note that water movement through the leaf also occurs via the apoplast and symplast pathways
