Transpiration Flashcards
What percentage of water is transported via the apoplast pathway?
90%
What percentage of water is transported via the symplast pathway?
10%
What is similar between apoplast and symplast pathways?
Both are routes for water to go from the soil to the epidermis to the cortex to the stele.
Why does less water travel by the symplast pathway?
It is harder to move through every cell than go round them.
Describe the apoplast pathway
Water moves from cell to cell by passing along the cell walls. Water fills space between cellulose fibres. This route is stopped at the endodermis because of the casparian strip so it joins the symplast pathway.
Describe the symplast pathway
Water moves into a cell, through the vacuole and cytoplasm and into the next cell via the plasmodesmata.
This route continues all the way to the xylem.
What is cohesion?
Water molecules are attracted to each other because of hydrogen bonds between them. They have hydrogen bonds because they are polar molecules.
What is adhesion?
Water molecules are attracted to the lignin in the xylem vessels creating tension in the xylem vessels.
What is cohesion tension theory?
Adhesion and cohesion ensures there is a constant stream of water running through the xylem vessels.
This is mass flow.
What pulls the water up the xylem?
Significant negative pressure made from water evaporating out the stomata and water molecules being pulled up to take their place.
What are the 6 steps of transpiration in xylem/leaves?
Water is evaporated out of the stomata because of the difference in water potential.
Spongy mesophyll losing water lowers water potential.
Water moves from cell to cell in leaves by apoplast/ symplast pathways.
Water moves out the xylem in leaves through pits.
Water molecules stick to each other due to H bonding (cohesion) so as water molecules leave, others are pulled up to take their place, this creates a significant negative pressure in the xylem due to the tension in the column of water.
Loading of water into xylem in roots causes a high hydrostatic pressure that help push water a little way up the stem.
What are the 4 stages of transpiration in roots?
Water moves into the endodermis by symplast pathway.
Water moves across the cortex by either symplast or apoplast pathways.
Water enters root hair cells by osmosis, down the water potential gradient.
This transpiration stream moves water by the cohesion tension hypothesis.
What 4 things increase transpiration?
Hot - increase evaporation
Windy - Water evaporates and is taken away quickly
Low humidity/dry - steeper water potential gradient
High light intensity - More photosynthesis, more stomata open, more water lost.
Definition of transpiration?
Water vapour being lost from the stomata in the leaves.
Definition of transpiration stream?
Whole journey of water from the roots, up the plant, to the leaf where it is then lost
