Water Transport in Plants Flashcards
What are the exchange surfaces in plants responsible for the absorption of water and mineral ions?
Root hair
Why are there root hairs on the bit of a root that absorbs water?
It increases surface area, speeding up water uptake.
How does water move?
From areas of higher water potential to areas of lower water potential- down a water potential gradient.
Why does water move from roots to leaves?
The soil around roots generally has a high water potential and leaves have a lower water potential because water constantly evaporates from them.
This creates a water potential gradient that keeps water moving through the plant in the right direction.
What are the two paths water can be transported through roots?
- The symplast pathway
2. The apoplast pathway
What is cohesion and tension in plants?
Cohesion and tension help water move up plants, from roots to leaves against the force of gravity.
1. Water evaporates from the leaves at the ‘top’ of the xylem.
2. This creates tension (suction) which pulls more water into the leaf.
3. Water molecules are cohesive (stick together) so when some are pulled into the leaf others follow.
This means the whole column of water in the xylem, from the leaves down to the roots, move upwards.
4. Water then enters the stem through the roots.
How does roots pressure help move water upwards?
When water is transported into the xylem in the roots, it creates a pressure and shoves water already in the xylem further upwards.
What is the transpiration?
The evaporation of water from a plants surface, especially the leaves.
Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf.
When the stomata open, it moves out of the leaf down the water potential gradient.
Describe the apoplastic pathway.
Lower water potential in the root hair cell compared to soil- water moves via osmosis along water potential gradient.
Water drawn into endodermal cells.
Cohesion tension between water and cellulose fibres as a result of h-bonding.
This creates a continuous column of water in a transpirational pull.
Water gets as far as casparian strip before symplastic pathway.
Describe the symplastic pathway.
Water enters root through cell surface membrane via osmosis along osmotic potential gradient.
Root hair cell now has a higher water potential than the first cell in the cortex- therefore moves to.
Next cell lower.. and so on.
Water moves via plasmodesmata.
What is the passage of water into the xylem.
When water reaches the endodermic, the waterproof band that makes up the Casparian strip in endodermal cells prevents it progressing further along the cell wall.
Water is instead forced into living protoplast of the cell, where it joins water that has arrived via symplastic pathway.
How are salts able to to get into the xylem?
Via active transport.
Endodermal cells actively transport salts into the xylem.
This requires energy and can therefore only occur within living tissue.
Takes place along carrier proteins in the cell surface membrane.
What is root pressure?
The active transport of ions into the xylem by the endodermal cells creates a lower water potential in the xylem.
Water now moves into the xylem, by osmosis, along a water potential gradient.
This creates a force that helps to move water up the plant- root pressure.
Evidence for the existence of root pressure due to the active pumping of salts into the xylem includes…
The pressure increases with a rise in temperature and decreases at lower temperatures.
Metabolic inhibitors, such as cyanide, prevent most energy release by respiration and also cause root pressure to cease.
A decrease in the availability of oxygen or respiratory substrates causes a reduction in root pressure.
The movement of water out through stomata.
The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
Open, water molecules diffuse out of air spaces into surrounding air.
Water lost from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
