Water Transport 9.2 Flashcards
Why do plants need water?
.Water maintains the hydrostatic pressure in plant cells which keeps them turgid so they can maintain the structure of the plant.
.It also enables cell expansion and extension which is why roots can push their way through concrete
. Water is a raw material used in photosynthesis
. It is used as a transport medium as most substances are carried in aqueous solutions
. It keeps plants cool when water is lost by evaporation
Root hair cells
These are the main exchange surfaces in the plant where water is taken in by osmosis from the soil. The root hair extensions are specialized to maximize intake
Adaptations of root hair cells
They have a SA:V ratio, because of their big surface area, many more substances can be taken up at the same time.
They are very small in size so can penetrate the soil very easily
They have very thin surfaces so that diffusion and osmosis can take place very quickly and easily.
There is a high concentration of solutes in the cytoplasm of the root hair cell so that the water potential is kept low and water will diffuse in by osmosis.
What does water do after it moves into the root?
It will move through the root using one of two different pathways, the symplast or apoplast pathway.
Symplast
The continuous cytoplasm between living plant cells that is possible because of the plasmodesmata connecting them.
Symplast pathway
This is the movement of water through the symplast by osmosis. After water has been taken up by the root, the water potential in the root hair cell will be higher than the water potential in the neighboring cell, so water will move into that cell by osmosis, lowering the water potential in the root hair cell and maintaining a concentration gradient. This will continue until the water reaches the xylem.
Apoplast
The cell wall and intercellular spaces
Apoplast Pathway
This is where water enters the cell wall through gaps in the loose cellulose fibres. It then moves through the apoplast and will pull more water molecules in with it because of the cohesive nature of water. This will continue until it reaches the xylem and the water molecules will still be pulled creating a pressure and a continuous flow of water
Movement of water into the endodermis
After the water has flown through all the cells, it will reach the endodermis which surrounds the vascular system. These cells are specialised and have a casparian strip which is made out of a waxy material called suberin and surrounds the endodermal cells. This will prevent movement of water through the cell wall, so the water in the apoplast pathway will move to the symplast pathway.
Why does the water move from the apoplast pathway?
It has to enter the symplast pathway so it can be in the cytoplasm. This means all the water will have to travel through selectively permeable membranes, so any big and toxic substances will not be transported into the vascular system
Movement of water into the xylem
Now that the water is in the cytoplasm of the epidermal cells, and it’s in the vascular bundles, some of it will move back into the apoplast pathway in order to move back into the xylem. The xylem itself has a very low water potential compared to the endodermal cells, so water will be moved into the xylem. Mineral ions needed to be transported are also actively pumped beforehand by the endodermal cells to increase the water potential gradient.
Root pressure
Because of the movement of mineral ions into the xylem, it results in root pressure as more water will move up the xylem. This gives it a little push, but it isn’t a major effect in transpiration.
Evidence for the role of active transport in root pressure
When cyanide is added to root hair cells, it will inhibit the production of ATP which means there is no energy supply for active transport and the root pressure will fall
As temperature increases, the root pressure increases as there are faster rates of reactions happening
As the substrate levels fall, the root pressure will fall
Guttation can happen when xylem sap is forced out of special pores at the ends of leaves that would only happen if there was root pressure
