Biology - Plant Cells and Water
How is water taken up from the soil?
The outermost layer of cells (epidermis) on the plant's roots contain root hair cells.These cells are surrounded by soilparticles.The root hair cells that make up the epidermis have large projections which increase the surface area of the root.
The root hair cells actively transport water and minerals from the soil into the cell using ATP produced by mitochondria within the cell.
The minerals that are absorbed reduce the water potential of the cell's cytoplasm. A gradient is then formed as the water potential in the cell is lower than that in the soil. Water is then absorbed and transported across the cell's membrane by osmosis.
The Pathways of Water
There are 3 ways that water can travel from cell to cell:
The Apoplast Pathway:
- The cell wall is made of cellulose.
- The cellulose molecules that make up the wall have lots of water-filled spaces in between them.
- The apoplast pathway carries dissolved mineral ions and salts with the water because it does not cross any plasma membranes.
The Symplast Pathway:
- Water enters the cell cytoplasm via the plasma membrane.
- The plamodesma is a small strip of cytoplasm that creates gaps in the cell wall connecting the cells.
- The symplast pathway crosses the plasmodemata in order to get to the next cell.
The Vacuolar Pathway:
- This is very similar to the symplast pathway in that it transports water through the plasmodesmata.
- However the water is not restricted to travelling just through the cytoplasm, it can travel through the vacuoles of the cells too.
The Xylem - It's Function and Adaptations
The Xylem is a hollow column made of dead cells that transports water and mineral ions up the plant from the roots.
The cell walls of the xylem have been reinforced with lignin (a complex organic polymer) which waterproofs the cells, strengthens them and prevents them from collapsing.
The lignin is not fully formed in some places along the xylem, this creates pits in the walls which allows water to leave the xylem and move to an adjacent vessel.
The ways in which the xylem is adapted to it's function are that:
- The cells are aligned to form a continuous column.
- The tubes are narrow so that the xylem is more stable and doesn't break as easily.
- The pits in the walls allow water to move in and out.
- The patterns that the lignin makes allows the xylem to stretch as the plant grows and increases the flexibility. This means the xylem doesn't break as easily.
How does water enter the Xylem?
They layer of cells surrounded the Xylem are called the endodermis cells. These cells contain a waterproof strip in some of their walls. This strip is known as the Casparian Strip.
The Casparian Strip blocks the route of the apoplast pathway and as a result forces the water into the symplast pathway.
The endodermal cells actively transport the minerals and ions into the xylem from the cortex which decreases the xylem's water potential.
Because the water potential is now lower in the xylem than it is in the cortex, the water moves from the cortex, through the endodermal cells and into the xylem.
When water enters the xylem by osmosis, the water potential becomes higher in the xylem and lower in the cells just outside the endodermal cells. This creates a water potential gradient across the whole cortex.
Because of this gradient, water is continuously moved from the root hair cells, through the symplast pathway, across the cortex and eventually into the xylem.