Water Transport in Multicellular plants
Why is Water Important in Plants:
- Turgor pressure makes cells rigid, providing support in the form of a hydrostatic skeleton.
- Turgor pressure makes root cells strong enough to force their way through hard surfaces such as concrete.
- Evaporation of water keeps the plant cool.
- Mineral ions and products of photosynthesis are transported in solution.
- Water is required for photosynthesis.
- Root hairs are long, thin extensions of root hair cells, and are the place where water osmoses into the root.
- Small size means they can grow between soil particles.
- Large SA:V so more space for diffusion.
- Surface layer is 1 cell thick, so osmosis happens quickly.
- Solutes in the root hair cells maintain a water potential gradient, so water from the soil continues to osmose in.
- Vacuolar sap and cytoplasm of root hair cells contains many different components so is very dilute, as a result water osmoses down a water potential gradient into the cell from the soil.
- Solutes are actively transported in, to create a water potential gradient.
Movement of Water Across the Root:
Apoplast Pathway:
- Water moves through the apoplast - the gaps between the loose network of cellulose fibres in the cell wall.
- Water molecules are pulled along the apoplast pathway by the cohesive forces between water molecules, so as one water molecule at the beginning of the chain is pulled into the xylem, water molecules at the end of the chain are pulled along and into the apoplast.
- Water moves through the symplast - the continuous cytoplasm connected by plasmodesmata.
- Each root hair cell has a higher water potential than the next cell along, so water travels along a concentration gradient until the xylem isreached.
- Water potential of each cell is maintained as the solutes move to the next cell, the water potential increases back to its starting potential..
- Once the water reaches the endodermis of the roots, all water if forced into the symplast pathway by an impermeable suberin band called the casparian strip.
- The casparian strip is important as this forces all of the water and its solutes to pass through selectively permeable membranes, which stops any toxins entering the plant.
- Ions are actively transported into the xylem, which creates a water potential gradient, so water osmoses into the xylem.
- Active transport of minerals creates root pressure, which helps push water and solutes up the xylem.
Evidence for the Role of Active Transport:
- When cyanide is added to root cells, root pressure drops. This is because cyanide kills mitochondria, and therefore stops the production of ATP, so there is no energy for active transport.
- Root pressure increases with temperature to a certain point, suggesting chemical reactions are involved.
- If levels of respiratory substrates lower, root pressure lowers, suggesting that movement of water into the xylem is an active process which requires energy.
- Xylem sap is forced out of the ends of cut stems, suggesting that is is not just the cohesive forces of water. In nature, guttation is where water is forced out of plants when transpiration rate is low.