The structure of a dicotyledonous root in terms of the up take of water.
The first cell is a root hair cell. This is an epidermal cell with a long extention which provides a large surface area for the diffusion of water.
There is a high water potential in the soil, because there are not many ions dissolved in it, and there is a low water potential in the cell, because the vacuole contains cell sap which has many ions dissolved in it, water moves by osmosis from the soil into the root hair cell.
The second cell is a parenchyma (packing) cell of the cortex, water moves between these cells in two ways.
- The symplastic pathway- cell walls have spaces in which water can move along
- The apoplastic pathway- there are strands of cytoplasm called plasmodesma which link the cytoplasm of different cells, water moves through these along the concentration gradient by osmosis.
The third cell is an endodermal cell. Water in the apoplastic pathway arrives in the ‘protoplast’ of the cell through the plamodesma, but water in the symplastic pathway has to be forced out of the cell wall by a waterproof strip called the Casparian strip.
Root pressure
Endodermal cells actively transport ions into the xylem, this means the xylem has loads of ions in it and so has a really low water potential- so water will move into the xylem as there is a concentration gradient. This helps move water through the plant and is known as ‘root pressure’.
Water moved through a leaf
Water moves from the roots to the leaves through the xylem. When it reaches the leaf it is moved into the mysophyll cells through the apoplastic and symplastic pathways. Water evaporates from the mysophyll cells into the air spaces in the leaf, it then leaves through the stomata- this is called transpiration.
Cohesion-tension theory
Water molecules stick together due to hydrogen bonds formed between them, this is called ‘cohesion’. When water molecules are moved through the leaf they pull other molecules up behind them- this means that as water molecules evaporate from the mysophyll they pull more molecules into the cell behind them, in turn this pulls molecules up in the xylem. In this way there is a pull on the water in the xylem which moves water in the stem.
Factors affecting transpiration
Temperature:
this evaporates more water (by increasing kinetic energy and so the space between molecules making them a gas.) It also decreases the humidity of the air.
Humidity:
humid air has many water molecules in and so it has a low water potential- this means less water can diffuse into it.
Air movement (wind speed):
the more air movement, the more quickly water vapour gets taken away from the stomata- this means that the air can be cleared of vapour and have a higher water potential and more water will diffuse out of the leaf into it.
Light:
photosynthesis happens when there is light, so the more light, the more photosynthesis, the more gas exchange is needed to happen. This means that when it is light the stomata will open and therefore water vapour will escape.
