One of the first problems faced, is moving the water up very tall plants, without a pump. (after all, it takes a big fire engine which makes a lot of noise and uses a lot of energy to move water up to the top of a hose pipe a few hundred feet up, but forest trees constantly do that, silently).
The root itself has a thing called root pressure. This root pressure can push water up to the height of a meter or two. But how does the water get up the rest of the way?
The answer is hydrostatic cohesion.
Because of the cohesive properties of water (H atoms explained in water essay), the water pulls itself up and keeps going as long as there is a constant pull.
This constant pull is achieved by use of the transpiration cycle.
Transpiration
Transpiration is the process in which water is absorbed by the root systems of a plant, moves up through the plant, passes through pores (stomata) in leaves, and then evaporates into the atmosphere as water vapour.
Transpiration in a plant is greatly controlled by the opening and closing of the stomata.
The stomata are special pores which allow the release of water. They consist of guard cells, and companion cells. Guard cells surround each stoma, and help regulate the rate of transpiration in the plant.
On the left is a diagram of stomata.
The way the stomata works is simply due to the basics of water potential. The guard cells will either become turgid or flaccid depending on the levels of concentration of solution in the cells.
This is why the vacuole plays such an important role in the guard cell.
The things that are used to change the concentration in the cell are potassium ions. E.g, as the potassium ions increase inside the guard cell, the water potential decreases, and water enters the guard cell.
The more water that is drawn out due to the transpiration stream, the more that is drawn in from the roots, and similarly, if less water is being drawn in from the roots, less water will be transpired out.
Factors affecting transpiration can be tested using a potometer set up (which in reality only measures the rate of water being taken up the stem).
A photometer set up looks like this.
From this you can find out via experiments, the many factors which affect the rate of transpiration.
A higher temperature increases the rate of evaporation and can reduce humidity which causes increased transpiration.
Light stimulates the stomata to open allowing gas exchange for photosynthesis to occur, which increases the rate of transpiration.
If wind around the plant is faster, then the saturated air will be blown away. This will be replaced by drier air, and transpiration will occur at a faster rate.
So we now know about the transpiration stream, how does the water flow through the stem?
The stem has a number of vessels running down it which are used to transport water around the plant. These consist of the following two. The Xylem and the Phloem.
Xylem
Xylem vessels are composed of dead cells which have hardened and have formed long empty tubes. Before the cells die, they produce lignin, which is set into helices. Lignin makes xylem vessels very strong. AT the end of some of the cells, there are holes, and at the end of others, there might be nothing, some are perforated.
Water is mainly carried thought he xylem vessels. It is very effective at carrying water up trees and tall plants.
Xylem are found on the inside of the plant cell. You can see in the stem picture where the xylem are located. They do not function until they die, which is one of the big differences between xylem and phloem.
Phloem
Phloem tissue is composed of sieve tube cells.
These form long columns with holes in their end walls called sieve plates.
Unlike xylem, the cells are alive, but they do not have a cell nucleus. Therefore, all of the work in keeping the cell alive is done by the companion cell. The companion cell is connected to the sieve cell via plasmodesmata, providing them with ATP, nutrients and proteins.
To keep the area for transport high, the cytoplasm is in strands around the edge of the cell. These cytoplasmic strands pass through holes in the sieve plates, forming continuous filaments.
These are used to transport sugars, so most likely what will be travelling up and down the phloem, will be the sugary sap of the plant.
These carry water throught the stem mainly, we haven’t looked at how water moves around the plant at other areas. The three main areas are the movement through the root, the stem and the leaves.
Movement through the roots is purely dependant on osmosis and diffusion.
There are two different ways, the symplast pathway, and the apoplast pathway.
As you can see, it is mainly diffusion and osmosis through the root hairs. The symplast route accounts for 10 percent of the water, and the apoplast accounts for the other 90 percent.
Movement through the leaves is carried out by the branching of xylem into leaf veins. This can be seen on the surface of the leaf, and the symplast pathway is used yet again. Water evaporates from the spongy cells into the sub-stomatal air space, and diffuses out through the stomata.
Finally, the best way to explain the movement of water and sugars in plants is to use the Munch model. This is self explanatory, and the best explanation of it is to simply label one of the tubes xylem, and another tube, phloem.
From Sunflower Biology Indiana
The diagram shows two water-filled compartments connected by a small tube. Another U-shaped tube with bags made of semipermeable membranes are attached to the ends. At the start, the tube and the bag on the left are filled with water while the bag on the right is filled with a concentrated sugar solution. The membrane that the bags are made from allow water but not sugar to diffuse across. Due to the difference in water potential between the contents of the bag on the left and the water it is immersed in, osmosis will occur and water will begin to accumulate in the bag. As this happens, pressure will build and a flow will develop that pushes the sugar solution through the tube to the other bag. This system will only function for a short time since the sugar will eventually be distributed evenly throughout the bag and tube.
In the phloem, there is a continuous input of solute from source tissues and a continuous efflux at the sink. This input and output at the two ends will maintain a pressure differential that will keep liquid flowing. Thus, the driving force for solute transport is a pressure gradient between the source and sink regions.
