Transpiration starts in the root hairs. Water and minerals are brought into the roots by active transport, which uses ATP. The water and minerals have to reach the top of the plant, and they are brought up in the xylem, continuous tubes formed from dead cells, called vessel members.
There are numerous factors in the water uptake or water translocation. Water coheres to itself, and adheres to the cell walls, so there is capillary action on the water column. When stomata are open, water evaporates out the leaves. This creates a negative leaf pressure, and water from the roots is drawn up the xylem. When stomata are closed, root pressure builds up, and pushes water up the xylem anyway.
Stomata open and close to let carbon dioxide in. As earlier said, water also evaporates out these pores. When a stoma ‘wants’ to close, the guard cells around the stoma get a large concentration of potassium ions and cause lower water potential. Through the wonders of osmosis, the neighboring cells give up some of their water for the guard cells. This causes an expansion of the cell, until the membrane is full, and then the stoma closes.
In the leaves, photosynthesis occurs. The carbon dioxide brought in by the stoma enters the spongy and palisade mesophyll. In the mesophyll, sucrose and oxygen are created. The oxygen goes out the stoma, and the sucrose mixes with water, and creates sap. There is a higher concentration in the leaves, so osmosis takes it down to the roots. The sap travels down the vein in the phloem, which consists of sieve cells and sieve-tube members. Each sieve-tube members is adjacent to a specialized parenchyma cell called a companion cell. It carries out some of the metabolic functions needed to keep the sieve-tube member alive. From the phloem, the sap travels down the stem until it reaches the places it is needed.