Transport systems and processes in plants

Plants have two systems for the transportation of substances - using two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, while phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

Involved in the movement of water through a plant from the roots to the leaves via the stem 

During this process:

• Water is absorbed from the soil through root hair cells
• Water moves by osmosis from root cell to root cell until it reaches the xylem 
• It is transported through the xylem vessels in the stem to the leaves
• It evaporates through the leaves (transpiration)

The xylem tubes are made from dead xylem cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. The rest of the xylem cell has a thick, reinforced cell wall which provides strength.

Phloem vessels are involved in translocation. Dissolved sugars, produced during photosynthesis, and other soluble food molecules are moved from the leaves to growing tissues (eg the tips of the roots and shoots) and storage tissues (eg in the roots).

In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down, but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Water on the surface of spongy and palisade cells (inside the leaf) evaporatesand then diffuses out of the leaf. This is called transpiration.

More water is drawn out of the xylem cells inside the leaf to replace what has been lost. Water molecules have a tendency to stick together – so as water leaves the xylem to enter the leaf, more water is pulled up behind it. This produces a continuous flow of water and dissolved minerals moving up the xylem tube from the roots, up the stem, and into the leaves. This is known as the transpiration stream.

FactorDescriptionExplanation Light Transpiration increases in bright light The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis. More water is therefore able to evaporate. Temperature Transpiration is faster in higher temperatures Evaporation and diffusion are faster at higher temperatures. Wind Transpiration is faster in windy conditions Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf. Humidity Transpiration is slower in humid conditions Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.