How does a tree pull water from its roots to leaves?

Water is very important for plants. Plants require water for the following activities: 

Water is needed for germination of seeds, photosynthesis by which plants prepare their food, transport of nutrients and minerals from the soil to the plants, maintenance of the plant structure by providing the appropriate pressure to the plant tissues, provides habitat in the form of ponds, rivers, lakes and sea for a large number of plants.

Plants are thriving in this planet Earth because they transport water and minerals necessary for their growth from the roots to the tips of their leaves.  Plants have to work against gravity to transport water and minerals to the tips of their leaves. How does the process take place? Plants especially, trees are very tall and water has to be pumped to great heights. Trees do not have any motors or circulation pump for pumping water. But still each shoot at great heights gets water to grow leaves. How?

Xylem is the specialised tissue of plants and trees that transports water and nutrients from the soil to the stems and leaves. Xylem cells also provide strength to tissues and organs to maintain plant architecture and provide resistance to bending. It also supports the weight of water transported upward in the plants and trees.

In trees the xylem cells are wood, which is an essential raw material for energy and wood products. By genetic engineering of the xylem cells we will be able to develop better energy sources and improved raw materials for wood products.

What are the driving forces behind water and mineral transport in plants?

Following are the different steps involved in the process:

Plants absorb water through roots, stems and leaves. But most of the water required by plants is absorbed by root hairs from the soil. The root hairs are thin-walled unicellular outgrowths of epidermis. They are in close contact with the thin film of water surrounding the soil particles. Water is absorbed by the root hairs through their cell membranes from the soil due to osmosis.  Water enters the root hairs if the concentration of water in the soil is greater than the concentration of water in the root hairs. Hence, there will be diffusion of water from the root hair to the adjacent cortical cells.

Water moves deep into the cortical cells and reaches the endodermis of the root due to osmotic pressure.  Water moves from the root hair into the root cells and the endodermis of the root forces water into the xylem tubes through the passage of cells.  This force is called root pressure and this force maintains the water column in the xylem tubes.  Xylem tubes carry water from the root hair to the stem and leaves. This upward movement of water from the root to leaf through stem is against the force of gravity and is called ascent of sap.

Water and minerals from the soil are pulled up by the plants against the force of gravity through xylem tubes that are thin, narrow and hard due to capillary action. Water is a chemical compound with special bonds. Water has hydrogen bond in addition to covalent bond. The hydrogen bond in water molecules are responsible for a. strong intermolecular force of attraction called surface tension. b. Stickiness of water molecules due to cohesion. They fill the xylem tube as a single molecule of water. c. Water molecules form hydrogen bond with other molecules like carbohydrates will be attracted to xylem tube due to adhesion.

There is a continuous loss of water caused by Transpiration in leaves. Transpiration is the evaporation of water from the plants. Water evaporates from the stomata of the leaves and creates a vacuum or a negative pressure in the leaves. So water is pulled from xylem tubes towards leaves where water is lost.  This is a suction force that aid in drawing water from the root to leaves and is known as Transpiration pull.

Leaves depend on this efficient delivery system for their supply of water. If the transpired water is not replaced by water transported from roots, the leaves will wilt. Photosynthesis cannot take place in the wilted leaves and the plants will die. So, Photosynthesis, Respiration, Translocation, Transportation, Transpiration are all important functions of plants and trees that are interdependent and necessary for survival of the plants.