Question

Describe the pathway of a H2O molecule from the soil solution from its absorption by a root hair to its exit to the atmosphere from a stomate 100 meters above the forest floor. Be sure to mention all tissues and cell types that it may pass through, their significance, as well as any relevant physical forces acting upon it.

Answer 1

Answer :

Transporting water

Plants require transport systems to move water, dissolved food and other substances around their structures in order to stay alive.

Plants require water for two major reasons:

• For photosynthesis. In most flowering plants this happens in mesophyll cells in the leaves.
• To transport materials, eg minerals.

Water taken up by the roots of a plant is transported through a plant to the leaves where some of it passes into the air. The stages of the process are:

1. Soil to xylem

• Water enters root hair cells: tiny hairs covering the ends of the smallest roots. They provide a large surface area for the absorption of water by the process of osmosis.
• Water then moves from cell to cell through the root cortex by osmosis down a concentration gradient. This means that each cell has a lower water concentration than the one before it.
• In the centre of the root the water enters the xylem vessels - vein-like tissues that transport water and minerals up a plant.

2. Xylem to leaf to air

Water molecules move up the xylem vessels to the leaves where they exit and move from cell to cell. Water moves from the xylem vessels into the mesophyll cells where it can be used for photosynthesis.

Some of the water evaporates into the surrounding air spaces inside the leaf and then diffuses out through the stomata into the surrounding air. The opening and closing of the stomata is controlled by guard cells in the epidermis.

The loss of water from the leaves of a plant is called transpiration, and the resulting flow of water through the plant is called the transpiration stream. The transpiration stream is important because:

• It carries water for photosynthesis to the mesophyll cells: the upper layer of cells where photosynthesis mostly takes place in the leaves.
• The water carries essential mineral salts dissolved in solution.

The rate of transpiration is affected by wind speed, humidity, temperature and surface area.

The primary forces that create the capillary action are adhesion and cohesion. Adhesion is the attraction that occurs between water and the surface of the xylem, and cohesion is the attraction between water molecules