Unit 3 - Xylem and Phloem

Water is absorbed from the soil by the root hair cells (which also obtain essential mineral ions) by active transport. The solution passes into the xylem of the plant and is carried to the leaves where the plant cells use the nutrients to make sugars and amino acids. The tubes in the xylem are made from elongated cells that are arranged end to end. The cell walls contain the substance lignin which gives strength to the xylem tubes, supporting the plant. The xylem cells are alive at first but as the plant grows, they die. The cytoplasm and cell walls between adjoining cells break down and the result is a dead, empty continuous tube, one cell in diameter that is impermeable to water. Movement of the water and minerals through the xylem tissue is caused by transpiration from the leaves of the plant.

The water and minerals that reach living plant cells are used for photosynthesis, producing food for the plant in the form of sugars. Plants also produce amino acids from nitrate and other ions. Both the sugars and the amino acids are required for cell growth and repair. These substances are carried to growing tissues and storage tissues by the phloem. This movement of fluid through plants is called translocation. The elongated cells that form the tubes of the phloem are alive. The cells are joined end to end and the cell walls between them develop holes that allow liquids to pass through.