Uptake of Water and Minerals in Roots

Uptake of Water and Minerals in the Roots

In the first section of this tutorial, we looked at the structure of the dicotyledonous root and stem and compared the different cells in the specialised tissues of the plant root and stem. Now we will look at how these specialised cells help the plant to absorb water from the soil and transport it to the stem, where it can then be transported to the rest of the plant.

Movement of water through the dicotyledonous root

Water is found in the spaces between the soil particles. Water and mineral salts first enter through the cell wall and cell membrane of the root hair cell by osmosis. Root hair cells are outgrowths at the tips of plants' roots (see figure below). They function solely to take up water and mineral salts. Root hair cells do not perform photosynthesis, and do not contain chloroplasts as they are underground and not exposed to sunlight. These cells have large vacuoles which allow storage of water and mineral salts. Their small diameter (5-17 micrometres) and greater length (1500 micrometres) ensure they have a large surface area over which to absorb water and mineral salts. Water fills the vacuole of the root hair cell.

The following list summarises how the root hair is adapted to absorb water from the soil:

• There are many, elongated root hairs to increase the total root surface area for water absorption.
• They have thin walls to speed up the intake of water by osmosis.
• They have large vacuoles to absorb water quickly and transport it to the next cells.
• The vacuoles have salts, which speed up water absorption from soil water.
• Root hairs do not have cuticles, as this would prevent water absorption.

Water can now move from the root hair cells and across the parenchyma cells of the cortex in two major ways. Some water passes through the cells by osmosis. Most water travels either in, or between the cell walls (of the parenchyma cells) by simple diffusion. The water must pass through the endodermis to enter the xylem. Once water is in the xylem of the root, it will pass up the xylem of the stem.

Step-by-step transport of water in plants, from the roots to the xylem.