17.12–17.14: Plants transport water, sugar, and minerals through vascular tissue.

A mature cypress sits beside a country road in Tuscany.

Figure 17.29: The root hair cells are permeable to water. The absorption of minerals requires an intervening step.

Plants absorb water and minerals from the soil through their roots. Water absorption relies on osmosis and is straightforward: the cell membranes of the root hair cells are permeable to water, and as long as the root reaches water in the soil that has a lower concentration of dissolved minerals than the fluid inside the cells, water will move into the root (FIGURE 17-29).

The absorption of minerals requires an intervening step (FIGURE 17-30). Because most minerals are chemically charged, they cannot pass through cell membranes unaided. For this reason, plants take up minerals from the soil only with the help of transport proteins embedded in their root cell membranes. If a mineral is at a higher concentration outside a root cell than inside, the mineral can move across the membrane by simple facilitated diffusion (a process that does not require the plant cell to expend any energy). If a mineral is at a higher concentration inside the cell, it can still be taken up by the plant, but only through active transport (see Section 3-10), in which the plant expends energy to move the mineral into the root cell against its concentration gradient.

When it comes to acquiring water and minerals, plants again benefit from collaboration with other organisms, in this case fungi (FIGURE 17-31). Most plants have fungi growing all around and even within their roots, forming mutualistic associations known as mycorrhizae. Tiny, thread-like fungi trap water like a sponge and hold it around the roots. The fungi’s huge surface area dramatically increases the amount of water and minerals that can be absorbed. The fungi don’t do all that work for free, of course. In exchange, they receive sugars, amino acids, and vitamins. The association between plants and fungi is so important to plants that many trees can’t grow at all if they don’t have fungi around their roots.

Figure 17.30: Taking up minerals into the plant. The absorption of minerals through the cell membranes of plant roots requires the aid of transport proteins and sometimes energy.

Figure 17.31: Mutualistic relationship between plants and fungi.