Roots expend a great deal of energy acquiring nutrients from the soil. We have already seen that energy is required to move nutrients across the cell membranes of the endodermis on their journey to the xylem. Energy is also required to make specific nutrients in the soil accessible to the root. The mitochondria of root cells must, therefore, respire at high rates to provide energy needed for nutrient uptake. Their high respiration rates explain why plants are harmed by flooding: Waterlogged soils slow the diffusion of oxygen, limiting respiration.

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To understand how roots acquire nutrients and why this requires energy, let’s first look at how nutrients move through the soil. Nutrients move by diffusion through the films of water that surround individual soil particles. In addition, when transpiration rates are high, the flow of water through the soil helps to convey nutrients toward the root. The active uptake of nutrients by root cells, a process that requires an expenditure of energy in the form of ATP, decreases nutrient concentrations at the root surface, creating a concentration gradient that drives diffusion from the bulk soil to the root. However, because diffusion transports compounds efficiently only over short distances, each root is able to acquire nutrients only from a small volume of soil. This limitation explains why plants produce so many roots and so many root hairs: Both structures increase the volume of soil from which a root can obtain nutrients. It also explains why roots elongate more or less continuously, exploiting new regions of soil to obtain nutrients required for growth.

Nutrients vary dramatically in the ease and speed with which they move through the soil. For example, nitrate (NO₃–) and sulfate (SO₄^2–) are extremely mobile. In contrast, interactions with clay minerals keep zinc and inorganic phosphate attached to soil particles. One way that roots gain access to highly immobile nutrients is by releasing protons or compounds that make the soil environment close to the root more acidic. This requires energy in the form of ATP, but the decrease in pH weakens the attachment of immobile nutrients to soil particles. As a result, these formerly immobile nutrients are able to move by diffusion to the root.

Soil microorganisms—both fungi and bacteria—are more adept than plants at obtaining nutrients. Fungi (Chapter 34) are particularly good at mobilizing phosphorus and at decomposing organic matter, releasing nitrogen, while some bacteria can convert gaseous nitrogen (N₂) into a chemical form that can be incorporated into proteins and nucleic acids (Chapter 26). What plants have at their disposal is a renewable source of carbohydrates. Using these carbohydrates, plants support the growth of symbiotic fungi and bacteria, in effect exchanging carbon for phosphorus or nitrogen.