Organisms have specific nutrient requirements
An organism's nutrient requirements are related to its physiology. The amounts and specific nutrients needed therefore vary according to the organism's mode of energy acquisition (autotrophs vs.
heterotrophs), mobility, and thermal physiology (ectotherms vs. endotherms). Mobile animals, for example, generally have higher rates of metabolic activity than plants or bacteria, and they therefore have higher requirements for nutrients such as nitrogen (N) and phosphorus (P) to support the biochemical reactions associated with movement. Differences in nutrient requirements are reflected in the chemical composition of organisms (TABLE 22.1). Carbon is often associated with structural compounds in plant cells and tissues, while nitrogen is largely found in enzymes. Accordingly, the ratios of carbon to nitrogen (C:N) in organisms can indicate the relative concentrations of biochemical machinery in cells. Animals and microorganisms typically have lower C:N ratios than plants: for example, humans and bacteria have C:N ratios of 6.0 and 3.0, respectively, whereas those of plants range from 10 to 40. This difference is one reason why herbivores must consume more food than carnivores to acquire enough nutrients to meet their nutritional demands.TABLE 22.1 Elemental Composition of Organisms (as Percentage of Dry Mass)
| Element (symbol) | Bacteria (in general) | Plant (corn, Zea mays) | Animal (human, Homo sapiens) |
| Oxygen (O) | 20 | 44.43 | 14.62 |
| Carbon (C) | 50 | 43.57 | 55.99 |
| Hydrogen (H) | 8 | 6.24 | 7.46 |
| Nitrogen (N) | 10 | 1.46 | 9.33 |
| Silicon (Si) | 1.17 | 0.005 | |
| Potassium (K) | 1-4.5 | 0.92 | 1.09 |
| Calcium (Ca) | 0.01-1.1 | 0.23 | 4.67 |
| Phosphorus (P) | 2.0-3.0 | 0.20 | 3.11 |
| Magnesium (Mg) | 0.1-0.5 | 0.18 | 0.16 |
| Sulfur (S) | 0.2-1.0 | 0.17 | 0.78 |
| Chlorine (Cl) | 0.14 | 0.47 | |
| Iron (Fe) | 0.02-0.2 | 0.08 | 0.012 |
| Manganese (Mn) | 0.001-0.01 | 0.04 | — |
| Sodium (Na) | 1.3 | — | 0.47 |
| Zinc (Zn) | — | 0.01 | |
| Rubidium (Rb) | — | 0.005 |
Sources: (Bacteria) S.
Aiba et al. 1973. Biochemical Engineering. 2nd ed. Academic Press: New York; (Plant and Animal) E. Epstein and A. J. Bloom. 2005. Mineral Nutrition of Plants: Principles and Perspectives. 2nd ed. Oxford University Press/Sinauer: Sunderland, MA; based on E. C. Miller. 1938; P. B. Hawk and B. L. Oser. 1965 (cited within).Note: Dashes indicate a negligible amount of an element; blank spaces indicate that the element has not been measured.
The nutrients essential for all plants, and the functions associated with them, are presented in TABLE 22.2. Some plant species have specific requirements for other nutrients not found in Table 22.2. For example, many, but not all, C4 and CAM plants (see Concept 5.3 for discussion of these photosynthetic pathways) require sodium, while most plants do not. In contrast, sodium is an essential nutrient for all animals, critical for maintaining pH and osmotic balances. Cobalt is required by some plants that host nitrogen-fixing symbionts (discussed later in this section). Selenium is toxic to most plants, but a small number of plants growing on soils rich in selenium may require it. In contrast, selenium is an essential nutrient for animals and bacteria.
TABLE 22.2 Plant Nutrients and Their Principal Functions
| Nutrients | Principal functions |
| Carbon, hydrogen, oxygen | Components of organic molecules |
| Nitrogen | Component of amino acids, proteins, chlorophyll, nucleic acids |
| Phosphorus | Component of ATP, NADP, nucleic acids, phospholipids |
| Potassium | IonicZosmotic balance, pH regulation, regulation of guard cell turgor |
| Calcium | Cell wall strengthening and functioning, ionic balance, membrane permeability |
| Magnesium | bgcolor=white>Component of chlorophyll, enzyme activation|
| Sulfur | Component of amino acids, proteins |
| Iron | Component of proteins (e.g., heme groups), oxidation-reduction reactions |
| Copper | Component of enzymes |
| Manganese | Component of enzymes, activation of enzymes |
| Zinc | Component of enzymes, activation of enzymes, component of ribosomes, maintenance of membrane integrity |
| Nickel | Component of enzymes |
| Molybdenum | Component of enzymes |
| Boron | Cell wall synthesis, membrane function |
| Chlorine | Photosynthesis (water splitting), ionic and electrochemical balance |
Sources: F.
B. Salisbury and C. Ross. 1992. Plant Physiology, 4th ed. Wadsworth: Belmont, CA; H. Marschner. 1995. Mineral Nutrition of Higher Plants. Academic Press: San Diego, CA.Plants and microorganisms usually take up nutrients from their environment in relatively simple, soluble chemical forms, from which they synthesize the larger molecules needed for their metabolism and growth. Animals, on the other hand, typically take up their nutrients through the consumption of living organisms or detritus, obtaining their nutrients in larger, more complex chemical compounds. Animals break down some of these compounds and resynthesize new molecules; others are absorbed intact and used directly in biosynthesis. For example, 9 of the 20 amino acids that are essential for metabolism in humans and other mammals must be absorbed intact, since we cannot synthesize them ourselves.