Learning objectives
By the end of this section, you will be able to do the following:
- Describe how plants obtain nutrients
- List the elements and compounds required for proper plant nutrition
- Describe an essential nutrient
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.
The Chemical Composition of Plants
Since plants require nutrients in the form of elements such as carbon and potassium, it is important to understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight. Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves (fig-ch31_01_01). Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.
View original figure (external; image not reproduced in this text edition)
Original figure description: Illustration shows a root tip. The tip of the root is bare, and hairs grow further up. A cross section at the top of the root reveals xylem tissue interspersed by four ovals containing phloem at the periphery.
Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An organic compound is a chemical compound that contains carbon, such as carbohydrates, lipids, proteins, and nucleic acids and is made by a living organism. Carbon that was obtained from atmospheric CO2 is incorporated into organic molecules by plants and as such, composes the majority of the dry mass within most plants. An inorganic compound does not contain carbon (except CO2) and is not part of, or produced by, a living organism. Inorganic substances, which form the majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and potassium (K) for structure and regulation.
Essential Nutrients
Plants require only light, water, and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients (tab-ch31_01_01). For an element to be regarded as essential, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.
| Essential Elements for Plant Growth | |
|---|---|
| Macronutrients | Micronutrients |
| Carbon (C) | Iron (Fe) |
| Hydrogen (H) | Manganese (Mn) |
| Oxygen (O) | Boron (B) |
| Nitrogen (N) | Molybdenum (Mo) |
| Phosphorus (P) | Copper (Cu) |
| Potassium (K) | Zinc (Zn) |
| Calcium (Ca) | Chlorine (Cl) |
| Magnesium (Mg) | Nickel (Ni) |
| Sulfur (S) | Cobalt (Co) |
| Sodium (Na) | |
| Silicon (Si) | |
Macronutrients and Micronutrients
The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called macronutrients. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 45 percent carbon. As shown in fig-ch31_01_02, carbon is a key part of plant biomolecules, followed by oxygen (45 percent) and hydrogen (6 percent), which are the next two most abundant elements in plants.
View original figure (external; image not reproduced in this text edition)
Original figure description: Three cellulose fibers and the chemical structure of cellulose is shown. Cellulose consists of unbranched chains of glucose subunits that form long, straight fibers.
The third most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. In addition to being macronutrients that are part of many organic compounds, hydrogen and oxygen also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration. Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.
Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, which are described below, also contribute to the plant’s ionic balance.
In addition to macronutrients, organisms require various elements in small amounts. These micronutrients, or trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).
Deficiencies in any of these nutrients—particularly the macronutrients—can adversely affect plant growth (fig-ch31_01_03); depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.
View original figure (external; image not reproduced in this text edition)
Original figure description: Photo (a) shows a tomato plant with two green tomato fruits. The fruits have turned dark brown on the bottom. Photo (b) shows a plant with green leaves; some of the leaves have turned yellow. Photo (c) shows a five-lobed leaf that is yellow with greenish veins. Photo (d) shows green palm leaves with yellow tips.
Section Summary
Plants can absorb inorganic nutrients and water through their root system, and carbon dioxide from the environment. The combination of organic compounds, along with water, carbon dioxide, and sunlight, produce the energy that allows plants to grow. Inorganic compounds form the majority of the soil solution. Plants access water though the soil. Water is absorbed by the plant root, transports nutrients throughout the plant, and maintains the structure of the plant. Essential elements are indispensable elements for plant growth. They are divided into macronutrients and micronutrients. The macronutrients plants require are carbon, nitrogen, hydrogen, oxygen, phosphorus, potassium, calcium, magnesium, and sulfur. Important micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, silicon, and sodium.
Review Questions
For an element to be regarded as essential, all of the following criteria must be met, except:
- No other element can perform the function.
- The element is directly involved in plant nutrition.
- The element is inorganic.
- The plant cannot complete its lifecycle without the element.
C
The nutrient that is part of carbohydrates, proteins, and nucleic acids, and that forms biomolecules, is ________.
- nitrogen
- carbon
- magnesium
- iron
B
Most ________ are necessary for enzyme function.
- micronutrients
- macronutrients
- biomolecules
- essential nutrients
A
What is the main water source for land plants?
- rain
- soil
- biomolecules
- essential nutrients
B
Critical Thinking Questions
What type of plant problems result from nitrogen and calcium deficiencies?
Deficiencies in these nutrients could result in stunted growth, slow growth, and chlorosis.
Research the life of Jan Babtista van Helmont. What did the van Helmont experiment show?
van Helmont showed that plants do not consume soil, which is correct. He also thought that plant growth and increased weight resulted from the intake of water, a conclusion that has since been disproven.
List two essential macronutrients and two essential micro nutrients.
Answers may vary. Essential macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Essential micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, sodium, and silicon.
Glossary
- inorganic compound
- chemical compound that does not contain carbon; it is not part of or produced by a living organism
- macronutrient
- nutrient that is required in large amounts for plant growth; carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur
- micronutrient
- nutrient required in small amounts; also called trace element
- organic compound
- chemical compound that contains carbon