Learning Objectives
In this section, you will explore the following questions:
- What are examples of macronutrients needs by prokaryotes, and what is their importance?
- How do prokaryotes obtain free energy and carbon for life processes?
- What are the roles of prokaryotes in the carbon and nitrogen cycles?
Connection for AP® Courses
Because prokaryotes are metabolically diverse organisms, they can flourish in many different environments using a wide range of energy and carbon sources. Some are decomposers that are essential to the cycling of nutrients in ecosystems, for example, carbon and nitrogen cycles. (Later, we will explore in more depth the role of these cycles in ecosystems.) Many bacteria form symbiotic relationships with other organisms; for example, nitrogen-fixing bacteria live on the roots of legumes. Other bacteria are disease-causing pathogens or parasites.
Like all cells, prokaryotes require macronutrients (including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) and micronutrients, such as metallic elements from growth and enzyme function.
Teacher Support
Three mechanisms—rapid reproduction, genetic recombination, and mutation—give rise to the extensive genetic variation found in prokaryotic populations. This genetic diversity is reflected in the broadly ranging nutritional and metabolic adaptations of prokaryotes. As is true for all organisms, prokaryotes can be organized by their nutritional and metabolic needs, or, how they obtain the energy and carbon needed to make the organic molecules that are the building blocks of cells. The nutritional diversity of prokaryotes is greater than that of eukaryotes. While all of nutritional modes observed in eukaryotes is also observed in prokaryotes, there are some nutritional modes that are unique to prokaryotic populations. These modes include chemoautotrophy and photoheterotrophy.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
| Big Idea 2 | Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. |
| Enduring Understanding 2.A | Growth, reproduction and maintenance of living systems require free energy and matter. |
| Essential Knowledge | 2.A.2 Prokaryotes have evolved multiple energy-capturing strategies, and photosynthesis first evolved in prokaryotes and was responsible for the production of an oxygenated atmosphere. |
| Science Practice | 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. |
| Science Practice | 3.1 The student can pose scientific questions. |
| Learning Objective | 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store and use free energy. |
| Enduring Understanding 2.A | Growth, reproduction and maintenance of living systems require free energy and matter. |
| Essential Knowledge | 2.A.2 Prokaryotes have evolved multiple energy-capturing strategies, and photosynthesis first evolved in prokaryotes and was responsible for the production of an oxygenated atmosphere. |
| Science Practice | 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. |
| Learning Objective | 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store or use free energy. |
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 4.7][APLO 4.10][APLO 4.23][APLO 2.28]
Needs of Prokaryotes
The diverse environments and ecosystems on Earth have a wide range of conditions in terms of temperature, available nutrients, acidity, salinity, and energy sources. Prokaryotes are very well equipped to make their living out of a vast array of nutrients and conditions. To live, prokaryotes need a source of energy, a source of carbon, and some additional nutrients.
Macronutrients
Cells are essentially a well-organized assemblage of macromolecules and water. Recall that macromolecules are produced by the polymerization of smaller units called monomers. For cells to build all of the molecules required to sustain life, they need certain substances, collectively called nutrients. When prokaryotes grow in nature, they obtain their nutrients from the environment. Nutrients that are required in large amounts are called macronutrients, whereas those required in smaller or trace amounts are called micronutrients. Just a handful of elements are considered macronutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. (A mnemonic for remembering these elements is the acronym CHONPS.)
Why are these macronutrients needed in large amounts? They are the components of organic compounds in cells, including water. Carbon is the major element in all macromolecules: carbohydrates, proteins, nucleic acids, lipids, and many other compounds. Carbon accounts for about 50 percent of the composition of the cell. Nitrogen represents 12 percent of the total dry weight of a typical cell and is a component of proteins, nucleic acids, and other cell constituents. Most of the nitrogen available in nature is either atmospheric nitrogen (N2) or another inorganic form. Diatomic (N2) nitrogen, however, can be converted into an organic form only by certain organisms, called nitrogen-fixing organisms. Both hydrogen and oxygen are part of many organic compounds and of water. Phosphorus is required by all organisms for the synthesis of nucleotides and phospholipids. Sulfur is part of the structure of some amino acids such as cysteine and methionine, and is also present in several vitamins and coenzymes. Other important macronutrients are potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na). Although these elements are required in smaller amounts, they are very important for the structure and function of the prokaryotic cell.
Micronutrients
In addition to these macronutrients, prokaryotes require various metallic elements in small amounts. These are referred to as micronutrients or trace elements. For example, iron is necessary for the function of the cytochromes involved in electron-transport reactions. Some prokaryotes require other elements—such as boron (B), chromium (Cr), and manganese (Mn)—primarily as enzyme cofactors.
The Ways in Which Prokaryotes Obtain Energy
Prokaryotes can use different sources of energy to assemble macromolecules from smaller molecules. Phototrophs (or phototrophic organisms) obtain their energy from sunlight. Chemotrophs (or chemosynthetic organisms) obtain their energy from chemical compounds. Chemotrophs that can use organic compounds as energy sources are called chemoorganotrophs. Those that can also use inorganic compounds as energy sources are called chemolithotrophs.
The Ways in Which Prokaryotes Obtain Carbon
Prokaryotes not only can use different sources of energy but also different sources of carbon compounds. Recall that organisms that are able to fix inorganic carbon are called autotrophs. Autotrophic prokaryotes synthesize organic molecules from carbon dioxide. In contrast, heterotrophic prokaryotes obtain carbon from organic compounds. To make the picture more complex, the terms that describe how prokaryotes obtain energy and carbon can be combined. Thus, photoautotrophs use energy from sunlight, and carbon from carbon dioxide and water, whereas chemoheterotrophs obtain energy and carbon from an organic chemical source. Chemolitoautotrophs obtain their energy from inorganic compounds, and they build their complex molecules from carbon dioxide. The table below (Table 22.3) summarizes carbon and energy sources in prokaryotes.
| Energy Sources | Carbon Sources | |||
|---|---|---|---|---|
| Light | Chemicals | Carbon dioxide | Organic compounds | |
| Phototrophs | Chemotrophs | Autotrophs | Heterotrophs | |
| Organic chemicals | Inorganic chemicals | |||
| Chemo-organotrophs | Chemolithotrophs | |||
Role of Prokaryotes in Ecosystems
Prokaryotes are ubiquitous: There is no niche or ecosystem in which they are not present. Prokaryotes play many roles in the environments they occupy. The roles they play in the carbon and nitrogen cycles are vital to life on Earth.
Prokaryotes and the Carbon Cycle
Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle (Figure 22.18). Carbon is cycled through Earth’s major reservoirs: land, the atmosphere, aquatic environments, sediments and rocks, and biomass. The movement of carbon is via carbon dioxide, which is removed from the atmosphere by land plants and marine prokaryotes, and is returned to the atmosphere via the respiration of chemoorganotrophic organisms, including prokaryotes, fungi, and animals. Although the largest carbon reservoir in terrestrial ecosystems is in rocks and sediments, that carbon is not readily available.
A large amount of available carbon is found in land plants. Plants, which are producers, use carbon dioxide from the air to synthesize carbon compounds. Related to this, one very significant source of carbon compounds is humus, which is a mixture of organic materials from dead plants and prokaryotes that have resisted decomposition. Consumers such as animals use organic compounds generated by producers and release carbon dioxide to the atmosphere. Then, bacteria and fungi, collectively called decomposers, carry out the breakdown (decomposition) of plants and animals and their organic compounds. The most important contributor of carbon dioxide to the atmosphere is microbial decomposition of dead material (dead animals, plants, and humus) that undergo respiration.
In aqueous environments and their anoxic sediments, there is another carbon cycle taking place. In this case, the cycle is based on one-carbon compounds. In anoxic sediments, prokaryotes, mostly archaea, produce methane (CH4). This methane moves into the zone above the sediment, which is richer in oxygen and supports bacteria called methane oxidizers that oxidize methane to carbon dioxide, which then returns to the atmosphere.
Prokaryotes and the Nitrogen Cycle
Nitrogen is a very important element for life because it is part of proteins and nucleic acids. It is a macronutrient, and in nature, it is recycled from organic compounds to ammonia, ammonium ions, nitrate, nitrite, and nitrogen gas by myriad processes, many of which are carried out only by prokaryotes. As illustrated in Figure 22.19, prokaryotes are key to the nitrogen cycle. The largest pool of nitrogen available in the terrestrial ecosystem is gaseous nitrogen from the air, but this nitrogen is not usable by plants, which are primary producers. Gaseous nitrogen is transformed, or “fixed” into more readily available forms such as ammonia through the process of nitrogen fixation. Ammonia can be used by plants or converted to other forms.
Another source of ammonia is ammonification, the process by which ammonia is released during the decomposition of nitrogen-containing organic compounds. Ammonia released to the atmosphere, however, represents only 15 percent of the total nitrogen released; the rest is as N2 and N2O. Ammonia is catabolized anaerobically by some prokaryotes, yielding N2 as the final product. Nitrification is the conversion of ammonium to nitrite and nitrate. Nitrification in soils is carried out by bacteria belonging to the genera Nitrosomas, Nitrobacter, and Nitrospira. The bacteria performs the reverse process, the reduction of nitrate from the soils to gaseous compounds such as N2O, NO, and N2, a process called denitrification.
Visual Connection
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Nitrogen-fixing bacteria exist on the root nodules of legumes and in the soil.
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Denitrifying bacteria convert nitrates (NO3-) into nitrogen gas (N2).
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Ammonification is the process by which ammonium ion (NH4+) is released from decomposing organic compounds.
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Nitrification is the process by which nitrites (NO2-) are converted to ammonium ion (NH4+).
Science Practice Connection for AP® Courses
Think About It
Prokaryotes inhabit many diverse environments. Think about the conditions (temperature, light, pressure, and organic and inorganic materials) that you may find in a deep-sea hydrothermal vent. What types of prokaryotes, in terms of their metabolic needs (autotrophs, phototrophs, chemotrophs, and so on) would expect to find there? What features of these prokaryotes would make it possible for them to inhabit such an extreme environment?
Teacher Support
Extremophiles or chemoautotrophs would inhabit deep-sea hydrothermal vents. This question is an application of AP® Learning Objective 2.5 and Science Practice 6.2 because students are explaining features and processes that allow prokaryotes to capture sufficient free energy to survive in an extreme environment.