Learning Objectives for this Section
Energy (def) is defined as the ability to do work. Organisms require energy for functions such as movement, active transport of nutrients into the cell, and the biosynthesis of cell components such as nucleotides, RNA, DNA, proteins, membranes. In other words, energy is required to drive various biosynthetic chemical reactions and do mechanical work.
Energy can exist as potential energy or kinetic energy. Potential energy (def) is stored energy such as the energy stored in the chemical bonds of carbohydrates, lipids, proteins, and ATP (def). Kinetic energy (def) is the energy of motion such as the energy being used to synthesize chemical compounds, transport substances across a membrane, or enable movement.
The first law of thermodynamics (def) states that energy can be neither created nor destroyed. It can, however, be converted from one form to another. For example, during photosynthesis, the energy in photons from the sun are converted into the energy in the chemical bonds of glucose and other organic molecules. During cellular respiration, the energy in the chemical bonds of glucose and other organic molecules can then be converted into energy in the chemical bonds of ATP, the form of energy most commonly used to do cellular work. During each of these energy conversions, however, some energy is always lost as heat.
The second law of thermodynamics (def) states that when energy is converted from one form to another, some energy is always lost as heat. In other words, no energy conversion is ever 100% efficient. Some usable energy, the energy available to do work, is always dispersed as heat into the surrounding environment. This is a part of what is called entropy, the concept that everywhere in the universe, disorder is always increasing. For example, a good portion of the orderly potential energy found in the chemical bonds of food is always being converted into the disorderly random molecular motion of heat during the production of ATP.
Metabolism (def) refers to all of the organized chemical reactions in a cell. Biochemical reactions that harvest energy during the breakdown of chemical compounds are called catabolic reactions (def), whereas biochemical reactions that require energy in order to synthesize chemical compounds are termed anabolic reactions (def). All of these biochemical reactions, as discussed earlier, are catalyzed of enzymes (def) . Frequently, metabolic processes occur as a result of a series of sequential biochemical reactions that make up what is called a metabolic pathway. Examples of metabolic pathways involved in cellular energy conversions include glycolysis, the citric acid (Krebs) cycle, and photosynthesis.
Free energy (def) refers to the amount of energy available during a chemical reaction to do cellular work. When the reactants of a chemical reaction have more free energy than the products of that reaction and energy is released, the reaction is said to be exergonic (def). On the other hand, if the products of a chemical reaction have more free energy than the reactants and the reaction requires an input of energy, the reaction is said to be endergonic (def).
All organisms in nature can be placed into one of four separate groups based on their nutritional patterns: photoautotrophs, photoheterotrophs, chemoautotrophs, and chemoheterotrophs.
a. Photoautotrophs (def) use light as an energy source and carbon dioxide as their main carbon source. They include photosynthetic bacteria (green sulfur bacteria, purple sulfur bacteria, and cyanobacteria), algae, and green plants. Photoautotrophs transform carbon dioxide and water into carbohydrates and oxygen gas through photosynthesis (def).
Cyanobacteria, as well as algae and green plants, use hydrogen atoms from water to reduce carbon dioxide to form carbohydrates, and during this process oxygen gas is given off (an oxygenic process). Other photosynthetic bacteria (the green sulfur bacteria and purple sulfur bacteria) carry out an anoxygenic process, using sulfur, sulfur compounds or hydrogen gas to reduce carbon dioxide and form organic compounds.
b. Photoheterotrophs (def) use light as an energy source but cannot convert carbon dioxide into energy. Instead they use organic compounds (def) as a carbon source. They include the green nonsulfur bacteria and the purple nonsulfur bacteria.
c. Chemolithoautotrophs (def) use inorganic compounds such as hydrogen sulfide, sulfur, ammonia, nitrites, hydrogen gas, or iron as an energy source and carbon dioxide as their main carbon source.
d. Chemooganoheterotrophs (def) use organic compounds (def) as both an energy source and a carbon source. Saprophytes (def) live on dead organic matter while parasites (def) get their nutrients from a living host. Most bacteria, and all protozoans, fungi, and animals are chemoorganoheterotrophs.
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Updated: June 26, 2001