Many organisms that use other oxidants (such as SO 4 2−, NO 3 −, or CO 3 2−) or oxidized organic compounds can live only in the absence of O 2, which is a deadly poison for them such species are called anaerobic organisms. Organisms that can use only O 2 as the oxidant (a group that includes most animals) are aerobic organisms that cannot survive in the absence of O 2.
Regardless of the nature of their energy and carbon sources, all organisms use oxidation–reduction, or redox, reactions to drive the synthesis of complex biomolecules. Phototrophs, such as plants, algae, and photosynthetic bacteria, use the radiant energy of the sun directly, converting water and carbon dioxide to energy-rich organic compounds, whereas chemotrophs, such as animals, fungi, and many nonphotosynthetic bacteria, obtain energy-rich organic compounds from their environment.
For example, a wood fire releases heat to its surroundings, but unless energy from the burning wood is also used to do work, there is no increase in order of any portion of the universe.Īlthough organisms employ a wide range of specific strategies to obtain the energy they need to live and reproduce, they can generally be divided into two categories: organisms are either phototrophs (from the Greek photos, meaning “light,” and trophos, meaning “feeder”), whose energy source is sunlight, or chemotrophs, whose energy source is chemical compounds, usually obtained by consuming or breaking down other organisms. Releasing heat to the surroundings is necessary but not sufficient for life: the release of energy must be coupled to processes that increase the degree of order within a cell. As long as Δ S surr is positive and greater than Δ S sys, the entropy of the universe increases, so the second law of thermodynamics is not violated. A cell releases some of the energy that it obtains from its environment as heat that is transferred to its surroundings, thereby resulting in an increase in S surr ( Figure 18.17 "Life and Entropy"). Consequently, a relatively new subdiscipline called nonequilibrium thermodynamics has been developed to quantitatively describe open systems such as living cells.īecause a cell cannot violate the second law of thermodynamics, the only way it can maintain a low-entropy, nonequilibrium state characterized by a high degree of structural organization is to increase the entropy of its surroundings. Cells use the energy obtained in these ways to maintain the nonequilibrium state that is essential for life.īecause cells are open systems, they cannot be described using the concepts of classical thermodynamics that we have discussed in this chapter, which have focused on reversible processes occurring in closed chemical systems that can exchange energy-but not matter-with their surroundings. In contrast, a cell is an open system that can exchange matter with its surroundings as well as absorb energy from its environment in the form of heat or light. With no external input, a clock will run down, a battery will lose its charge, and a mixture of an aqueous acid and an aqueous base will achieve a uniform intermediate pH value. One implication of the first and second laws of thermodynamics is that any closed system must eventually reach equilibrium. Energy Flow between a Cell and Its Surroundings
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