Photosynthesis (discussed in detail in Chapter 14) and respiration are complementary processes (Figure 2–18). This means that the transactions between plants and animals are not all one way. Plants, animals, and microorganisms have existed together on this planet for so long that many of them have become an essential part of the others’ environments. The oxygen released by photosynthesis is consumed in the combustion of organic molecules during aerobic respiration. And some of the CO2 molecules that are fixed today into organic molecules by photosynthesis in a green leaf were yesterday released into the atmosphere by the respiration of an animal—or by the respiration of a fungus or bacterium decomposing dead organic matter. We therefore see that carbon utilization forms a huge cycle that involves the biosphere (all of the living organisms on Earth) as a whole (Figure 2–19). Similarly, atoms of nitrogen, phosphorus, and sulfur move between the living and nonliving worlds in cycles that involve plants, animals, fungi, and bacteria.
The cell does not oxidize organic molecules in one step, as occurs when organic material is burned in a fire. Through the use of enzyme catalysts, metabolism takes these molecules through a large number of reactions that only rarely involve the direct addition of oxygen. Before we consider some of these reactions and their purpose, we discuss what is meant by the process of oxidation.
Figure 2–18 Photosynthesis and respiration as complementary processes in the living world. photosynthesis converts the electromagnetic energy in sunlight into chemical-bond energy in sugars and other organic molecules. plants, algae, and cyanobacteria obtain the carbon atoms that they need for this purpose from atmospheric Co2 and the hydrogen from water, releasing o2 gas as a by-product. The organic molecules produced by photosynthesis in turn serve as food for other organisms. many of these organisms carry out aerobic respiration, a process that uses o2 to form Co2 from the same carbon atoms that had been taken up as Co2 and converted into sugars by photosynthesis. in the process, the organisms that respire obtain the chemical-bond energy that they need to survive.
The first cells on the earth are thought to have been capable of neither photosynthesis nor respiration (discussed in Chapter 14). however, photosynthesis must have preceded respiration on the earth, since there is strong evidence that billions of years of photosynthesis were required before o2 had been released in sufficient quantity to create an atmosphere rich in this gas. (The earth’s atmosphere currently contains 20% o2.)
HUMUS AND DISSOLVED ORGANIC MATTER SEDIMENTS AND FOSSIL FUELS RESPIRATION PHOTOSYNTHESIS FOOD CHAIN CO2 IN ATMOSPHERE AND WATER PLANTS, ALGAE, BACTERIA ANIMALS
Figure 2–19 The carbon cycle. individual carbon atoms are incorporated into organic molecules of the living world by the photosynthetic activity of bacteria, algae, and plants. They pass to animals, microorganisms, and organic material in soil and oceans in cyclic paths. Co2 is restored to the atmosphere when organic molecules are oxidized by cells or burned by humans as fuels.
Chapter 2: Cell Chemistry and Bioenergetics enzymes lower the activation-energy Barriers That Block Chemical Reactions