Photosynthesis allows some organisms to capture energy from the sun

Living cells require energy to function. The earliest prokaryotes supplied their energy needs—their metabolism—by taking in small molecules from their environment, breaking the chemical bonds of these molecules, and using the energy released from those chemical bonds to do cellular work. Many modern prokaryotes still function this way, and they function very successfully. But about 2.5 billion years ago, the emergence of photosynthesis changed the nature of life on Earth.

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Photosynthesis transforms the energy of sunlight into a form of *chemical energy that can be used to do work such as the synthesis of large molecules. These large molecules can then be used to build cell structures or can be broken down to provide metabolic energy. Photosynthesis is the basis of much of life on Earth today because its energy-capturing processes provide food for other organisms. Early photosynthetic cells were probably similar to present-day prokaryotes called cyanobacteria (Figure 1.4). Over time, photosynthetic prokaryotes became so abundant that vast quantities of oxygen gas (O2), which is a by-product of photosynthesis, began to accumulate in the atmosphere.

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Figure 1.4 Photosynthetic Organisms Changed Earth’s Atmosphere (A) Cyanobacteria are aquatic and photosynthetic: they live in the water and can manufacture their own food. Although they are quite small, they often grow in colonies large enough to see. (B) Colonies of photosynthetic cyanobacteria and other microorganisms produced structures called stromatolites that were preserved in the ancient fossil record. This section of fossilized stromatolite reveals layers representing centuries of growth. (C) Living stromatolites can still be found in appropriate environments.

*connect the concepts The pathways that harvest chemical energy to do all the kinds of biological work necessary to support metabolism are presented in Chapter 9.

During the early history of prokaryotic life, there was no O2 in Earth’s atmosphere. In fact, O2 was toxic to many of the prokaryotes living at that time, and its buildup in the atmosphere resulted in a huge mass extinction. But those organisms that could tolerate O2 proliferated. Atmospheric O2 opened up vast new avenues of evolution because aerobic metabolism—a biochemical process that uses O2 to extract energy from nutrient molecules—is far more efficient than anaerobic metabolism (which does not use O2). Most organisms today use aerobic metabolism.

Oxygen in the atmosphere also made it possible for life to move onto land. For most of life’s history, UV radiation falling on Earth’s surface was so intense that it destroyed any organism that was not shielded by water. The atmospheric accumulation of photosynthetically generated O2 over a period of more than 2 billion years gradually produced a thick layer of ozone (O3) in the upper atmosphere. By about 500 million years ago, the ozone layer was sufficiently dense and absorbed enough of the sun’s UV radiation to make it possible for organisms to leave the protection of the water and live on land.