The Basic Chemicals of Life

1. The Basic Chemicals of Life • In the 1920s, scientists Aleksandr I. Oparin and John B. S. Haldane suggested that Earth’s early oceans contained large amounts of organic molecules. • They proposed these molecules formed spontaneously in chemical reactions that were activated by energy from the sun, volcanic eruptions, and lightning.

2. The Basic Chemicals of Life, continued • Oparin and other scientists hypothesized that the early atmosphere was rich in hydrogen gas (H2) and hydrogen-containing gases, such as water vapor (H2O), ammonia (NH3), and methane (CH4). • They thought that if the atmosphere lacked oxygen, a variety of organic compounds made up of these elements could form. • The hypothesis was tested in the 1950s by American scientists Harold Urey and Stanley Miller.

3. The Basic Chemicals of Life, continued The Miller-Urey Experiment • In this experiment, Miller placed hydrogen-containing gases into a device. • He used electrical sparks to simulate lightning. • After a few days, Miller found organic molecules in his device, including some of life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons (molecules made of carbon and hydrogen).

4. Miller-Urey Experiment

5. The Basic Chemicals of Life, continued • The Miller-Urey experiment showed that, under certain conditions, organic molecules could form from inorganic molecules. • We now know that the molecules used in Miller’s experiment could not have existed in abundance on early Earth. • Four billion years ago, shortly after it formed, Earth did not have a protective ozone layer.

6. The Basic Chemicals of Life, continued • Ultraviolet radiation from the sun would have destroyed any methane and ammonia in the atmosphere. • When ammonia and methane gases were absent from the Miller-Urey experiment, key biological molecules were not made. • However, the experiment shows that complex biological compounds can form from simple building blocks.

7. Life’s Building Blocks • One hypothesis that addresses the origin of life states that early biological molecules formed close to hydrothermal vents. • The heat from hydrothermal vents could have provided energy for chemical reactions. • Within the sea, biological molecules would have been protected from harmful solar radiation.

8. Life’s Building Blocks, continued • Another hypothesis that addresses the origin of life states that organic molecules arrived on Earth in meteorites or comets. • Some meteorites contain amino acids. • Such molecules could have arrived on early Earth, when frequent meteorite impacts were common.

9. The First Cells • Research continues to discover how biological molecules grouped together to form cells. • Scientists must figure out how amino acids first linked together to form proteins. Forming a Cell • Scientists have conducted research to determine how molecules became packaged together inside a cell membrane.

10. The First Cells, continued • When studying the behavior of organic molecules in water, scientists have observed that lipids tend to combine in water. • Certain lipids, when combined with other molecules, can form a tiny droplet that has a surface that resembles a cell membrane. • Further research has shown that short chains of amino acids can form tiny spherical structures called microspheres.

11. The First Cells, continued • Many scientists think that the formation of microspheres may have been the first step toward cellular organization. • Microspheres could not be considered cells unless they had the characteristic of heredity. Origins of Heredity • Scientists have studied the origins of heredity by studying the formation of proteins.

12. The First Cells, continued • In the laboratory, scientists have not been able to make proteins or DNA form spontaneously in water. • They have been able to form short chains of RNA, the nucleic acid that helps to carry out the instructions of DNA. • RNA molecules may have been the first self-replicating molecule.

13. The First Cells, continued • In the 1980s, American scientists Thomas Cech and Sidney Altman found a certain type of RNA molecule, called a ribozyme, that can act like an enzyme. • They also showed that RNA can form spontaneously in water, without DNA. • They hypothesized that RNA was the first self-replicating molecule that stored information and catalyzed the formation of the first proteins.

14. The First Cells, continued • One idea of how RNA could have been involved in protein synthesis is shown here.

15. The First Cells, continued • Cech and Altman further hypothesized that RNA could have changed—evolved—from one generation to the next. • Scientists hypothesize that DNA and proteins eventually took over these roles in the cell.

16. Summary • The Miller-Urey experiment showed that, under certain circumstances, organic compounds could form from inorganic molecules. • Among the scientific theories that address the origin of life, one suggests that life began close to hydrothermal vents, and another proposes that organic molecules arrived on early Earth from a meteorite. • The formation of microspheres might have been the first step toward cellular organization.