Pathways that Harvest and Store Chemical Energy

Agriculture was a key invention in the development of human civilizations. The planting and harvesting of seeds began about 10,000 years ago. One of the first plants to be turned into a reliable crop was barley, and one of the first uses of barley was to brew beer. Living in what is now Iraq, ancient Sumerians learned that partly germinated and then mashed-up barley seeds, stored under the right conditions, could produce a potent and pleasant drink. An ancient king, Hammurabi, laid down the oldest known laws regarding an alcoholic beverage: the daily beer ration was 2 liters for a normal worker, 3 liters for a civil servant, and 5 liters for a high priest. Alcoholic beverages were not just a diversion to these people; their health depended on them. Drinking water from rivers and ponds caused diseases, and whatever caused these diseases was not present in liquids containing alcohol.

Early chemists and biologists were interested in how mashed barley seeds (or grapes, in the case of wine) were transformed into alcoholic beverages. By the nineteenth century there were two theories. Chemists claimed that these transformations were simply chemical reactions, not some special property of the plant material. Biologists, armed with their microscopes and cell theory (see Chapter 4), said that the barley and grape extracts were converted to beer and wine by living cells.

The great French scientist Louis Pasteur tackled the question in the 1860s, responding to a challenge posed by a group of distillers who wanted to use sugar beets to produce alcohol. Pasteur found that (1) nothing happened to beet mash unless microscopic yeast cells were present; (2) in the presence of fresh air, yeast cells grew vigorously on the mash, and bubbles of CO₂ were formed; and (3) without fresh air, the yeast grew slowly, less CO₂ was produced, and alcohol was formed. So the biologists were right: living cells produced alcohol from ground-up, sugary extracts. Later, biochemists broke open yeast cells and unraveled the sequence of chemical transformations from sugar to alcohol. It turned out that the chemists were right too: the production of alcohol involves a series of chemical reactions that require energy transfers. The flow of energy in living systems (such as yeast cells) involves the same chemical principles as energy flow in the inanimate world.