{01} Microorganisms (1674) Microscope lens grinder Anton Van Leeuwenhoek accidentally discovers microorganisms in a drop of water. Using his own microscopes, he observes sperm, bacteria and red blood cells. His observations lay the foundation for the sciences of bacteriology and microbiology. {02} The Cell Nucleus (1831) While studying an orchid, botanist Robert Brown identifies a structure within the cells that he terms the "nucleus." {03} Archaea (1977) Carl Woese discovers bacteria are not the only simple celled prokaryotes (unicellular organisms without a nucleus) on Earth. Many of the organisms classified in the new kingdom of Archaea are extremophiles. Some live at very high or low temperatures, others in highly saline, acidic or alkaline water. Some have been found in environments like marshland, sewage and soil. Archaea are usually harmless to other organisms and none are known to cause disease. {04} Cell Division (1879) Walther Flemming carefully observes that animal cells divide in stages and calls the process mitosis. Eduard Strasburger independently identifies a similar process in plant cells. {05} Sex Cells (1884) August Weismann identifies that sex cells must have divided differently to end up with only half of a chromosomal set. This very special division of sex cells is called meiosis. Weismann's experiments with reproduction in jellyfish lead him to the conclusion that variations in offspring result from the union of a substance from the parents. He refers to this substance as "germ plasm." {06} Cell Differentiation (late 19th century) Several scientists participate in the discovery of cell differentiation, eventually leading to the isolation of human embryonic stem cells. During differentiation, a cell turns into one of the many cell types that make up the body, such as a lung, skin or muscle cell. Certain genes are activated and others are inactivated, so the cell develops structures to perform a specific function. Cells that are not yet differentiated and have the potential to become any type of cell are called stem cells. {07} Mitochondria (late 19th century to the present) Scientists discover mitochondria, the powerhouses of the cell. These small structures within animal cells are responsible for metabolism and convert food into chemicals that cells can use. Scientists now believe they are specialized bacteria with their own DNA. {08} The Krebs Cycle (1937) Hans Krebs identifies the many steps the cell takes to convert sugars, fats and proteins into energy. Also known as the citric acid cycle, it is a series of chemical reactions using oxygen as part of cellular respiration. The cycle contributes to the breakdown of carbohydrates, fats and proteins into carbon dioxide and water. {09} Neurotransmission (late 19th to early 20th century) Discovery of neurotransmitters, how they tell the body what to do by passing signals from one nerve cell to another via chemical substances or electrical signals. {10} Hormones (1903) William H. Bayliss and Ernest H. Starling give hormones their name and reveal their role as chemical messengers. The team specifically describes secretin, a substance released into the blood from the duodenum (between the stomach and small intestine) that stimulates secretion of pancreatic digestive juice into the intestine. {11} Photosynthesis (1770s) Jan Ingenhousz discovers that plants react to sunlight differently than shade. The underpinnings of the understanding of photosynthesis are born. Photosynthesis is a process in which plants, algae and certain bacteria convert the energy of light into chemical energy. In plants, leaves take in carbon dioxide and roots absorb water. Sunlight runs a reaction that yields glucose (food for the plant) and oxygen (a waste product released into the environment). Nearly all living things on Earth are dependent on this process.
