Chapter 7 Section 1: Life is CellularWhat’s the smallest part of any living thing that still counts as being “alive?”Can we just keep dividing living things into smaller and smaller parts, or is there a point at which what’s left is no longer alive?As you will see, there is such a limit. The smallest living unit of any organism is the cell.What is the cell theory?The cell theory states:- All living things are made up of cells.- Cells are the basic units of structure and function in living things.- New cells are produced from existing cells.It was not until the mid-1600s that scientists began to use microscopes to observe living things.In 1665, Englishman Robert Hooke used an early compound microscope to look at a nonliving thin slice of cork, a plant material.Under the microscope, cork seemed to be made of thousands of tiny, empty chambers that Hooke called “cells”. The term cell is used in biology to this day.Today we know that living cells are not empty chambers, but contain a huge array of working parts, each with its own function.In Holland, Anton van Leeuwenhoek examined pond water and other things, includinga sample taken from a human mouth. He drew the organisms he saw in the mouth—which today we call bacteria.Soon after Leeuwenhoek, observations made by other scientists made it clear that cells were the basic units of life.In 1838, German botanist Matthias Schleiden concluded that all plants are made of cells.The next year, German biologist Theodor Schwann stated that all animals were made of cells.In 1855, German physician Rudolf Virchow concluded that new cells could be produced only from the division of existing cells, confirming a suggestion made by German Lorenz Oken 50 years earlier. These discoveries are summarized in the cell theory, a fundamental concept of biology. How do microscopes work?Most microscopes use lenses to magnify the image of an object by focusing light or electrons.A typical light microscope allows light to pass through a specimen and uses two lenses to form an image.The first set of lenses, located just above the specimen, produces an enlarged image of the specimen.The second set of lenses magnifies this image still further.Because light waves are diffracted, or scattered, as they pass through matter, light microscopes can produce clear images of objects only to a magnification of about 1000 times.