4.1 Cell TheoryCells are characteristically microscopic in size. Although there are exceptions, a typical eukaryotic cell is 10 to 100 micrometers ( m) (10-100 millionths μof a meter) in diameter, although most prokaryotic cells are only 1 to 10 m in μ diameter.Because cells are so small, they were not discovered until the invention of the microscope in the 17th century. English natural philospher Robert Hooke was the first to observe cells in 1665, naming theshapes he saw in cork cellulae. This is known to us as cells. Another early microscopist, Dutch Anton van Leeuwenhoek, first observed living cells, which he termed "animalcules," or little animals. After these early efforts, a century and a half passed before bioogists fully recognized the importance of cells. In 1838, German botonist Matthias Shcleiden stated that all plants "are aggreagates of fully individualized , independent, separate beings, namely the cells themselves." In 1839, German physiologist Theodor Schwann reported that all animal tissues also consist of individual cells. Thus, the cell theory was born.Cell theory is the unifying foundation of cell biologyIn its modern form, the cell theory include the following three principles:1. All organisms are composed of one or more cells, and the life processes of metabolism and heredity occur within these cells.2. Cells are the smallest living things, the basic units of organization of allorganisms.3. Cells arise only by division of a previously existing cellCell size is limitedMost cells are relatively small for reasons related to the diffusion of substances into and out of them. The rate of diffusion is affect by a number of variables, including (1) surface area available for diffusion, (2) temperature, (3) concentration gradient of diffusing substances, and (4) the distance over which diffusion must occur. As the size of a cell increases, the length of time for diffusion from outside membrane to the interior of the cell increases as well. Larger cells need to synthesize more macromolecules, have correspondingly higher energy requirements, and produce a greater quantity of waste. Molecules used for energy and biosynthesis must be transported through the membrane. Anymetabolic waste produced must be removed, also passing through the membrane. The rate at which this transport occurs depends on both thedistance to the membrane and the area of membrane available. For this reason, an organism made up of many relatively small cells has an advantage over one composed of fewer, larger cells.The advantage of small cell size is readily apparent in terms of the surface area-to-volume ratio. As a cell's size increases, its volume increases much more rapidly than its surface area. For a spherical cell, thesurface area is proportional to the square of the radius, whereas the volume is proportional to the cube of the radius. Thus, if the radii of two cells differ by a factor of 10, the larger cell will have 100 times the surface area,but 1000 time the volume of the smaller cell.Microscopes allow visualization of cells and componentsOther than egg cells, not many cells are visible to the naked eye. Most are less than 50 m in diamter, far smaller than the period at the end of this sentence.μSo, to visualize cells we need the aid of technology. The development of microscopes and their refinement over the centuries has allowed us tocontinually explore cells in greater detail.The resolution problemHow do we study cells if they are too small to see? The key is to understand why we can't see them. The reason we can't see such small objects is the limited resolution of the human eye. Resolution it the minimum distance two points can be apart and still be distinguished at two separate points. When two objects are closer together than about 100 m, the light μreflected from each strikes the same photreceptor cell at the rear of theeye. Only when the objects are farther than 100 m, the light reflected from μeach strikes the same photoreceptor cell at the rear of the eye. only when the objects are farther than 100 m apart can the light from μeach strike different cells, allowing your eye to resolve them as two distinct objects rather than one. Types of microscopesModern light microscopes, which operate with visible light, use two magnifying lenses (and a variety of correcting lenses) to achieve very high magnification and clarity. The first lens focuses the image of the object on the second lens, which magnifies it again and focuses it on the back of the eye . Microscopes that magnify in stages using several lenses are called compound microscopes. They can resolve structures that are separated by at least 200 nanometers (nm). Light microscopes, even compound ones, are not powerful enough to resolve many of the structures within cells. For example, a cell membrane is only 5 nm thick. Why not just add another magnifying stage to the microscope to increase its resolving power? This doesn't work because when two objects are closer than a few hundred nanometers, the light beams reflecting from the two images start to overlap each other. The only way two light beams can get closer together and still be resolved is if their wavelengths are shorter. One way to avoid overlap is by using a beam of electrons rather than a beam of light. Electrons have a much shorter wavelngths, and an electron microscope, employing electron beams, has 1000 times the resolving power of a light microscope. Transmission electron microscopes, so called because the electrons used to visualize the specimentsare transmitted through the material, are capable of resolving objects only 0.2 nm apart--which is only twice the diameter of a hydrogen atom!A second kind of electron microscope, the scanning electron microscop,beams electrons onto the surface of the specimen. The electrons reflected back from the surface, together with other electrons that the specimen itself emits as a result of the bombardment, are amplified and transmitted to a screen, where the image can be viewed and photographed. Scanning electron microscopy yields striking three-dimensional images. This technique has improved our understanding of many biological and physical phenomena.Using stains to view cell structureAlthough resolution remains a physical limit, we can improve the images we see by altering the sample. Certain chemical stains increase the contrast between different cellular components. Structures