4 1 Cell Theory Cells are characteristically microscopic in size Although there are exceptions a typical eukaryotic cell is 10 to 100 micrometers m 10 100 millionths of a meter in diameter although most prokaryotic cells are only 1 to 10 m in 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 the shapes 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 biology In 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 all organisms 3 Cells arise only by division of a previously existing cell Cell size is limited Most 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 Any metabolic waste produced must be removed also passing through the membrane The rate at which this transport occurs depends on both the distance 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 the surface area is cells differ by a but 1000 is proportional to the square of the radius whereas the volume proportional to the cube of the radius Thus if the radii of two factor of 10 the larger cell will have 100 times the surface area time the volume of the smaller cell Microscopes allow visualization of cells and components Other 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 So to visualize cells we need the aid of technology The development of microscopes and their refinement over the centuries has allowed us to continually explore cells in greater detail The resolution problem How 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 reflected from each strikes the same photreceptor cell at the rear of the eye Only when the objects are farther than 100 m the light reflected from each 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 each strike different cells allowing your eye to resolve them as two distinct objects rather than one Types of microscopes Modern 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 speciments are 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 structure Although 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 within the cell absorb or exclude the stain differentially producing contrast that aids resolution Stains that bind to specific types of molecules have made these