THE LIVING WORLD
Unit two. The Living Cell
This alarming-looking creature is the single-celled organism Dileptus, magnified a thousand times. Too small to see with the unaided eye, Dileptus is one of hundreds of inhabitants of a drop of pond water. Everything that a living organism does to survive and prosper, Dileptus must do with only the equipment this tiny cell provides. Just as you move about using legs to walk, so Dileptus uses the hairlike projections (called cilia) that cover its surface to propel itself through the water. Just as your brain is the control center of your body, so the compartment called the nucleus, deep within the interior of Dileptus, controls the many activities of this complex and very active cell. Dileptus has no mouth, but it takes in food particles and other molecules through its surface. This versatile protist is capable of leading a complex life because its interior is subdivided into compartments, in each of which it carries out different activities. Functional specialization is the hallmark of this cell’s interior, a powerful approach to cellular organization that is shared by all eukaryotes.
Hold your finger up and look at it closely. What do you see? Skin. It looks solid and smooth, creased with lines and flexible to the touch. But if you were able to remove a bit and examine it under a microscope, it would look very different— a sheet of tiny, irregularly shaped bodies crammed together like tiles on a floor. Figure 4.1 takes you on a journey into your fingertip. The crammed bodies you see in panels 3 and 4 are skin cells, laid out like a tiled floor. As your journey inward continues, you travel inside one of the cells and see organelles, structures in the cell that perform specific functions. Proceeding even farther inward, you encounter the molecules of which the structures are made, and finally the atoms shown in panels 8 and 9. While some organisms are composed of a single cell, your body is composed of many cells. A single human being has as many cells as the stars in a galaxy, between 10 and 100 trillion (depending on how big you are). All cells, however, are small. In this chapter we look more closely at cells and learn something of their internal structure and how they communicate with their environment.
Figure 4.1. The size of cells and their contents.
This diagram shows the size of human skin cells, organelles, and molecules. In general, the diameter of a human skin cell 4 is a little less than 20 micrometers (pm), of a mitochondrion 5 is 2 ^m, of a ribosome 7 is 20 nanometers (nm), of a protein molecule 8 is 2 nm, and of an atom 9 is 0.2 nm.
The Cell Theory
Because cells are so small, no one observed them until microscopes were invented in the mid-seventeenth century. Robert Hooke first described cells in 1665, when he used a microscope he had built to examine a thin slice of nonliving plant tissue called cork. Hooke observed a honeycomb of tiny, empty (because the cells were dead) compartments. He called the compartments in the cork cellulae (Latin, small rooms), and the term has come down to us as cells. For another century and a half, however, biologists failed to recognize the importance of cells. In 1838, botanist Matthias Schleiden made a careful study of plant tissues and developed the first statement of the cell theory. He stated that all plants “are aggregates of fully individualized, independent, separate beings, namely the cells themselves.” In 1839, Theodor Schwann reported that all animal tissues also consist of individual cells.
The idea that all organisms are composed of cells is called the cell theory. In its modern form, the cell theory includes three principles:
1. All organisms are composed of one or more cells, within which the processes of life occur.
2. Cells are the smallest living things. Nothing smaller than a cell is considered alive.
3. Cells arise only by division of a previously existing cell. Although life likely evolved spontaneously in the environment of the early earth, biologists have concluded that no additional cells are originating spontaneously at present. Rather, life on earth represents a continuous line of descent from those early cells.
Most Cells Are Very Small
Most cells are relatively small, but not all are the same size. The cells of your body are typically from 5 to 20 micrometers (a micrometer, gm, is one-millionth of a meter) in diameter, too small to see with the naked eye. Bacteria cells are even smaller than yours, only a few micrometers thick. However, there are some cells that are larger; individual marine alga cells, for example, can be up to 5 centimeters long—as long as your little finger.
Why Aren't Cells Larger?
Why are most cells so tiny? Most cells are small because larger cells do not function as efficiently. In the center of every cell is a command center that must issue orders to all parts of the cell, directing the synthesis of certain enzymes, the entry of ions and molecules from the exterior, and the assembly of new cell parts. These orders must pass from the core to all parts of the cell, and it takes them a very long time to reach the periphery of a large cell. For this reason, an organism made up of relatively small cells is at an advantage over one composed of larger cells.
Another reason cells are not larger is the advantage of having a greater surface-to-volume ratio. As cell size increases, volume grows much more rapidly than surface area. For a round cell, surface area increases as the square of the diameter, whereas volume increases as the cube. To visualize this, consider the two single cells in figure 4.2. The large cell to the right is 10 times bigger than the small cell, but while its surface area is 100 times greater (102), its volume is 1,000 times (103) the volume of the smaller cell. A cell’s surface provides the interior’s only opportunity to interact with the environment, with substances passing into and out of the cell across its surface. Large cells have far less surface for each unit of volume than do small ones.
Figure 4.2. Surface-to-volume ratio.
As a cell gets larger, its volume increases at a faster rate than its surface area. If the cell radius increases by 10 times, the surface area increases by 100 times, but the volume increases by 1,000 times. A cell's surface area must be large enough to meet the needs of its volume.
Some larger cells, however, function quite efficiently in part because they have structural features that increase surface area. For example, cells in the nervous system called neurons are long, slender cells, some extending more than a meter in length. These cells efficiently interact with their environment because although they are long, they are thin, some less than 1 micrometer in diameter, and so their interior regions are not far from the surface at any given point.
Another structural feature that increases the surface area of a cell are small “fingerlike” projections called microvilli. The cells that line the small intestines of the human digestive system are covered with microvilli that dramatically increase the surface area of the cells.
With few exceptions however, cells don’t usually grow much larger than 50 micrometers. For organisms to get much larger, they are usually composed of many cells. By grouping together many smaller cells, these multicellular organisms vastly increase their total surface-to-volume ratio.
An Overview of Cell Structure
All cells are surrounded by a delicate membrane, called a plasma membrane, that controls the permeability of the cell to water and dissolved substances. A semifluid matrix called cytoplasm fills the interior of the cell. It used to be thought that the cytoplasm was uniform, like Jell-O™, but we now know that it is highly organized. Your cells, for example, have an internal framework that both gives the cell its shape and positions components and materials within its interior. In the following sections, we explore the membranes that encase all living cells and then examine in detail their interiors.
How many cells are big enough to see with the unaided eye? Other than egg cells, not many are (figure 4.3). Most are less than 50 micrometers in diameter, far smaller than the period at the end of this sentence.
Figure 4.3. A scale of visibility.
Most cells are microscopic in size, although vertebrate eggs are typically large enough to be seen with the unaided eye. Prokaryotic cells are generally 1 to 2 micrometers (pm) across.
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 is defined as the minimum distance two points can be apart and still be distinguished as two separated points. On the visibility scale in figure 4.3 below, you can see that the limit of resolution of the human eye (the blue bar at the bottom) is about 100 micrometers. This limit occurs because when two objects are closer together than about 100 micrometers, the light reflected from each strikes the same “detector” cell at the rear of the eye. Only when the objects are farther apart than 100 micrometers will the light from each strike different cells, allowing your eye to resolve them as two objects rather than one.
Microscopes. One way to increase resolution is to increase magnification so that small objects appear larger. Robert Hooke and Anton van Leeuwenhoek used glass lenses to magnify small cells and cause them to appear larger than the 100-micrometer limit imposed by the human eye. The glass lens adds additional focusing power. Because the glass lens makes the object appear closer, the image on the back of the eye is bigger than it would be without the lens.
Modern light microscopes 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 more than 200 nanometers (nm). The six entries in the upper portion of table 4.1 are images viewed through various types of light microscopes.
Increasing Resolution. Light microscopes, even compound ones, are not powerful enough to resolve many structures within cells. For example, a membrane is only 5 nanometers thick. Why not just add another magnifying stage to the microscope and so increase its resolving power? Because when two objects are closer than a few hundred nanometers, the light beams reflecting from the two images start to overlap. 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 wavelength, and a microscope employing electron beams has 1,000 times the resolving power of a light microscope. A transmission electron microscope (TEM), so called because the electrons used to visualize the specimens are transmitted through the material, is capable of resolving objects only 0.2 nanometer apart—just twice the diameter of a hydrogen atom! The entry on the left under electron microscopes in table 4.1 is an example of an image captured using a TEM.
A second kind of electron microscope, the scanning electron microscope (SEM), beams the electrons onto the surface of the specimen. The electrons reflected back from the surface of the specimen, 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 threedimensional images and has improved our understanding of many biological and physical phenomena. The entry on the right in table 4.1 under electron microscopes is an SEM image.
TABLE 4.1. TYPES OF MICROSCOPES
Visualizing Cell Structure by Staining Specific Molecules. A powerful tool for the analysis of cell structure has been the use of stains that bind to specific molecular targets. This approach has been used in the analysis of tissue samples, or histology, for many years and has been improved dramatically with the use of antibodies that bind to very specific molecular structures. This process, called immunocytochemistry, uses antibodies generated in animals such as rabbits or mice. When these animals are injected with specific proteins, they will produce antibodies that specifically bind to the injected protein, which can be purified from their blood. These purified antibodies can then be chemically bonded to enzymes, stains, or fluorescent molecules that glow when exposed to specific wavelengths of light. When cells are washed in a solution containing the antibodies, the antibodies bind to cellular structures that contain the target molecule and can be seen with light microscopy. The image produced using fluorescence microscopy in table 4.1 shows the cytoskeleton made of cablelike structures inside the cell. This approach has been used extensively in the analysis of cell structure and function.
Key Learning Outcome 4.1. All living things are composed of one or more cells, each a small volume of cytoplasm surrounded by a plasma membrane. Most cells and their components can only be viewed using microscopes.