Cells are as important to biology's living systems as the atom is to chemistry. Many distinct sorts of cells are now functioning for you. As you read this phrase, the contraction of muscle fibers moves your eyes.

Cells form connections between nerve cells, allowing memories to be solidified and learning to take place. Cells are the building blocks of all creatures.

The cell is the simplest collection of materials that may be called a living thing in the biological organization hierarchy. Indeed, many kinds of life survive as single-celled creatures, such as the eukaryote Paramecium seen above, which thrives in pond water.

Multicellular creatures are larger, more sophisticated organisms, such as plants and mammals.

Even when cells are organized into higher levels of the organization, such as tissues and organs, the cell remains the basic structural and functional unit of the organism.

All cells are connected by their lineage back to older cells. Cells have been changed in a variety of ways during the long evolutionary history of life on Earth.

However, while cells differ greatly from one another, they do share several characteristics. In this chapter, we'll look at the tools and approaches that help us comprehend cells, then take a tour of the cell and get to know its components.

The discovery and early study of cells were made possible by the invention of tools that extended the human senses. Microscopes were created in 1590 and improved during the 1600s.

In 1665, Robert Hooke used a microscope to examine dead cells from the bark of an oak tree and discovered cell walls for the first time. But it needed Antoni van Leeuwenhoek's exquisitely constructed lenses to see live cells.

Imagine Hooke's delight when, in 1674, he visited van Leeuwenhoek and was introduced to the world of microbes, which his host referred to as "very tiny animalcules."

All of the microscopes used by Renaissance scientists were light microscopes. Visible light is used in a light microscope (LM).

The lenses refract (bend) the light so that the image of the specimen is enlarged when it is projected into the eye or into a camera (see Appendix D).

Magnification, resolution, and contrast are three essential factors in microscopy. The ratio of an object's picture size to its true size is known as magnification. Light microscopes can successfully magnify a specimen to roughly 1,000 times its original size; at higher magnifications, extra features cannot be seen well.

The term Resolution is a measure of picture clarity; it is the shortest distance between two points that may still be identified as distinct points.

Confocal. The top picture is a conventional fluorescence micrograph of fluorescently tagged nervous tissue (nerve cells are green, support cells are orange, and overlap regions are yellow); the image below is a confocal view of the same tissue.

Using a laser, this "optical sectioning" approach removes out-of-focus light from a thick sample, resulting in a picture with a single plane of fluorescence.

A 3-D reconstruction may be constructed by collecting crisp pictures at several distinct planes. Because out-of-focus light is not eliminated, the typical picture appears fuzzy.

Super-resolution. The image placed on top is a confocal picture of a nerve cell with a fluorescent label that attaches to a molecule concentrated in tiny sacs (vesicles) in the cell that are 40 nm in diameter.

Because 40 nm is less than the 200-nm resolution limit for conventional light microscopy, the greenish-yellow dots appear fuzzy. The picture below shows the same section of the cell as seen using a new super-resolution method.

Individual fluorescent molecules are illuminated and their positions are recorded using sophisticated technology.

Combining information from many molecules in various locations “breaks” the resolution limit, resulting in the bright greenish-yellow dots shown above. (Each dot represents a 40-nm vesicle.)

• The term cytoplasm refers to the interior of either type of cell; in eukaryotic cells, this term refers only to the region between the nucleus and the plasma membrane

A number of specialized organelles exist in the cytoplasm of a eukaryotic cell, suspended in cytosol. Another contrast between prokaryotic and eukaryotic cells is the absence of these membrane-bounded structures in virtually all bacterial cells.

Despite the lack of organelles, the prokaryotic cytoplasm is not a formless soup. Some prokaryotes, for example, have areas enclosed by proteins (rather than membranes) in which particular processes occur.

Eukaryotic cells are far bigger than prokaryotic cells. The size of a cell is a common characteristic of its structure that is related to its function. Cell size is limited by the logistics of carrying out cellular metabolism.

The tiniest cells known are bacteria termed mycoplasmas, which have dimensions ranging from 0.1 to 1.0 m. These are possibly the tiniest packages, containing enough DNA to train metabolism as well as enzymes and other cellular machinery to carry out the tasks required for a cell to sustain itself and reproduce.

Bacteria are typically 1–5 m in diameter, roughly ten times the size of mycoplasmas. Eukaryotic cells have a diameter of 10–100 m.

Metabolic needs also put theoretical upper limitations on the size of a single cell that is feasible. The plasma membrane acts as a selective barrier at the cell's border, allowing sufficient oxygen and nucleic acid to flow through.

The requirement for a surface area big enough to handle the volume explains most cells' tiny size and the thin, elongated forms of others, such as nerve cells.