The Cell

It would be difficult to imagine life on earth without cells. In fact, it is not very easy to explain whether or not life would be possible without cells. The cell is to the organism what the nephron is to the kidney or the atom is to a chemical molecule or the individual employee is to a thriving coroporation, it is the smallest unit of the whole—in this case, the smallest unit of life. As we have already learned, below the cellular level, the parts of biological systems are usually thought of as non-living. Thus, it is the cell which has the capacity to be the smallest living thing. I have described how life forms, including our human bodies, are made up of millions of different cells comprising different levels of our form (tissues, organs, etc.). Now, we turn our attention to where we think cells came from, what cells are, what kinds of cells make up living things, what structures differentiate different cells, how these structural variations correlate with functional plasticity of cells, and how biologists study cells.




Figure 2. Generallized prokaryotic (left) and eukaryotic cells (right). (coming soon)

Today, cells are broadly classified into two different groups, (1) eukaryotic cells and (2) prokaryotic cells (Fig. 2). Further, eukaryotic cells may be subdivided into plant cells and animal cells based on different characteristics. Eukaryotic cells and prokaryotic cells—all cells in all living things—share several features, including DNA, cytoplasm, ribosomes, and a cell wall. Eukaryotic cells, thus eukaryotes (organisms comprised of eukaryotic cells), are differentiated from prokaryotes by possessing membrane-bound organelles, including a membrane bound nucleus, a kind of command center for the cell where the hereditary information is stored, and mitochondria, the “powerhouse of the cell.” Also, prokaryotes are single-celled organisms lacking distinct nuclei.

Cellular Evolution, Form and Function

Most biologists agree the evidence suggests eukaryotic cells originated through a series of events described by what is known as the serial endosymbiosis theory (SET), which explains the organellar (sp?) differences in prokaryotic and eukaryotic cells, hence the origin of eukaryotic cells. Serial endosymbiosis theory is founded upon the established idea that prokaryotic cells arose first in the evolution of life, then exploited and changed the early earth environment. A symbiosis is a cohabitation of living things, one joined to the other physically, as in the case of rumen bacteria-cow and gut living protist-termite relationships, or ecologically, in which both organisms benefit. SET suggests ancestral prokaryotes formed a symbiotic relationship with aerobic heterotrophic prokarytes after engulfment as undigestable food or infiltration of the original prokaryote by the second through parasitic means. It is also believed that photosynthetic proto-plastids, the ancestors of today’s chloroplasts, were also prokaryotes similarly engulfed by ancestral prokaryotes which had developed membrane-bound nuclei, forming photosynthetic eukaryotes. Since all eukaryotes possess mitochondria but many eukaryotes do not possess plastids, we infer that mitochondria preceded plastids in eukaryote evolution. Recent DNA sequence data support the earlier versions of SET by illustrating a monophyletic origin of mitochondria and plastids (Gray 1993, more later). So, this provides a background for understanding the origin of cells, but what is a cell anyway?

Cells were discovered in the 17th Century A.D. Specifically, this feat is attributed to the English scientist Robert Hooke, who used rudimentary light microscopy to view and describe the cellular components of cork (the first major discovery of cells, 1665) and other living tissues, plants and animals (NS). Most prokaryotic cells fall within a statistical range of 1-10 micrometers (μm), while most plant and animal cells are 10 μm to 100 μm in size. These cells are usually too small to be viewed with the naked human eye. Therefore, modern biology has developed a variety of microscope applications for looking at cellular and sub-cellular structures. These include light and electron microscopes.




Prokaryotic cells are interesting and the subject of study in numerous medical and evolutionary studies in modern biology. However, the bulk of our discussion will focus heretofore upon animal and plant cells, their form, function, and evolution.




Animal cells differ from plant cells by several features. First, virtually all animal cells contain lysosomes, centrioles, and flagella, while plant cells lack these. Alternatively, plant cells possess chloroplasts, central vacuoles, tonoplasts, rigid cell walls, and plasmodesmata which animal cells do not.




It is one thing to know the difference in animal and plant cells based on their distribution of organelles; however, it is another thing altogether to understand the interaction of these cellular components and to be able to say something about their different functions within plants and animals, especially their importance in reproduction and survival, thus evolution of populations. Thus, what follows is a list of the different plant and animal organelles and their different functions (Fig. 3).


Figure 3. Animal and plant cell organelles and their functions.

Together, the nuclear envelope, rough and smooth endoplasmic reticulum, golgi apparatus, lysosomes and, in plants, central vacuoles, comprise what is known as the endomembrane system. This system regulates the flow of membrane lipids and proteins through the cell and performs metabolic functions. All organelles in this system have membranes. Subsequently, all membranes in the endomembrane system are linked through direct or indirect transfer of membranes and sacs, such as small vesicles, which are tiny spheres of membranes that move through the cell for different purposes. It will be helpful to look at each piece of this system in more detail.



The endoplasmic reticulum exists in two forms, smooth and rough, segregated by the absence or presence of ribosomes on the surface of the membrane, respectively. These two kinds of ER may also be thought of differently in terms of function. For example, the smooth ER is heavily involved in lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification processes. As a result, tissues functioning in detoxification, such as liver tissues, often display a higher density of smooth ER than tissues which carry out detoxification processes to lesser degrees. An important point about smooth ER function is that we be clear about the distinction between different kinds of lipids—steroids and other hormones are kinds of lipids, as are oils and the phosphate-containing lipids making up the bulk of cellular plasma membranes. Because smooth ER is active in the synthesis of hormones, cells of tissues that specialize in hormone synthesis, such as human gonads and adrenal glands, are rich in smooth ER. On the other hand, rough ER has its own set of form with different functions. Many cellular and super-cellular processes rely upon proteins produced by rough ER ribosomes, including insulin, which is a secretory protein. Rough ER is also responsible for membrane production.



Golgi apparatus can be thought of as the manufacturing and shipping center of the cell. Golgi has no special meaning, other than being the name of this structure’s discoverer (NS). But golgi apparatus has a central function integral to cell function: the golgi provide a crucial link between structures in the endomembrane system by modifying carbohydrates, manufacturing macromolecules, and shipping different cellular products by budding vesicles from its surface and sending them out into the cytoplasm to be directed toward different locations.

Lysosomes, peroxisomes, and vacuoles are some of the simplest compartments in the endomembrane system. These are basically bags containing hydrolytic enzymes, hydrogen peroxide, and cellular wastes and water, respectively. Lysosomes digest engulfed food particles and degraded organic material beaten by cellular wear and tear. This latter function is called autophagy.
Campbell and Reece summarize well, “The cytoskeleton is a network of fibers that organizes structures and activities in the cell.” The cytoplasm actively provides structural support to cells. Also, it plays a role in cellular locomotion, as in pseudopodia formation and movement in amoebas, and cell motility, which may not mean whole-cell movement, but rather indicates movement of cellular parts. There are three main structural elements in cytoplasm: microtubules, microfilaments, and intermediate filaments. Microtubules are formed by coiled strands of tubulin dimers, polymers formed by alpha and beta tubulin subunits, which are hollow in the center. Microtubules are the only cytoskeletal elements considered to be organized and originate from the centrosome (pair of centrioles). Microtubules function in cell motility as parts of cilia and flagella and facilitate chromosome and organelle movement by providing substrates for motor proteins to move along. Microfilaments are composed of two strands of actin subunits wrapped around one another. Microfilaments are smaller in size than microtubules and help cells, tissues, and organisms by facilitating shape change, providing a substrate for cytoplasmic streaming, aiding in cell motility (pseudopodia) and cell division (cleavage formation), and providing a substrate for myosin ratcheting during muscle contraction. Intermediate filaments receive their name because they exist in diameters which are smaller than microtubules but larger than microfilaments. These structures are composed of proteins in the keratin family and vary from cell type to cell type; however, they maintain certain cell functions wherever they are found, including structural support, nucleus anchorage, and formation of the nuclear lamina, a netlike mixture of proteins which maintains the shape of the nucleus. For all of these reasons, tubulin, actin, and keratin represent some of the most important cellular proteins know to cell biologists.



Cilia and flagella, as I mentioned, are locomotory organelles of cells extending beyond the main plasma membrane. These exist in different sizes, are found in different types of cells, and exhibit different types of stroke patterns; however, the fine structure or ultrastructure of these organs are homologous. Each is formed by microtubules connected by an orderly arrangement of proteins, all bound by a plasma membrane continuous with that of the remainder of the cell. Through the main body of cilia and flagella, microtubule sets are arranged in a “9 + 2” configuration of doublets; however, microtubule triplets form the anchoring region of cilia and flagella, known as the basal body, and are analogous in form to the centrioles. The motion of cilia and flagella cannot be explained by these microtubule-based structures joined by structural proteins alone. Instead, a large protein complex known as dynein is responsible for the bending and stroking motions of cilia and flagella. When fueled with adenosine triphosphate (ATP), dynein molecules can perform work in the form of a “walking” motion, a complex series of steps resulting in their movement along a microtubule doublet and subsequent movement of anything the polypeptides are bound to, such as another microtubule. In the absence of constraining forces, this motion would move microtubule doublets found in the main bodies of cilia and flagella in a parallel fashion. However, because microtubules of these structures are bound by cross-linking proteins, they are cut off from their potential range of parallel motion in such a way that causes cilia and flagella to bend. It is believed that synchronized dynein movements are responsible for orderly bending and stroking of cilia and flagella, although this is not very clearly understood.

THE IMPORTANCE OF CELL WALLS, EXTRACELLULAR MATRIX, AND INTERCELLULAR JUNCTIONS ARE NOT DOWNPLAYED, BUT SKIPPED.