The Cell

After completing your study of this section, you should be able to:

1. Describe the contributions of the following scientists to the development of the cell theory: Robert Hooke, Anton van Leeuwenhoek, Robert Brown, Matthias Schleiden, Theodore Schwann, and Rudolf Virchow.

2. State the cell theory and explain its importance.

All living things are made up of small, individual units called cells. Some organisms consist of just one cell. Others contain billions of cells. In all cases, however, the life processes of the organism are actually carried on by its cells. To understand the workings of living things, we must first understand what goes on inside the cell.

The individual cells of most organisms cannot be seen with the naked eye. It was not until the mid-1800s that microscopes were used to study biological materials. Robert Hooke examined thin slices of cork and other plant tissues with a microscope and found that they were made up of boxlike structures, which he called cells. What Hooke saw was only the walls of dead cells. He never studied living materials in which the content of the cells were to be seen. Hooke's findings were published in 1665 in his book Micrographia. The microscopes that Hooke used were compound microscopes, similar in principle to those we use today (see Figure 5-1). However, they produced magnifications of only about 30x.

Figure 5-1 Robert Hooke's Compound Microscope

At the same time that Hooke was making and using compound microscopes, Anton van Leeuwenhoek (lay-ven-huk) was making single-lens microscopes of amazing power. Several of his microscopes still exist, and some have magnifying powers of more than 200x. Looking at drops of pond water with these microscopes, Leeuwenhoek saw living organisms that no one else had ever seen. We now know that many of them were one-celled organisms. Leeuwenhoek also observed and described human sperm cells and blood cells. In 1683 he described what must have been bacteria- the smallest kind of living cell. However, Leeuwenhoek did not know that he was seeing single cells, and he drew no conclusions about the cellular nature of organisms.

It was not until the early 1800s that the cellular nature of biological materials began to receive attention. In 1824, Henry Dutrochet (doo-troh- shay proposed that all living things were composed of cells. However, the actual nature of living cells was still not known.

In 1831, Robert Brown noted that the small, dense, round body that had been observed in cells my other microscopists was a common feature of all plant cells. He called this structure the nucleus. The major role of the nucleus in cell function was not recognized at this time.

In 1838, Matthias Schleiden ( shly-den) theorized that all plants were made up of cells. In the following year, Theodor Schwann (shvahn) proposed that all animals were also made up of cells. In the same year, Johannes Purkinje (per-kin-jee) used the term "protoplasm" to refer to lhe jellylike material that fills the cell. The last part of the cell theory was expressed by Robert Vichow (vihr-koh) in 1855, when he stated that all new cells arise only from existing cells.

In 1861, Max Schultze (shults) defined protoplasm as "the physical basis of life," and proposed that it was found in the cells of all types of organisms. At about the same time, Felix Dujardin (doo-zhar-dahn) recognized the existence of one-celled organisms. He also expressed the idea that protoplasm was associated with all forms of life. Justus von Liebig (lee-big) described protoplasm as consisting mainly of water, with the rest of the substance being proteins, fats, and carbohydrates.

By the end of the 1800s, biologists had discovered many of the structures within the cell, including the chromosomes, which contain the hereditary information. They were also able to describe in detail the events of cell division, in which one cell divides, forming two cells.

Figure 5-2. Generalized Structure of an Animal Cell

The ideas that are generally called thecell theory are:

1. All organisms are made up of one or more cells and the products of those cells. An organism may be a single cell. Examples are protozoans, such as the ameba and paramecium, and bacteria. In many-celled organisms there may be intercellular material made by the cells.

2. All cells carry on their own life activities. The life activities of a many-celled organism are the combined effect of the activities of its individual cells.

3. New cells can arise only from the other living cells by the process of cell division or reproduction. Reproduction of a many-celled organism is brought about by reproduction of certain of its cells.

After completing your study of this section, you should be able to:

1. Describe the structures and functions of the following cell parts: cell wall, cell membrane, nucleus, and cytoplasm.

2. Describe the structures and functions of the following cell organelles: endoplasmic reticulum, ribosomes, lysosomes; mitochondria, plastids, vacuoles, centrioles, microfilaments, microtubes, and cilia and flagella.

Figure 5-3. Generalized Structure of a Plant Cell

3. Contrast and compare the general structures of an animal cell and a plant cell.

All processes necessary for life are carried on by the cells. In addition, the cells of many-celled organisms are generally highly specialized to perform specific functions. There are nerve cells that can carry messages, muscle cells that can shorten, glandular cells that produce certain substances, and so on. Figure 5-2 shows the general structure of an animal cell and Figure 5-3 shows the general structure of a plant cell. However, actual animal and plant cells will show variations from these structures.

Cells vary in size and shape as well as in internal structure. Most cells are between 10 and 30 micrometers in diameter. However, chicken egg cells may be 6 centimeters across, and certain nerve cells may have extensions that are more than 1 meter in length.

All cells are surrounded by a cell membrane that separates the cell contents from the environment. All but the simplest cells contain a membrane-bounded nucleus. In all cells the space between the cell membrane and the nucleus is filled with a fluid material called cytoplasm.

Figure 5-4. The primary cell wall, which is formed by the young plant cell, stretches as the cell grows. In the woody parts of the plant, full-grown cells produce a thick secondary cell wall inside the primary cell wall. The middle lamella is a layer that forms between adjacent cells and holds them together.

The cells of plants and various microorganisms are enclosed by a rigid cell wall , which Is outside the cell membrane. The cell wall gives the cell its shape and also provides protection (see Figure 5-4) In plants, this wall is composed largely of cellulose (sel-yuh-lohs). In other organisms it may contain other compounds. The cell wall has many small openings that allow the free passage of materials to and from the cell membrane. Thin strands of cytoplasm sometimes extend through the walls of adjacent cells, possibly allowing the direct passage of materials from one cell to another. Animal cells do not have a cell wall.

Figure 5-5. Structure of the Cell Membrane.

p The cell, or plasma (plaz-muh), membrane separates the interior of the cell from the surrounding environment. It controls the movement of materials into and out of the cell, thus making it possible for the cell contents to be chemically different from the environment. Its function is also to keep the internal conditions of the cell constant- to maintain homeostasis.

Permeability of the cell membrane. The cell membrane is selectively permeable, or semipermeable (sem-ee-per-mee-uh-bul). That is, some substances pass freely through it. Other substances can pass through only to some slight extent or only at certain times. And still other substances cannot pass through at all. Through its selective permeability the cell membrane regulates the chemical composition of the cell. The semipermeable nature of the membrane results from the chemical and electrical properties of its molecules. The passage of materials through cell membranes is discussed in detail later in this chapter (page 79).

Structure of the cell membrane. The cell membrane is a two-layered structure composed of lipids and proteins )see Figure 5-5). The two layers are lipids, and the proteins are embedded in them. Some of the proteins are on the outer surface of the double membrane, some are on the inner surface, and some are thought to extend through the membrane. In electron micrographs there appears to be a light middle layer. This middle layer consists of the "tail ends" of the lipid molecules.

These ends are chemically different from the rest of the lipid molecule.

The proteins of the membrane are believed to be important in controlling the passage of substances through the membrane. In some cells the membrane proteins are thought to be involved in the pumping of various ions into and out of the cell. In nerve cells, they are involved in the transmission of messages, or impulses, along the cell membranes.

Figure 5-6. Pinocytosis. In pinocytosis, inpocketings in the cell membrane close over, leaving vacuoles in the cytoplasm.

Pinocytosis and phagocytosis. Materials that cannot pass through the cell membrane may be taken into the cell by processes called pinocytosis and phagocytosis. In pinocytosis (pin-uh-sy-toh-sis), or "cell drinking," liquid from the surrounding medium or very small particles are taken into the cell. Where the substance is in contact with the surface of the cell membrane, the membrane forms an inpocketing or pouch (see Figure 5-6). The outer surface of the cell membrane closes over, aand the pouch pinches off, forming a sac, or vacuole, within the cell. Inside the cell, the vacuole may open, releasing its contents.

In phagocytosis (fag-uh-sy-toh-sis), large particles or even small organisms are ingested into the cell. In this process, extensions of the cell called pseudopods (sood-uh-pahdz) flow around the particle to be taken in. When the particle has been surrounded, the membrane pinches off, forming a vacuole within the cell. Both phagocytosis and pinocytosis require the use of energy by the cell.

Figure 5-7. Electron Micrograph of the Cell Nucleus. The nuclear membrane and its pores are clearly visible. The round, dark structure within the nucleus is the nucleolus.

The cell nucleus (noo-klee-us) is a round, dense body surrounded by a membrane (see Figure 5-7). The nucleus serves as the control center for cell metabolism and reproduction. If it is removed, the cell dies.

The nuclear membrane, like the cell membrane, is a selectively permeable double membrane. Unlike the cell membrane, the nuclear membrane has pores that can be seen in electron micrographs. The opening and closing of these pores allow the passage of certain substances into and out of the nucleus. The selective permeability of the nuclear membrane allows the contents of the nucleus to remain chemically different from the rest of the cell.

Careers Cytotechnologists are members of a medical laboratory team. Using microscopic and other techniques, they examine tissue sections obtained from patients for signs of cellular abnormalities. The cytotechnologist generally works under the supervision of a pathologist, a physician whose specialty is the effects of disease and injury on body tissues. Most cytotechnologists work in hospitals or in private medical laboratories. A career as a cytotechnologist requires two years of college study, including biology courses, and a one-year course at an approved school of cytotechnology.

Within the nucleus are one or more nucleoli (noo-kleeuh-ly) (singular, nucleolus). These are dense, granular bodies that disappear at the beginning of cell division and reappear at the end. They are made up of DNA and protein. Nucleoli are the sites of production of ribosomes (see below).

During the periods between cell divisions, much of the nucleus id filled with chromatin. Chromatin (kroh-muh-tin) is the material of the chromosomes (kroh-muh-sohmz) in the form of long, very thin threads. During cell division, the chromatin shortens by coiling and becomes thick enough to be clearly visible as separate chromosomes. The chromosomes contain the hereditary material of the cell. Nucleoli are often formed at a particulaar location on a specific chromosome. This area is called the nuclear organizer.

All the material within the cell between the cell membrane and the nucleus is the cytoplasm (syt-uh-plaz-um). The cytoplasm is a watery material in which are dissolved many of the substances involved in cell metabolism. Many of the chemical reactions of cell metabolism take place in the cytoplasm. Also found in the cytoplasm are a variety of specialized structures called organelles (or-guh-nelz). Each type of organelle carries out a specific function in cell metabolism. We will discuss the various organelles on the following pages.

The endoplasmic reticulum (en-duh-plaz-mik rih-tik-yuh-lim) consists of a system of fluid-filled canals or channels enclosed by membranes. These canals generally form a continuous network throughout the cytoplasm (see Figure 5-8). The canals of the endoplasmic reticulum serve as a path for transport of materials through the cell. In addition, the membranes of the network provide a large surface area on which many biochemical reactions are thought to occur. Also, the endoplasmic reticulum divides, or partitions, the cell into compartments, making it possible for a number of different reactions to be going on at the same time.

Figure 5-8. Rough Endoplasmic Reticulum. The membranes of the rough endoplasmic reticulum are lined with ribosomes.

The membranes of the endoplasmic reticulum are similar in structure to the cell membrane and nuclear membrane. It has been observed that in places the membranes of the endoplasmic reticulum are continuous with the outer portion of the nuclear membrane. There are two types of endoplasmic reticulum- rough and smooth. In rough endoplasmic reticulum, the outer surfaces of the membranes are lined with tiny particles called ribosomes. The ribosomes give the membrane a granular appearance. On smooth endoplasmic reticulum there are no ribosomes.

Ribosomes (ry-buh-sohmz) are the sites of protein synthesis in the cell. They are found both free in the cytoplasm and lining the membranes of the endoplasmic reticulum. In cells involved in the synthesis of proteins that are to be transported out of the cell, the ribosomes are mainly attached to the membranes of the endoplasmic reticulum. The proteins pass through these membranes into the canals, which carry them to the cell membrane and out of the cell. Where the products of protein synthesis are used within the cell, the ribosomes are generally free in the cytoplasm. Proteins synthesized on free ribosomes are usually enzymes that function in the cell cytoplasm.

Figure 5-9. Golgi Bodies

Golgi (gohl-jee) bodies consist of a stack of membranes forming flattened sacs and small spherical sacs, or vesicles (see Figure 5-9). Golgi bodies serve as packaging and storage centers for the secretory products of the cell. Animal cells generally have only one Golgi body, which is usually located near the nucleus. Plants may have up to several hundred Golgi bodies.

In some studies, connections between the Golgi body and the endoplasmic reticulum have been found. There is evidence that proteins synthesized on the ribosomes attached to the endoplasmic reticulum pass through the canals of the endoplasmic reticulum into the Golgi bodies. The vesicles migrate to the cell surface, where their membranes fuse with the cell membrane. The materials in the vesicle are then released outside the cell. Cell secretory products other than proteins may also be packaged in the Golgi body. In plant cells, the Golgi bodies are thought to be involved in assembling materials for the cell wall.

Lysosomes (ly-suh-sohmz) are small, saclike structures surrounded by a single membrane. These organelles contain strong digestive, or hydrolitic, enzymes. Lysosomes are thought to be produced by the Golgi bodies. They are found in most animal cells and in some plant cells. In one-celled organisms, lysosomes are involved in the digestion of food within the cell. In multicellular organisms, lysosomes serve several different functions. They break down worn-out cell organelles. In some animals they are part of the body's defense against disease. Lysosomes are present in white blood cells, which ingest disease-causing bacteria by phagocytosis. The lysosomes within the white cells break down the bacteria. Lysosomes are also involved in certain developmental processes. For example, as a frog develops from a tadpole to a mature frog, it loses its tail. Lysosomes are involved in the digestion and absorption of the tail.

Figure 5-10. A Mitochondrion.The mitochondrion is surrounded by two membranes. Infoldings of the inner membrane form cristae.

Mitochondria (myt-uh-kahn-dree-uh) (singular, mitochondrion) are round or slipper-shaped organelles surrounded by two membranes (see figure 5-10). The inner membrane is highly folded, forming cristae (kris-tee) that extend into the mitochondrion itself. The cristae of the mitochondria provide a large surface area on which many biochemical reactions occur. Active cells, such as muscle cells, which use a lot of energy, contain large numbers of mitochondria. Because most of the energy needed by the cells is released in the mitochondria, the organelle is often called "the powerhouse of the cell". The process by which energy is released in mitochondria is called cellular respiration. Typical cells contain from 300 to 800 mitochondria, depending on their activity. Within the cell, the mitochondria are usually in motion, moving individually or in groups. They may also be found at specific locations within the cell. For example, in muscle cells, the mitochondria are found along the fibers that cause the muscle cell to contract. Mitochondria contain DNA and are capable of duplicating themselves.

Figure 5-11. Microtubules.Microtubules are found in the cytoplasm and also in centrioles, cilia, and flagella.

Microtubules (my-kroh-toob-yoolz) are long, hollow, cylindrical structures. They are found in the cell cytoplasm, where they serve as a sort of "skeleton" for the cell, giving it shape (see Figure 5-11). They are found in centrioles, cilia, and flagella, and may also be involved in movement of the chromosomes during cell division. Microtubules are composed of a protein called tubulin (toob-yuh-lin). The molecules of this protein consist of two subunits that stack alternately in a helix. This gives the microtubule its form.

Microfilaments (my-kroh-fil-uh-ments) are long, solid, threadlike organelles found in some types of cells. Most are composed of the protein actin (ak-tin) and are generally associated with cell movement. Microfilaments are thought to have the capacity to contract and to be involved in the movement of cytoplasm within the cell, a phenomenon known as cyclosis, or cell streaming. Actin microfilaments are also found in skeletal muscle cells and are involved in muscle contraction. Some microfilaments are not made of actin, and may serve as supporting structures for the cell.

Figure 5-12. A Centriole.A centriole consists of a ring of nine groups of three microtubules.

Near the nucleus in animal cells there is a pair of cylindrical centrioles (sen-tree-oles) which lie at right angles to each other (see Figure 5-12). Each centriole consists of a ring of nine groups of three microtubules. Centrioles are involved in cell division in animal cells. They are also found in some algae and fungi, but not in plants.

Figure 5-13. Structure of Cilia.Cilia arise from organelles called basal bodies that are structurally similar to centrioles. Each cilium contains a ring of nine pairs of microtubules with one pair of microtubules in the center.

Cilia (sil-ee-uh) and flagella (fluh-jel-uh) are hairlike organelles with the capacity for movement (see Figure 5-13). They extend from the surface of many different types of cells. Their structure is identical except that flagella are longer than cilia. There are usually only a few flagella on a cell, but cilia often cover the entire cell surface. In one-celled organisms, cilia and flagella are involved in cell movement. In larger, many-celled animals, ciliated cells serve to move substances over the surface of the cells.

Cilia and flagella arise from the structures called basal (bay-sul) bodies. The structure of a basal body is similar to that of a centriole. The cilia and flagella are slightly different in structure from the basal body. They have a ring of nine pairs of microtubes, and in the center of the ring is another pair of microtubes.

Vacuoles (vak-yuh-wohlz) are fluid-filled organelles enclosed by a membrane. Those found in plant cells are filled with a fluid called cell sap. In mature plant cells, there may be a single, very large vacuole that occupies most of the interior of a cell. In various microorganisms and simple animals, food is digested in special food vacuoles within the cells. Many of these organisms also have contractile vacuoles in which excess water from the cell collects. The water is periodically excreted from the cell directly into the environment. Vacuoles may also serve as storage sites for certain cell products.

Figure 5-14. Electron Micrograph of Chloroplast. Within the chloroplast are grana, which are stacks of membranes, and large starch granules.

Chloroplasts, leucoplasts, and chromoplasts are all types of plastids. Plastids (plas-tidz) are membrane-enclosed organelles found in the cells of some protists and almost all plants. They are not present in the cells of animals or fungi. Like mitochondria, plastids are bounded by a double membrane and have systems of membranes within the organelle. There are three types of plastids. Leucoplasts (loo-kuh-plasts) are colorless plastids in which glucose is converted to starch, and in which starch and other plant nutrients are stored. Chromoplasts (kroh-muh-plasts) contain the pigments that give bright colors to fruits, flowers, and leaves. The pigments are synthesized within the chromoplasts. The most important type of plastids are chloroplasts, which contain the green pigment chlorophyll (klor-uh-fil). The chloroplasts are the site of photosynthesis, the food-making process of plants.

The inside of the chloroplast contains a system of double membranes called lamellae (luh-mel-ee) (see Figure 5-15). The lamellae form stacks called grana (gray-nuh). The pigments involved in photosynthesis are located in the membranes of the grana. The protein-containing material that fills the rest of the chloroplast is called the stroma (stroh-muh). Chloroplasts, like mitochondria, contain their own DNA and have the ability to duplicate themselves.