Chapter 2

Examining Cell Biology Basics

IN THIS CHAPTER

Reviewing the structure of eukaryotic cells

Checking out macromolecules and genetic material

Familiarizing yourself with the cell cycle

Knowing the kinds of tissues that form your body

Biologists see life as existing at five levels of organization, of which the cellular level is the first (see Chapter 1 for more on the levels of organization). A basic principle of biology says that all organisms are made of cells and that anything that has even one cell is an organism. Understanding the basics of cell biology is necessary for understanding any aspect of biology, including human anatomy and physiology.

This chapter clues you in to cell biology basics so you have some context for the various physiological processes we describe in later chapters.

How Cells Spend Their Time

Almost all the structures of anatomy are built of cells, and almost all the functions of physiology are carried out in cells. A comprehensive list of cell functions would be impossible, but we can group cell functions into a few main categories, which we do in the following sections.

Creating new cells

Cells arise from other cells and nowhere else. Once in an organism’s lifetime, at the beginning, two cells fuse to form a new cell. As a result, all the cells in an organism’s lifetime ultimately derive from the first one.

This process is how an organism builds itself from one single generic cell to a complex organism comprising trillions of highly differentiated, highly specialized, and highly efficient cells all working together in a coordinated way. Here’s a look at how a cell goes from one to many.

• Fusing: The organism’s first cell is the zygote, made by the fusion of sex cells: an ovum (egg cell) from the female parent and a sperm cell from the male parent.
• Dividing: In the form of cell division called mitosis, one cell divides into two daughter cells, each of them complete but smaller than the original cell.
• Differentiating: After mitosis is complete, each daughter cell goes on to its own separate life. One or both may start or continue down a path of differentiation, the name for processes that give cells their particular structures and functions. A cell destined to become a nerve cell starts down one path of differentiation; a cell destined to become a muscle cell starts down another path.

Manufacturing tissues

All tissues are made of and built by cells and are maintained by them, too. Cells in a tissue are, to one degree or another, differentiated or specialized for their anatomical or physiological function in the tissue.

For the purposes of anatomy and physiology, keep in mind that all cells have some important features in common, but they also differentiate into a vast array of shapes and sizes, containing vastly diverse structures and having different functions and life cycles.

Constructing and moving products

A great many types of cells make special chemicals that are incorporated into tissues and participate in metabolic reactions. Cellular products include thousands of specific proteins and polypeptides, signaling chemicals such as neurotransmitters and hormones, small molecules and ions, lipids of many kinds, and many types of cellular matrices.

Some cells specialize in transporting the products of other cells around the body or in transporting metabolic waste products out of the body. Some of these transporting cells have other functions as well. Others do nothing but that one job through their entire life cycle. Red blood cells are an extreme example of the one-job model. They lose their nuclei during differentiation and thereafter do nothing but transport gas molecules from one place to another.

Transmitting signals around the body

Some cells transmit various signals while remaining in one place in the body. Some nerve cells perform only the functions of generating and conducting electrical signals and maintaining themselves, and they typically live for years, or even until the death of the organism itself. Other cells produce various signaling molecules, such as hormones and neurotransmitters, or receive and react to those signaling molecules.

Exploring Eukaryotic Cells

Although they’re astoundingly varied, cells are also remarkably alike. All cells, at least all eukaryotic cells, are alike. Plants, animals, and fungi are eukaryotes (organisms made up of eukaryotic cells), and all their cells, in all their enormous complexity and variation, are fundamentally alike.

Here’s a simplistic description of a eukaryotic cell: It’s a membrane-bound sac containing smaller but distinctive structures, called organelles (“little organs”), suspended in a gel-like matrix called cytoplasm. As their name suggests, organelles are functional subunits of a cell, just as organs are functional subunits of an organism.

Figure 2-1 shows the general structure of a eukaryotic cell. Refer to this figure as you read about the various cellular structures in the following sections. Table 2-1 gives an overview of the structures found within a eukaryotic cell.

FIGURE 2-1: A cutaway view of a basic animal cell and its organelles.

TABLE 2-1 Organelles of All Animal Cells

Organelle	Function
Nucleus	Controls the cell; houses the genetic material
Mitochondrion	Cell powerhouse
Endoplasmic reticulum	Plays an important role in protein synthesis; participates in transporting cell products; involved in metabolizing fats
Ribosome	Binds amino acids together under the direction of mRNA to make protein
Golgi apparatus	Packages cellular products in sacs called vesicles so some of the products can cross the cell membrane to exit the cell
Vacuoles	Membrane-bound spaces in the cytoplasm that sometimes serve in the active transport of materials to the cell membrane for discharge to the outside of the cell
Lysosomes	Contain digestive enzymes that break down harmful cell products and waste materials and actively transport them out of the cell

Enclosing it all with the cell membrane

A cell is bound by the cell membrane (also called the plasma membrane). The cell membrane of all eukaryotes is made of phospholipid molecules. These molecules are made by cells, a process that requires energy. The molecules assemble spontaneously (without input of energy) into the membrane, obeying the forces of polarity.

The structure of the cell membrane and the various proteins inserted in it is important for cell function.

Explaining the fluid-mosaic model

The phospholipid bilayer is embedded with structures of many different kinds. Though the bilayer itself is essentially similar in all cells, the embedded structures are as various and specialized as the cells themselves. Some structures identify the cell to other cells (which is important in immune system functioning), and others control the movement of certain substances in or out of the cell across the membrane.

Figure 2-2 is a diagrammatic representation of the phospholipid bilayer and embedded structures. This model of the cell membrane is called the fluid-mosaic model. Fluid describes the ability of molecules in the bilayer to move; mosaic pertains to the embedded structures.

FIGURE 2-2: The fluid-mosaic model of the cell membrane.

The chemical properties of the phospholipid bilayer and the embedded structures contribute to an important feature of the cell membrane: It controls which substances pass through it and which don’t. This control means the membrane is semipermeable.

Moving across the membrane passively

Some substances, mainly small molecules and ions, cross the membrane by a passive transport mechanism, meaning they more or less flow unimpeded across the bilayer, driven by the forces of ordinary chemistry, such as concentration gradients, random molecular movement, and polarity.

Here are some ways that substances cross a membrane passively:

• Diffusion: A substance moves spontaneously down a concentration gradient (from a highly concentrated area to a less concentrated area). For example, if you drop a teaspoon of salt into a jar of water, the dissolved sodium and chloride ions will, in time, diffuse (spread themselves evenly through the water).
• Osmosis: The diffusion of water molecules across a selectively permeable membrane gets a special name: osmosis.
• Filtration: This form of passive transport occurs during capillary exchange. (Capillaries are the smallest blood vessels that bridge arterioles and venules; see Chapter 4 for details on the circulatory system.) Capillaries are only one cell layer thick, and the capillary cell membrane acts as a filter, controlling the entrance and exit of small molecules.

Passing through the membrane actively

Active transport allows a cell to control which big, active, biological molecules move in and out of the cytoplasm. Active transport is a fundamental characteristic of living cells.

Like many matters in cell biology, active transport mechanisms are numerous and widely varied. A simple active import mechanism has a molecule outside the cell, which helps the cell function, as well as a membrane-embedded structure that can identify the molecule with unerring specificity. The membrane-embedded structure frequently uses a kind of lock-and-key mechanism and can communicate its presence to another membrane-embedded structure. The second structure then opens a channel that only the specific molecule can pass through, and the channel closes until the structure gets another reliable message to open up.

A slightly more complex variation of the preceding process involves a transport molecule that brings the molecule from the cell where it was made to the cell where it’s used.

Relinquishing control to the nucleus

The defining characteristic of a eukaryotic cell is the presence of a nucleus, which directs the cell’s activity. The largest organelle, the nucleus is oval or round and is plainly visible under a microscope. Refer to Figure 2-1 to see the relationship of the nucleus to the cell.

Giving the organelles a place to hang in the cytoplasm

Within the cell membrane, between and around the organelles, is a fluid matrix called cytoplasm (or cytosol). The cytoplasm also contains internal scaffolding made of microfilaments and microtubules that support the cell, give processes the space they need, and protect the organelles. The organelles are suspended in the cytoplasm.

Seeing how mitochondria provide energy to cells

A mitochondrion (plural, mitochondria) is an organelle that transforms energy into a form that can be used to fuel the cell’s metabolism and functions. It’s often called the cell’s powerhouse. We describe the role of the mitochondrion in cellular respiration in Chapter 1.

The number of mitochondria in a cell depends on the cell’s function. Cells whose function requires only a little energy, such as nerve cells, have relatively few mitochondria; muscle cells may contain several thousand individual mitochondria because of their function in using energy to do work. A mitochondrion can divide, like a cell, to produce more mitochondria, and it can grow, move, and combine with other mitochondria, all to support the cell’s need for energy.

Producing protein in the endoplasmic reticulum (ER)

The process of protein construction begins in the nucleus. In response to many different kinds of signals, certain genes become active, setting off the production of a specific protein molecule (gene expression). Think of the nucleus as a factory’s administrative department.

The endoplasmic reticulum (or ER) is a chain of membrane-bound canals and cavities that runs in a convoluted path, connecting the cell membrane with the nuclear envelope. The ER brings all the components required for protein synthesis together. Think of the ER as a factory’s logistics function.

The ribosomes are the site of protein synthesis where the binding reactions that build a chain of amino acids are performed. Ribosomes are organelles that may float in the cytoplasm or adhere to the outer surface of some parts of the ER membrane, sticking out into the cytoplasm. These areas are called rough ER (in contrast to smooth ER, where no ribosomes adhere). Ribosomes are tiny, even by the standard of organelles, but they’re highly energetic, and a typical cell contains thousands of them. Think of the ribosomes as the production machinery.

The Golgi body forms a part of the cellular endomembrane system, which includes the nuclear membrane and the ER. It functions in the storage, modification, and secretion of proteins and lipids. Think of it as the shipping department of a factory.

Taking out the trash with lysosomes

Old, worn-out cell parts need to be removed from the cells; if they aren’t, they can become sources of toxins or severe energy drains. Lysosomes are organelles that do the dirty work of autodigestion. Lysosomal enzymes destroy another part of the cell, say an old mitochondrion, through a digestive process. Molecules that can be recovered from the mitochondrion are recycled in that cell or in another. Waste products are excreted from the cell in a membrane-bound vacuole.

Macromolecules, the Building Blocks of Life

Macromolecules, molecules that are many thousands of times larger than water or carbon dioxide, are constructed in cells and react together in seemingly miraculous ways. These building blocks of life — nucleic acids (DNA and RNA), polysaccharides, and proteins — are all made mainly of carbon, with varying proportions of oxygen, hydrogen, nitrogen, and phosphorous.

Macromolecules are made of molecular subunits called, generically, monomers (“one piece”); each type of macromolecule has its own kind of monomer. Macromolecules are, therefore, polymers (“many pieces”).

The following sections focus on these large molecules and their complex interactions. The amazing chemistry of these polymeric macromolecules is the chemistry of life.

Naming the nucleic acids and nucleotides

The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers that are made of monomers called nucleotides and are arranged in chains one after another. And another and another…. Although both DNA and RNA molecules are long, DNA molecules are thousands of nucleotides long. The functioning of genes is inseparable from the chemical structure of the nucleic acid monomers.

The structural similarities and differences between DNA and RNA allow them to work together to produce proteins within cells. The DNA molecule remains stable in the nucleus during normal cell functioning and is protected from damage by the nuclear envelope. An RNA molecule is built on demand to transmit a gene’s coded instructions for building proteins, and then it disintegrates. Some of its nucleotide subunits remain intact and are recycled into new RNA molecules.

Energizing organisms with polysaccharides

The simple carbohydrate molecule called glucose is the main energy molecule in physiology. Polysaccharides, which are polymers of carbohydrate monomers like glucose, are useful in animal physiology, including human physiology, as fuel storage (glycogen).

Polysaccharides play more roles in plant, fungal, and bacterial physiology by forming much of the (connective) structural tissue, a role taken largely by proteins in animals. However, polysaccharides are as vital (required for life) for animals as they are for plants.

Peeking at proteins

Proteins, also called polypeptides, are polymers of amino acids. The amino acid monomers are arranged in a linear chain and may be folded and refolded into a globular form. Structural proteins comprise about 75 percent of your body’s material. The integument, the muscles, the joints, and the other kinds of connective tissue are made mostly of structural proteins, such as collagen, keratin, actin, and myosin. In addition, the enzymes that catalyze all the complex chemical reactions of life in all organisms (plants, fungi, bacteria, and animals) are also proteins.

Amino acids

Twenty different amino acids exist in nature. Amino acids themselves are, by the standards of nonliving chemistry, huge and complex. A typical protein comprises hundreds of amino acid monomers that must be attached in exactly the right order for the protein to function properly.

Enzymes

Enzymes are protein molecules that catalyze, or change, the chemical reactions of life. Enzymes can only speed up a reaction that’s otherwise chemically possible. How effective are enzymes in speeding up reactions? A reaction that may take a century or more to happen spontaneously happens in a fraction of a second with the right enzyme.

Enzymes are involved in every physiological process, and each enzyme is extremely specific to one or a few individual reactions. Your body has tens of thousands of different enzymes.

Discovering How Genetic Material Makes You Who You Are

Your anatomical structures are specified in detail, and all your physiological processes are controlled by your own unique set of genes. Unless you’re an identical twin, this particular set of genes, called your genome, is yours alone, created at the moment your mother’s ovum and your father’s sperm fused. The genome itself is incorporated in the DNA in the nucleus of every one of your cells.

Your genes are responsible for your traits. If your genes specify that you’ll grow to 6 feet tall (a trait), they cause bone and tissue to grow until your body reaches that height, and they maintain it thereafter (assuming a favorable environment), until the cells the genes work through age and die. Similarly, if you have the genes for brown eyes (a trait), your genes direct the production of pigments that color the eyes.

How do genes actually produce your traits? A gene that’s active sends messages to its own cell or to other cells, ordering them to produce molecules of its particular protein, the only one it’s capable of making. The message is sent from DNA through the intermediary of mRNA (messenger RNA), which places an order at the protein factory of the cell and stays around for a while to supervise production. This process is referred to as gene expression.

Here’s a brief rundown of the three parts of the process:

• Transcription: The first part of the process, where DNA writes the order in the form of a sequence of nucleotides on mRNA, takes place in the nucleus and is called transcription (“writing across”).
• Translation: The next part of the process, where RNA places the order at the protein factory of the cell, is called translation (“carrying across”).
• Posttranslational modifications: The last part, where the amino acid monomers are sorted out and assembled into the polypeptide, starts in the ribosome. The finishing touches are put on the protein molecule in the endoplasmic reticulum and the Golgi body.

Cycling through Life, Cell-Style

The life cycle of an individual cell is called the cell cycle. The moment of cell cleavage, when a cell membrane grows across the “equator” of a dividing cell, is considered to be the end of the cycle for the mother cell and the beginning of the cycle for the daughter cells. We walk you through the phases of the cell cycle in the following sections.

Progressing through interphase

Interphase begins when the cell membrane fully encloses the new cell and lasts until the beginning of mitosis or meiosis. The duration of interphase may be anywhere from minutes to decades.

Generally speaking (exceptions always exist in cell biology), cells do most of their differentiating and most of their routine metabolizing during interphase. Stem cells grow in size and reduplicate organelles during interphase, in preparation for mitosis. Some other cells enter mitosis after an extended period of steady-state metabolism. Sometimes, a cell remains in interphase, carrying out its physiological function for years and years until it dies.

Pushing ahead to DNA replication

DNA replication is an early event in cell division, occurring during interphase, just prior to the beginning of mitosis or meiosis but within the protected space in the nuclear envelope. Maintaining the integrity of the DNA code is vital.

During DNA replication, the double helix must untwist and “unzip” so the two strands of DNA are split apart. As shown in Figure 2-3, each strand becomes a template for building the new complementary strand. This process occurs a little at a time along a strand of DNA. The entire DNA strand doesn’t unravel and split apart all at once. When the top part of the helix is open, the original DNA strand looks like a Y. This partly open/partly closed area where replication occurs is the replication fork.

FIGURE 2-3: The DNA replication process.

Entering mitosis

A cell enters a process of mitosis (division) in response to signals from the nucleus. As you can see in Figure 2-4, mitosis is a multistage process.

FIGURE 2-4: The stages of mitosis: prophase, metaphase, anaphase, and telophase.

When each of the two identical nuclei is at opposite poles of the cell, mitosis is technically over. However, the cell’s cytoplasm still has to split into two masses, a process called cytokinesis. The center of the mother cell indents and squeezes the cell membrane across the cytoplasm until two separate cells are formed. The two daughter cells are then in interphase. At this point, they may go on to differentiation, depending on the instructions to the cell from the genome.

All cells arise from the division of another cell, but not all cells go on to divide again. The following list describes cells that divide and some that don’t:

• Zygote: The zygote is the diploid cell that comes into existence when the sex cells (ovum and sperm, both haploid) fuse at conception. Almost immediately, the zygote divides into two somatic cells.
• Somatic cells: The somatic cells include all the cells of the body except the sex cells. Somatic cells may be relatively differentiated (somewhat specialized) or terminally differentiated (they never divide again), or they may be stem cells.

• Stem cells: Stem cells are special kinds of generic somatic cells that divide to produce one new stem cell and one new somatic cell that goes on to differentiate into a particular type of cell in a particular type of tissue.

  An embryo (an organism in the early stages of development) has special stem cells, called pluripotent (“many powers”) stem cells, which have the ability to give rise to just about any kind of cell an organism needs, given the right chemical environment.

• Sex cells (gametes): Sex cells form when specialized somatic cells in the reproductive system divide by a process called meiosis.

Forming Tissues from Cells

A tissue is an assemblage of cells, not necessarily identical but from the same origin, that together carry out a specific function. As you discover in Chapter 1, tissue is the second level of organization in organisms, above (larger than) the cell level and below (smaller than) the organ level.

Like just about everything else in anatomy, tissues are many and various, and they’re grouped into a reasonable number of types to make talking about them and understanding them a little simpler. The tissues of the animal body are grouped into four types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. All body tissues are classified into one of these groups. We explain the first two in the following sections. We cover muscle and nervous tissue in Chapters 3 and 9, respectively.

Linking structures with connective tissue

Connective tissues connect, support, and bind body structures together. Generally, connective tissue is made of cells that are spaced far apart within a gel-like, semisolid, solid, or fluid matrix. (A matrix is a material that surrounds and supports cells. In a chocolate chip cookie, for example, the dough is the matrix for the chocolate chips.)

Connective tissue has many functions, and thus many forms. In some parts of the body, such as the bones, connective tissue supports the weight of other structures, which may or may not be directly connected to it. Other connective tissue, such as adipose tissue (fat pads), cushions other structures from impact. Every organ system in the human body has some kind of connective tissue.

Aside from bones and cartilage (see Chapter 3) and blood (see Chapter 4), other types of connective tissue found in the human body are

• Areolar tissue: This loose connective tissue surrounds and separates structures in every part of the body.
• Dense regular connective tissue: The secretory cells of this type of connective tissue produce dense bands and sheets of parallel protein fibers, such as those in ligaments and tendons.
• Dense irregular connective tissue: The protein fibers in this tissue type are arranged in thick, tough, irregularly oriented bundles.
• Adipose tissue: Composed of fat cells, this loose connective tissue provides fuel storage as well as support and protection to its underlying structures.
• Reticular tissue: This type of loose connective tissue forms a web and functions as a filter in such organs as the spleen, the lymph nodes, and the bone marrow.

Reviewing types of epithelial tissue

Epithelial tissue forms the epidermis of the integument (the skin and the accessory structures such as sweat glands, oil glands, nails, and hair follicles), a continuous covering of the outside of the body (described in Chapter 3), and the endothelium, a continuous lining of the internal surfaces of the blood vessels. Epithelial tissue comes in ten types; each one is defined by the way epithelial cells are combined and shaped (see Figure 2-5).

• Simple squamous epithelium: This tissue, a single layer of flat cells, functions in rapid diffusion and filtration.
• Simple cuboidal epithelium: This tissue, which is a single layer of cuboidal cells, functions in absorption and secretion. It’s typically found in glands.
• Simple columnar epithelium: This tissue is made of a single layer of cells that are elongated in one dimension (like a column) and usually densely packed. Like simple cuboidal epithelium, this tissue functions in secretion and absorption.
• Simple columnar ciliated epithelium: This tissue possesses a type of organelle called cilia — hairlike structures that act to move substances along in waves.
• Pseudostratified columnar epithelium: This tissue is made from a single layer of columnar cells. Note that the prefix pseudo means “false.” The tissue appears stratified, or layered, because the cells’ nuclei don’t line up in a row as they do in simple columnar epithelium.
• Pseudostratified columnar ciliated epithelium: These cells are the same as the pseudostratified columnar epithelium, but they also feature cilia.
• Stratified squamous epithelium: This tissue consists of several layers of cells — squamous epithelial cells on the outside with deeper layers of cuboidal or columnar epithelial cells. It’s found in areas where the outer layer is subject to wear and needs to be replaced continuously.
• Stratified cuboidal epithelium: This tissue is made of several layers of cuboidal cells that mainly act as protection.
• Stratified columnar epithelium: This tissue is a multilayered absorptive and secretive tissue.
• Transitional epithelium: The cells of this tissue can change (or transition) from dome-shaped or scalloped-shaped to squamous (flat) and back again, as needed by the tissue.

FIGURE 2-5: The cellular composition of epithelial tissue.