The cell is the basic structural unit of a living organism. Cells are very tiny, visible only under a microscope. Some organisms such as bacteria are unicellular, consisting of a single cell. All plants, animals, algae, and fungi are multicellular organisms, organisms made up of more than one cell. A human, for example, consists of about 100 trillion cells. In multicellular organisms, cells differ in structure depending on their roles in the body; nerve cells, skin cells, and blood cells all have different characteristics.
Prokaryotes and Eukaryotes
There are two main types of cells, prokaryotes and eukaryotes. These terms also refer to the organisms that possess those types of cells. Prokaryotes do not have a nucleus or organelles wrapped in membranes. The nucleus is the portion of the cell that contains its DNA, or genetic code. An organelle is a small structure that performs a specific set of functions within the eukaryotic cell. Eukaryotes do have a nucleus and organelles. Eukaryotic cells are much bigger than prokaryotic cells.
Prokaryotes include bacteria and archaea. All other living organisms are eukaryotes. Fungi, plants, animals, and even many single-celled organisms are eukaryotes.
The Structure and Function of Cells
The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have an outer covering called a plasma membrane. The plasma membrane or cell membrane holds the cell together and permits the passage of substances into and out of the cell. With a few minor exceptions, plasma membranes are the same in prokaryotes and eukaryotes.
The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles. Both types of cells contain small structures called ribosomes that produce proteins, large molecules that are essential to the structure and functioning of all living cells. Ribosomes are not bound by membranes and are therefore not considered organelles.
The Structure of Prokaryotes
Bacteria and archaea are prokaryotes. All prokaryotes are enclosed by a plasma membrane. The plasma membrane contains the cell’s cytoplasm. Within the cytoplasm of prokaryotes is a nucleoid, a region where the cell’s genetic material is stored, and many ribosomes. The nucleoid is not a true nucleus because it is not surrounded by a membrane.
Surrounding the plasma membrane is a cell wall, which gives the cell its shape and structure; bacteria come in several shapes, including spheres, rods, and corkscrews. In some bacteria, a jelly-like material known as a capsule coats the cell wall. Attached to the cell wall of some bacteria are flagella, whiplike structures that make it possible for the bacteria to move. Some bacteria also have pili (short, fingerlike projections that help the bacteria to attach to tissues).
The Structure of Eukaryotes
Like prokaryotes, eukaryote cells are enclosed by plasma membranes. Unlike prokaryotes, their cytoplasm contains several organelles bound in membranes. The organelles found in eukaryotes include the membrane system, consisting of the plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; the cytoskeleton; and the mitochondria. In addition, plant cells have special organelles not found in animal cells called chloroplasts, cell wall, and vacuoles. (See the drawing of a plant cell.)
The plasma membrane of the cell is selectively permeable, which means that some substances can pass through the membrane, but others cannot. For example, the products formed by the breakdown of foods can pass into a cell, and the waste products formed within the cell can pass out of the cell. Cells control this process by either adjusting the concentration of fluids inside and outside the membrane or by actively moving substances from one side of the membrane to the other.
The endoplasmic reticulum (ER) consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell’s cytoplasm. The ER looks something like a very complex subway or highway system. That analogy is not a bad one, because a major function of ER is to transport materials throughout the cell.
Cells contain two kinds of ER, rough ER and smooth ER. Rough ER contains ribosomes on its outside surface, giving it a rough or grainy appearance. Smooth ER does not. Rough ER is involved in the process of protein synthesis (production) and transport. Proteins made on the ribosomes attached to rough ER are modified, “packaged,” and then shipped to various parts of the cell for use. Some are sent to the plasma membrane, where they are moved out of the cell and into other parts of the organism’s body for use. Smooth ER has many different functions, including the manufacture of lipids (fatlike materials), the transport of proteins, and the transmission of nerve messages.
The Golgi Body
The Golgi body is named for its discoverer, the nineteenth century Italian scientist Camillo Golgi (1843–1926). It looks somewhat like a stack of pancakes. The Golgi body consists of a pile of membrane-bound, flattened sacs. Surrounding the Golgi body are numerous small membrane-bound vesicles (particles). The function of the Golgi body and its vesicles is to sort, modify, and package large molecules that are secreted by the cell or used within the cell for various functions.
The Golgi body can be compared to the shipping and receiving department of a large company. Each Golgi body within a cell has a cis face, which is similar to the receiving division of the department. Here, the Golgi body receives molecules manufactured in the endoplasmic reticulum. The trans face of the Golgi body can be compared to the shipping division of the department. It is the site from which modified and packaged molecules are transported to their destinations.
Vesicles are small spherical particles that contain various kinds of molecules. Some vesicles are used to transport molecules from the endoplasmic reticulum to the Golgi body and from the Golgi body to various destinations. Lysosomes are vesicles that contain enzymes involved in cellular digestion. Some protists, for instance, engulf other cells for food. In a process called phagocytosis (pronounced FA-go-sy-t o-sis), the protist surrounds a food particle and engulfs it within a vesicle. This food-containing vesicle is transported within the protist’s cytoplasm until it is brought into contact with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break down the food into smaller parts for use by the protist.
The nucleus is the control center of the cell. Under a microscope, the nucleus looks like a dark blob, with a darker region, called the nucleolus, centered within it. The nucleolus is the site where parts of ribosomes are manufactured. Surrounding the nucleus is a double membrane called the nuclear envelope. The nuclear envelope is covered with tiny openings called nuclear pores.
The nucleus directs all cellular activities by controlling the synthesis of proteins. Proteins are critical chemical compounds that control almost everything that cells do. In addition, they make up the material from which cells and cell parts themselves are made.
The instructions for making proteins are stored inside the nucleus in a helical molecule called deoxyribonucleic acid, or DNA. DNA molecules differ from each other on the basis of certain chemical units, called nitrogen bases. The way nitrogen bases are arranged within any given DNA molecule carries a specific genetic “message.” One arrangement of nitrogen bases might carry the instruction “Make protein A,” another arrangement of bases might carry the message “Make protein B,” yet a third arrangement might code for the message “Make protein C,” and so on.
The first step in protein synthesis begins in the nucleus. Within the nucleus, DNA is translated into a molecule called messenger ribonucleic acid (mRNA). mRNA then leaves the nucleus through the nuclear pores. Once in the cytoplasm, mRNA attaches to ribosomes and initiates protein synthesis. The proteins made on ribosomes may be used within the same cell or shipped out of the cell through the plasma membrane for use by other cells.
The cytoskeleton is the skeletal framework of the cell. The cell’s skeleton consists of three kinds of protein filaments that form networks. These networks give the cell shape and provide for cellular movement. The three types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.
Microtubules are very thin, long tubes that form a network of “tracks” over which various organelles move within the cell. Microtu-bules also form small, paired structures called centrioles within animal cells. These structures are not considered organelles because they are not bounded by membranes. Centrioles are involved in the process of cell division (reproduction).
Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Cells with cilia also perform important functions in the human body. The airways of humans and other animals are lined with such cells that sweep debris and bacteria upwards, out of the lungs and into the throat. There, the debris is either coughed from the throat or swallowed into the digestive tract, where digestive enzymes destroy harmful bacteria.
Actin filaments are especially prominent in muscle cells, where they provide for the contraction of muscle tissue. Intermediate filaments are relatively strong and are often used to anchor organelles in place within the cytoplasm.
The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane. The inner membrane of a mitochondrion is folded into compartments called cristae. The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy.
Plant cells have several organelles not found in animal cells. These include plastids, vacuoles, and a cell wall.
Plastids are vesicle-type organelles that perform a variety of functions in plants. For example, amyloplasts store starch and chromoplasts store pigment molecules that give some plants their vibrant orange and yellow colors. Chloroplasts are plastids that carry out photosynthesis, a process in which water and carbon dioxide are transformed into sugars.
Vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90 percent of the cellular space. They perform a variety of functions in the cell, including storage of organic compounds, waste products, pigments, and poisonous compounds as well as digestive functions.
All plant cells have a cell wall that surrounds the plasma membrane. The cell wall of plants consists of a tough carbohydrate substance called cellulose laid down in a medium or network of other carbohydrates. (A carbohydrate is a compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.) The cell wall provides an additional layer of protection between the contents of the cell and the outside environment. The crunchiness of an apple, for instance, is attributed to the presence of these cell walls.