The cell is the smallest living component of organisms and is the basic unit of life. A cell contains the genetic material that supplies the coded instructions for the manufacture of a new cell, as well as the other materials necessary for the cell’s growth and survival.
In multicellular living organisms, a collection of cells that performs a similar function is called a tissue. Various tissues that perform coordinated functions form organs, and organs can work together to perform the general processes of body systems.
Background and Scientific Foundations
Types of cells
Multicellular organisms contain an array of highly specialized cells. Each kind of cell is structured to perform a highly specialized function. Examining a cell’s structure can reveal a great deal about its function within the organism. For instance, epithelial cells in the small intestine are specialized for absorption due to the numerous microvilli that crowd their surfaces. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long processes, called axons, that conduct nerve impulses. Dendrites are shorter processes that receive nerve impulses.
Sensory cells—the cells that detect sensory information from the outside environment and transmit this information to the brain—often have unusual shapes and structures that contribute to their function.
Cells also exist as single-celled organisms. Protists are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas. Bacteria are also single-celled organisms.
Prokaryotes and eukaryotes
Prokaryotes (literally, “before the nucleus”) are cells that have no distinct nucleus. Instead, the genetic material is dispersed in the interior material of the cell (the cytoplasm). Most prokaryotic organisms are single-celled, such as bacteria and algae. In contrast, eukaryotic (literally, “true nucleus”) organisms have a distinct nucleus and a highly organized internal structure. Also present are membrane-bound structures that perform specific functions (an example is the lysosome; compartmentalized enzymes that digest compounds). Prokaryotes lack these membrane-bound regions; digestive enzymes and other functional molecules are dispersed in the cytoplasms of the prokaryotic cell.
Cells transport substances from one place to another, reproduce themselves, and produce various enzymes and chemicals for export to the extracellular environment. All of these activities are accomplished under the direction of the nucleus.
Cell structure and function
Prokaryotic and eukaroytic cells share structural similarities. Both cell types have a plasma membrane through which substances pass into and out of the cell. With the exception of a few minor differences, plasma membranes are the same in prokaryotes and eukaryotes. The interior of both kinds of cells is called the cytoplasm. Finally, both types of cells contain small structures called ribosomes that function in protein synthesis. Composed of two protein subunits, ribosomes are not bounded by membranes; therefore, they are not considered organelles. In eukaryotes, ribosomes are either bound to an organelle, the endoplasmic reticulum, or exist as “free” ribosomes in the cytoplasm. Prokaryotes contain only free ribosomes.
An example of a typical prokaryote is the bacterial cell. Bacteria can be shaped like rods, spheres, or corkscrews. All prokaryotes are bounded by a plasma membrane and contain a rigid layer called the peptidoglycan. Some bacteria also contain another membrane, which sandwiches the peptidoglycan between the membranes. As well, some bacteria have a sugary outer coating called a capsule. Many bacteria that cause illness in animals have capsules. The capsule provides an extra layer of protection against host immune molecules like antibodies.
Attached to the cell wall of some bacteria are flagella, whip-like structures that provide for movement. Some bacteria also have pili, which are short, finger-like projections that assist the bacteria in attaching to tissues.
The organelles found in eukaryotes include the membrane system, consisting of plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; cytoskeleton; and mitochondria. In addition, plant cells have special organelles not found in animals cells. These organelles are the chloroplasts, cell wall, and vacuoles.
The membrane system of a cell performs many important functions. This system controls the entrance and exit of substances into and out of the cell and also provides for the manufacture and packaging of substances within the cell. The membrane system of the cell consists of the plasma membrane, which encloses the cell contents; the endoplasmic reticulum, which manufactures lipids and proteins; the Golgi body, which packages substances manufactured within the cell; and various vesicles, which perform different functions.
The plasma membrane of the cell is selectively permeable; that is, the membrane is designed so that only certain substances are allowed to cross unaided. Other compounds that cross require specialized transport proteins or pass through specific channels. The plasma membrane is composed of two layers of molecules called phospholipids. Each phospholipid molecule consists of a phosphate “head” and two fatty acid chains that dangle from the head.
The orientation of these two sections of the phospholipid molecule is crucial to the function of the plasma membrane. The phosphate region is hydrophilic (literally, “water-loving”). The fatty acid region is hydrophobic (literally, “water-hating”) and repels water. In the phospholipid bilayer of the plasma membrane, the phospholipid layers are arranged so that the two phosphate hydrophilic regions face outward, towards the watery extracellular environment, and inward, towards the cellular cytoplasm, which also contains water. The two hydrophobic fatty acid portions of the chains face each other, forming a watertight shield. The plasma membrane functions both as a boundary between the cell’s contents and the external cellular environment, yet also allows the transport of water-containing and other substances across its boundaries.
Embedded within the plasma membranes of eukaryotes are various proteins. These proteins serve several distinct functions in the cell. Some proteins are pumps or channels for the import and export of substances. Other proteins, called antigens, serve as identification markers for the cell. Still other proteins help the cell form attachments with other cells. Because these membrane proteins often protrude out of the cell membrane into the extracellular environment, they too have hydrophobic and hydrophilic regions. Portions of the proteins that are embedded within the plasma membrane are hydrophobic, and portions of the proteins that extend outward into the extracellular environment are hydrophilic.
Scientists studying plasma membranes use the term “fluid-mosaic model” to describe the structure of plasma membranes. The “mosaic” portion of the model describes the way proteins are embedded within the plasma membrane. The membrane proteins also may drift within the plasma membrane, albeit more slowly than the phospholipids.
The endoplasmic reticulum (meaning “within the cytoplasm” and “net”) consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell’s cytoplasm. This internal system of membrane is continuous with the double membrane that surrounds the cell’s nucleus. Therefore, the encoded instructions that the nucleus sends out for the synthesis of proteins flow directly into the endoplasmic reticulum. Within the cell, the endoplasmic reticulum synthesizes lipids and proteins. The proteins that the endoplasmic reticulum synthesizes, such as enzymes, are exported from the cell to perform various functions in the body.
Two types of endoplasmic reticulum are found in the eukaryotic cell. Rough endoplasmic reticulum is studded with ribosomes on its outer face. These ribosomes are the sites of protein synthesis. Once a protein is synthesized on a ribosome, it is enclosed within a vesicle, a bubble-type structure. The vesicle travels to another organelle called the Golgi body. By fusing with the Golgi body membrane, the contents of the vesicle can be released into the Golgi body. Within the Golgi body, the proteins within the vesicle are further modified before they are exported from the cell. Cells that specialize in protein secretion contain large amounts of rough endoplasmic reticulum. Plasma cells, white blood cells that secrete immune proteins called antibodies, are so crowded with rough endoplasmic reticulum it is difficult to distinguish other organelles within the cytoplasm.
The other type of endoplasmic reticulum is smooth endoplasmic reticulum. Smooth endoplasmic reticulum does not have ribosomes and is the site of lipidmetabolism. Here, macromolecules containing lipids are broken down into their constituent parts. In addition, smooth endoplasmic reticulum functions in the synthesis of lipid-containing macromolecules. Smooth endoplasmic reticulum is not as common in cells as rough endoplasmic reticulum.
Named for its discoverer, the nineteenth century Italian scientist Camillo Golgi (1843–1926), the Golgi body is one of the most unusually shaped organelles. Looking somewhat like a stack of pancakes, the Golgi body consists of stacked, membrane-bounded, flattened sacs. Surrounding the Golgi body are numerous, small, membrane-bounded vesicles. The Golgi body and its vesicles function in the sorting, modifying, and packaging of macromolecules that are secreted by the cell or used within the cell for various functions.
One portion of the Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles. A portion of the Golgi body located on the opposite side is the site from which modified and packaged macromolecules are transported to their destinations.
Within the Golgi body, as the macromolecules move from the receiving to the transporting faces of the Golgi body, various chemical groups are added to the macromolecules to ensure that they reach their proper destination.
Vesicles are small, membrane-bounded spheres that contain various macromolecules. Lysosomes are vesicles that contain enzymes involved in cellular digestion and the digestion of cellular components that are chemically targeted for destruction. In a process called phagocytosis, protists surround a food particle and engulf it within a vesicle. This food containing vesicle is transported within the protist’s cytoplasm until it is contiguous with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break the food down into smaller parts for use by the protist.
Peroxisomes contain hydrogen peroxide. Peroxisomes function in the oxidation of many materials, including fats.
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 (meaning “box”). 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.
Cells contain hundreds to thousands of mitochondria. An interesting aspect of mitochondria is that they contain their own DNA sequences, although not in the profusion that the nucleus contains. The presence of this separate DNA, along with the resemblance of mitochondria to single-celled prokaryotes, has led to a theory that postulates that mitochondria were once free-living prokaryotes that became engulfed within other prokaryotes. Instead of being digested, the mitochondrial prokaryotes remained within the engulfing cell and performed its energy-releasing functions. Over millions of years, this symbiotic relationship fostered the evolution of the eukaryotic cell.
Plant cells have several organelles not found in animal cells. Plastids are vesicle-type organelles that perform a variety of functions in plants. Amylopasts 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. The interior of chloroplasts contains an elaborate membrane system. Thylakoids bisect the chloroplasts; attached to these platforms are stacks of membranous sacs called grana. Each granum contains the enzymes necessary for photosynthesis. The membrane system within the chloroplasts is bathed in a fluid called stroma, which also contains enzymes.
Like mitochondria, chloroplasts resemble some ancient single-celled prokaryotes and also contain their own DNA sequences.
Plant vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90% 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.