During the 1950s, scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes. The cells of all prokaryotes and eukaryotes possess two basic features: a plasma membrane, also called a cell membrane, and cytoplasm. However, the cells of prokaryotes are simpler than those of eukaryotes. For example, prokaryotic cells lack a nucleus, while eukaryotic cells have a nucleus. Prokaryotic cells lack internal cellular bodies (organelles), while eukaryotic cells possess them. Examples of prokaryotes are bacteria and archaea. Examples of eukaryotes are protists, fungi, plants, and animals (everything except prokaryotes).
All prokaryote and eukaryote cells have plasma membranes. The plasma membrane (also known as the cell membrane) is the outermost cell surface, which separates the cell from the external environment. The plasma membrane is composed primarily of proteins and lipids, especially phospholipids. The lipids occur in two layers (a bilayer). Proteins embedded in the bilayer appear to float within the lipid, so the membrane is constantly in flux. The membrane is therefore referred to as a fluid mosaic structure. Within the fluid mosaic structure, proteins carry out most of the membrane’s functions.
The “Movement through the Plasma Membrane” section later in this chapter describes the process by which materials pass between the interior and exterior of a cell.
Cytoplasm and organelles
All prokaryote and eukaryote cells also have cytoplasm (or cytosol), a semiliquid substance that composes the volume of a cell. Essentially, cytoplasm is the gel-like material enclosed by the plasma membrane.
Within the cytoplasm of eukaryote cells are a number of membrane-bound bodies called organelles (“little organs”) that provide a specialized function within the cell.
One example of an organelle is the endoplasmic reticulum (ER). The ER is a series of membranes extending throughout the cytoplasm of eukaryotic cells. In some places, the ER is studded with submicroscopic bodies called ribosomes. This type of ER is called rough ER. In other places, there are no ribosomes. This type of ER is called smooth ER. The rough ER is the site of protein synthesis in a cell because it contains ribosomes; however, the smooth ER lacks ribosomes and is responsible for producing lipids. Within the ribosomes, amino acids are actually bound together to form proteins. Cisternae are spaces within the folds of the ER membranes.
Another organelle is the Golgi apparatus (also called Golgi body). The Golgi apparatus is a series of flattened sacs, usually curled at the edges. In the Golgi body, the cell’s proteins and lipids are processed and packaged before being sent to their final destination. To accomplish this function, the outermost sac of the Golgi body often bulges and breaks away to form droplike vesicles known as secretory vesicles.
An organelle called the lysosome (see Figure 3-1) is derived from the Golgi body. It is a droplike sac of enzymes in the cytoplasm. These enzymes are used for digestion within the cell. They break down particles of food taken into the cell and make the products available for use; they also help break down old cell organelles. Enzymes are also contained in a cytoplasmic body called the peroxisome.
Figure 3-1 The components of an idealized eukaryotic cell. The diagram shows the relative sizes and locations of the cell parts.
The organelle that releases quantities of energy to form adenosine triphosphate (ATP) is the mitochondrion (the plural form is mitochondria). Because mitochondria are involved in energy release and storage, they are called the “powerhouses of the cells.”
Green plant cells, for example, contain organelles known as chloroplasts, which function in the process of photosynthesis. Within chloroplasts, energy from the sun is absorbed and transformed into the energy of carbohydrate molecules. Plant cells specialized for photosynthesis contain large numbers of chloroplasts, which are green because the chlorophyll pigments within the chloroplasts are green. Leaves of a plant contain numerous chloroplasts. Plant cells not specializing in photosynthesis (for example, root cells) are not green.
An organelle found in mature plant cells is a large, fluid-filled central vacuole. The vacuole may occupy more than 75 percent of the plant cell. In the vacuole, the plant stores nutrients, as well as toxic wastes. Pressure within the growing vacuole may cause the cell to swell.
The cytoskeleton is an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell. The main components of the cytoskeleton are microtubules, microfilaments, and intermediate filaments. All are assembled from subunits of protein.
The centriole organelle is a cylinderlike structure that occurs in pairs. Centrioles function in cell division.
Many cells have specialized cytoskeletal structures called flagella and cilia. Flagella are long, hairlike organelles that extend from the cell, permitting it to move. In prokaryotic cells, such as bacteria, the flagella rotate like the propeller of a motorboat. In eukaryotic cells, such as certain protozoa and sperm cells, the flagella whip about and propel the cell. Cilia are shorter and more numerous than flagella. In moving cells, the cilia wave in unison and move the cell forward. Paramecium is a well-known ciliated protozoan. Cilia are also found on the surface of several types of cells, such as those that line the human respiratory tract.
Prokaryotic cells lack a nucleus; the word prokaryotic means “primitive nucleus.” Eukaryotic cells, on the other hand, have a distinct nucleus.
The nucleus of eukaryotic cells is composed primarily of protein and deoxyribonucleic acid, or DNA. The DNA is tightly wound around special proteins called histones; the mixture of DNA and histone proteins is called chromatin. The chromatin is folded even further into distinct threads called chromosomes. Functional segments of the chromosomes are referred to as genes. Approximately 21,000 genes are located in the nucleus of all human cells.
The nuclear envelope, an outer membrane, surrounds the nucleus of a eukaryotic cell. The nuclear envelope is a double membrane, consisting of two lipid layers (similar to the plasma membrane). Pores in the nuclear envelope allow the internal nuclear environment to communicate with the external nuclear environment.
Within the nucleus are two or more dense organelles referred to as nucleoli (the singular form is nucleolus). In nucleoli, submicroscopic particles known as ribosomes are assembled before their passage out of the nucleus into the cytoplasm.
Although prokaryotic cells have no nucleus, they do have DNA. The DNA exists freely in the cytoplasm as a closed loop. It has no protein to support it and no membrane covering it. A bacterium typically has a single looped chromosome.
Many kinds of prokaryotes and eukaryotes contain a structure outside the cell membrane called the cell wall. With only a few exceptions, all prokaryotes have thick, rigid cell walls that give them their shape. Among the eukaryotes, some protists, and all fungi and plants, have cell walls. Cell walls are not identical in these organisms, however. In fungi, the cell wall contains a polysaccharide called chitin. Plant cells, in contrast, have no chitin; their cell walls are composed exclusively of the polysaccharide cellulose.
Cell walls provide support and help cells resist mechanical pressures, but they are not solid, so materials are able to pass through rather easily. Cell walls are not selective devices, as plasma membranes are.