Eukaryotic cells (as opposed to prokaryotic cells) have internal, membrane-bound organelles and a distinct nucleus that physically separates the genetic material of the cell from the all of the other parts of the cell. All protists, fungi, plants, and animals are composed of eukaryotic cells.
The cells of all organisms can be divided into two broad categories: prokaryotic cells and eukaryotic cells. Prokaryotic cells are cells with a relatively simple structure, having no internal, membrane-bound organelles. The most striking feature of prokaryotic cells is that they lack a distinct nucleus, hence the name prokaryotic, literally translated from its Greek roots as “before nucleus.”
The prokaryotic organisms comprise two domains of the three domains of life: the ancient bacteria, Archaea; and the modern bacteria, Bacteria or Eubacteria. The Archaea are single-celled organisms that often inhabit extreme environments, such as hot springs. The remainder of bacteria are classified as Eubacteria.
All other organisms, including fungi, plants, and animals, are composed of eukaryotic cells and belong to the domain Eukarya. Eukaryotic cells are more structurally complex than prokaryotic cells, having internal, membrane-bound organelles and a distinct nucleus that physically separates the genetic material of the cell from the all of the other parts of the cell.
Based on genetic analysis, the Archaea and Eukarya are more closely related to each other than they are to the Bacteria, suggesting that eukaryotic cells may have arisen from a single ancestral archaean cell.
Eukarya includes the traditional kingdoms Plantae, Animalia, Fungi, and Protista. Protists include a diverse assemblage of single-celled eukaryotic organisms including algae, amoebas, and paramecia. Because algae are photosynthetic, they have often been included in the study of plants, although they are not members of the plant kingdom.
Fungi include such organisms as smuts, rusts, molds, and mushrooms. Fungal cells have external cell walls and because of this have often been included in the study of plants.
However, fungal cell walls have a completely different structure and composition from those of plant cell walls, and fungi lack plastids and photosynthetic pigments. Fungi represent a unique evolutionary line. They too, however, tend to be studied in botany courses, even though they are not plants.
Eukaryotic cells are surrounded by a cell membrane, or plasma membrane, that is composed of a lipid structure in which other molecules, such as proteins and carbohydrates, are embedded. The cell membrane serves as a semipermeable, or selective, barrier between the cell and its environment.
Some small, uncharged molecules can freely cross the cell membrane; others must be transported across the membrane before they can enter the cell. The cell membrane serves to protect the cell and to receive signals from the environment and other cells that help to direct cell activities.
In addition to the cell membrane, plant cells also have external cell walls. The presence of the external cell wall is one of the major characteristics that distinguishes plant cells from animal cells.
The cell wall limits the size of the internal protoplast (the internal cytoplasm and nucleus) and prevents the plasma membrane from breaking when the protoplast enlarges following the uptake of water by the cell.
Cell walls are not merely static support structures, however. They contain enzymes that are important in bringing essential molecules into the cell and in secreting molecules. They may also play important roles in the defense of the plant against bacterial and fungal pathogens.
Eukaryotic cells also have a prominent, membrane-bound organelle called the nucleus. The nucleus contains the genetic information of the cell that directs the cellular activity. A double membrane called the nuclear envelope surrounds the nucleus.
Inside the nucleus, deoxyribonucleic acid (DNA) is transcribed to make molecules of ribonucleic acid (RNA), copies of the genetic information that can be delivered to the cytoplasm, where the RNA molecules serve to direct the manufacture of proteins.
DNA in the eukaryotic nucleus exists as linear molecules that are associated with many proteins, and the DNA is packaged into a highly organized chromosomal structure by proteins called histones.
In addition to the nucleus, eukaryotic cells contain a number of internal membrane-bound organelles that help the cell carry out the functions necessary for life.
The types of organelles found inside a eukaryotic cell reflect the function of that cell and the processes that it must carry out. Some of these organelles, such as mitochondria and chloroplasts, are important in capturing and releasing energy for cell function.
Some, like the Golgi complex and the endoplasmic reticulum (ER), are involved in the manufacture, processing, and transport of proteins and other molecules within the cell. Others, such as peroxisomes, are involved in detoxifying chemicals and breaking down molecules.
The cell cytoskeleton is a highly dynamic structure that provides support and motility to cells as well as providing some of the apparatus that is used in the transduction of signals from the cell membrane to the nucleus.
In plant cells, cytoskeletal elements form tracks for the movement of internal cellular organelles, such as the cytoplasmic streaming of chloroplasts, which can be observed by light microscopy.
Work of the cytoskeleton is also necessary for the opening and closing of the stomata in plant leaves. The cytoskeleton consists of a variety of filament like proteins as well as proteins that serve as anchor points for filaments.
Origins of Mitochondria and Chloroplasts
The nucleus of the eukaryotic cell is not the only organelle that contains DNA and is enclosed by two membranes: The mitochondria of all cells and the chloroplasts of plant cells contain DNA and are surrounded by two membranes.
The DNA of these organelles directs the synthesis of certain proteins that are necessary for the function of the organelles. This DNA is similar to DNA found in bacteria.
Mitochondria and chloroplasts are thought to have evolved by a process known as endosymbiosis, in which bacteria were engulfed in the primitive eukaryotic cell, where they manufactured adenosine triphosphate (ATP), the nucleotide responsible for most of the chemical energy needed for metabolism, or captured energy from sunlight for the eukaryotic cell, establishing a mutually beneficial, or symbiotic, relationship with the eukaryotic cell.
Several lines of evidence support the endosymbiotic theory for the origin of mitochondria and chloroplasts. First, these organelles have areas of specialized cytoplasm called nucleoids that contain the DNA, much as bacteria do.
The DNA molecules of the chloroplasts and mitochondria are circular and are associated with few proteins, like bacterial DNA, rather than linear and associated with histone proteins like most eukaryotic DNA.
Chloroplasts and mitochondria also have ribosomes, structures that translate the genetic material into proteins, that are more similar to bacterial ribosomes than they are to eukaryotic ribosomes. These ribosomes are even sensitive to some of the same antibiotics, such as chloramphenicol and streptomycin, that inhibit the function of bacterial ribosomes.
The internal membranes of eukaryotic cells are dynamic, constantly changing structures. The concept of the endo membrane system describes all internal cytoplasmic membranes,with the exception of mitochondrial and plant plastid membranes, as a single continuum.
In this model, the ER, generally the largest membrane system of eukaryotic cells, is the initial source of most other membranes. The ER is a network of interconnected, closed, membrane-bound vesicles that is contiguous with the nuclear envelope.
Vesicles from the ER carry proteins from the ER to the Golgi complex, fusing with its membranes. The Golgi complex can be described as a series of flattened membrane sacs, like a stack of hollow pancakes.
The side closest to the nucleus receives vesicles from the ER, and the proteins inside these vesicles are processed and modified as they pass through the Golgi complex. Eventually, membrane vesicles containing the modified proteins will bud from the opposite surfaces of the Golgi complex and fuse with the cell membrane or the membranes of other organelles.