A eukaryotic cell or organism is that possesses a clearly defined nucleus. The eukaryotic cells are the true cells that occur in the plants (from algae to angiosperms) and the animals (from Protozoa to mammals).
Though the eukaryotic cells have a different shape, size, and physiology: All eukaryotic cells are composed of organelles, including mitochondria (cellular energy exchangers), a Golgi apparatus (secretory device), an endoplasmic reticulum (a canal-like system of membranes within the cell), and lysosomes (digestive apparatus within many cell types).
Here the nuclear contents, such as DNA, RNA, nucleoproteins, and nucleolus remain separated from the cytoplasm by the thin, perforated nuclear membranes.
There are several exceptions to this, however; for example, the absence of mitochondria and a nucleus in red blood cells and the lack of mitochondria in the oxymonad Monocercomonoides species.
Before going into the details of the cell and its various components, it will be advisable to consider the general features of different types of eukaryotic cells which are as follows.
The basic shape of the eukaryotic cell is spherical; however, the shape is ultimately determined by the specific function of the cell. Thus, the shape of the cell may be variable (i.e., frequently changing the shape) or fixed.
Variable or irregular shape occurs in Amoeba and white blood cells or leucocytes (In fact, leucocytes are spherical in the circulating blood, but in other conditions, they may produce pseudopodia and become irregular in shape).
The fixed shape of the cell occurs in almost all protists (e.g., Euglena, Paramecium), plants, and animals. In unicellular organisms, the eukaryotic cell shape is maintained by a tough plasma membrane and exoskeleton.
In a multicellular organism, the shape of the cell depends mainly on its functional adaptations and partly on the surface tension, a viscosity of the protoplasm, the cytoskeleton of microtubules, microfilaments and intermediate filaments, the mechanical action exerted by adjoining cells, and the rigidity of the plasma membrane (i.e., presence of rigid cell wall in plant cells).
The shape of the cell may vary from animal to animal and from organ to organ. Even the cells of the same organ may display variations in the shape. Thus, cells may have diverse shapes such as polyhedral (with 8, 12, or 14 sides; e.g., squamous epithelium); flattened (e.g., squamous epithelium, endothelium and the upper layers of the epidermis); cuboidal (e.g., in thyroid gland follicles); columnar (e.g., the cells lining the intestine); discoidal (e.g., red blood cells or erythrocytes); spherical (e.g., eggs of many animals); spindle-shaped (e.g., smooth-muscle fibers); elongated (e.g., nerve cells or neurons); or branched (e.g., chromatophores or pigment cells of the skin).
Among plants, the cell shape also depends upon the function of the cell. For example, cells such as glandular hairs on a leaf, the guard cells of stomata, and root hair cells have their special shape.
The eukaryotic cells are typically larger (mostly ranging between 10 to 100 μm) than the prokaryotic cells (mostly ranging between 1 to 10 μm). The size of the cells of the unicellular organisms is larger than a typical multicellular organism’s cells.
For example, Amoeba proteus is the biggest among the unicellular organisms; its length being 1 mm (1000 μm). One species of Euglena is found up to 500 μm ( 0.5 mm) in length. Euplotes (a freshwater ciliate) is 120 μm in length.
Another ciliate, Paramecium caudatum is from 150 to 300 μm (0.15 to 0.3 mm) in length. Diatoms have a length of 200 μm or more. The single-celled alga, Acetabularia which consists of a stalk and a cap is exceptionally large-sized and measures up to 10 cm in height.
The size of the cells of multicellular organisms ranges from 20 to 30 μm. Among animals, the smallest cells have a diameter of 4 μm (e.g., polocytes); human erythrocytes being 7 to 8 μm in diameter.
The largest animal cell is the egg of an ostrich, having a diameter of 18 cm (its yolk or deutoplasm is about 5 cm in diameter); though, some nerve cells of human beings have a meter long “tails” or axons. Among the multicellular plants, the largest cell is the ovule of Cycas (see Dnyansagar, 1988). The fibre cells (i.e., sclerenchyma cells) of Manila hemp are over 100 cm in length.
The volume of a eukaryotic cell is fairly constant for a particular cell type and is independent of the size of the organism. (This is called the law of constant volume.) For example, kidney or liver cells are about the same size in the bull, horse, and mouse. The difference in the total mass of the organ or organism depends on the number, not on the volume of the cells.
Thus, the cells of an elephant are not necessarily larger than those of other tiny animals or plants. The large size of the elephant is due to the larger number of cells present in its body. If a cell is to be efficient, the ratio of volume to the surface should be within a limited range.
An increase in cell volume is accompanied by a much smaller expansion in the surface area of the cell (In fact, volume increases as the cube of the radius, while surface area increases as the square of radius). In other words, a large cell has a proportionately smaller surface and a higher volume: surface ratio than a smaller cell.
Further, a large cell volume has to accommodate many organelles simultaneously limiting the exchange of information and materials through the surface. This problem is partially overcome by developing a cylindrical shape or by forming numerous extensions (e.g., microvilli) of the plasma membrane. It is also for this reason that metabolically active cells, tend to be smaller in size.
The number of eukaryotic cells present in an organism varies from a single cell in a unicellular organism (Protists such as protozoa and protophyta) to many cells in multicellular organisms (Most plants, fungi, and animals).
The number of eukaryotic cells in the multicellular organisms usually remains correlated with the size of the organisms and, therefore, the small-sized organism has less number of cells in comparison to large-sized organisms.
For example, a human being weighing about 80 kg may contain about 60 thousand billion cells in his body. This number would be more in certain other multicellular organisms.
Further, the number of cells in most multicellular organisms is indefinite, but the number of cells may be fixed in some multicellular organisms. For example, in rotifers, the number of nuclei in the various organs are found to be constant in any given species.
This phenomenon of cells or nuclear constancy is called eutely. In one species of rotifer, Martini (1912) always found 183 nuclei in the brain, 39 in the stomach, 172 in the cornea epithelium, and so on (see Hickman, Sr., et al. 1979).
Among plants, colonial green algae exhibit cell constancy. For example, the green alga, Pandorina has a colony consisting of 8, 16, or 32 cells. Likewise, another green alga, Eudornia, has 16, 32, or 64 cells in its colony.