The eukaryotic cells occur in all eukaryotes votes like protists, plants, fungi and animals.
The eukaryotic cells are too complex than prokaryotic cells and evolved from them about 1.5 billion years ago (BYA).
Eukaryotic cell size varies greatly from 10 mm to 500 mm. Ostrich egg is the largest eukaryotic cell known measuring 170 mm X150 mm. In plants, the longest cells are the sclerenchyma fibers of Ramie (Boehmeria nivea) of Utricaceae. But in animals, neurons are the longest cells reaching up to 3 mt. in elephants and whales.
The size of a cell depends upon:
i. Surface area to volume ratio
ii. Nucleo-cytoplasmic or kern-plasmic ratio
iii. Size and number of chromosomes.
The shape of eukaryotic cell may be variable or fixed. Variable shape occurs in Amoeba and white blood cells. Fixed shape of cell occurs in most plants and animals. In unicellular organisms, the cell shape is maintained by plasma membrane and exoskeleton (e.g. Polystomella or Elphidium).
In multicellular organisms, the shape of a cell depends mainly on its functional adaptations and partly by external pressure, surface tension, internal stress, viscosity of protoplasm and cytoskeleton. Thus, the eukaryotic cells have diverse shapes such as spherical, elongated, spindle-shaped, discoidal, polyhederal, branched, oval and so on.
Types of Eukaryotic Cells:
In multicellular organisms cells can be classified into three main types — Undifferentiated, differentiated and. dedifferentiated cells.
1. Undifferentiated Cells:
These unspecialized cells are capable of undergoing division and development. For example, Zygote, stem cells (in animals) and meristematic cells (in plants)
2. Differentiated cells (- Post-mitotic cells):
These are specialized cells which perform a specific function and exhibit division of labour. For example, mesophyll cells carry out photosynthesis, RBCs transport O2 and CO2 etc. However, in animals the cellular differentiation is an irreversible phenomenon.
3. Dedifferentiated cells:
Sometimes, the differentiated cells revert back to undifferentiated cells and carry out cell division. The process by which differentiated cells lose their specialization is called dedifferentiation. For example, parenchyma dedifferentiated into meristematic cells for wound healing, regeneration and secondary growth. Tissue culture and colonial propagation depends on dedifferentiation of cells. If any dedifferentiated cells again become specialize then process is called redifferentiation.
Though eukaryotic cells vary in shape, size and functions, all show some basic structural plan. A generalized/typical eukaryotic cell as seen under Electron Microscope (EM) consists of cell wall (absent in animal cells and some protists), plasma membrane. Cytoplasm, nucleus, other organells and inclusions. Except nucleus and plastids all other cytoplasmic structures can be seen under EM. Eukaryotic cell is, by definition, possess a nucleus containing nuclear material enclosed by a double layered nuclear envelope.
EM has revealed that the cytoplasm has extensive compartmentalization due to presence of membrane bound organelles. In addition, eukaryotic cells are supported by cytoskeleton and have locomotory appendages (cilia or flagella) in motile cells. Since the functions of ER, golgi complex, lysosomes and vacuoles are coordinated, these are considered together as an endomembrane system.