Eukaryotic cell: Definition, Size, Shape, and Structure.

What is Eukaryotic cell ?

eukaryotic cell contains membrane-bound organelles such as a nucleus, mitochondria, and an endoplasmic reticulum. Organisms based on the eukaryotic cell include protozoa, fungi, plants, and animals. These organisms are grouped into the biological domain Eukaryota.

A eukaryotic cell or organism has a clearly defined nucleus. The eukaryotic cells are the true cells found in plants (from algae to angiosperms) and animals (from protozoa to mammals).

Although the eukaryotic cells have a different shape, size, and physiology: All eukaryotic cells are made up of organelles, including mitochondria (cellular energy exchangers), a Golgi apparatus (secretion device), an endoplasmic reticulum (a canal-like membrane system inside the cell) and lysosomes (Digestive system 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 membrane.

There are several exceptions, however; For example, the absence of mitochondria and a nucleus in red blood cells and the absence of mitochondria in the Oxymonad Monocercomonoides species. Before going into the details of the cell and its various components, it is advisable to consider the general characteristics of different types of eukaryotic cells, which are as follows.

Shape of Eukaryotic Cell

Eukaryotic cells show a variety of different cell morphologies. Possible shapes are spherical, ovoid, cuboid, cylindrical, flat, lenticular, fusiform, disc-shaped, crescent-shaped, ring-shaped, and polygonal. Some eukaryotic cells are irregular in shape; others can change shape.

However, the shape is ultimately determined by the specific function of the cell. Thus, the shape of the cell can be variable (i.e. often changing shape) or fixed. A variable or irregular shape occurs in Amoeba and white blood cells or leukocytes (in fact, leukocytes in circulating blood are spherical, but under other conditions, they can produce pseudopodia and assume an irregular 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 an exoskeleton.

In a multicellular organism, the shape of the cell depends mainly on its functional adaptations and, in part, on the surface tension, a viscosity of the protoplasm, the cytoskeleton of microtubules, microfilaments and intermediate filaments, the mechanical action of neighboring cells, and the rigidity of the plasma membrane (i.e. the presence of a rigid Cell wall in plant cells).

The shape of the cell can vary from animal to animal and from organ to organ. Even the cells of the same organ can differ in shape. Thus cells can have various shapes, such as E.g., polyhedral (e.g., squamous epithelium); flattened (e.g., squamous epithelium); cuboid (e.g. in thyroid gland follicles); columnar (e.g., the cells lining the intestines); disc-shaped (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).

In plants, the cell shape also depends on the cell’s function. For example, cells like glandular hairs on a leaf, protective cells of stomata, and root hair cells have their special shape.

Eukaryotic Cell Size

Typical eukaryotic cells size ranging from 10 to 100 micrometers (μm) in diameter and are significantly larger than prokaryotic cells, which usually have diameters ranging from 0.1 to 5.0 μm. The size of the cells of the unicellular organisms is larger than a typical multicellular organism’s cells.

For example, amoebic proteus is the largest among the unicellular organisms; its length is 1 mm (1000 μm). A species of Euglena is up to 500 μm (0.5 mm) long. Euplotes (a freshwater ciliate) are 120 μm long.

Another ciliate, Paramecium caudatum, is 150 to 300 μm (0.15 to 0.3 mm) long. Diatoms are 200 μm or more in length. The unicellular alga Acetabularia, which consists of a stem and a cap, is exceptionally large and up to 10 cm high.

The size of the cells of multicellular organisms is between 20 and 30 μm. Among the animals, the smallest cells are 4 µm in diameter. m (e.g. polocytes); human erythrocytes with a diameter of 7 to 8 μm.

The largest animal cell is an ostrich egg, 18 cm in diameter (its yolk, or deutoplasm, is about 5 cm in diameter); However, some human nerve cells have “tails” or axons one meter long. Among the multicellular plants, the largest cell is the Cycas egg cell. The fiber cells (i.e., sclerenchyma cells) of Manila hemp are over 100 cm long.

Volume of a Eukaryotic Cell

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.

Structure Of Eukaryotic Cell

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes. a membrane-bound nucleus. numerous membrane-bound organelles (including the endoplasmic reticulum, Golgi apparatus, chloroplasts, and mitochondria) several rod-shaped chromosomes.

cell structure of animal cell and plant cell

The eukaryotic cell structure comprises the following:

Plasma Membrane: The plasma membrane separates the cell from the outside environment. It comprises specific embedded proteins, which help in the exchange of substances in and out of the cell.

Cell Wall: A cell wall is a rigid structure present outside the plant cell. It is, however, absent in animal cells. It provides shape to the cell and helps in cell-to-cell interaction. It is a protective layer that protects the cell from any injury or pathogen attacks. It is composed of cellulose, hemicellulose, pectins, proteins, etc.

Cytoskeleton: The cytoskeleton is present inside the cytoplasm, which consists of microfilaments, microtubules, and fibers to provide perfect shape to the cell, anchor the organelles, and stimulate the cell movement.

Endoplasmic Reticulum: It is a network of small, tubular structures that divides the cell surface into two parts: luminal and extraluminal. Endoplasmic Reticulum is of two types:

  • Rough Endoplasmic Reticulum contains ribosomes.
  • Smooth Endoplasmic Reticulum that lacks ribosomes and is therefore smooth.

Nucleus: The nucleoplasm enclosed within the nucleus contains DNA and proteins. The nuclear envelop consists of two layers- the outer membrane and the inner membrane. Both the membranes are permeable to ions, molecules, and RNA material. Ribosome production also takes place inside the nucleus.

Golgi Apparatus: It is made up of flat disc-shaped structures called cisternae. It is absent in the red blood cells of humans and sieve cells of plants. They are arranged parallel and concentrically near the nucleus. It is an important site for the formation of glycoproteins and glycolipids.

Ribosomes: These are the main site for protein synthesis and are composed of proteins and ribonucleic acids.

Mitochondria: These are also known as the “powerhouse of cells” because they produce energy. It consists of an outer membrane and an inner membrane. The inner membrane is divided into folds called cristae. They help in the regulation of cell metabolism.

Lysosomes: They are known as “suicidal bags” because they possess hydrolytic enzymes to digest protein, lipids, carbohydrates, and nucleic acids.

Plastids: These are double-membraned structures and are found only in plant cells. These are of three types: Chloroplast contains chlorophyll and is involved in photosynthesis. Chromoplast contains a pigment called carotene that provides the plants with yellow, red, or orange colors. Leucoplasts are colorless and store oil, fats, carbohydrates, or proteins.