CENTRIOLE, CYTOSKELETON AND VACUOLE

CENTRIOLES:

A minute cylindrical organelle near the nucleus in animal cells, occurring in pairs and involved in the development of spindle fibers in cell division.

Animal cells and cells of some microorganisms and lower plants contain two centrioles located near the exterior surface of the nucleus. In cross section each centriole consists of a cylindrical array of nine microtubules. However, each of the nine microtubules is further composed of three tubules. The two centrioles are usually placed at right angle to each other. Just before a cell divides, its centrioles duplicate and one pair migrates to the opposite sides of the nucleus. The spindle then forms between them. They are absent in higher plants. Centrioles play important role in the location of furrowing during cell division, and in the formation of cilia.

CYTOSKELETON:

A microscopic network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and coherence.

Cytosol contains cytoskeletal fabric formed of microtubules, microfilaments and intermediate filaments. The main proteins that are present in cytoskeleton are tubulin (in microtubules), actin, myosin, tropomyosin and others which are also found in muscles. Several cell organelles are derived from special assemblies of microtubules, for examples cilia, flagella, basal bodies and centrioles. The movement of cyclosis and amoeboid movements are because, of microfilaments, whereas intermediate filaments are involved in determination of cell shape and integration of cellular compartments.

Microtubules:

These are long, un-branched, slender tubulin protein structures. One very important function of mirotubules is their role in the assembly and disassembly of the spindle structure during mitosis.

Microfilaments:

These are considerably more slender cylinders made up of contractile actin protein, linked to the inner face of the plasma membrane. They are involved in internal cell motion.

Intermediate filaments:

They have diameter in between those of microtubules and microfilaments. They play role in the maintenance of cell shape. 

Vacuole:

A space or vesicle within the cytoplasm of a cell, enclosed by a membrane and typically containing fluid.

Although vacuoles are present both in animal and plant cells, they are particularly large and abundant in plant cells often occupying a major portion of the cell volume and forcing the remaining intracellular structures into a thin peripheral layer. These vacuoles are bounded by a single membrane and are formed by the coalescence of smaller vacuoles during the plant's growth and development. Vacuoles serve to expand the plant cell without diluting its cytoplasm and also function as sites for the storage of water and cell products or metabolic intermediates. The plant vacuole is the major contributor to the turgor that provides support for the individual plant cell and contributes to the rigidity of the leaves and younger parts of the plants.

BRIEF REVIEW OF PROKARYOTIC AND EUKARYOTIC CELL

Life scientists have separated cells into two sorts: Prokaryotic and Eukaryotic.

The distinction between these two categories of cells is primarily based upon the structure of their core i.e. nuclei. 

In prokaryotic cells, the heritable material (DNA) is without any membrane covering and is openly submerged in the cytoplasm. 

image of prokaryotic cell

Eukaryotes have a very well defined core i.e. nucleus, in which nuclear material (chromosomes or DNA) is encased in two fold nuclear membrane.

image of eukaryotic cell

Living organisms acquiring prokaryotic cells are called Prokaryotes and those possessing eukaryotic cells are called Eukaryotes. Prokaryotes take account of bacteria and blue green alga. Eukaryotes comprise all other unicellular or multicellular creatures for instance animals, plants, fungi and Protista.

Prokaryotic cells for the most part are a shortage of many of the membrane bounded structures found in eukaryotic cells. For instance, mitochondria, endoplasmic reticulum, chloroplasts and Golgi apparatus are not present in prokaryotic cells. As there is no nuclear membrane, a prokaryotic cell has no distinct core nucleus and its DNA molecule is directly submerged in cytoplasm. Prokaryotes have little estimated ribosomes 70s contrasted with eukaryotes 80s.

70S prokaryotic ribosome

Eukaryotes have larger sized ribosomes of 80S. They are composed of two parts i.e. 60S and 40S.

80S eukaryotic ribosome

In prokaryotes, mitosis is missing and the cell reproduces by binary fission. As a consequence of their simpler structure, it was generally accepted for quite a while that prokaryotic cells speak to a more primitive phase of development than eukaryotic cells. Perhaps the most distinguishing feature of the prokaryotic cell is that its cell divider composed of polysaccharide chains which are covalently bounded to shorter chains of amino acids structuring peptidoglycan or murein. The entire cell wall is viewed as a single and solitary enormous particle or sub-atomic complex called sacculus. The cell wall of plants for the most part is composed of cellulose and is distinctively structured than that of a bacterium.

CELL; A UNIT OF STRUCTURE AND FUNCTION

A cell is a unit of structure and function in living organisms. In multicellular organisms there is a division of labor among cells. Different cells are specialized for different functions. The function of the organism as a whole is the result of the sum of activities and interactions of different cells and of different components of the cell. In animals e.g. muscle cells, contract and relax, nerve cells transmit impulses, gland cells secrete, red blood cells carry oxygen and some stomach cells secrete gastric juice.

Similarly in plants xylem cells conduct water and mineral salts from soil to the aerial parts of the plant. Phloem cells translocate food, sclerenchymatous cells give support to the plants, chlorenchymatous cells carry out photosynthesis, parenchymatous cells store surplus food and Meristematic cells produce new cells for growth and development of the plant. As they perform different functions they show great variation in shape and size. Despite the structural and functional diversity, the plant cells as well as animal cells have a common plan of organization.

HISTORY AND DISCOVERY OF CELL AND EMERGENCE OF CELL THEORY

The cell can be defined as the structural and functional unit of life. It is the smallest unit that can carry out all activities of life. Cells are building blocks of complex multicellular organisms.

EMERGENCE AND IMPLICATION OF CELL THEORY: 

Study of cell (cell biology) began with the discovery of cell by Robert Hooke (1665), who reported his work in his famous publication “Micrographia”. He prepared and studied thin sections of cork (of dead plant material) under his self-made compound microscope. He observed that the cork is composed of minute honey comb like compartments which he termed as Cells. According to Hooke, cell is an empty space bounded by thick walls. Very little information was added to this idea in the following century. The work again started in the beginning of 19th century.

cell discovered by Robert Hooke

Lorenz Oken (1805) A German scientist believed that, “all living being originate from or consists of cells”.

Jean Baptist de-Lamarck (1809) expressed similar idea and said,"no body can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue".

In 1831 Robert Brown reported the presence of nucleus in the cell. Due to this discovery Hooke's idea about the cell as an empty space was changed. It was later established that cell is not an empty space.

A German zoologist Theodor Schwann (1839) and a German botanist Schleiden (1838), working independently, came out with a theory called the Cell Theory.

They found that the cell consisted of 3 basic parts, viz nucleus, the fluid (cytoplasm) surrounding the nucleus, and an outer thin covering or membrane (plasma membrane). The cell wall, they said, was an additional structure, present only in plant cell. Keeping in view this definition of cell, the cells could be observed in plant as well as in animal according to cell theory; all living organisms are composed of cells and cell products.

The cell theory is one of the most fundamental generalizations in Biology. It has wide ranging effect in all fields of biological sciences. After the cell theory was presented, many details of cell were studied, as a result of which the cell theory was extended.

Rudolph Virchow (1855), a German physician, hypothesized that new cells were formed only by the division of previously existing living cells (to put it in Virchow's words: "omnis cellula e cellula"). It was contrary to the idea of "abiogenesis" (living things arise spontaneously from non-living things), one of the prevailing but controversial ideas about origin of life, at that time.

Louis Pasteur (1862), one of the greatest scientists of all times, supplied experimental proof for Virchow's hypothesis by demonstrating that micro-organisms (bacteria) could be formed only from existing bacteria. Original cell theory and Virchow's hypothesis gave us the basis for working definition of living things: living things are chemical organizations composed of cells and capable of reproducing themselves.

August Weismann (1880) said, “All presently living cells have a common origin because they have basic similarities in structure and molecules etc. It was shown that there are certain similarities in chemical composition, metabolic activities and structure, although they differ in many respects. Cells are basically similar but extraordinarily versatile. Cell is not only the structural but also the functional unit of living organisms. So cell theory is a very important unifying concept.

The salient features of Cell Theory in its present form are: 

• All organisms are composed of one or more cells.
•  All cells arise from pre-existing cells.
• Cell is the basic structural as well as functional unit for all organisms.