EUKARYOTIC CELL



General Organization of Eukaryotic Cells:

 The eukaryotic cells have a more elaborate internal organization than the prokaryotic cells.A eukaryotic cell is essentially a double membrane or two _ envelope system .Primary membrane  surrounds the cell.Secondary membranes envelope the nucleus and certain other subcellular organelles, and also pervade the cytoplasm, dividing the latter into compartments. These membranes participate directly in the  cell's metabolism, many  enzymes are built right into the membranes. Cell's compartments provide different local environments that facilitate specific metabolic functions and allow incompatible processes to occur simultaneously inside the same cell.The eukaryotic cells occur in ,protists, fungi,  plants and animals. These organisms are called eukaryotes.They are said to form the superkingdom Eukaryotae.





Structure of a Eukaryotic Cell :

Main States: 

A eukaryotic cell is seen in one of the two main states. It may be undergoing division to form daughter cells.Such a cell is said to be in the dividing state, or mitotic phase.When not in the process of division, the cell is said to be in the nondividing state,or in the interphase.An interphase eukaryotic cell is described in this book.

Size 

 Cells vary greatly in  size.Most of the cells are microscopic. Eukaryotic cells usually range from 10 to 1PM ųm.A single_ celled alga, Acetabularia, is about 10cm long. Internodal 
cell of Chara may reach 10 cm. in length. A hemp fibre may be over a meter long. Among protozoans, a sporozoite of material parasite, plasmodium vivax,is just 2 ųm long.The ostrich egg measures 17×150 mmr.Nerve cells are the longest, their fibres, which are their integral part, may be over a metre in length. Human cells generally range from 20 to 30 ųm.


Q. What is a prokaryote glant?

Ans. Largest prokaryote is a rod_ shaped bacterium ,Epulopiscium Fishelsoni.It is about 600 ųm ( over half a millimere) long and 80 ųm wide. It lives as a symbiote in the gut of the brown sturgeon, a fish .

Relationship between Size and Function. 

The size of a cell  is generally related to its function. A cell draws nutrients and oxygen from the environment and turns out waste products of metabolism  through its surface.Thus, the surface area of a cell determine its capacity to exchange materials with the environment. Small cells have more surface area per unit volume than the large cells. This is so because the surface area of a sphere ( a general cell shape) increases as a square of the radius and the volume increases as the cube of the radius. The amount of nutrients a cell requires for metabolism depends on its volume. Therefore, a metabolically active cell must have small volume so that its surface area is large enough to cope with its needs. This is why the cells maintain microscopic dimensions. 

The size of  a cell is determined by two main factors: 
(i) its requirement of nutrients and oxygen, and 

(ii) the regulating ability of its nucleus.







Shape 

Cells vary a good deal in shape not only in different organisms but also in different tissues of the same organism. They may be spherical, oval, disc_ like, cuboidal, columnar, polygonal, spindlelike, irregular or bizarre. The shape of a cell is often correlated with its function.  Amoebaand leucocytes can change their form so that they may engulf materials. Muscle and nerve cells are greatly elongated, being well adapted to their function.The form of a cell also depends on many other factors such as function, age, viscosity, cell wall,  external pressure, and internal or external skeleton.

Number

Some organisms have a microscopic body made of a single cell.Such forms are called unicellular ,or acellular organisms. Majority of organisms have a large to very large body composed of numerous cells. Such forms are termed multicellular organisms. The number of cells in a multicellular organism is correlated to the size of the body.The human blood contains about 30 quadrillion ( 30× 10¹⁵) corpuscles, and a adult human being weighing about 60 kg.has about 100 trillion ( 100 ×10¹²) cells.

It is noteworthy that all multicellular organisms begin life as a single cell, the zygote, and become  multicellular by mitotic divisions of the cell during development. 

Gross Structure of a Cell 

No cell is completely unspecialized so that it may be considered a typical cell. However, there are certain subcellular structures common to most  cells. A generalized eukaryotic cell with the common components is described here. A cell has three main components: cell membrane  cytoplasm and nucleus.The  cytoplasm and the nucleus further have several components. All the components of a cell taken together keep it living. No part can survive if separated from the cell.



1); Cell Membrane: 

All eukaryotic cells are surrounded by a thin, elastic, living covering called the cell membrane,or plasma membrane plasmalemma.It keeps the cell contents in place, gives form to the cell, and controls the entry and exit of materials into and out of the cell. It is composed of lipid __ protein complex.It lacks respiratory enzymes unlike the  plasma membrane of prokaryotic cells. 

Certain protists, most fungi and all plant cells have a  thick, rigid, nonliving additional covering outside the cell membrane. It is called the cell wall.It protects and supports the cell. 

2. Cytoplasm. 

The cytoplasm  consists of a semi fluid, homogeneous, translucent, colloidal ground substance, formerly called the cytoplasmic matrix, or cytoplasm,  now termed cytosol.In free cells e.g., protozoans, the hyaloplasm has an outer narrow, relatively firm zone, the ectoplasm, around the central relatively fluid mass, the endoplasm. The cells lying in contact with one another, as in the multicellular animals, lack ectoplasm. The cells lining the tubular organs, such ad the alimentary canal, have ectoplasm on their free side.The cytoplasmic matrix, along with the structures it contains, is often in constant motion,called streaming movement, or cyclosis.The hyaloplasm contains a variety of structures suspended in it. These structures are of two main kinds: organelles, or organoids and inclusions. 





(a) Organelles ¹:The organelles are organized structures in the cytoplasm having specific functions necessary for the metabolism of the cell. They are capable of growth and in some cases  of multiplication also.They are often described as the  living, or protoplasmic, structures. of these are visible through a light microscope. These include  mitochondria, Golgi apparatus,  centrosome, plastids, and Vacuoles. Others are revealed by an electron microscope. These include endoplasmic reticulum,  ribosomes, microfilaments, microtubules, intermediate fibres, and   microbodies. Certain cells bear  cilia or flagella and have basal granules in the cytoplasm at the bases of these organelles. Most of the cell organelles are enclosed by membrane but some are without membrane. 

(b) Inclusions:  The  inclusions are incapable of growth and multiplication. They are often described as nonliving, or deutoplasmic ,structures. The common Inclusions are reserves food materials ( glycogen granules, protein  grains, fat globules), pigment granules, crystals, engulfed materials such as carbon or other foreign substances, secretions and excretions. 

3. Nucleus. 

The nucleus  is a prominent spherical organelle in eukaryotic cells. It is bounded by a thin double membrane, the nuclear envelope.Within the envelope is a jelly _ like  nucleoplasm, or nuclear sap.Suspended in the nuclear sap are some thread __ like organelles, the chromosomes,and one or more rounded bodies, the nucleoli.The  chromosomes bear Gene's which consist of hereditary material ( DNA ) and control  the functions of the cell. A cell devoid of nucleus dies.The nucleolus stored  RNA. 




Biological Activities : 

(i) Cytoplasm often shows streaming movements or cyclosis.

(ii) Phagocytosis, pinocytosis and exocytosis are common in protists and animal cells.

(iii) Mitotic apparatus is regularly formed during cell division. The plasma membrane has no role in the segregation of chromatids into daughter cells.

(iv) Replication of DNA occurs only in the S phase of the cell cycle.

(v) Transcription and translation occur in the nucleus and in the cytoplasm respectively. The mRNA needs processing. 

(vi) The eukaryotes are mostly sexual organisms. Meiosis, gamete formation and fertilization occur as essential requirements of sexual reproduction. Asexual reproduction also occurs in many forms.

(vii) No eukaryotes cell has the ability of nitrogen fixation. 

(viii) Respiration may be aerobic or anaerobic. Respiratory enzymes occur in the mitochondria. 

(ix) Nutrition may be autotrophic ( photosynthetic) or heterotrophic ( saprotrophoc, parasitic or holozoic).

Viroids and prions

Both Viroids and Prions are infectious.These have been clearly identified in plant cells and the animals respectively. 

Viroids¹ are tiny bits of nucleic acid ( single_ stranded closed circular RNA) without a protein coat. They have been clearly identified in plant cells only, where they replicate  with the help of host cell enzymes. They are infectious pathogens and cause plant diseases. 

Prionsare tiny protein particles without nucleic acid. They are infectious particles that cause neurodegenerative diseases such as Kuru in humans and scrapie in sheep.


































































Stomach               Pancreas

RNA                    Nucleic Acids








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