CELL – THE FUNDAMENTAL UNIT OF LIFE

 

The cell (from Latin cella, meaning "small room"] is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.

                                 

All unicellular organisms are capable of                                                                                 Independent existence.        Performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.

CELL THEORY

    • Formulated by Schleiden and Schwann.

    • Modified by Rudolf Virchow – he explained that new cells develop from pre existing cells by cell division (Omnis cellula-e cellula).

    • Exception of cell theory – virus, viriods,

All living organisms are composed of cells and products of cells.                                                  Cell is structural unit of life.                               All cells arise from pre-existing cells.

 

There are two types of cells: plant cell and animal cell.

              

 

Cell Wall

Discovery: It was first discovered by Robert Hooke in 1665.  Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists.

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It is not found in animal cells.

 

Plasma Membrane

Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane.

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It is quasi fluid in nature.  According to Singer and Nicolson it is “protein iceberg in a sea of lipid”.

A cell wall lies external to plasmalemma in plant cells, many monerans, some protists and fungal cells.

Membranes also occur inside the cells.

They are collectively called biomembranes.

The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast.

It was replaced by the term plasmalemma or plasma membrane by Plowe (1931).

Protoplasm

Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable.

It is considered as “Polyphasic colloidal system”.

Cytoplasm

The substance occurs around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.

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Mitochondria

(Gk – mito = thread ; chondrion = granule) Mitochondria are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem.

These are absent in all prokaryotes like bacteria and cyanobacteria.

Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

Plastids

Definition: Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H+ etc. These are largest cell organelles in plant cell.

Endoplasmic Reticulum

It is well developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the endoplasm. Garnier (1897) was first to observe the ergastoplasm in a cell. The ER was first noted by Porter, Claude, and Fullman in 1945 as a network. It was named by Porter in 1953.

Golgi Complex

Golgi complex is made up of various membranous system e.g. cisternae, vesicles and vacuoles. These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or Baker’s body. These are also called “traffic police” of the cell. It was first observed by George (1867) but it’s morphological details were given by Camillo Golgi (1898), in nerve cells of barn fowl and cat.

 

Lysosomes                                                                                                                       These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

 

Vacuoles                                                                                                                                Membrane-bound space found in the cytoplasm. Membrane known as tonoplast.                 It contains water, sap, excretory product and other materials not useful for the cell.   In plant cells the vacuoles are very large.In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole. In Amoeba the contractile vacuole is important for excretion.                        In many cells food vacuoles are formed by engulfing the food particles.

 

Ribosome                                                                                                                                     

First observed under the electron microscope by George Palade.                                       They are composed of ribonucleic acid (RNA) and proteins. Not Bounded by any membrane.The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.

 

Cilia and Flagella                                                                                                                 They are hair like outgrowths of cell membrane responsible for locomotion and movement of cell. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer. Eukaryotic cilium and flagellum are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. (9+2) Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

Centrosome and centriole

Centrosome is an organelle usually containing two perpendicularly lying centrioles surrounded by amorphous pericentriolar materials. Centriole has an organisation like the cartwheel. They are made up of nine evenly spaced triplet peripheral fibrils of tubulin. The central part of the centriole is also proteinaceous and called the hub, connected with peripheral tubules by radial. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

 Nucleus                                                                                                                                    First described by Robert Brown. The material of the nucleus stained by the basic dyes was given the name chromatin by Flemming. The interphase nucleus has nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called. The nuclear envelope is consists of two parallel membranes with a space inbetween called perinuclear space. Normally, there is only one nucleus per cell.Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants. The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.


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