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2 Cell : The Unit of Life NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Cell theory is understood as (i) All living organisms are composed of cells and products of cells. (ii) All cells arise from pre-existing cells. (iii) Activities of an organism are the outcome of sum total of activities and interactions of its constituent cells. Viruses are an exception to cell theory as they are not made up of cells. They are composed of nucleoprotein particles. Therefore, they are not considered either living or non-living. AN OVERVIEW OF CELL Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Besides nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the Endoplasmic Reticulum (ER), the Golgi complex, lysosomes, mitochondria, microbodies and vacuoles. The prokaryotic cells lack such membrane-bound organelles. Ribosomes are non-membrane bound organelles found in all cells – both eukaryotic as well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER. Animal cells contain another non-membrane bound organelle called centriole which helps in cell division. Cells differ greatly in size, shape and activities. For example Size (i) Mycoplasma - 0.3 m in length (Smallest cell) (ii) Bacteria - 3-5 m (iii) Human RBC - 7 m in diameter The largest isolated single cell - egg of ostrich. Nerve cells are some of the longest cells. Columnar epithelial cells (long and narrow) White blood cells (amoeboid) Red blood cells (round and biconcave) Nerve cell (Branched and long) A tracheid (elongated) Mesophyll cells (round and oval) Fig. : Diagram showing different shapes of the cells
NEET Cell : The Unit of Life 3 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 PROKARYOTIC CELLS The prokaryotic cells are represented by bacteria, blue-green algae, Mycoplasma and PPLO (Pleuro pneumonia- like organisms). They are generally smaller in size and multiply much faster than the eukaryotic cells. They may vary greatly in shape and size but exhibit a similar basic cellular organisation. A typical eukaryotic cell (10-20 )  Viruses (0.02-0.2 )  PPLO (about 0.3 )  Typical bacteria (1-2 )  Fig. : Diagram showing comparison of eukaryotic cell with other organisms Most prokaryotes have a cell wall surrounding the cell membrane. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. The plasmid DNA is used to monitor bacterial transformation with foreign DNA. Cell Envelope and its Modifications Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope. The cell envelope consists of a tightly bound three-layered structure i.e., the outermost glycocalyx followed by the cell wall and the plasma membrane. 1. Glycocalyx is the outermost layer comprising a coating of mucous or polysaccharides macromolecules, which protects the cells and also helps in adhesion. This layer differs in thickness and chemical composition in different bacteria. Some have a loose sheath called slime layer, which protects the cell from loss of water and nutrients. Others may have a thick and tough covering known as capsule. The capsule and slime layer are made up of polysaccharides, but may sometimes contain proteins also. The capsule is responsible for giving gummy and sticky character to the cell. It allows bacterium to hide from host’s immune system. 2. The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing. This layer is rigid due to a special macromolecule called peptidoglycan (murein or mucopeptide). A number of antibiotics (e.g., penicillin) inhibits cross-linking of peptidoglycan strands. Therefore, cells undergo lysis in the presence of these antibiotics. Cell wall is single layered and smooth in gram positive bacteria. It is two layered and wavy in gram negative bacteria.
4 Cell : The Unit of Life NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Glycogen Lipid globule Mesosome Respiratory enzymes Pilus Filament Hook Basal body Flagellum Mucilage Cytoplasm Plasma membrane Nucleoid Polyribosome Ribosome RNA Cell wall Fig. : Cell structure of bacteria under electron microscope Gram staining (developed by Christian Gram) is a special technique, which is used to classify bacteria into two groups, viz. Gram-positive and Gram-negative bacteria. Those bacteria that take up the Gram stain are Gram positive and the others that do not are called Gram negative bacteria. 3. The plasma membrane is selectively permeable in nature and interacts with the outside world. This membrane is similar structurally to that of the eukaryotes. A special membranous structure is the mesosome which is formed by the invagination of plasma membrane into the cell. They help in the cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion process, to increase the surface area of the plasma membrane and enzymatic content. Mesosome is found in gram positive bacteria. In photosynthetic prokaryotes like cyanobacteria, and purple bacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments. Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagella. The flagellum is composed of three parts – filaments, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. It is a hollow rigid cylindrical structure made up of the protein called flagellin. Basal body is a rod-like structure which consists of rings. Hook Filament L-ring P-ring Rod S-ring M-ring Cell wall Distal set Plasma membrane Proximal set Fig. : A sectional view of a bacterial cell showing the detailed structure of the flagellum and attachment of flagellum to the bacterial cell