Cell : The Unit Of Life Revision Notes For NEET


        Cell is the basic structural and functional unit of life.

        Cell theory Schleiden and Schwann formulated cell theory which includes three statements, i.e.

1)     All organisms are composed of one or more cell and cell products.

2)     Cells are the structural and functional unit of living organisms.

3)     Cells arise from pre-existing cells (Omnis-cellula-e-cellula) (given by Virchow)

        Types of cells these are the two basic types

1)     Prokaryotic cell

2)     Eukaryotic cell

Prokaryotic cell These cells have incipient nucleus that is divoid of membrane. They do not have membrane bound cell organelles.

Eukaryotic cell these cells consist of outer cell wall (only in plants and fungi), plasma membrane, cytoplasm, membrane bound nucleus within cytoplasm cell organelles are suspended.

        Cell wall It is the outermost, rigid protective covering present in plants and fungi. It is composed of structural polysaccharides such as cellulose, hemicellulose or pectin. It consists of three layers

1)     Middle lamella

2)     Primary wall

3)     Secondary wall

Middle lamella It is the connecting layer between primary cell walls of neighbouring cells.

Primary cell wall It is thin, elastic and found in younger cells.

Secondary cell wall It is present inner to primary cell wall.

        Plasma membrane It is the selectively permeable outer covering that limits the cytoplasm of cell.

        Fluid mosaic model This was proposed by Singer and Nicholson to explain the structure of plasma membrane. According to this, a biological membrane is composed of fluid phospholipid bilayer with loosely held peripheral proteins on its surface and firmly held integral proteins embedded in it.

        Functions of plasma membrane includes transport of substances which can be of two types

  Passive transport Molecules move without the involvement of ATP.

  Active transport It utilises ATP to move substances, e.g. Na+ /K+ pump.

        Cytoplasm It is a jelly-like homogenous ground substance. It contains cell organelles, microbodies and cell inclusions.

        Nucleus It is the main controller of cell activities that is bounded by a nuclear envelope. It also consists of nucleoplasm, chromatin and nucleolus.

        Nuclear envelope It is composed of two membranes, i.e. outer and inner membrane separated by perinuclear space.

        Nucleoplasm It is the fluid part of nucleus that contains DNA, associated proteins, enzymes, etc.

        Nucleolus It is a spherical membraneless body within nucleus. It acts as a site of synthesis of ribosomes and rRNA.

        Chromatin These are fine fibres composed of DNA, and histone proteins.

        Histone proteins These are basic proteins mainly composed of lysine and arginine.

        Chromosome During cell division, chromatin condense into thick chromosomes. The centre is composed of outer covering called pellicle, inside which a matrix is present. In the centre of chromosome, centromere is present at which chromatids, arms of chromosome meet.

  Centromere It is the narrow, non-chromatid part of chromosome. Based on the position of centromere, chromosomes can be of following types

  Metacentric It has centromere in middle and equal arms.

  Submetacentric Centromere is slightly away from the middle of the chromosome.

  Acrocentric Centromere is situated close to end.

  Telocentric Centromere is terminal.

        Satellites It is a short, sphere-like structure present beyond the secondary constriction-I.

        Kinetochores These are disc-like structures present on centromere. They act as sites of attachment of spindle fibres during cell division.

        Semiautonomous organelles These cell organelles possess their own DNA and protein synthesising machinery but their activities are under the control of both nuclear and organeller DNA. These include mitochondria and chloroplast.

        Mitochondria These are semiautonomous organelles involves in ATP generation in cell. They contain double membrane, i.e. an outer and inner membrane. The inner membrane is folded to form cristae that increase the surface area of mitochondria.

        Plastids These are double membrane-bound organelles used for trapping radiations and storage.

        Chloroplast a plastid containing chlorophyll. It is divided in photosynthesis. It contains an outer and inner membrane. The matrix inside the membrane is stroma in which membranous sacs, i.e. thylakoids are present. In these thylakoids chlorophyll is found. Chloroplasts are semiautonomous.

        Chromoplast These plastids contain carotenoids.

        Leucoplast These are colourless plastids that perform storage of substances. They can be elaioplasts, amyloplasts and aleuroplasts.

        Ribosomes These organelles are smallest non-membranous RNA-protein complexes. These are involved in protein synthesis.

        Endomembrane system It consists of organelles such as Golgi apparatus, Endoplasmic Reticulum and Lysosome (GERL).

        Endoplasmic reticulum It consists of interconnected membranes in the form of cisternae, vesicles and tubules. It is of two types

1)     Rough endoplasmic reticulum

2)     Smooth endoplasmic reticulum

        Rough endoplasmic reticulum Bears ribosome on surface and synthesises secretory and membrane proteins.

        Smooth endoplasmic reticulum It is free of ribosomes and prepares cholesterol and steroids hormones.

        Golgi apparatus It is also a membranous organelle consisting of flattened sac-like structures cisternae. These are concerned with modifying, sorting and packaging of cell products into secretory vesicles.

        Lysosomes These are single-membrane bound small-spherical organelles that contain hydrolytic enzymes in them. Due to their function of digesting cell products these are also called as suicidal bags of cell.

        Vacuoles These are single membrane bound organelles limited by a membrane called tonoplast.

        Sap vacuole They maintain osmotic and turgor pressure.

        Contractile vacuole They help in osmoregulation.

        Food vacuole These possess hydrolases along with food.

        Microbodies These are single membrane bound structures.

        Peroxisomes The help in hydrolysis of peroxides.

        Glyoxysomes These are found in endosperm of germinating seeds.

        Spherosome They synthesise and store lipids.

        Lomasomes These bodies are associated with cell wall formation.

        Centrosome These are short paired cylindrical structures found in animal cells. These are made up of 9+2 microtubular arrangement. They form the basal bodies of cilia and flagella.

        Cytoskeleton It is a network of interconnected, proteinaceous filaments and tubules. It maintains the shape of the cell.

        Cilia and flagella These are filamentous outgrowths extending from the cell surface. Cilia are smaller in size. They show 9+2 microtubular arrangement in which there are 9 peripheral doublets of microtubules and 2 central singlet microtubules.

        In order to study cellular structures various technique are used

        Microscope It is an instrument to visualise objects and organisms too small to be seen by naked eyes. For magnification, it may utilise glass lenses or electromagnetic lenses.

        Microtome It is an instrument used to cut thin sections for electron microscopy.

        Cell fractionation It is a process of isolating subcellular structures. It includes homogenisation and centrifugation.

        Centrifugation It involves separating different components of a homogenate using centrifugal force.

        Chromatography It is the process of separation of constituents of a mixture based on partition coefficient.

        X-ray crystallography It is used to determine the arrangement of atoms in a molecule by using x-ray beam.

        Cytological staining It is the procedure of colouring specimens with a dye to get a clear view of certain structures.

        Spectrophotometry It is the quantitative method for estimating protein or nucleic acid.

        Electrophoresis It is the technique of separating molecules from their mixture under the influence of an applied electric field.

        Autoradiography This technique is used to study metabolic pathways, functions of cell organelles, etc.

        Flow cytometry This technique is used to detect abnormalities in genes such as aneuploidy, duplication, deletion, etc.

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