Cell : The Unit of Life

Cilia & Flagella, Centrosome, Nucleus, Microbodies


  • Both cilia and flagella are structurally similar and possess similar parts - basal body, rootlets, basal date and shaft.
  • Shaft is the hair like projecting part of cilia and flogella which remains outside the cytoplasm. It has 9 doubt of microtubules in radial symmetry. They are called axonema. Each axonema has 11 fibrils, 9 in the periphery and 2 in the centre. The arrangement is called as 9 + 2 pattern.
  • Cilia are small in size (5 - 10 μm) and flagella are long (upto 150 μm). Cillia are more in number (upto 14000 cell) and flagella are few in number (1-8).
  • Cilia beat in coordinated manner and flagella beats independently cilia take part in locomotion and attachment and flagella is involved only in locomotion.
  • Centrosome, is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriole meterials. Both the centriole in a centrosome lie perpendicular to each other, in which each has organisation like the cartwheel.
  • Centrioles are made up of nine evenly spaced peripheral fibrils of tubulin, each peripheral fibril is a triplet. The adjacent triplets are also linked. The central past of centriole is also proteinaceous and called hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein. It has (a + o) pattern.
  • The centriole helps in organising the spindle fibres and astral rays during cell division.
  • Nucleus is the most important part of the cell which directs and controls all the cellular functions.
  • Nuclear membrane / nuclear envelope, consists of two parallel membranes with a space of about 10 - 50 nm called the perinuclear space.
  • Nucleus allows the passage of inorganic ions, small organic molecules, ribosomal subunits, RNAs and proteins through nuclear pores.
  • Nucleolus is a conspicuous, darkly stained spherical body found in nucleoplasm. It is composed of large amount of ribosomal proteins and ribosomal RNA. It is the site of biogenesis of rRNA and also stores rRNA.
  • Chromatin fibres are thread like structure which are uniformly distributed in the nucleoplasm. These are observed only in the interphase stage.
  • They form chromosomes for equitable distribution of genetic information during cell division and reproduction.
  • Based on the position of the centromere, the chromosomes can be classified into four types :- namely metacentric, sub-metacentric, acrocentric and telocentric chromosomes.
  • Metacentric chromosomes has centromere at the middle forming two equal arms of the chromosome. The submetacentric chromosome has centromere nearer to one end of chromosome resulting into one shorter arm and one longer arm.
  • In acrocentric chromosome, the centromere is situated close to its end forming one extremely short and one very long arm. The telocentric chromosome has a terminal centromere.
  • Sometimes a few chromosome have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.
  • Many membrane bound minute resicles called microbodies contain various enzymes, which are present both in plants & animals
  • Sphaerosomes found in endosperm and cotyledon of oil seeds which contains hydrolytic enzymes like pronease, ribonuclease phosphatase, esterase etc.
  • Peroxisomes (uricosomes) contain peroxide producing enzymes (oxidase) and peroxide destroying enzymes (catalases). Urate oxidase, d - amino oxidase, ∝ - hydroxy acid oxidase produce H2O2 whereas the catalases degsade H2O2 because H2O2 is toxic for cells.
  • Glyoxysomes contain enzymes for metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. They contain enzymes for β - oxidation of fatty acids, producing acetyl CoA.
  • Lomasomes are sac like structure found between cell wall and plasmalemma in the haustoria of fungal hyphae and spore producing structure, algal cells and in some cells of higher plants.
  • Non membrane organelles → "RCC", R - Ribosome, C - Centriole, C - Cytoskeleton.
    Single membrane organelles → "LPV" , L - Lysosome, V - Vacuole, P - Peroxisome (microbodies)
    double membrane organelles → "MEN - GC"
                                                  M - Mitocondria, E - ER, N - Nucleus,
                                                  G - Golgibody, C - Chloroplast,

View this video for the topics Cilia & Flagella from 10:19 to 18:17

View this video for the topic Microbodies from 0:07 to 8:49

View this video for the topic Centrosome from 0:06 to 5:05

View this video for the topic Nucleus part-1 from 0:13 to 19:00

View this video for the topic Nucleus part-2 from 0:12 to 9:09

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