Friday, July 31, 2020

Cell The Unit of Life - Notes | Class 11 | Part 4: Eukaryotic Cell - 2

8. CELL: THE UNIT OF LIFE


EUKARYOTIC CELLS

4. Mitochondria

  • Mitochondria are clearly visible only when stained.
  • Number, shape and size of mitochondria per cell are variable depending on the physiological activity.
  • It is sausage-shaped or cylindrical having a diameter of 0.2-1.0 mm (average 0.5 mm) and length 1.0-4.1 mm.

  • A mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane. It divides lumen into 2 aqueous compartments, i.e., the outer compartment and the inner compartment (matrix).
  • Inner membrane forms many infoldings (cristae) towards the matrix. They increase the surface area.
  • The two membranes have their own specific enzymes associated with the mitochondrial function.
  • Matrix possesses a circular DNA, a few RNA molecules, ribosomes (70S) and components for protein synthesis.
  • The mitochondria divide by fission.
  • Function: Mitochondria are the sites of aerobic respiration. They produce energy in the form of ATP. So they are called ‘power houses’ of the cell.

5. Plastids

  • Plastids are found in all plant cells and in euglenoides.
  • Large sized. Easily observable under the microscope.
  • They contain some pigments.
  • Based on the type of pigments, plastids are 3 types:
    1. Chloroplasts: Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.
    2. Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour.
    3. Leucoplasts: These are colourless plastids of varied shapes and sizes with stored nutrients. They include:
      • Amyloplasts: Store starch. E.g. potato.
      • Elaioplasts: Store oils and fats.
      • Aleuroplasts: Store proteins.

Chloroplasts:

  • These are double membrane bound organelles mainly found in the mesophyll cells of the leaves.
  • These are lens-shaped, oval, spherical, discoid or ribbon-like organelles.
  • Length: 5-10 mm. Width: 2-4 mm.
  • Their number varies from 1 (e.g. Chlamydomonas) to 20-40 per cell in the mesophyll.
  • Inner membrane of chloroplast is less permeable.
  • The space limited by the inner membrane of the chloroplast is called stroma. It contains many organized flattened membranous sacs called thylakoids.
  • Membrane of thylakoids encloses a space called lumen.
  • Chlorophyll pigments are present in the thylakoids.
  • Thylakoids are arranged in stacks called grana or the intergranal thylakoids.
  • There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
  • The stroma contains small, double-stranded circular DNA molecules, ribosomes and enzymes for the synthesis of carbohydrates and proteins.
  • The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

6. Ribosomes


  • They are non-membranous granular structures composed of ribonucleic acid (RNA) & proteins.
  • It is first observed by George Palade (1953).
  • Eukaryotic ribosome has 2 subunits- 60S (large subunit) and 40S (small subunit). They together form 80S.

7. Cytoskeleton

  • It is a network of filamentous proteinaceous structures present in the cytoplasm.
  • It provides mechanical support, motility, maintenance of the shape of the cell etc.

8. Cilia and Flagella

  • They are hair-like outgrowths of the cell membrane.
  • Cilia: Small structures which work like oars. Causes the movement of the cell or surrounding fluid.
  • Flagella: Longer. Responsible for cell movement. Flagella of prokaryotes and eukaryotes are structurally different.



  • Cilium and flagellum are covered with plasma membrane. Their core (axoneme) has many microtubules running parallel to the long axis.
  • The axoneme has 9 pairs of doublets of radially arranged peripheral microtubules and a pair of central microtubules. This is called 9+2 array.
  • The central tubules are connected by bridges and are enclosed by a central sheath. It is connected to one of the tubules of each peripheral doublet by a radial spoke. Thus, there are 9 radial spokes. The peripheral doublets are also interconnected by linkers.
  • Cilium and flagellum emerge from centriole-like structure called the basal bodies.

9. Centrosome and Centrioles

  • Centrosome is an organelle usually containing two non-membrane bound cylindrical structures called centrioles.
  • They are surrounded by pericentriolar materials.
  • The centrioles lie perpendicular to each other. They are made up of 9 evenly spaced peripheral fibrils of tubulin. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked.
  • The central part of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
  • 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.

10. Nucleus


  • Nucleus was first described by Robert Brown (1831).
  • The material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.
  • Normally, a cell has only one nucleus. Some cells have more than one. Some mature cells lack nucleus. E.g. mammalian RBC and sieve tube cells of vascular plants.
  • The interphase nucleus contains
    • Nuclear envelope: Double layered membrane with a space between (10 - 50 nm) called perinuclear space. It is a barrier between the materials present in nucleus & cytoplasm. Outer membrane usually remains continuous with ER and also bears ribosomes on it. Nuclear envelope has minute pores formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between nucleus and cytoplasm.
    • Nuclear matrix (nucleoplasm).
    • Chromatin: A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), non-histone proteins and RNA. During cell division, chromatins condense to form chromosomes.
    • Nucleolus: One or more non-membranous spherical bodies. It is continuous with the nucleoplasm. It is a site for ribosomal RNA synthesis.

Chromosomes:

  • A human cell has 2 m long thread of DNA distributed among its 46 (23 pairs) chromosomes.
  • Every chromosome has a primary constriction (centromere). On the sides of centromere, disc shaped structures called kinetochores are present.
  • Based on position of centromere, chromosomes are 4 types:
    • Metacentric chromosome: Middle centromere forming two equal arms of the chromosome.
    • Sub-metacentric chromosome: Centromere is nearer to one end forming one shorter arm and one longer arm.
    • Acrocentric chromosome: Centromere is close to its end forming one very short and one very long arm.
    • Telocentric chromosome: Terminal centromere.

  • Some chromosomes have non-staining secondary constrictions at a constant location. It is called satellite.

11. Microbodies

  • These are membrane bound minute vesicles that contain various enzymes.
  • Present in both plant and animal cells.

Differences between Plant and animal cells

Plant cell

Animal cell

1.Cell wall present

Absent

2.Plastids are present

Absent

3.A large central vacuole

Many small vacuoles

4.Centrioles are absent

Present


COMPARISON BETWEEN PROKARYOTIC & EUKARYOTIC CELLS

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