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Thursday, 23 February 2017

Cell Structure and Functions IV

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 0.2-1.0m (average 0.5 m) and length 1.0-4.1 m
  • 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 a number of 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:
Chloroplasts: Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.
Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour.
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
  • 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, a green alga) 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 the stroma. It contains a number of 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
  • These are the non-membranous granular structures composed of ribonucleic acid (RNA) and 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 are small structures which work like oars, causing the movement of either the cell or the surrounding fluid.
  • Flagella are comparatively longer and responsible for cell movement.
  • Flagella of prokaryotic bacteria and eukaryotes are structurally different.
  • Cilium and flagellum are covered with plasma membrane. Their core (axoneme) possesses a number of microtubules running parallel to the long axis.
  • The axoneme usually has 9 pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. This is known as 9+2 array.
  • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers.
  • Both the 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).
  • Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.
  • Normally, there is only one nucleus per cell. Variations in the number of nuclei are also observed. Some mature cells lack nucleus. E.g. mammalian erythrocytes and sieve tube cells of vascular plants.
  • The interphase nucleus contains
Nuclear envelope: Double layered membrane with a space between (10 - 50 nm) called the perinuclear space. It forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the ER and also bears ribosomes on it. The nuclear envelope has minute pores, which are formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between the nucleus and the cytoplasm
Nuclear matrix (nucleoplasm)

Chromatin: A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), some 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

A single human cell has about 2 m long thread of DNA distributed among its 46 (23 pairs) chromosomes
Every chromosome has a primary constriction (centromere) on the sides of which disc shaped structures called kinetochores are present
Based on the position of the centromere, chromosomes are 4 types.
Metacentric chromosome: It has middle centromere forming two equal arms of the chromosome
Sub-metacentric chromosome: It has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm
Acrocentric chromosome: Centromere is situated close to its end forming one extremely short and one very long arm
Telocentric chromosome: It has a terminal centromere
Some chromosomes have non-staining secondary constrictions at a constant location. It is called satellite

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

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