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Thursday, 23 February 2017

Cell Structure and Function II


Cells are of 2 types:
• Prokaryotic cells
• Eukaryotic cells

PROKARYOTIC CELLS
  • They have no membrane bound nucleus and organelles.
  • They include bacteria, blue-green algae, mycoplasma & PPLO (Pleuro Pneumonia Like Organisms).
  • They are generally smaller and multiply more rapidly than the eukaryotic cells.
  • They vary in shape & size. E.g. Bacteria have 4 basic shapes: Bacillus (rod like), Coccus (spherical), Vibrio (comma shaped), and Spirillum (spiral).
Cell organelles in prokaryotic cells
1. Cell Envelope
  • It is a chemically complex protective covering.
  • It is made of 3 tightly bound layers.

Glycocalyx: Outer layer. It differs in composition and thickness among different bacteria. It may be a slime layer (loose sheath) or capsule (thick & tough).
Cell wall: Middle layer. It is seen in all prokaryotes. It gives shape to the cell and provides a structural support to prevent the bacterium from bursting or collapsing.
Plasma membrane: Inner layer. It is semi-permeable in nature and interacts with the outside. This is structurally similar to that of the eukaryotes.

Based on the types of the cell envelopes and response to Gram staining (developed by Gram), bacteria are 2 types:
Gram positive: They take up and retain the gram stain.
Gram negative: They do not retain the gram stain.

2. Mesosome & Chromatophores (Membranous structures)
Mesosome is formed by the infolding of plasma membrane. It includes vesicles, tubules & lamellae
Functions: Mesosome helps
• In cell wall formation
• In DNA (chromosome) replication
• In Distribution of chromosomes to daughter cells
• In respiration and secretion processes
• To increase the surface area of the plasma membrane and enzymatic content
Chromatophores are membranous infoldings in some prokaryotes (e.g. cyanobacteria). They contain pigments.

3. Nucleoid

  • It is formed of non-membranous (naked) circular genomic DNA (single chromosome/ Genetic material) & protein
  • Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives some unique phenotypic characters (e.g. resistance to antibiotics) to bacteria
4. Flagella
  • These are thin filamentous extensions from the cell wall of motile bacteria. Their number and arrangement are varied in different bacteria
  • Bacterial flagellum is made of 3 parts – filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside.
5. Pili and Fimbriae
  • These are surface structures that have no role in motility
  • The pili (sing. Pilus) are elongated tubular structures made of a special protein (pilin)
  • The fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they help to attach the bacteria to rocks in streams and also to the host tissues
6. Ribosomes
  • In prokaryotes ribosomes are associated with the plasma membrane of the cell
  • They are about 15 nm by 20 nm in size
  • They are made of two subunits - 50S and 30S units. They together form 70S prokaryotic ribosomes. (S= sedimentation coefficient; a measure of density & size).
Function:
• Ribosomes are the site of translation (protein synthesis).  Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome). Ribosomes of a polysome translate the mRNA into proteins.

7. Inclusion Bodies

  • These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells.  E.g. phosphate granules, cyanophycean granules and glycogen granules, gas vacuoles etc.
  • Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.
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