Page 11 - Biology_F5

Biology for Advanced Level Secondary Schools

Based on the level of organisation, cells (g) Cilia and flagella, if present do not
can be divided into two main types, namely arise from basal bodies, and they lack
prokaryotic cells and eurykaryotic cells. microtubules of a “9+2” arrangement
Prokaryotic cells have simple cellular pattern.
organisation, while eukaryotic cells have
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complex cellular organisation. Structure of prokaryotic cells
The prokaryotic cell does not have a defined
1.3.1 The prokaryotic cells nucleus, as it lacks a nuclear membrane.
The word prokaryotes comes from two Almost all prokaryotes have a protective
Greek words pro meaning ‘primitive’ cell wall that prevents them from bursting
or ‘before’ and karyo (karyon) meaning in hypotonic conditions. Such cell walls
‘nucleus.’ From this basic implication, have different components depending on
prokaryotic cells refer to those cells which the type of organism.
lack true or well organised nuclei such that However, most of the prokaryotic cells
the nuclear materials are freely suspended have cell walls containing major organic
in the cytoplasm. The common examples molecules of proteins, carbohydrates and
of prokaryotic cells are bacteria and blue- lipids. Bacterial cell walls have a unique
green algae. Prokaryotic cells have the molecule known as peptidoglycan. This
following general characteristics: component of the cell walls allows scientists
(a) They are microscopic with an average to classify bacteria as either Gram-positive
diameter of between 0.5 nm and10 nm. or Gram-negative. The Gram–positive
(b) They lack well organised nuclei. bacterium contains many layers of
That is, their nucleus has no nuclear peptidoglycan in the cell wall and lacks the
membrane, therefore, the nuclear outer membrane, while the Gram-negative
materials are freely suspended in the bacterium contains a thin cell wall made
cytoplasm. up of a few layers of peptidoglycan and
(c) They have small, circular and naked possesses the outer membrane.
DNA. That is, the DNA is not The glycocalyx or capsule is a layer
associated with histone protein coat to surrounding the cell wall of some bacteria.
form chromosomes. This layer protects the bacteria from drying
(d) They have few and small ribosomes of out, especially in hypertonic conditions. The

70s sedimentation coefficient. fimbriae are structures that help bacteria to
(e) They lack membrane bound organelles, adhere to target cells. They thus play a major
such as mitochondria, Golgi bodies, role in bacterial virulence. The flagella are
and plastids. long whip-like extensions that help bacteria
to move about in the environment. The axial
(f) The cell wall is chemically composed filaments or endoflagella are long structures
of a carbohydrate-protein complex which move in waves enabling the bacteria
called peptidoglycan or murein. to spin.

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