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Prokaryotic Cell Structure
Organisms may be divided into two main forms :prokaryotic and eukaryotic
This is a typical prokaryotic bacterial cell and shows the following features all of
which are mentioned in the monograph P2
Plasmids
Nucleoid – This is highly condensed DNA in the form of a single circular
molecule. It is often referred to as a bacterial chromosome
Plasmid – these are smaller rings of DNA which contain genes
Cytosol – This is the watery gel making up the majority of the cell content and
is the site of bacterial metabolism
Ribosomes – Bacteria have many ribosomes suspended in the cytosol. These
as smaller and less complex than the ones found in Eukaryotic cells
Cell wall – Tough outer coat made of peptidoglycan (linked dissacharide and
peptide) and is used as the basis of the gram stain
Capsule – The bacterial has a mucilaginous coating known as the capsule.
This protects the cell against chemicals such as acids
Pili – These can be used to help bacteria attach to surfaces or to help them
transfer their plasmids.
Flagella – Both aid bacterial movement
Nucleus and DNA
Eukaryotic cells have a true nucleus, bound by a double membrane.
Prokaryotic cells have no nucleus.
Prokaryotic cells do have DNA and DNA functions. Biologists describe the central
region of the cell as its "nucleoid" (-oid=similar or imitatingbut the nucleoid is
essentially an imaginary "structure." There is no physical boundary enclosing the
nucleoid.
Eukaryotic DNA is linear; prokaryotic DNA is circular (it has no ends).
Eukaryotic DNA is complexed with proteins called "histones," and is organized into
chromosomes; prokaryotic DNA is "naked," meaning that it has no histones
associated with it, and it is not formed into chromosomes.
A eukaryotic cell contains a number of chromosomes; a prokaryotic cell contains
only one circular DNA molecule and a varied assortment of much smaller circlets of
DNA called "plasmids." The smaller, simpler prokaryotic cell requires far fewer
genes to operate than the eukaryotic cell.
Ribosomes
Both cell types have many, many ribosomes, but the ribosomes of the eukaryotic
cells are larger and more complex than those of the prokaryotic cell. Ribosomes
are made out of a special class of RNA molecules (ribosomal RNA, or rRNA) and a
specific collection of different proteins. A eukaryotic ribosome is composed of five
kinds of rRNA and about eighty kinds of proteins. Prokaryotic ribosomes are
composed of only three kinds of rRNA and about fifty kinds of protein.
Membrane bound organelles
The cytoplasm of eukaryotic cells is filled with a large, complex collection of
organelles, many of them enclosed in their own membranes; the prokaryotic cell
contains no membrane-bound organelles which are independent of the plasma
membrane. This is a very significant difference, and the source of the vast majority
of the greater complexity of the eukaryotic cell. There is much more space within a
eukaryotic cell than within a prokaryotic cell, and many of these structures, like the
nucleus, increase the efficiency of functions by confining them within smaller
spaces within the huge cell, or with communication and movement within the cell.
Structure of a typical Eukaryotic animal cell
Plant and animal cells eukoryotic cells have a nucleus and other
membrane bound organelles. Common to both are :a) Cell membrane – controls entry and exit of molecules
b) Cystol – Fluid part of cytoplasm in which many reactions take place
c) Ribosomes – site of protein synthesis. These are bound to rough ER
d) Endomembrane system – various membrane bound organelles such as:
• Nucleus – Contains genetic information e.g. DNA
• Rough ER – Transport of proteins
• Golgi – packages protein for secretion
• Mitochondria – the sites of oxidative phosphorylation
• Lysosomes – sub cellular digestion using powerful protease enzymes
• Peroxisomes – breakdown of hydrogen peroxide
• Cytoskeleton – System of fibres inside the cell which add strength,
allow cell movement and allow cell to change shape
Certain features are only found in animal cells e.g.
a) Intermediate filaments – these are part of the cytoskeleton which is discussed
in more detail in the next section. These filaments are found under the cell
membrane and add mechanical strength. Plant cell don’t need them as they
have a cell wall.
b) Centrioles – These orginise the spindle fibres during cell division
c) Microvili – the lack of a cell wall allows the animal cell to form fold called microvilli which further increase the surface area in specialised cells e.g. epithelial
cells of gut.
Middle Lamellae This is the layer
between two adjacent
cells
Plasmodesmata
These are cytoplasmic
connections between
plants which all the
movement of
substances.
Cellulose Cell Wall –
Flexible, strong and
elastic wall which
adds mechanical
strength and prevent
cell from bursting
Chloroplasts Synthesis of
carbohydrates using
light energy
Large permanent
vacuole - Contains
cell sap
Feature
Prokaryote
Eukaryote
Size
small, mean
diameter
0.5 - 5 m
Up to 40 m common
Genetic
material
Contained in
nucleoid and
plasmids
DNA associated with proteins to form a
chromosome, found within a nucleus
Few present and
none surrounded
Organelles
by a plasma
membrane
Cell Walls
Many organelles;
Some with double membranes e.g.
nucleus, mitochondria and chloroplasts.
Many with a single membrane e.g. golgi
apparatus, endoplasmic reticulum and
lysosomes
Fungi: Rigid, formed from polysaccharide
Rigid formed from
chitin
glycoproteins
Plants: Rigid, formed from polysaccharide
(mainly murein)
cellulose