Cell Structure and Organisation Flashcards
An organism may comprise just a single cell (______________), a collection of cells
that are not morphologically or functionally differentiated (____________), or several distinct cell types with specialised functions (____________). Among microorganisms, all bacteria and protozoans are ___________; fungi may be _____________ or _________, while algae may exist in all three forms.
An organism may comprise just a single cell (unicellular), a collection of cells
that are not morphologically or functionally differentiated (colonial), or several distinct
cell types with specialised functions (multicellular). Among microorganisms, all bacteria and protozoans are unicellular; fungi may be unicellular or multicellular, while algae may exist in all three forms.
Similarities and differences between procaryotic and eucaryotic cell structure?
IMAGE 29
The most fundamental difference between procaryotic and eucaryotic cells is reflected
in their names; eucaryotic cells possess a true nucleus, and several other distinct subcellular organelles that are bounded by a membrane. The procaryotes comprise the simpler and more primitive types of microorganisms; they are generally single celled, and arose much earlier in evolutionary history than the eucaryotes.
Note that the viruses do not appear in Table 3.2, because they do not have a
cellular structure at all, and are not therefore considered to be living organisms.
Describe the Archaea group.
The Archaea comprise a wide range of mostly anaerobic bacteria, including many of those that inhabit extreme environments
such as hot springs.
Despite their differences, Archaea and Bacteria are both procaryotes.
Describe the Procaryotic cell structure.
IMAGE 30
The only internal structural features are:
- a bacterial chromosome or nucleoid, comprising a closed loop of double stranded, supercoiled DNA. In addition, there may be additional DNA in the form of a plasmid.
- thousands of granular ribosomes
- a variety of granular inclusions associated with nutrient storage.
All of these are contained in a thick aqueous soup of carbohydrates, proteins, lipids and
inorganic salts known as the cytoplasm, which is surrounded by a plasma membrane.
This in turn is wrapped in a cell wall, whose rigidity gives the bacterial cell its characteristic shape. Depending on the type of bacterium, there may be a further surrounding layer such as a capsule or slime layer and/or structures external to the cell associated with motility (flagella) or attachment (pili/fimbriae).
Describe the Genetic material of a bacteria.
Although it occupies a well defined area within the cell, the genetic material of procaryotes is not present as a
true nucleus, as it lacks a surrounding nuclear membrana. The nucleoid or
bacterial chromosome comprises a closed circle of double stranded DNA, many times the length of the cell and highly folded and compacted.
The DNA may be associated with certain bacterial proteins, but these are not the same as the histones found in eucaryotic chromosomes.
Some bacteria contain additional DNA in the form of small, self-replicating extrachromosomal elements called plasmids. These do not carry any genes essential for growth and reproduction,
and thus the cell may survive without them. They can be very important however, as
they may include genes encoding toxins or resistance to antibiotics, and can be passed
from cell to cell.
Describe ribosomes of procaryotic cells.
IMAGE 31 and 32
Apart from the nucleoid, the principal internal structures of procaryotic cells are the ribosomes. These are the site of protein synthesis, and there may be many thousands of these in an active cell, lending a speckled appearance to the cytoplasm. Ribosomes are composed of a complex of protein and RNA, and are the site of protein synthesis in the cell.
Although they carry out a similar function, the ribosomes of procaryotic cells
are smaller and lighter than their eucaryotic counterparts. Ribosomes are measured
in Svedberg units (S), a function of their size and shape, and determined by
their rate of sedimentation in a centrifuge; procaryotic ribosomes are 70S, while
those of eucaryotes are 80S. Some types of antibiotic exploit this difference by targeting the procaryotic form and selectively disrupting bacterial protein synthesis.
What is a polyribosome?
A polyribosome (polysome)
is a chain of ribosomes
attached to the same molecule of
mRNA.
What are Inclusion bodies in bacteria?
Within the cytoplasm of certain bacteria may be found granular structures known as
inclusion bodies. These act as food reserves, and may contain organic compounds such as starch, glycogen or lipid. In addition, sulphur and polyphosphate can be stored as
inclusion bodies, the latter being known as volutin or metachromatic granules. Two
special types of inclusion body are worthy of mention. Magnetosomes, which contain
a form of iron oxide, help some types of bacteria to orientate themselves downwards into favourable conditions, whilst gas vacuoles maintain bouyancy of the cell in blue greens and some halobacteria.
What are Endospores in bacteria?
Certain bacteria such as Bacillus and Clostridium produce endospores. They are dormant forms of the cell that are highly resistant to extremes of temperature, pH
and other environmental factors, and germinate into new bacterial cells when conditions become more favourable.
The spore’s resistance is due to the thick coat that surrounds it.
Endospores of pathogens such as Clostridium botulinum can resist boiling
for several hours. It is this resistance that makes it necessary to autoclave at 121◦C in order to ensure complete sterility.
Describe the The plasma membrane in bacteria.
IMAGE 33
The cytoplasm and its contents are surrounded by a plasma membrane, which can be thought of as a bilayer of phospholipid arranged like a sandwich, together with associated proteins. The function of the plasma membrane is to keep the contents in, while at the same time allowing the selective passage of certain substances in and out of the cell (it is a semipermeable membrane).
Phospholipids comprise a compact, hydrophilic (= water-loving) head and a long
hydrophobic tail region; this results in a highly ordered structure when
the membrane is surrounded by water. The tails ‘hide’ from the water to form the inside
of the membrane, while the heads project outwards. Also included in the membrane
are a variety of proteins; these may pass right through the bilayer or be associated with the inner (cytoplasmic) or outer surface only. These proteins may play structural or
functional roles in the life of the cell. Many enzymes associated with the metabolism
of nutrients and the production of energy are associated with the plasma membrane in procaryotes. this is fundamentally different from eucaryotic cells, where these reactions are carried out on specialised internal organelles.
Proteins involved in the active transport of nutrients are also to be found
associated with the plasma membrane. The model of membrane structure must not be thought of as static; in the widely accepted fluid mosaic model, the lipid is seen as a fluid state, in which proteins float around, rather like icebergs in an ocean.
The majority of bacterial membranes do not contain sterols (c.f. eucaryotes: see
below), however many do contain molecules called hopanoids that are derive from the same precursors. Like sterols, they are thought to assist in maintaining membrane stability. A comparison of the lipid components of plasma membranes reveals a distinct difference between members of the Archaea and the Bacteria.
What is the function of bacterial cell wall?
Bacteria have a thick, rigid cell wall, which maintains the integrity of the cell, and
determines its characteristic shape. Since the cytoplasm of bacteria contains high concentrations of dissolved substances, they generally live in a hypotonic environment (i.e. one that is more dilute
than their own cytoplasm). There is therefore a natural tendency for water to flow into the cell, and without
the cell wall the cell would fill and burst (you can demonstrate this by using enzymes to strip off the cell wall, leaving the naked protoplast).
What is the major component of the cell wall of a bacteria?
IMAGE 34
The major component of the cell wall, which is responsible for its rigidity, is a substance unique to bacteria, called peptidoglycan (murein). This is a high molecular
weight polymer whose basic subunit is made up of three parts: N-acetylglucosamine, N-acetylmuramic acid and a short peptide chain. The latter comprises the amino acids l-alanine, d-alanine, d-glutamic acid and
either l-lysine or diaminopimelic acid (DAP). DAP is a rare amino acid, only found in
the cell walls of procaryotes. Note that some of the amino acids of peptidoglycan are found in the d-configuration. This is contrary to the situation in proteins, and confers protection against proteases specifically directed against l-amino acids.
Precursor molecules for peptidoglycan are synthesised inside the cell, and transported
across the plasma membrane by a carrier called bactoprenol phosphate before being incorporated into the cell wall structure. Enzymes called transpeptidases then covalently bond the tetrapeptide chains to one another, giving rise to a complex network; it is this cross-linking that gives the wall its mechanical strength. A number of
antimicrobial agents exert their effect by inhibiting cell wall synthesis; β-lactam antibiotics such as penicillin inhibit the transpeptidases, thereby weakening the cell wall, whilst bacitracin prevents transport of peptidoglycan precursors out of the cell.
What is the Gram stain technique?
Although all bacteria (with
a few exceptions) have a cell wall containing peptidoglycan, there are two distinct structural types. These are known as Gram-positive and Gram-negative.
The names derive from the Danish scientist Christian Gram, who, in the 1880s developed a rapid staining technique that could differentiate bacteria as belonging to one of two basic types.
Describe Gram-positive cell walls.
IMAGE 35
Gram-positive cell walls are relatively simple in structure, comprising several layers
of peptidoglycan connected to each other by cross-linkages to form a strong, rigid
scaffolding. In addition, they contain acidic polysaccharides called teichoic acids; these
contain phosphate groups that impart an overall negative charge to the cell surface.
Describe Gram-negative cell walls.
IMAGE 36
Gram-negative cells have a much thinner layer of peptidoglycan, making the wall
less sturdy, however the structure is made more complex by the presence of a
layer of lipoprotein, polysaccharide and phospholipid known as the outer membrane. This misleading name derives from the fact that it superficially resembles
the bilayer of the plasma membrane; however, instead of two layers of phospholipid, it has only one, the outer layer being made up of lipopolysaccharide. This has three parts: lipid A, core polysaccharide and an O-specific side chain. The lipid A component may act as an endotoxin, which, if released into the bloodstream, can lead to serious conditions
such as fever and toxic shock. The O-specific antigens are carbohydrate chains
whose composition often varies between strains of the same species.
Serological methods
can distinguish between these, a valuable tool in the investigation, for example, of
the origin of an outbreak of an infectious disease.
Proteins incorporated into the outer
membrane and penetrating its entire thickness form channels that allow the passage of water and small molecules to enter the cell. Unlike the plasma membrane, the outer membrane plays no part in cellular respiration.
