Download Note 19

South Tuen Mun Government Secondary School
Biology Revision Note 19
General features of Virus (also called phage)
 extremely small, in average 50 times smaller than bacteria; cannot be seen under light microscope
 made up of a protein coat [not cell wall / cell membrane] surrounding genetic materials (nucleic acid)
 non-living, virus is not a cell! Because it has no cellular structure e.g. cell membrane, cytoplasm
 obligate parasite – can reproduce inside a host cell (remain inactive outside the host cell, no metabolism)
Diversity of virus:
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Some have envelope, some have not.
Nucleic acid can be DNA or RNA.
Some have only lytic cycle; some both lytic
cycle and temperate cycle.
Viruses have different specific hosts. Some
attack bacteria cell (called bacteriophage),
some attack plant cell, some attack animal cell,
some attack human cell.
Lytic cycle
Attachment is specific. The
shapes of receptor and
surface protein are
complementary to each
other.
Temperate cycle
Sometimes, after stage 2,
viral nucleic acid goes into
the nucleus and replicates
with the host cell’s DNA. It
will not go to stage 3 and
lyse the cell. When the host
cell divides, the viral
nucleic acid replicates and
stays with the host cells.
The virus is not harmful.
Upon activation (due to
unknown reason), the viral
nucleic acid goes out of the
nucleus and starts stage 3,
many host cells are lysed at
the same time (lytic cycle).
The period when the virus is
not harmful, is called latent
period.
General features of bacteria
 unicellular prokaryotes (no nucleus / no nuclear membrane; no membrane bounded organelles)
 have cell wall (not made of cellulose)

1-5 µm in length or diameter
Diversity of bacteria:
General features of Protist
 eukaryotes (true nucleus with nuclear membrane, membrane bounded organelles e.g. chloroplasts,
mitochondria) e.g. protozoa, algae
 5-10 µm in length or diameter
Diversity of Protist:
Protozoa
Algae
Unicellular
Unicellular or forms in colony / filamentous
No cell wall
Has cell wall
No photosynthetic pigment, nutrition by phagocytosis
Photosynthetic pigment present (some brown, red) –
nutrition by photosynthesis
e.g. paramecium, amoeba
e.g. euglena, spirogyra
General features of Fungi
 eukaryotes, presence of cell wall (not made of cellulose), no photosynthetic pigment – no photosynthesis
Diversity of fungi:
Yeast
Moulds e.g. bread mould, penicillium
Unicellular
Made up of hyphae that are multinucleated; hyphae
forms into mycelium
Reproduced asexually by budding
Reproduced asexually by spore formation
Growth Requirement of microorganisms:
Conditions
Diversity of microorganisms
Nutrient supply A few microorganisms can make their own food. (e.g. algae can carry out photosynthesis,
nitrogen fixing bacteria can carry out nitrogen fixation.) Most microorganisms must rely on
a carbon source and a nitrogen source.
Common carbon source can be carbohydrate and / or lipid. Common nitrogen source can be
nitrate, ammonium salts, protein, nucleic acids.
Temperature
Different microorganisms have different optimum temperature for growth. Most
microorganisms cannot grow / are inactive in very low temperature e.g. below 0℃ and are
killed in extreme high temperature (120℃) because enzymes are not active at low
temperature and are denatured at high temperature.
pH
Different microorganisms have different optimum pH for growth. Most microorganisms are
killed in extreme low and high pH because enzymes are denatured at extreme pHs.
Oxygen
availability
Obligate aerobes: microorganisms that cannot live without oxygen; most microorganisms
belong to this.
Facultative anaerobes: microorganisms that can live with or without oxygen.
Obligate anaerobes : microorganisms that cannot live with oxygen; oxygen is toxic to them.
Water
availability
Water is essential for survival of microorganisms.
In hypotonic solution, there is a net movement of water into the cell by osmosis, rigid cell
wall of microorganisms can prevent the cell from bursting or some microorganisms develop
contractile vacuole to remove excess water. Most microorganisms can survive.
In hypertonic solution, there is a net movement of water out of the cell by osmosis. The cell
becomes dehydrated. Most microorganisms die or become inactive / dormant.
Growth Curve of microorganisms:
Most microorganisms grow by mitotic cell division / binary fission.
Measurement of growth:
The microorganisms can be grown in liquid culture medium using nutrient broth or in solid culture medium
using nutrient agar plate.
(i)
Cell count
 Total cell count using a special microscopic slide
Limitation of the method : dead cells are also measured; small cells are difficult to be seen; if the cell
density is too low, there will be sampling error (sampling error = the number measured depends on the 10
selected squares); if the cell density is too large, cells will overlap each other making counting difficult.
 Viable cell count by counting the number of colonies on solid culture medium
Only living cells can grow and be counted. One colony corresponds to one living cell. Normally, 30 – 300
colonies can be taken. There may be sampling error if the number of colonies is too small; and it is
difficult to count if the number of colonies is too large.
Number of living cells per ml. of the sample = number of colonies counted x dilution factor x 1/0.1
(ii)
Biomass – in both liquid culture medium and solid culture medium, the microorganisms are dried and
weighed to obtain the dry mass
Optical method – turbidity in a liquid culture medium is taken as an estimate of growth
Turbidity is measured by the optical density using spectrophotometer.
(iii)
Precaution when working with microorganisms, must be taken to avoid contamination of the culture medium
by other microorganisms in the environment and to avoid contamination of the microorganisms in the culture
medium to the workers
 Workers must wear laboratory coat, gloves. All wounds must be covered. Hands must be washed
thoroughly after the experiment.
 Equipments must be autoclaved (heated at 121℃ under higher pressure of 103 kPa) and sterilized
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before and after the experiment.
The workplace must be sterilized with disinfectants e.g. alcohol before and after the experiment.
Between each transfer of microorganisms, the inoculating loop must be flamed and sterilized.
Uses of microorganisms:
(i) Beer brewing
(ii) Wine making
(iii) Bread making
(iv) Microorganisms as food e.g. single cell protein in spirulina, mycoprotein made from mould
(v) Antibiotic making e.g. penicillin made from mould Penicillium – antibiotics are used to inhibit the growth
of bacteria or to kill bacteria by inhibiting the formation of cell wall and inhibiting protein synthesis
(vi) Vaccine making
(vii) Making enzymes,
 protease and lipase, in biological washing powder – the enzymes break down protein / lipid in the
stain into soluble products that can be washed away by water;
 pectinase for making clear fruit juice, pectinase breaks down pectin and removes the debris in the
fruit juice
(viii)Pollution control
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E. coli is used as an indicator of water pollution because it is found in faeces, water contaminated by
faeces / organic pollutants has more E. coli
 Microorganisms act as decomposer in sewage treatment –under aerobic condition , to break down and
reduce the amount of organic pollutant in the sewage; and under anaerobic condition, to break down
solid sludge for the formation of biogas, e.g. methane, used as fuel and the solid residue is used as
fertilizer
(ix) Recombinant DNA technology
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Microorganisms act as the donor cell when the gene of interest is found in the microorganisms.
Microorganisms act as the vector as viral DNA and bacterial plasmid are both suitable for the
insertion of the gene of interest
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Restriction enzyme and ligase are enzymes used in the technology, they are found and easily
extracted from microorganisms.
Microorganisms act as the host cell when they take in the genetically modified (GM) DNA fragments.
They can then act as a factory to produce the desired gene products or they can be used to infect a
higher living organisms e.g. maize to produce a genetically modified organism (GMO).
Some examples of products made by the technology: insulin, pectinase, hepatitis B vaccine
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Advantage of using microorganisms to make commercial products:
 Microorganisms can grow and reproduce very fast, with simple nutritive requirements. Thus the products
can be made at a very fast rate. The products can also be extracted and purified easily because very few
unwanted bi-products are present.
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Some microorganisms can work effectively under extreme environmental conditions which are required in
some industrial processes.
Potential hazards of using GM microorganisms:
 To human health
 Spread harmful gene of microorganisms to man
 GM microorganisms produce unknown substances that cause allergy in man
 GM microorganisms are made to produce toxins / diseases that are harmful to man e.g. anthrax
 To the environment
 Leakage of GM microorganisms to the nature may bring the foreign gene to the other microorganisms
forming “superbugs” which are difficult to kill
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Leaked GM microorganisms may mutate to give unpredictable effect to the environment.
Measures to minimize the potential hazard of GM microorganisms:
 Strict guidelines have been set for investigation using GM microorganisms, to prevent their leakage and
their infection on the workers.
 GM products are tested to prevent possible allergy in man.
 GM microorganisms are further modified to prevent the leakage of their genes to other microorganisms.
Harmful effects of microorganisms:
 Some microorganisms and virus are pathogens.
 After they gain entry into the body, they grow and reproduce at the expense of the human body
tissues; they can spread through food contamination (food-borne infection), e.g. Salmonella (spread
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through contaminated, undercooked or precooked meat products such as sausages, ham, raw egg.)
When they grow, they produce enzymes and toxins that affect the normal functioning of the human
body tissues (food poisoning), e.g. bacterial toxins from Staphylococcus aureus (spread through
manual handling of food without further cooking, such as salad, precooked meat, sandwiches), fungal
toxins (present in contaminated peanut oil, corn oil), algal toxins such as ciguatoxins (present in
shellfish)
 Symptoms of infection: abdominal pain, vomiting, diarrhoea, fever, fatigue
Some microorganisms spoil our food (microbial deterioration). They change the taste and appearance of
food, give a bad smell, thus making the food not suitable to be eaten.
Some microorganisms spoil our leather, wood, metal, painted surface under warm and humid conditions.
Method to control microbial growth
Biological principle(s)
Heating, autoclave (heat under pressure)
High temperature kills most microorganisms.
Ultra high temperature (UHT) treatment (heat to 132
℃ for 1 sec)
High temperature kills most microorganisms. The
Pasteurization (heat to 71℃ for a short period of
time – 15 sec, then cool quickly to about 10℃, keep
High temperature kills many microorganisms. The
short duration prevents the heat damage on the
quality of food. Low temperature inhibits the growth
of microorganisms.
at low temperature / in refrigerator)
Canning (heat over 100℃ under pressure and place
the product in sterilized, sealed vacuum can)
Refrigeration (keep at 2 – 7℃)
Freezing (keep at -18℃)
short duration prevents the heat damage on the
quality of food.
High temperature kills most microorganisms and
most microorganisms cannot live without oxygen.
Low temperature inactivates enzymes and inhibits the
growth of microorganisms.
Drying by (i) evaporation of water under the sun, (ii)
freeze drying, (iii) addition of salt and sugar
Most microorganisms cannot grow without water.
Exposure to radiations, disinfectants (e.g. chlorine
Radiation, disinfectants, antiseptics, preservatives,
gas / bleach), antiseptics (e.g. alcohol, iodine
solution), preservatives (e.g. sulphur dioxide in fruit
juice; nitrites in hams, sausages), antibiotics, drugs.
antibiotics can kill most microorganisms.
Drugs and antibiotics are used to kill infectious
microorganisms that cause diseases in man.
Low pH / vinegar
Low pH inactivates enzymes and inhibits the growth
of microorganisms.