Module Two: Life at a Molecular,
Cellular and Tissue Level
Paper One
• Owing to evolution, we have a large diversity of life on planet
Earth.
• The earliest organisms that were thought to have developed were
viruses, protists, bacteria and fungi. These are the organisms we
will be focussing on. They:
– Are an extremely diverse group found in virtually all of the planet’s
environments.
– Occur in vast numbers.
– Vary in size. Some are micro-organisms (microbes) while others are
macro-organisms (e.g. fungi and some algae).
– Cellular forms can be prokaryotes (bacteria) or eukaryotes (protists and
fungi)
• Micro-organisms can be useful.
• Pathogenic/non-pathogenic:
– Pathogenic (disease-causing), i.e. germs
– Non-pathogenic (harmless to plants and animals).
• Viruses can infect animals, plants and even bacteria.
• Viruses were discovered after bacteria, as they are smaller
and can only be seen by an electron microscope.
• The word ‘virus’ is Latin, meaning ‘poison’.
• Exist in two distinct states: a virus (when active) and a virion
(when dormant and not in contact with a host cell).
• Are not really living: they don’t respire, eat, excrete, grow or
move on their own.
• Viruses are obligate intracellular parasites and can only survive
inside the cells.
• Associated with disease. After the virus leaves the host cell, the
host cell is destroyed.
• Viruses can have different shapes:
– Polyhedral
cubical
e.g. adenovirus
– Helical
spiral cylinder
e.g. tobacco virus
– Complex
tadpole-like
e.g. bacteriophage
• The immune system becomes activated in order to fight the
infection. This may lead to fever, tiredness or an opportunistic
disease.
• HIV/Aids is the most serious disease facing the world today.
South Africa has the highest prevalence of HIV in the world;
18% of the adult population is infected with HIV.
• Human Immunodeficiency Virus (HIV) affects the immune
system
Physical effects:
• Weakened immune system, resulting in fatigue, weight loss,
frequent fevers and sweats, fungal infections, shingles (nerve
disease), skin rashes and flaky skin, diarrhoea, spots or
blemishes on tongue, in mouth or throat.
• At this point, many viruses, bacteria, fungi and protozoa are
able to invade the body. Other infections could include drug
resistant TB, cancer and pneumonia as well as memory loss,
depression and other neurological disorders.
• Awareness of one’s HIV status is the first step.
• Treatment with a combination of anti-retrovirals (ARV’s).This
doesn’t cure HIV but slows down viral replication, allowing the
immune system to recover and helping the individual to feel
healthy again.
• Monitoring the viral load (amount of HIV in one’s blood) enables
medical staff to adjust the combination of ARV’s.
• Treat opportunistic infections immediately.
• Boost the immune system.
• Care for the mental and emotional health of sufferers.
• Sensible financial planning.
• Giving ARV’s to pregnant women to reduce the chance of
infecting the baby.
• Drug-resistance is the ability of disease-causing viruses to
continue multiplying, despite the presence of drugs which
usually kill them.
• ARV’s consist of a ‘cocktail’ of various classes of drugs, specific
to the stage of the disease, to attack the viruses at different
stages of it’s replication.
• Ancestors of current bacteria were unicellular organisms
• They were the first forms of life to develop on earth 4 billion years
ago.
• They inhabit practically all environments on earth. Some,
extremophiles, thrive in environmental extremes, e.g. intense
heat or intense cold.
• Bacteria are unicellular and are among the smallest organisms.
• In favourable conditions, bacteria will reproduce rapidly by binary
fission, an asexual process whereby one bacteria can split into
two every 10-15 minutes.
• As prokaryotes, they do not have a true nucleus. This is because
they do not have a true nuclear membrane and the DNA isn’t
combined with the protein found in eukaryotes.
• Single chromosome of DNA – closed loop.
• Some bacteria have flagella to enable them to move around.
• Bacteria are mostly useful, but some cause
disease.
• They can survive for thousands of years and
can cause diseases such as tetanus and
anthrax.
How are Bacteria Useful to Humans?
• Many bacteria in the intestines are considered
“good” bacteria
- Some create an acidic environment in
intestines that prevents the growth of bad
bacteria
- They help break down sugars, protein, and
fat so body can absorb their energy
How are Bacteria Useful to Humans?
- Bacteria support the immune system
They increase white blood cells
White blood cells – help immune system’s
response to harmful bacteria.
• Yogurt has probiotics
- Probiotics consist of good bacteria that help
with stomach issues
Pathogenic bacteria infect our bodies in order to get food.
They may:
• Destroy body cells which are then used as a source of food.
• Absorb material directly from body fluids such as blood.
• Release harmful substances, called toxins. Toxins may:
– Cause symptoms like a rash or a high temperature.
– Block metabolic pathways in the host cells which can have a
devastating effect on the body.
• TB is a chronic infection caused by myocardium tuberculosis, aka
the TB bacillus. It is covered in a waxy layer which protects it
from drying out, heat, and being destroyed by the immune
system.
• It affects all organs, including the brain and bones, but especially
the lungs. This is called pulmonary tuberculosis.
• It is highly contagious and is spread by moisture droplets.
• After infection one of the following might occur:
– If the person has a strong immune system, the infection would be
contained. It will be walled off and can lie dormant for years. The person
will display little/no symptoms of TB.
– If the person has a weak immune system, the bacilli will multiply further
and within four to six weeks, the individual will be ill with pulmonary TB.
• The sufferer feels tired and weak. They may cough up blood,
lose weight rapidly, and have fevers and night sweats.
• The patient could lose earnings while ill and unable to work.
• The government has to cover the cost of medication, clinics
and medical staff.
• There is a stigma attached to this disease. Thus, an infected
person could feel ostracized and lonely.
• If left untreated, a person could infect on average between
10-15 people each year.
• TB can be cured by an anti-biotic which is free and easily
available. The treatment is a six-month regime, known as DOTS
(Directly Observed Therapy, Short Course).
• The patients are carefully monitored to ensure they take the full
course of antibiotics. Within days of starting the treatment, the
patient will be unable to infect others.
• Many patients stop taking medication when they start to feel
better. As a result:
– illness is prolonged
– the patient becomes infectious again
– they can develop multi-drug-resistant TB, which is difficult to treat and can
be fatal.
• Areas most affected by TB are poverty stricken and overcrowded.
Malnutrition and bad sanitation exacerbate the situation.
• To prevent the spread of TB:
– screen those at risk
– treat the infected ASAP
– reduce overcrowding, bad sanitation and malnutrition
• Quality education and information about TB must be given out via
schools, clinics, pamphlets, posters etc.
• Health care workers must be passionate, trained and wellequipped.
• People should be immunised against TB with the BCG vaccine.
Bacteria can be used to:
• Replace pesticides
• Degrade herbicides
• Eat or neutralize toxic waste
• Synthesise riboflavin
• Decompose sewage waste
• Produce food by fermentation processes
• Make probiotics
• Separate the fibres of jute, hemp and flax in preparation for
making sacks and ropes.
The Protista do not form part of any other kingdom. They share the
following characteristics:
– All are eukaryotic (have a true nucleus)
– All live in moist environments as they have no means of preventing
themselves from drying out.
However, there is a lot of variety in this group. The following
characteristics are variable:
– Can be unicellular or multicellular
– Can be microscopic or over 100 metres long
– Can be heterotrophs or autotrophs.
The Protista Kingdom is grouped into the following subcategories:
– Plant-like protista
– Animal-like protista
Plant-like Protista are known as algae.
• Algae are simple, aquatic eukaryotes (have true nuclei)
• Contain chlorophyll. They photosynthesis, thus they are
autotrophic.
• Release large chains of oxygen during photosynthesis.
• Mostly free-floating but some, e.g. kelp, are attached.
• Can be unicellular: e.g. diatoms, or multicellular: e.g. seaweeds.
• Usually reproduce asexually.
• Grouped into: green algae, brown algae, red algae, diatoms and
dinoflagellates.
Algae can be divided into the following groups:
• Green algae
– e.g. spirogyra (slime found in fresh water) and green seaweeds (found
in shallow water – for photosynthesis)
• Brown algae,
– e.g. kelp (found in deep water)
• Red algae,
– e.g. red seaweeds (found in deep water)
• Diatoms
– microscopic, unicellular algae whose cell walls contain silica.
• Dinoflagellates
– microscopic, unicellular algae with flagella. Found in surface waters.
All animal-like protista, or protozoa, are:
• Unicellular.
• Heterotrophs.
– They cannot make their own food. Some kinds engulf their food, e.g.
Amoeba, while others absorb it directly through their cell membranes,
e.g. plasmodium.
• Swim around actively looking for food.
• Groups consist of: amoeboids, cilliates, flagellates, parasitic.
• Malaria is a chronic, life-threatening parasitic disease
transmitted through a bite of a female Anopheles mosquito. The
mosquito acts as a vector.
• It affects mainly poor countries as the mosquitoes thrive in
tropical environments.
• In 1880, scientists discovered it originates from a unicellular
protozoan called Plasmodium.
• Plasmodium is carried in the salivary glands of the mosquito.
• Physical effects:
– Flu-like. Fever, chills, headache, muscle ache, vomiting, tiredness,
anaemia, brain disease, kidney failure.
• Can result in the destruction of red blood cells (anaemia) or
clogging of capillaries surrounding the brain (cerebral
malaria), which results in death.
• Can be costly to individuals and government, impeding
economic growth.
• Treat the illness:
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with a multi-drug called Coartem.
It contains sweet wormwood which is an important natural herb.
It is unfortunately quite expensive.
It is a 3 day course.
• Kill the mosquitoes:
– insecticide-treated mosquito nets act as a physical barrier and actively
kill the mosquitoes.
– Spraying with DDT, a repellent. This is, however, harmful to the
environment.
Extracts from algae can be used as the following:
• Plant growth regulators.
• Phycocolloids are used:
– As gelling agents
– As thickening and stabilizing agents
– To make agar plates
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Natural pigments, used as an alternative to chemical dyes.
Nutrient extracts.
Iodine from kelp is used in mineral salt tablets.
Used in food, e.g. in sushi and in Eastern soups.
Kelp feeds harvested abalone.
• Fungi were put into a single kingdom because of their:
– Similar ecological roles – i.e. they’re decomposers of
organic matter.
– Similar anatomical and biochemical features.
• The kingdom contains about 1 million species.
• Fungi are found everywhere, usually invisible to the naked eye.
• They are mostly free-living, living in the soil, air and dead matter.
• They are heterotrophs; they obtain food from other organisms.
– Most feed on dead organisms, i.e. are saprotrophic.
– Some are parasitic and cause ill health – e.g. thrush and athlete’s foot in
humans, and rust in plants.
• Some live in symbiotic relationships with plants and animals, e.g.
lichen (a combination of fungi and algae), and mycorrhizae.
• Most are useful
• Reproduce:
– Asexually by means of reproductive spores
– Sexually, when in unfavourable conditions, by resistant zyrospores.
– By budding or binary fission (e.g. yeast)
• Mostly multicellular
• Multicellular fungi consist of a mass of branched filaments
(hyphae), enclosed by a rigid cell wall of chitin and glucan.
• The hypha is a multi-branched tubular cell that could be
separated or continuous.
• When the fungi produce asexually they form sporeproducing bodies, e.g. mushrooms or sporangia.
• Caused by Candida albicans, a yeast or fungus.
• Affects the mucous membranes in the mouth (babies) or vagina
(adult females)
• Makes one more vulnerable to HIV, as it affects mucous
membranes.
• Can occur harmlessly in the intestine.
• The immune system usually keeps this in check, but in certain
cases overgrowth may occur:
– Excessive taking of antibiotics
– Weak immune system due to AIDS, stress, lack of sleep, poor diet or
illness.
– Warmth and moisture from tight underwear and being obese.
75% of women at child-bearing age would have suffered from
this condition at least once.
• Oral thrush is treated with an antifungal mouth wash.
• Vaginal yeast infections can be treated with a topical
medication.
• Caused by a fungus called Tinea which grows in damp areas.
• It is quite common. It usually affects the skin between the
toes and makes the skin crack and flake.
• It feeds on keratin, found in skin cells.
• Treated with dryness and an antifungal powder.
• As a food source, i.e. mushrooms.
• Produce biological compounds, e.g. alcohol, plant growth
regulators and enzymes.
• Produce anti-biotics, e.g. penicillin.
• Control haemorrhage after birth.
• Control cholesterol levels
• For fermentation processes, e.g. yeast.
Most micro-organisms are extremely useful. They can be:
• Plant-degraders. Cellulose-degrading fungi and bacteria clean
up dead plant matter.
• Micro-regulators of nutrients. They further break down the
nutrients left behind by scavengers.
• Algae such as cyanobacteria and phytoplankton regenerate
oxygen in the atmosphere.
• Transform nitrogen.
• Biological control agents – they control pests, pathogens and
weeds.
• Escherichia coli and humans
– this is found in the colon and is part of the normal gut flora.
– Bacteria benefit by obtaining glucose and other nutrients from
undigested food remains in the colon.
– Humans gain vitamin K2, which helps blood clotting. We are also
protected from pathogenic bacteria within the gut.
• Nitrogen-fixing bacteria and plants
– bacteria converts nitrogen into useful forms for living organisms. Lives
in root nodules of legumes.
– Rhizobium benefits by gaining glucose and other nutrients as well as a
favourable habitat.
– Plant gains ammonia – forms amino acids, which form protein.
• Mycorrhizal fungi and plant roots
– The fungus gets glucose.
– The plant has an increased surface area, improving nutrient uptake, as
well as protection against pets and diseases.
Plants have an innate immune system. The infected cells will:
• Produce salicylic acid
• Activate resistance genes – e.g. to resist viral replication.
• Self-destruct so that the disease cannot spread.
• The SA moves around the plant to activate the immune
system, making the cells become resistant to the pathogen.
The closing of stomata can play an active role in limiting bacterial
infection.
• First line of defence is the skin. Prevents entry of pathogen.
• The second like of defence involves two responses.
– The primary response is to prevent pathogens from
spreading, by:
• Inducing inflammation (if it is a local infection)
• Inducing fever (raised body temperature.)
– The secondary response is to activate the immune system.
Essentially involves:
• The destruction of the invading germs
• Holding a memory of this response.
The immune processes involve two groups of WBC:
• Lymphocytes
• Phagocytes
B-lymphocytes destroy germs by:
• Reading the antigen on the pathogen, which tells it that it is
dangerous. There are many B-lymphocytes, all specific to their
own antigen.
• If a B-lymphocyte recognizes its pathogen, it replicates itself
many times.
• They are then stimulated to produce antibodies which are
secreted into the plasma.
• The antibodies combine with antigens on the germ’s surface,
thus neutralizing or destroying the pathogen. Some cause the
cell to burst, others label the germs for phagocytosis, others
clump the germs together, thus weakening them.
• Some of the B-lymphocytes stay on in the lymph glands as
memory cells. If the antigen is encountered again, the
immune response will be faster. This is the basis of immunity.
• Natural immunity occurs when an attack of a disease is
prevented by antibodies and memory cells