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GCSE Biology Revision Notes
A. 2. Five Kingdoms
THE FIVE KINGDOMS
Bacteria
Bacteria are small, single celled organisms, typically 1-2 μm in diameter. Their cell structure
is far simpler than most organisms with which you are familiar. Critically, bacterial cells lack
a nucleus or any other “membrane bound organelles” such as mitochondria or chloroplasts.
The term “Prokaryotic” is applied to bacteria, which means “before the nucleus”, a reference
to the fact that bacteria successfully occupied the Earth for nearly 2 billion years before more
complex cells arose (the “Eukaryotes” with nuclei”).
A Typical Bacterial Cell
Important features of Bacteria
DNA – a long molecule divided into regions called genes which determine the
characteristics of bacteria and control the activities of the cell. In a eukaryote these would be
formed into linear (straight) chromosomes and held in the nucleus, neither of which exist in
bacteria. In bacteria the DNA is a single long loop.
Plasmid – a small, independent loop of DNA. Very important in genetic engineering.
Cytoplasm – a complex chemical mixture but mainly water. This is where important
chemical reactions take place.
Plasma membrane – another word for cell surface membrane. Semi-permeable, it controls
what enters or exits the cell.
Cell wall – made of a material called proteoglycan (not cellulose as found in plant cells).
Provides support.
Capsule – not always present – a layer of protective slime.
Flagellum – plural flagella. Not always present. A swimming device.
Useful bacteria
Skin bacteria
A lot of the harmless bacteria living on your skin take up space and prevent other, harmful
or pathogenic bacteria colonising your skin.
Gut bacteria
Some prevent undesirable bacteria growing in your gut, others assist the digestive process
by producing enzymes.
Decomposing bacteria
Break down dead organic waste (dead organisms, faeces etc) as a source of food. Important
recyclers in ecosystems and vital for sewage treatment.
Edible bacteria
Lactobacillus is used to turn milk into yoghurt. It breaks down the sugar lactose into lactic
acid which thickens the milk and makes it sour (acidic). The leaves less lactose available for
other bacteria and the acidity stops them from growing. There are hundred of other
examples of bacteria in foods or food production including sauerkraut, chorizo sausage, soy
sauce and vinegar. Be careful not to confuse bacteria with yeast, used to make bread and
alcohol, which is a fungus.
Industrial bacteria
Many bacteria make useful biological compounds like proteins. They can easily be
genetically modified to make proteins from other organisms, including the human hormone
insulin. Industrial bacteria are grown in fermenters.
The fermenter contains a nutrient broth the bacteria grow in.
It is stirred to prevent them settling to the bottom, and air is bubbled in to provide oxygen.
The bacteria grow and respire very rapidly and produce a lot of waste heat, so the fermenter
is cooled by a water jacket around the outside.
pH and temperature are carefully monitored to maintain ideal conditions.
Harmful Bacteria
Most bacteria are either harmless or useful, however a small minority are pathogenic
(disease causing) and cause a wide variety of diseases.
Pneumonia
Pneumonia is caused by the bacterium Streptococcus pneumoniae, commonly called
‘pneumococcus’. Pneumonia occurs when this bacterium infects the lungs, leading to a
build up of fluid, difficulty breathing, chest pain and a cough. Pneumonia is particularly
common in young children, the elderly and in other people with weak immune systems,
such as AIDS patients.
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Pneumonia is spread by droplet infection. An infected person coughs out droplets of water
or mucus which contain the bacteria. These may be inhaled by someone else, or land on a
surface which someone else then touches. The bacteria are then likely to enter their body
when the person eats, rubs their eyes etc.
Streptococcus infections can be treated with antibiotics like penicillin, although there is an
increasing problem with drug-resistant bacteria.
Protoctista
Note: if you have a copy of the IGCSE text book with a blue cover then you won’t find much
information about the Protoctista. However, they are on your syllabus. Ask JRB or your
teacher for a handout on the Protoctista.
These are a group of usually single celled misfits which are hard to classify into any other
kingdom. Some seem plant-like (the algae), others more animal-like (like Paramecium) and
some seem to have characteristics of both. They are all Eukaryotes, however, and so contain
mitochondria and a membrane-bound nucleus. The photosynthetic members also contain
chloroplasts.
Amoeba
A single celled Protoctist. It feeds by
engulfing small food particles and hunts
actively, so is ‘animal like’.
Chlorella
Another single celled Protoctist, Chlorella is
a type of algae. It has a nucleus, large
vacuole and a chloroplast surrounded by a
cellulose cell wall, but doesn’t qualify as a
plant as it isn’t multicellular. Plants
evolved from green algae, but green algae
aren’t plants.
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Plasmodium
Another single celled Protoctist. This one
has a complicated life cycle including
several different looking stages. You don’t
need to know about these, but you should
recall that Plasmodium causes malaria and
that it is spread by mosquitoes.
Plants
Plants are multicellular organisms – they contain chloroplasts and are able to carry out
photosynthesis; they have cellulose cell walls; they store carbohydrate as starch. Examples
include flowering plants, such as a cereals (for example maize) and legumes (for example
peas or beans).
Plant cell walls are structural and prevent the cell from bursting under its own internal
pressure (see osmosis notes). The cell wall is fully permeable, it does NOT control what goes
in and out, that job is done by the membrane.
Chloroplasts are the small green organelles within the cell that carry out photosynthesis.
They contain the green pigment chlorophyll. (NB students often talk about chloroplasts and
chlorophyll interchangeably but they are not the same thing).
Animals
These are multicellular organisms; they do not contain chloroplasts and are not able to carry
out photosynthesis; they have no cell walls; they usually have nervous coordination and are
able to move from one place to another; they often store carbohydrate as glycogen.
Examples include mammals (for example humans) and insects (for example housefly and
mosquito).
Fungi
Linnaeus classified fungi as plants, and you can probably understand why, but the
similarity is only superficial. Fungi are more closely related to animals – for instance they
both use glycogen as a store of carbohydrate.
Fungi are Eukaryotic (have a nucleus) and are heterotrophic (they gain food from other
organisms). They have no chloroplasts and cannot photosynthesis, so clearly are not plants.
They have cell walls made of chitin, not cellulose, and store glucose as glycogen rather than
starch.
Fungi feed by extracellular digestion; they secrete digestive enzymes into their food and
absorb the small nutrient molecules, such as glucose and amino acids, which are released.
They are usually saprotrophic (digesting dead things and helping the process of
decomposition) but some are parasitic and will live in or on live plants or animals. Athlete’s
foot is a fungal disease.
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Fungi form some of the largest organisms in the world by mass, though they are usually
invisible underground. They grow a network (a “mycelium”) of fine, branching hair like
structures called hyphae which can extend over many square miles. They do NOT construct
sophisticated root structures like a plant. Periodically they send up fruiting bodies
containing reproductive spores which may have been produced sexually or asexually. Some
of these fruiting bodies are very large and we know them as mushrooms and toadstools.
Not all fungi are multicellular, there are thousands of different kinds of single celled fungi
which are collectively called yeasts.
Yeasts reproduce asexually by budding, in which a new small cell grows out of the side of
an existing one.
Yeasts are very important due to their role in brewing and breadmaking. If allowed to grow
in the presence of glucose but no oxygen, they carry out anaerobic respiration in order to
produce energy.
𝑨𝒏𝒂𝒆𝒓𝒐𝒃𝒊𝒄 𝑹𝒆𝒔𝒑𝒊𝒓𝒂𝒕𝒊𝒐𝒏
𝑮𝒍𝒖𝒄𝒐𝒔𝒆 →
𝑬𝒕𝒉𝒂𝒏𝒐𝒍 + 𝑪𝒂𝒓𝒃𝒐𝒏 𝑫𝒊𝒐𝒙𝒊𝒅𝒆 + 𝑬𝒏𝒆𝒓𝒈𝒚
Ethanol is what makes beer and wine alcoholic, whilst carbon dioxide gas puffs up bread to
make it rise. The alcohol evaporates from bread when you bake it in a hot oven.
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