Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
438
Figure 18.19 Volvox globator is a spherical colonial green
protoctist. The body is composed of thousands of cells with
flagella. These cells work together in a coordinated way but
there is little specialisation of cells. Cells at one pole detect
light so the colony swims towards the light. Cells at the other
pole are specialised for reproduction. Inside there are new
colonies that are just about to be released (× 60).
The three domains are each divided into kingdoms.
The following section describes the features of the four
kingdoms of the Eukarya.
Kingdom Protoctista
The Protoctista is made up of a very diverse range of
eukaryotic organisms, which includes those that are often
called protozoans (‘simple animals’) and algae, such as
seaweeds. Any eukaryote that is not a fungus, plant or
animal is classified as a protoctist (Figures 18.18 and
18.19). The characteristic features of protoctists are:
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eukaryotic
mostly single-celled, or exist as groups of similar cells
some have animal-like cells (no cell wall) and are
sometimes known as protozoa
others have plant-like cells (with cellulose cell walls and
chloroplasts) and are sometimes known
as algae.
Many organisms in this kingdom may actually be more
closely related to organisms in other kingdoms than they
are to each other. For example, there are strong arguments
for classifying algae as plants.
Kingdom Fungi
Fungi have some similarities with plants, but none of
them is able to photosynthesise. They are all heterotrophic,
obtaining energy and carbon from dead and decaying
matter or by feeding as parasites on living organisms.
There is a vast range in size from the microscopic yeasts
to what may be the world’s largest organism. A specimen
of the honey fungus, Armillaria bulbosa, grows in a forest
in Wisconsin, USA and spreads over 160 000 m 2 . Not only
is it possibly the largest organism in the world, but it may
also be the oldest at 1500 to 10 000 years old; its estimated
mass is 100 tonnes.
Characteristic features of fungi are:
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eukaryotic
do not have chlorophyll and do not photosynthesise
heterotrophic nutrition – they use organic compounds
made by other organisms as their source of energy and
source of molecules for metabolism
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reproduce by means of spores (Figure 18.20)
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simple body form, which may be unicellular or made
up of long threads called hyphae (with or without cross
walls) (Figure 18.21); large fungi such as mushrooms
produce large compacted masses of hyphae known as
‘fruiting bodies’ to release spores
cells have cell walls made of chitin or other substances,
not cellulose
never have cilia or flagella.
Figure 18.20 A puffball fungus, Lycoperdon sp., releasing
millions of microscopic spores. Their method of feeding on
dead and decaying matter means that eventually the food is
all used up. A few of these spores may land on a suitable food
source and be able to grow.