Page 9 - Biology Form Two

Biology for Secondary Schools
used for various activities. Digestion,
which takes place by the release of
enzymes outside the cell, is also called
extracellular digestion. An example of
a saprophyte is a mushroom growing on
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a log. The mushroom releases enzymes
externally through their root-like
structures, called rhizoids. The enzymes
cause decomposition and decay of the
log from which the mushroom gets
its nutrients. Another example of a
saprophyte is bread mould that grows on
the surface of decaying bread to obtain
the nutrients, as shown in Figure 1.1
Growing bread moulds
Figure 1.1: Bread mould growing on
decaying bread
Symbiotic nutrition
This is a feeding relationship or an
association between two species in
which one or both benefit. Symbiosis
can be in the form of commensalism,
mutualism, or parasitism.
Commensalism: This is a feeding
relationship or an association between
individuals of two different species in
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which one species benefits, while the
other is neither harmed nor benefitted.
In most cases, the host individual is
larger than the commensal individual.
Commensal organisms benefit in
various ways such as in getting
nutrients, shelter, support, or transport
from the host. For example, small
fishes like remora attach to the body of
a large fish such as a shark. The shark
enables remora to move in different
areas without using its own energy, but
the shark is not harmed, and it does
not benefit from such a relationship.
Other examples are epiphytes which
are small plants that grow on different
trees but do not harm the trees (Figure
1.2). The epiphytes get access to
nutrients, exposure to sunlight, and
support from the trees while the trees
are neither harmed nor benefitted by the
relationship.
Host plant
Epiphytic plant
Figure 1.2: Host and epiphytic plants
exhibiting commensalism
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