Parasites and mutualists
LO: To explain how the survival of
one organism may depend on the
presence of another species
You have two minutes in pairs to write down
what you think a parasite and a mutualist
Hink: For the mutualist think about the word
In most feeding relationships a predator kills
and eats a prey and then moves on to find
a more prey.
Parasitism is a feeding relationship in which
two organisms live together with one
feeding off the other.
The organism doing the feeding is a parasite
and the organism it feeds on is its host.
Parasites usually harm their host but do
not kill it.
Can you think why this is?
Headlice and fleas
Headlice and fleas are parasites. They bite
other animals (hosts) such as humans so
they can feed off their blood.
Other parasites, such as the tapeworm, live
inside their hosts. Tapeworms live inside
vertebrate intestines. Their eggs leave the
host in its faeces. Eggs swallowed by
other animals hatch and grow inside their
Tapeworms absorb nutrients from the host’s
gut which can cause the host to lose
The worm can also grow large enough to
block the host’s intestines.
• The head has hooks and suckers to attach
it to the small intestine wall.
• It has no eyes.
• The flat body gives it a large surface area
for food absorption
• It has no circulatory system, intestines or
much of a nervous system.
• The outside of the worm contain
substances that stop it being digested by
enzymes in the small intestine.
• Each segment contains male and female
reproductive organs so the worm can self
fertilise its eggs if other tapeworms are not
The world's longest parasite is the tapeworm
so the next question is 'What is the world's
The longest tapeworm ever removed from a
human came out of Sally Mae Wallace on
September 05, 1991.
In all, doctors pulled 37 feet of tapeworm out
of Sally Mae Wallace's body through her
If you find all of this disgusting, be glad
you're not a whale as tapeworms in whales
can grow up to 120 feet long.
Believe it or not mistletoes is also a
It has leaves that can photosynthesise but
its roots grow into the veins of the host
tree and absorb water and mineral salts.
Some organisms live in close relationships
where both organisms benefit. This is
Oxpeckers have a mutualistic relationship
with large herbivores in Africa.
Both the oxpecker and the herbivore benefit
from the relationship. The oxpecker eats
parasitic insects that live on the skin of the
Cleaner fish eat dead skin and parasites
from the skin of larger fish, such as
You could say a fish pedicure is a
mutualistic behaviour!!- why?
Nitrogen –fixing bacteria
Some organisms live in mutualistic relationships
inside other organisms.
Bacteria that turn nitrogen in the air into nitrogen
compounds are called nitrogen-fixing bacteria.
Some live inside the roots of legumes (plants
like peas that produce pods).
What do the bacteria gain?
they are protected from the environment
They gain chemical substances from the
plant they use as food.
What do the legumes gain?
It gets nitrogen compounds which it needs to
grow well from the bacteria.
You may remember these from the work on
deep sea volcanic vents in topic 1- what
They are giant tubeworms.
Chemosynthetic bacteria are producers that
get their energy from chemical substances
and not from light.
There are chemosynthetic bacteria inside
giant tube worms.
The tubeworms gather
substances that the
bacteria need for
by the bacteria.
1. In the human-headlouse feeding relationship,
i) the parasite
ii) the host
2. Explain fully the difference between parasitic
relationships and mutualistic relationships.
3. The tubeworm-bacteria relationship is
mutualistic. Explain why each organism needs
4. With the mutualistic relationship of the nitrogenfixing bacteria and the legume what do both
‘What am I?’ game!
Write down on your mini
whiteboard what the
description is describing