Symbiosis by dfhdhdhdhjr



Learning outcomes
To know the following interspecies relationships and
examples of each:
1. parasitism
2. mutualism.
Detailed learning outcomes

     Symbiotic relationships are co-evolved and specific intimate
     relationships between members of two different species.

1.   Parasitism
     A parasite benefits in terms of energy or nutrients, whereas a
     host is harmed by their loss. Parasites often have a limited
     metabolism so cannot survive out of contact with a host.

2.   Mutualism
     Both mutualistic partner species benefit from the relationship.
     One example is that of cellulose-digesting protozoa/bacteria
     living in the guts of many herbivores.
Symbiotic relationships


  Parasitism               Mutualism
Symbiosis is the close relationship between
two different species, eg the relationship
between nitrogen-fixing soil bacteria (Rhizobia)
and leguminous plant roots (peas, beans and
There are two types of symbiosis:
• parasitism
• mutualism.
A parasite lives on another organism
(the host) from which it obtains food.   A       B
This:                                  Harmed Benefits
• benefits the parasite
• harms (or kills) the host.
Some parasites (ectoparasites) live on the outer skin
of the host:

    ticks              leech            fleas
Some parasites (endoparasites) live inside the
host (usually in vertebrates):

 Living with parasites

 Only poor parasites kill their host as this is not
  to their advantage, especially for obligate
 There is a balance between parasitic damage
  and host defence, so the relationship is
  relatively stable.
 Diseases often occur in cycles as parasite and
  host co-evolve.
Secondary host species or vectors

 Vectors are organisms that transmit parasites,
  usually insects and ticks. These vectors are
  essential for the development of the infectious
  stage of the lifecycle of some parasites before
  they are transmitted to the next host (eg
  mosquitoes transmit the malarial parasite).
 Some of these vectors therefore have a double
  aspect of parasitism.
Host–parasite co-evolution

 As a host evolves defence mechanisms against
  the parasite, so the parasite evolves
  mechanisms to overcome the defence.

 This has led to many host–parasite interactions
  being species specific, in other words most
  parasites only infect a particular host.
 Types of parasite

 Parasites may be animals or plants.
 Viruses are parasites.
 Unicellular organisms, such as some
  bacteria, are parasites.
 Multicellular organisms, such as worms
  and insects, may be parasites.
Mutualism is a specialist
relationship between two                A                                 B
organisms that is beneficial to both. Benefits                        Benefits

                                            Herbivores have special
Lichens are composed of fungus              cellulose-digesting micro-
and algae in a beneficial                   organisms in their stomachs.
Leguminous plants and
Rhizobium bacteria

        These bacteria live freely in
        They are able to infect the
         roots of leguminous plants
         and form nodules.
        Inside the nodules bacteria
         obtain energy from their
         host and supply nitrogen
         compounds in return.
Fungi and algae (lichens)

                  Lichens are able to colonise
                   areas where other plants are
                   unable to grow.
                  The fungal part of the lichen
                   produces tough cellulose walls
                   and secretes mineral-digesting
                   chemicals while receiving
                  The algal part of the lichen
                   photosynthesises and receives
                   protection and essential
Herbivores and gut bacteria

                 Herbivores cannot
                  produce cellulose-
                  digesting enzymes.
                 Bacteria which can do
                  this are found in the
                  alimentary tract of
                 These bacteria receive
                  food and shelter, and
                  aid plant digestion.
Ants and aphids

               Some species of ants ‘farm’
                aphids. They move them from
                plant to plant and protect them
                from predators.
               In return the aphids secrete a
                sugary liquid that the ants
Terrestrial plants and pollinating

                   Many plants have evolved
                    special flower shapes which
                    allow only one kind of animal
                    near them.
                   The animal depends on the
                    nectar it receives as a reward
                    for its visit.
                   The plant depends on the
                    animal to transport its pollen
                    between flowers.

            Parasitism   Mutualism

Species A      +            +
Species B       -           +
 Many thanks to R. Purdie from Sanquhar
  Academy for the use of some materials.

To top