ka5: symbiosis

• Symbiosis is an ecological relationship between organisms of two different species that live in direct contact with one another.  These are intimate relationships that have co-evolved over millions of years. 

   

  There are two types of symbiosis:

  • Parasitism – one organism benefits at the expense of the other organism.
  • Mutualism – both organisms benefit from the relationship.
• One organism, the parasite, obtains energy or nutrients from another organism, the host.  The host is harmed by the loss of these resources to the parasite. 

   

  Parasites often possess a limited metabolism and cannot survive outside the body.  For example, tapeworms do not have a digestive system since the host has already digested the food.  Therefore an adult tape worm cannot survive if it loses contact with its host.
• Transmission of parasites to new hosts

  1. Direct contact

  2. Release of resistant stages

  3. Use of a vector
• Direct contact


  Passed from person to person during physical contact
• Release of resistant stages


  During certain stages of the life cycle the parasite is able to survive adverse environmental conditions for long periods until they come into contact with a new host
• Release of resistant stages

  • Cat flea larva and pupa are resistant but the adult and eggs can only survive on a host
• Use of a vector

  Another organism, the vector, carries the parasite and transmits it to the host
• Use of a vector

  • Mosquitoes (vector) carry Plasmodium (causing malaria) from human to human
• There are two types of parasitic life cycle:

  • Direct: Eggs are shed and pass to a new member of the host species.  This involves only one species of host, e.g. fleas and lice.

   

  • Indirect: involves two different host species. Use a primary host species as the site for sexual reproduction. It then employs an intermediate (secondary) host species to allow them to complete their lifecycle.  A new primary host is infected when it is invaded or consumes the infected intermediate host.
• Mutualistic relationship


  An interdependent relationship where both partner species benefit
• Cellulose digestion in herbivores

  A mutualistic relationship between herbivores and cellulose-digesting microbes
• Herbivores


  • Lack the genes required for the synthesis of cellulose-digesting enzymes
  • Have gut containing special chambers with cellulose-digesting microbes
• Cellulose-digesting microbes

  • Produce enzymes that digest cellulose into simple sugars
  • Receive protection, warmth and a constant supply of food from herbivores
• Coral polyps


  An animal that resembles a sea anemone and has a mutualistic relationship with a unicellular alga called zooxanthella
• Coral polyps

  • Feed on organic debris and microbes in the sea water
  • Use as much as 80% of the sugar produced by the alga for energy
• Zooxanthella

  • A unicellular alga that lives within and between coral polyp cells
  • Produce sugar through photosynthesis
  • Receive a secure habitat and a supply of the polyp’s nitrogenous waste
• Symbiosis — co-evolved intimate relationships between members of two different species.
• Types of symbiotic relationship — parasitism and mutualism.
• A parasite benefits in terms of energy or nutrients, whereas its host is harmed by the loss of these resources.
• Parasites often have limited metabolism and cannot survive out of contact with a host.
• Transmission of parasites to new hosts using direct contact, resistant stages and vectors.
• Some parasitic life cycles involve intermediate (secondary) hosts to allow them to complete their life cycle.
• Mutualism Both mutualistic partner species benefit in an interdependent relationship.