Describe how the survival of some organisms is dependent on other species,…

The idea

Some organisms cannot survive alone: their survival depends on another species. In parasitism, one organism (the parasite) lives on or in a host and gains nutrients from it, while the host is harmed. Examples include a tapeworm living in a mammal's gut and taking digested food, or mistletoe drawing water and minerals from a tree. In mutualism, both species benefit. Examples include bees collecting nectar while transferring pollen between flowers, and nitrogen-fixing bacteria in root nodules supplying usable nitrogen compounds while receiving sugars. Recognise the direction of benefit: parasite gains and host loses; both mutualists gain. Read each option and ask which species benefits and which is harmed.

What earns the marks

• Identifies parasitism as a relationship in which the parasite benefits and the host is harmed.
• Identifies mutualism as a relationship in which both species benefit.
• Selects an example where one organism lives on or in a host and takes nutrients from it.
• Selects an example where each species gains something it needs, such as food or a place to live.
• Distinguishes a mutualistic pairing from a parasitic one by stating which species gains and which loses.

Common mistakes and how to put them right

• Treats mutualism as a one-way benefit.

  Put it right: Check that both species gain before choosing the option.

• Assumes any close relationship is parasitism.

  Put it right: Look for harm to the host before deciding.

• Reverses the roles so the host appears to gain.

  Put it right: State clearly that the parasite gains and the host is harmed.

• Ignores the named examples and picks a familiar animal.

  Put it right: Match the option to the benefit pattern described.

Exam tips

• Underline the words parasitism or mutualism in the question before reading the options.
• For each option, write a quick plus or minus beside each species to show who gains and who loses.
• Eliminate options where both species gain but the question asks about parasitism, and vice versa.
• Check that the chosen example involves two different species, not one organism alone.