HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
Diseases > Parasites
Welcome, Guest!
Login with Facebook | Login
Video tutorials

Supplemental Videos

Click here to watch video tutorials for exam board specific videos by A* students

Parasites and Parasitism

  • Parasites (endo-) or (ecto-) feed on living organisms while causing harm
  • They gain benefits from them (e.g. unlimited supply of nutrients, H2O, constant temp)
  • Can be bacteria, protoctists, viruses, fungi, arthropods, platyhelminthes
  • Have structural and functional adoptions to their lifecycle
    • Specialised reproductive strategies
    • Modification of mouthparts, digestive enzymes and enzymes to allow attachment to the host and utilisation of host's food supply, blood or tissues
    • Resist attack by immune system
    • Reduction of unnecessary sensory organs and locomotory organs in the adult stage, as they live in protected, optimum conditions

Plasmodium (Malarial parasite)

  • Single celled, intracellular parasite of the kingdom Proctista (class protozoa)
    • Development requires 2 hosts for its lifecycle
    • Female mosquito (unaffected) and human (affected)
    • Parasite is transferred by bites from the female mosquito
    • Mosquito releases anti-coagulants when piercing skin / prevents blot clotting
    • Parasites is injected into human in the form of sporozoites
    • If the human is infected with the malarial parasite, the mosquito takes up Plasmodium gametes in the blood on which the mosquito feeds
  • Asexual reproduction phase
    • Occurs in human liver and red blood cells
    • Produces enormous quantities of parasites, merozoite stage
  • Sexual phase
    • Occurs in the female mosquito
  • [EXAM] Lives inside the liver and red blood cells
    • Survive for long periods because it is protected from the immune system
      • Inside the blood or liver cell; and
      • Surface antigen changes rapidly
    • Has no (need for) locomotory structures because
      • It is transported via blood stream
      • No need to move to find food
    • Has no (need for) mechanism for regulating its water content because
      • cytoplasm has same water potential as blood cell
  • Symptoms and Treatment
    • Red blood cells are destroyed causing anaemia and fever
    • Temp peaks correspond to bursting of red blood cells
    • Anti-malarial drug chloroquine lowers fever; reduces number of parasites
    • Increasing resistance to the drug / combined drugs used
    • High mutation rate / memory cells or vaccines useless


  • Infection of Human
    • Infected female mosquitoes are feeding human blood
    • Secretes salvia which contains anticoagulants (anti blood-clotting agents)
    • Sporozoites (young malarial parasites) in the salvia enter blood
  • Latent Period
    • Parasites migrate to liver cells / undergo asexual reproduction / develop into merozoites
    • Merozoites infect red blood cells / undergo asexual reproduction / produce more merozoites
  • Outbreak
    • Merozoites burst out of red blood cells to infect more cells
    • This outburst is associated with bouts of fever
    • Some merozoites develop into gametocytes
  • Infection of Female Mosquitos
    • Gametocytes are taken up by female mosquitos that feed on the infected person's blood
    • Inside the female mosquito, gametes produced by those gametocytes fuse (sexual reproduction phase)
    • Results in zygotes which develop into sporozoites
    • Sporozoites move into the salivary glands of the female mosquito where they can be injected into another person
  • Cycle starts again

Schistosoma (endoparasite)

  • Name of a genus of parasitic flatworms; causes the disease Bilharzia in humans
  • Two hosts for lifecycle: fresh water snails (vector) and humans
  • Penetrate the skin via enzymes damaging the host cell membrane
  • Invades blood vessels, veins of the bladder region, abdomen and pelvis (Schistosoma haematobium) or the intestine (S. mansoni and S. japanicum)
  • Adult schistosomes exist as separate males and females usually found attached to one another, to ensure mating and sexual reproduction
  • Fertilised eggs are deposited in the blood vessels of the host
    • Huge number of eggs cause the vessels to rupture
    • Eggs are discharged into the intestine to reach the outside of the body
    • They work their way into adjacent organs (bladder, large intestine) for their way out
    • Within tissues they cause severe inflammation, blood in the urine (anaemia)
  • Adult worm lacks locomotion and sensory organs
    • Unnecessary inside the host's body
    • Surrounded by nutrients in constant optimum conditions
  • Adult worm manipulates host's immune system
    • Parasite coats itself with molecules from the host's red blood cells
    • The host recognises those cells as its own


  • Eggs pass out of an infected human via the urine or faeces
  • Hatch into minute ciliated larvae (miracidia) / capable of swimming until they find and burrow into a water snail and grow into a structure called sporocyst
  • Asexual reproduction occurs, producing free swimming larvae, cercariae, which borrow, aided by the secretion of digestive enzymes, into the human via the skin or feet
  • Larvae migrate to the bladder or gut region where they may exist for many years producing a vast number of parasites