A-Level Biology October/November 2024 Q9(a): Usnea is composed of a mixture of two types of cell: • photosynthetic cells that are cl…

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  Both Protoctista and Fungi belong to the domain Eukarya, meaning their cells are eukaryotic. This means they possess a true, membrane-bound nucleus and other membrane-bound organelles such as mitochondria and 80S ribosomes.

  Kingdom Protoctista: This is a diverse group of organisms that do not fit into the other eukaryotic kingdoms. A key feature is their variation; they can be either unicellular or multicellular. Their mode of nutrition also varies, with some being autotrophic (e.g., algae) and others being heterotrophic (e.g., Amoeba).

  Kingdom Fungi:

  • The cell wall is made of chitin.
  • The body of a fungus is typically composed of a network of thread-like structures called hyphae, which collectively form a mycelium.
  • Fungi are heterotrophic, obtaining nutrients via saprotrophic or parasitic methods. They secrete extracellular enzymes to digest food externally before absorbing the nutrients.
  • They store carbohydrates as glycogen.

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  How the marks are awarded

  • B1 — Stating that both kingdoms are composed of eukaryotic cells, which directly addresses the 'both, eukaryotic' marking point.
  • B1 — Providing an example of a eukaryotic feature, such as having a 'true, membrane-bound nucleus' or 'membrane-bound organelles'.
  • B1 — Describing a way in which Protoctista vary, giving both sides of the variation, for example, that they can be 'unicellular or multicellular'.
  • B1 — Stating a correct feature of Fungi, such as the cell wall being 'made of chitin' or that they are 'heterotrophic'.

  Common mistakes

  • Confusing features of Fungi with Plants, for example, stating that fungal cell walls are made of cellulose or that they store starch.
  • Providing vague descriptions for Protoctista, such as 'they are simple organisms', instead of specific, contrasting features like 'unicellular or multicellular' or 'autotrophic or heterotrophic'.
  • Incorrectly stating that Fungi are prokaryotic or lack a nucleus.
  • Only describing features for one of the two kingdoms, thereby limiting the maximum marks achievable to 2 or 3.

  Examiner tip: When learning the characteristics of the five kingdoms, create a comparative table to clearly organise and contrast key features like cell type, cell wall composition, nutrition, and body organisation.

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