IGCSE Biology / The variety of living organisms
Fungi, protoctists and bacteria revision notes
Edexcel International GCSE Biology (4BI1), chapter 2: The variety of living organisms, specification point 1.2, 1.3. Free lesson.
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Fungi, protoctists and bacteria are separated by their cell structures and how they obtain food. Knowing whether a cell has a nucleus is a useful first step, especially when identifying bacteria.
How fungi are built
Most fungi have a body made from hyphae, which are long, thin threads. A mass of hyphae is called a mycelium. Their cell walls contain chitin, not cellulose, and they have no chloroplasts, so they cannot photosynthesise. Hyphae have many nuclei in a shared cytoplasm, rather than being divided neatly into separate cells. Fungi store carbohydrate as glycogen, as animals do, rather than as starch. Mucor has a thread-like body. Yeast is a fungus but is single-celled and has no hyphae, so not all fungi are multicellular.
How fungi feed
Fungi obtain organic food from other material by saprotrophic nutrition. They digest food outside their body, then absorb the dissolved products.
What makes an organism a protoctist
Protoctists are mostly single-celled organisms that have a nucleus. That nucleus separates them from bacteria. Many live in water or damp places. Protoctists are a varied group that do not fit into the plant, animal or fungus kingdoms. Plant-like protoctists, such as Chlorella, have chloroplasts and photosynthesise. Animal-like protoctists, such as Amoeba, have no chloroplasts and take in food. Some protoctists cause disease. Plasmodium causes malaria.
How to identify a bacterial cell
Bacteria are single-celled prokaryotes: their DNA is not enclosed in a nucleus. Fungi and protoctists have nuclei, so they are eukaryotes, like plants and animals. Bacteria still have DNA, but it lies loose in the cytoplasm. The circular chromosome is one loop of DNA. Plasmids are small, separate rings of DNA. The cell wall is not made of cellulose. The cell membrane controls what enters and leaves, and the cytoplasm is where reactions happen. Bacteria have no mitochondria or chloroplasts. They respire using the cell membrane. Lactobacillus is a named example that turns milk…
How to tell the groups apart
Check for a nucleus first: if there is none, the organism is a bacterium. Fungi and bacteria both have cell walls, but their walls differ. Protoctists are the third group and do not belong in either comparison column. Fungi: usually hyphae forming a mycelium, except for single-celled yeast; chitin cell wall; saprotrophic feeding; Mucor and yeast are examples. Bacteria: always single-celled; no nucleus, but a circular chromosome; cell wall that is neither cellulose nor chitin; feeding varies and some photosynthesise; Lactobacillus is an example.