Organisms I

Biologists need a way to group the ~2 million named species. The simplest model at MYP level is the five-kingdom system.

Kingdom	Cell type	Key features	Examples
Animals	Eukaryote, no cell wall	Multicellular, eat (heterotrophic), can usually move	Humans, fish, insects
Plants	Eukaryote, cellulose cell wall	Multicellular, photosynthesise (chloroplasts), don't move	Oak tree, moss, grass
Fungi	Eukaryote, chitin cell wall	Decomposers / saprotrophs, don't photosynthesise, hyphae	Mushroom, yeast, mould
Protists	Eukaryote	Mostly single-celled. Mixed bag — some plant-like (algae), some animal-like (amoeba)	Amoeba, paramecium
Prokaryotes	PROKARYOTE — no nucleus	Single-celled, very small (1-10 μm), no membrane-bound organelles	E. coli, Streptococcus

Eukaryote vs Prokaryote is the deepest distinction:

• Eukaryote = has a NUCLEUS and other membrane-bound organelles (mitochondria, chloroplasts).
• Prokaryote = NO nucleus. DNA floats free in the cytoplasm.

Bacteria are the only prokaryotes. The other four kingdoms are all eukaryotes.

A dichotomous key (Greek 'dichotomos' = cut in two) helps identify an unknown organism. Each step gives TWO choices.

Example key for 4 organisms:

1. Does it have a backbone?
   • Yes → go to 2
   • No → INSECT (or other invertebrate)
2. Does it have feathers?
   • Yes → BIRD
   • No → go to 3
3. Does it have fur?
   • Yes → MAMMAL
   • No → FISH or REPTILE (next question would split them)

To use a key:

• Start at question 1.
• Pick the answer that matches your specimen.
• Follow the direction it sends you.
• Continue until you reach the name.

A good MYP investigation: design a key for 5 of your classmates using observable, neutral, unambiguous features (e.g. hair length above shoulder vs below; wears glasses or not). Test it on your partner.

Before DNA sequencing, biologists grouped organisms by appearance and anatomy. With DNA we can now compare GENES directly. Sometimes this confirms the old groupings; sometimes it overturns them:

• Birds were once a class of their own. DNA + fossil evidence now shows that birds are technically dinosaurs (theropod dinosaurs that survived the extinction).
• Fungi were once grouped with plants (because they don't move and live in soil). DNA shows fungi are actually more closely related to ANIMALS than plants.
• Archaea, a group of bacteria-like single cells, was found to be so genetically different from bacteria that scientists now use SIX kingdoms (or three DOMAINS: Bacteria, Archaea, Eukarya).

MYP criterion D: classification is a HUMAN-MADE model that we update as new evidence arrives. It's a great example of how science is provisional, not 'final truth'.