What are the main differences between prokaryotic and eukaryotic organisms?

Prokaryotic organisms, such as bacteria, lack a nucleus and membrane-bound organelles, while eukaryotic organisms, including plants and animals, have a defined nucleus and organelles. This distinction is fundamental in the study of organisms in the IB MYP Science curriculum, as it helps students understand the complexity and diversity of life forms.

How do organisms adapt to their environments?

Organisms adapt to their environments through evolutionary processes that enhance their survival and reproduction. These adaptations can be structural, such as the thick fur of polar bears for insulation, behavioral, like birds migrating to warmer areas during winter, or physiological, such as the ability of some desert plants to store water. Understanding these adaptations is crucial in the IB MYP Science curriculum to appreciate the diversity of life on Earth.

What role do organisms play in ecosystems?

Organisms play various roles in ecosystems, including producers, consumers, and decomposers. Producers, like plants, convert sunlight into energy through photosynthesis. Consumers, such as animals, feed on other organisms, and decomposers, like fungi and bacteria, break down dead matter, recycling nutrients back into the ecosystem. This interconnectedness is a key concept in the IB MYP Science curriculum, highlighting the balance and sustainability of ecosystems.

Why is biodiversity important in ecosystems?

Biodiversity is crucial because it ensures ecosystem resilience, providing stability and the ability to recover from disturbances. High biodiversity increases the variety of resources and habitats, supporting a wide range of organisms. In the IB MYP Science curriculum, students learn that biodiversity contributes to ecosystem services, such as pollination, water purification, and climate regulation, which are vital for human survival.

How do human activities impact organisms and their habitats?

Human activities, such as deforestation, pollution, and climate change, significantly impact organisms and their habitats by altering natural environments and reducing biodiversity. These changes can lead to habitat loss, species extinction, and ecosystem imbalance. The IB MYP Science curriculum emphasizes the importance of understanding these impacts to promote conservation efforts and sustainable practices that protect organisms and their ecosystems.

Why is "Saying whales/dolphins are fish." a common mistake in Organisms II?

Why it happens: They live in water. How to avoid it: They produce MILK and give LIVE BIRTH → mammals. Habitat is not what defines a class — body covering and reproduction do.

Why is "Saying bats are birds." a common mistake in Organisms II?

Why it happens: They fly. How to avoid it: Bats have FUR and produce MILK → mammals. Birds are defined by FEATHERS.

Why is "Writing binomial as 'Homo Sapiens' or 'homo sapiens'." a common mistake in Organisms II?

Why it happens: Forgetting capitalisation rule. How to avoid it: ALWAYS: Capital G in genus, lowercase species, italics. Example: *Homo sapiens*.

OrganismsIB MYP Sciences (Years 1-5)

Organisms II

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Short Study Notes — Organisms II

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Organisms II — IB MYP Sciences: vertebrate groups and invertebrate basics

Year-3 spiral on classification within the Animal kingdom. This MYP Sciences note covers the five vertebrate classes, key invertebrate groups, and the binomial naming system.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — List the five vertebrate classes and one feature of each.
MYP Sciences A — Distinguish vertebrate from invertebrate.
MYP Sciences A — Apply binomial naming convention correctly.
MYP Sciences C — Classify a new organism using its features.

The five vertebrate classes

Fish, amphibians, reptiles, birds, mammals.

Within the Animal kingdom, animals with a BACKBONE (vertebrae running down the spine) are called vertebrates. There are five main classes.

Class	Body covering	Eggs	Body temp	Examples
Fish	Scales	Soft eggs, in water	Cold-blooded (ectotherm)	Salmon, shark
Amphibians	Moist, smooth skin	Soft eggs, in water	Cold-blooded	Frog, salamander
Reptiles	Dry scales	Leathery eggs, on land	Cold-blooded	Snake, lizard, turtle
Birds	Feathers	Hard-shelled eggs	WARM-blooded (endotherm)	Eagle, sparrow
Mammals	Hair / fur	Live young (most); produce MILK	WARM-blooded	Human, dog, whale

The five vertebrate classes. The most reliable distinguishing trait is the body covering and method of reproduction.

Cold-blooded (ectotherm): body temperature matches the surroundings. Has to bask in the sun to warm up. Uses little food energy. Most fish, amphibians, reptiles.

Warm-blooded (endotherm): keeps its body at a constant temperature using metabolism. Needs lots of food. Birds and mammals.

Mammals: hair + milk (always — even whales and dolphins).
Birds: feathers (always) — only group with feathers.
Reptiles: dry scaly skin, lay eggs on land.
Amphibians: moist skin, eggs in water.
Only birds and mammals are warm-blooded.

Invertebrates

97% of animal species have no backbone.

Most animal species — about 97% — are INVERTEBRATES. The main groups at MYP level:

Arthropods (by far the biggest group). Jointed legs, exoskeleton (chitin). Includes:
- Insects (6 legs, 3 body parts — head/thorax/abdomen)
- Arachnids (8 legs — spiders, scorpions)
- Crustaceans (often 10 legs — crabs, lobsters)
- Myriapods (lots of legs — centipedes, millipedes)
Molluscs: soft body, often with a shell. Snails, slugs, octopuses (no shell), clams.
Annelids (segmented worms): earthworm, leech.
Cnidarians (stinging cells): jellyfish, coral, sea anemone.
Echinoderms (spiny-skinned): starfish, sea urchin.

The arthropod group is so successful because the EXOSKELETON gives protection, supports the body on land, and prevents drying out.

Arthropods = jointed legs + exoskeleton (insects, spiders, crabs).
Insects: 6 legs, 3 body parts.
Molluscs (soft body), Annelids (segmented worms), Cnidarians (stinging), Echinoderms (spiny).

Binomial naming

Every species has a 2-part Latin name.

Common names are unreliable — a 'robin' in the UK is a different species from a 'robin' in the US. Scientists use a Latin two-part name that is the same worldwide. This is binomial nomenclature (introduced by Linnaeus in 1758).

Rules:

First word = GENUS. Capital letter (e.g. Homo).
Second word = species. Lowercase (e.g. sapiens).
Italicise the whole name (or underline if writing by hand).
After first use, abbreviate the genus to the first letter — H. sapiens.

Examples:

Homo sapiens — modern humans.
Panthera leo — lion. Panthera tigris — tiger. Same genus.
Canis lupus — wolf. Canis familiaris (or C. lupus familiaris) — domestic dog.
Quercus robur — English oak.

The genus tells you the close relative; the species name uniquely identifies the organism. Species in the same genus often share more features (and DNA) than species in different genera.

Two words: Genus (capital) + species (lowercase).
Italicise the whole name.
Same name worldwide — avoids common-name confusion.

How it’s examined

Criterion A: name vertebrate classes / write binomial correctly. Criterion C: classify a new organism using features given.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Organisms II

Step-by-step solutions to past-paper-style questions on organisms ii, written exactly the way a tutor would explain them at the board.

1Place the organism in a vertebrate class
Getting started• vertebrate
Question
Dolphins live in the sea, have smooth skin, give birth to live young and produce milk. Which class are they?
Step-by-step solution
1. Step 1
  Live young + milk = the signature feature of MAMMALS.
2. Step 2
  Smooth skin (not scales, not feathers) consistent with mammals (hair is reduced in marine mammals).
Answer
Mammals. Living in water doesn't make them fish — milk-feeding is the deciding feature.
2Write the binomial name correctly
Getting started• binomial
Question
A student writes 'homo Sapiens' for humans. List two errors and write the correct form.
Step-by-step solution
1. Step 1
  Error 1: 'homo' should be Capitalised → genus is ALWAYS capitalised.
2. Step 2
  Error 2: 'Sapiens' should be lowercase → species is ALWAYS lowercase.
3. Step 3
  Also italicise (or underline when handwritten).
Answer
Correct: Homo sapiens (Genus capital, species lowercase, italics).
3Insect or spider?
Building confidence• invertebrate
Question
Explain how to tell an insect from a spider just by looking at one.
Step-by-step solution
1. Step 1
  Count the LEGS. Insects = 6. Spiders = 8.
2. Step 2
  Count BODY PARTS. Insects have 3 (head, thorax, abdomen). Spiders have 2 (cephalothorax + abdomen).
3. Step 3
  Insects usually have antennae; spiders don't.
Answer
Insect = 6 legs, 3 body parts, antennae. Spider = 8 legs, 2 body parts, no antennae.

Key Definitions and Keywords — Organisms II

Definitions to memorise and the exact keywords mark schemes credit for organisms ii answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Vertebrate
Examiner keyword
An animal with a backbone (vertebral column). 5 classes: fish, amphibians, reptiles, birds, mammals.
Invertebrate
Examiner keyword
An animal without a backbone. ~97% of animal species.
Ectotherm (cold-blooded)
Examiner keyword
An organism whose body temperature depends on its surroundings (fish, amphibians, reptiles).
Endotherm (warm-blooded)
Examiner keyword
An organism that maintains a constant body temperature using its own metabolism (birds, mammals).
Binomial nomenclature
Examiner keyword
The two-part Latin naming system. Genus (capitalised) + species (lowercase), italicised.

Common Mistakes and Misconceptions — Organisms II

The traps other students keep falling into on organisms ii questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying whales/dolphins are fish.
Why it happens
They live in water.
How to avoid it
They produce MILK and give LIVE BIRTH → mammals. Habitat is not what defines a class — body covering and reproduction do.
✕Saying bats are birds.
Why it happens
They fly.
How to avoid it
Bats have FUR and produce MILK → mammals. Birds are defined by FEATHERS.
✕Writing binomial as 'Homo Sapiens' or 'homo sapiens'.
Why it happens
Forgetting capitalisation rule.
How to avoid it
ALWAYS: Capital G in genus, lowercase species, italics. Example: Homo sapiens.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Organisms II — frequently asked questions

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Organisms II

Short Study Notes in the form of Slides

Short Study Notes — Organisms II

Detailed Notes

What you’ll learn

How it’s examined

1Place the organism in a vertebrate class

2Write the binomial name correctly

3Insect or spider?

Vertebrate

Invertebrate

Ectotherm (cold-blooded)

Endotherm (warm-blooded)

Binomial nomenclature

✕Saying whales/dolphins are fish.

✕Saying bats are birds.

✕Writing binomial as 'Homo Sapiens' or 'homo sapiens'.

Practice questions

Past paper style quiz

Assess your understanding

Organisms II — frequently asked questions