# Variety of living organisms

> Edexcel International GCSE Biology · Edexcel IGCSE Biology (4BI1)
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s1-variety-of-living-organisms/

This guide covers classification, key traits and mark-scheme required named examples of eukaryotes, prokaryotes, viruses and pathogens for Edexcel IGCSE Biology 4BI1 Section 1.2-1.4, including all core defining features tested in exams.

**Prerequisites:** [Knowledge of the 8 life processes (Edexcel IGCSE Biology S1_T01)](https://www.owlsprep.com/study/edexcel-igcse-biology-s1-life-processes/)

## Learning objectives

- Describe common features and named examples of the four eukaryotic organism groups
- Describe key traits and specification examples of prokaryotic bacteria
- Define pathogen and list all classes of pathogenic organisms
- Explain why viruses are not classified as living and list their core features

## Eukaryotic Organism Groups

**Eukaryote** — Organism whose cells contain a membrane-bound nucleus and other membrane-bound organelles (e.g. chloroplasts, mitochondria). There are four core eukaryotic groups tested in this specification.

| Group | Key Features | Mark Scheme Approved Examples |
| --- | --- | --- |
| Plants | Multicellular, have chloroplasts for photosynthesis, cellulose cell walls, store carbohydrate as starch or sucrose | Maize (cereal), pea/bean (herbaceous legume) |
| Animals | Multicellular, no chloroplasts, no cell walls, have nervous coordination, can move independently, store carbohydrate as glycogen | Human (mammal), housefly/mosquito (insect) |
| Fungi | Cannot photosynthesise, body is mycelium made of hyphae (or single-celled), chitin cell walls, use saprotrophic nutrition, store carbohydrate as glycogen | Mucor (hyphal structure), yeast (single-celled) |
| Protoctists | Microscopic single-celled organisms, may have animal-like or plant-like features | Amoeba (animal-like, pond water), Chlorella (plant-like, has chloroplasts), Plasmodium (pathogenic, causes malaria) |

> **tip**
>
> Cell wall composition is a very common 1-mark exam question: plants = cellulose, fungi = chitin, animals = no cell wall. Memorise these exactly.

**Worked example:** A student observes a multicellular organism with no cell walls that stores carbohydrate as glycogen. Identify the eukaryotic group it belongs to, and give one mark-scheme approved named example of this group.

1. 1. Match the observed features to group traits: no cell walls, glycogen storage, multicellular = animal group.
2. 2. Select a valid specification example for animals: e.g. human, housefly or mosquito.
3. Final answer: Animal group, example = human.

*Calculator:* allowed

## Prokaryotic Organisms (Bacteria)

**Prokaryote** — Microscopic single-celled organism that lacks a membrane-bound nucleus, and instead has a single circular chromosome of DNA in the cytoplasm, plus small plasmids. All prokaryotes in this specification are bacteria.

Bacteria have a cell wall, cell membrane and cytoplasm. Some can carry out photosynthesis, but most feed off living or dead organic matter. You must memorise the two named examples: *Lactobacillus bulgaricus* (rod-shaped, used to make yoghurt) and *Pneumococcus* (spherical, pathogenic, causes pneumonia).

> **warning**
>
> Never state that bacteria have a nucleus. This is one of the most common mark-loss mistakes for this topic, explicitly tested in multiple choice questions.

**Worked example:** State three defining features of prokaryotic bacteria, and name one specification example of a pathogenic bacterium.

1. 1. List three valid defining features: no membrane-bound nucleus, single circular DNA chromosome, plasmids present, cell wall present, single-celled.
2. 2. Select the pathogenic bacterial example from the specification: *Pneumococcus*.
3. Final answer: Features = no nucleus, circular DNA, plasmids present; example = *Pneumococcus*.

*Calculator:* allowed

## Pathogens and Non-Living Viruses

**Pathogen** — Disease-causing organism or particle. Pathogens may be fungi, bacteria, protoctists or viruses.

Viruses are **not classified as living organisms**. They are small particles (smaller than bacteria), parasitic, and can only reproduce inside living host cells. They have no cellular structure, only a protein coat surrounding one type of nucleic acid (either DNA or RNA, never both). Named examples: tobacco mosaic virus (TMV, discolours tobacco leaves by preventing chloroplast formation), influenza virus (causes flu), HIV (causes AIDS).

> **warning**
>
> You must explicitly state that viruses are non-living in exam answers. Mark schemes will reject answers that refer to viruses as living organisms, and will also reject claims that viruses contain both DNA and RNA.

**Worked example:** Explain why viruses are not classified as living organisms, and give one named example of a virus that infects humans.

1. 1. Reference key non-living traits: viruses have no cellular structure, cannot carry out life processes independently, and can only reproduce inside a living host cell.
2. 2. Select a valid human virus example from the specification: influenza virus or HIV.
3. Final answer: Viruses lack cellular structure and cannot perform life processes without a host, so they are non-living. Example = HIV.

*Calculator:* allowed

## Recap and Concept Check

**Check your understanding**

1. Which group of organisms uses saprotrophic nutrition?

   - Plants
   - Fungi
   - Protoctists
   - Animals

   *Why:* Fungi secrete digestive enzymes onto food extracellularly then absorb the broken-down products, which is saprotrophic nutrition.

2. Which of the following is a prokaryote?

   - Amoeba
   - Yeast
   - *Lactobacillus bulgaricus*
   - TMV

   *Why:* *Lactobacillus bulgaricus* is a bacterium, a prokaryote. Amoeba and yeast are eukaryotes, TMV is a non-living virus.

3. Which of the following is a correct feature of viruses?

   - They have a cellulose cell wall
   - They contain both DNA and RNA
   - They are living organisms
   - They can only reproduce inside host cells

   *Why:* Viruses are parasitic and require a living host cell to reproduce. They have no cell wall, only one type of nucleic acid, and are non-living.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Referring to Mucor as a parasitic fungus
  - Why it fails: Mucor feeds on dead organic matter via saprotrophic nutrition, not living hosts. Mark schemes reject the term parasitic for Mucor.
  - Correct: Explicitly refer to Mucor's nutrition as saprotrophic.
- **Wrong:** Stating bacteria have a nucleus
  - Why it fails: Prokaryotes like bacteria lack a membrane-bound nucleus; their DNA is a circular chromosome in the cytoplasm. This is a frequent MCQ discriminator.
  - Correct: State bacteria have no nucleus, but have a single circular DNA chromosome and plasmids.
- **Wrong:** Classifying viruses as living organisms
  - Why it fails: Viruses have no cellular structure and cannot carry out life processes independently, so they are not classified as living.
  - Correct: Explicitly note viruses are non-living particles, not organisms, in all exam answers.
- **Wrong:** Stating viruses contain both DNA and RNA
  - Why it fails: Each virus only contains one type of nucleic acid, either DNA or RNA, never both. Mark schemes deduct marks for this error.
  - Correct: Specify viruses have a protein coat surrounding either DNA or RNA.
- **Wrong:** Using non-approved examples for organism groups
  - Why it fails: Edexcel mark schemes only award credit for the named examples listed in the specification.
  - Correct: Memorise the exact specification examples (e.g. maize/peas for plants, Mucor/yeast for fungi) and use these exclusively in answers.

## Cheatsheet

| Group Type | Key Defining Features | Mark Scheme Approved Examples |
| --- | --- | --- |
| Plants | Multicellular, chloroplasts, cellulose cell walls, store starch/sucrose | Maize, pea/bean |
| Animals | Multicellular, no cell walls, no chloroplasts, store glycogen | Human, housefly/mosquito |
| Fungi | Hyphal mycelium (or single-celled), chitin cell walls, saprotrophic nutrition, store glycogen | Mucor, yeast |
| Protoctists | Microscopic single-celled | Amoeba, Chlorella, Plasmodium |
| Bacteria (Prokaryotes) | Single-celled, no nucleus, circular DNA, plasmids | *Lactobacillus bulgaricus*, *Pneumococcus* |
| Viruses | Non-living, protein coat, DNA/RNA only, reproduce in host cells | TMV, influenza, HIV |
| Pathogens | Disease-causing, may be fungi, bacteria, protoctists, viruses | *Pneumococcus*, Plasmodium, HIV |

## What's next

Now that you have mastered the variety of living organisms, you are ready to move on to cell structure and organisation, where you will explore the ultrastructure of cells from each of the groups you have just learned, including the function of key organelles like chloroplasts, cell walls and nuclei. This content links directly to Section 2 of the Edexcel IGCSE Biology specification, and is frequently tested alongside classification questions in both multiple choice and extended response exam questions. You can also test your knowledge with topic-specific past paper questions to reinforce your recall of mark-scheme approved terms and examples.

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