The Nature and Variety of Living Organisms

Characteristics of Living Organisms – Revision Notes

Edexcel IGCSE Biology | Topic 1: The Nature and Variety of Living Organisms

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Key Definitions and Terminology

• **Movement** – an action by an organism or part of an organism causing a change of position or place
• **Respiration** – the chemical reactions in cells that break down nutrient molecules and release energy for metabolism
• **Sensitivity** – the ability to detect and respond to changes in the internal or external environment (stimuli)
• **Growth** – a permanent increase in size and dry mass by an increase in cell number, cell size, or both
• **Reproduction** – the process of producing new organisms of the same species, either sexually or asexually
• **Excretion** – the removal from organisms of the waste products of metabolism (chemical reactions in cells), including toxic substances and substances in excess of requirements
• **Nutrition** – the taking in of materials (nutrients) for energy, growth, and development; organisms either make their own food (autotrophic) or take in organic substances (heterotrophic)

> 💡 Memory aid: Use the mnemonic MRS NERG (or MRS GREN) — Movement, Respiration, Sensitivity, Nutrition, Excretion, Reproduction, Growth — to remember all seven life processes.

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Main Concepts

1. All Living Organisms Share Seven Characteristics

All organisms — whether they are plants, animals, fungi, bacteria, or protoctists — must carry out all seven life processes. If something carries out only some of these, it is not classified as a living organism (e.g., a virus does not carry out all seven independently).

2. Movement Is Not Just Locomotion

• **Animals** move their whole bodies from place to place (locomotion).
• **Plants** show movement too, but it is usually movement of **parts** of the organism (e.g., roots growing towards water, shoots growing towards light).
• Movement does not require legs or muscles — it simply requires a change in position.

3. Respiration Is NOT Breathing

• Respiration is a **chemical process** that occurs inside every living cell to release energy from glucose.
• It should not be confused with **gas exchange** (breathing/ventilation), which is the movement of gases into and out of the body.
• All living cells respire continuously, including plant cells.

4. Sensitivity Involves Detecting and Responding to Stimuli

• A **stimulus** is a change in the environment (e.g., light, temperature, sound, chemicals).
• Organisms detect stimuli and produce a **response** (e.g., a plant bending towards light; a human pulling a hand away from a hot surface).

5. Excretion ≠ Egestion

• **Excretion** is the removal of waste products of **metabolism** (e.g., carbon dioxide from respiration, urea from the breakdown of amino acids in the liver).
• **Egestion** is the removal of **undigested food** from the body (faeces). This material was never part of metabolism, so it is **not** excretion.
• This is a very common exam distinction.

6. Growth Is Measured as a Permanent Increase in Dry Mass

• Wet mass can fluctuate (e.g., after drinking water), so **dry mass** is a more reliable measure of true growth.
• Growth results from cells dividing (increasing cell number) and/or cells getting larger (increasing cell size).

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Worked Examples and Real-World Applications

Example 1: Is a Car Alive?

A car can move, it uses fuel (nutrition of a sort), and it produces exhaust gases (excretion of a sort). However, a car cannot reproduce, grow, or show true sensitivity. It does not carry out respiration at a cellular level. Because it does not display all seven characteristics of life, a car is not a living organism.

> This type of question tests whether you can apply MRS NERG to distinguish living from non-living things.

Example 2: Are Viruses Living Organisms?

Viruses can reproduce, but only inside a living host cell. They do not carry out nutrition, respiration, excretion, movement, sensitivity, or growth on their own. According to the Edexcel IGCSE specification, viruses are not considered true living organisms because they do not independently perform all seven life processes. (The specification notes that viruses are classified separately and are described as pathogenic agents rather than living organisms.)

Example 3: Plants and Movement

Students sometimes state that plants do not move. However, a sunflower turning to face the sun (phototropism) and the Venus flytrap closing its leaves to trap insects are clear examples of plant movement. This demonstrates that movement applies to all kingdoms of living organisms, not just animals.

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Exam Technique Tips (Edexcel-Specific)

✅ Tip 1: Use Precise Edexcel Definitions

Edexcel mark schemes award marks for specific key words. For example:

• When defining excretion, you **must** include the phrase **"waste products of metabolism"** — simply writing "getting rid of waste" will not score the mark.
• When defining respiration, include **"chemical reactions in cells"** and **"release energy"** — do not write "breathing in and out."

Always learn the exact specification wording for each definition.

✅ Tip 2: "State" vs. "Explain" Commands

• If a question says **"State one characteristic of living organisms"**, a single word or short phrase (e.g., "respiration") is enough for the mark.
• If a question says **"Explain how an organism shows a named characteristic"**, you must give a **specific example** and describe what the organism does (e.g., "The plant shows sensitivity because its roots grow towards water in response to the stimulus of gravity").

Read the command word carefully — unnecessary extra detail in a "state" question wastes time, while too little detail in an "explain" question loses marks.

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Quick-Check Summary Table

| Characteristic | Definition (Key Phrase) | Example |

|---|---|---|

| Movement | Change of position or place | Plant shoots growing towards light |

| Respiration | Chemical reactions in cells that release energy | All cells respire to release energy from glucose |

| Sensitivity | Detect and respond to stimuli | Pupil constricting in bright light |

| Growth | Permanent increase in size/dry mass | A seedling developing into a tree |

| Reproduction | Producing new organisms of the same species | A bacterium dividing into two |

| Excretion | Removal of metabolic waste products | CO₂ removed via the lungs |

| Nutrition | Taking in of nutrients for energy and growth | Plants photosynthesising; animals eating food |

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*These notes are aligned to the Edexcel International GCSE (9–1) Biology specification, Section 1: The Nature and Variety of Living Organisms — Subtopic: Characteristics of Living Organisms.*