Characteristics of Living Organisms

Every living organism — from a single bacterium to a blue whale — shares seven core processes. Edexcel use these as the diagnostic test for life. The mnemonic is MRS GREN:

• Movement — an action by an organism, or part of an organism, causing a change of position or place. Animals usually move their whole bodies; plants show slower movements like tropisms (e.g. roots growing down towards gravity, stems growing up towards light).
• Respiration — the chemical reactions in cells that break down nutrient molecules to release energy in the form of ATP. Respiration is NOT breathing — that's ventilation. Examiners reject 'taking in oxygen' as the definition.
• Sensitivity — the ability to detect and respond to changes (stimuli) in the internal or external environment (e.g. eye responds to light; a Venus flytrap closes on contact).
• Growth — a permanent increase in size and dry mass by an increase in cell number, cell size, or both. (Temporary changes like a turgid leaf swelling with water do not count.)
• Reproduction — the processes that make more of the same kind of organism. Sexual reproduction involves two parents and produces genetically varied offspring; asexual reproduction involves one parent and produces genetically identical offspring (clones).
• Excretion — the removal from the body of the waste products of metabolism (CO₂ from respiration, urea from amino acid breakdown) and substances in excess of requirements. NOT faeces — that's egestion.
• Nutrition — taking in materials for energy, growth and development. Two broad strategies: autotrophic (plants and algae make their own food by photosynthesis) and heterotrophic (animals, fungi and most bacteria take in ready-made organic substances).

Edexcel tip. When the question says 'list four characteristics of living organisms' the mark scheme typically awards 1 mark per correctly NAMED AND DEFINED characteristic. The name alone scores 0 — always pair it with a definition.

All known living organisms fall into one of five kingdoms. Edexcel ask you to know structural and nutritional features for each, plus named examples.

1. Animals (e.g. humans, fish, insects)

• Multicellular eukaryotes.
• No cell walls — only a cell-surface membrane.
• Specialised tissues (muscle, nerve) → complex movement and rapid responses.
• Heterotrophic — ingest organic food, digest it internally.
• Store carbohydrate as glycogen.

2. Plants (e.g. Hibiscus, rice, oak)

• Multicellular eukaryotes.
• Cell walls made of cellulose.
• Contain chloroplasts with chlorophyll → photosynthesise.
• Large permanent vacuole containing cell sap.
• Autotrophic — make their own food by photosynthesis.
• Store carbohydrate as starch.

3. Fungi (e.g. Mucor, Penicillium, yeast Saccharomyces)

• Mostly multicellular eukaryotes. Yeast is the exception — unicellular.
• Body is made of thread-like hyphae forming a network called a mycelium.
• Cell walls made of chitin (NOT cellulose).
• Saprotrophic nutrition — secrete enzymes onto dead organic matter, then absorb the soluble products (extracellular digestion).
• Store carbohydrate as glycogen.

4. Protoctists (e.g. Amoeba, Chlorella, Plasmodium)

• Mostly unicellular eukaryotes.
• A diverse "rag-bag" kingdom: some plant-like (Chlorella — has chloroplasts), some animal-like (Amoeba — feeds by phagocytosis), some pathogenic (Plasmodium — causes malaria).

5. Prokaryotes / bacteria (e.g. Lactobacillus, Pneumococcus)

• Unicellular.
• NO nucleus — DNA is a single circular chromosome free in the cytoplasm; small extra rings of DNA called plasmids may also be present.
• Cell walls are present but NOT made of cellulose (they are peptidoglycan / murein).
• No membrane-bound organelles (no mitochondria, no chloroplasts).
• Some have flagella for movement.
• Nutrition is varied — some are autotrophic, most are heterotrophic.

Viruses present a classification problem. They have genetic material (DNA or RNA), and they cause many diseases — but they fail the MRS GREN test for life:

• Not cellular. A virus particle is just genetic material inside a protein coat (capsid), sometimes surrounded by a lipid envelope. No nucleus, no cytoplasm, no cell-surface membrane.
• Cannot respire, grow, sense or excrete on their own. Outside a host, a virus particle is metabolically inert.
• Reproduce only by infecting a host cell — they hijack the host's machinery to make more copies (lytic / lysogenic cycles).

For these reasons, viruses are NOT classified in any of the five kingdoms. They are studied as a separate group.

Named examples to remember:

• HIV (human immunodeficiency virus) → AIDS in humans.
• Influenza virus → flu in humans.
• Tobacco mosaic virus (TMV) → mottling disease of tobacco / tomato leaves.

Edexcel pitfall. Don't write 'viruses are too small to be living' — size is not a classification criterion. The reason is that they are acellular and cannot perform life processes independently.

A pathogen is any organism (or virus) that causes disease. Pathogens are NOT a separate group — they come from across the biological world:

Group	Named pathogen	Disease
Virus	HIV	AIDS
Virus	Influenza virus	Flu
Virus	Tobacco mosaic virus	Mosaic disease in tobacco / tomato
Bacterium (prokaryote)	Pneumococcus	Pneumonia
Bacterium	Vibrio cholerae	Cholera
Protoctist	Plasmodium	Malaria
Fungus	Tinea (ringworm fungus)	Athlete's foot

Important: the SAME kingdom may also include harmless or beneficial species. For example Lactobacillus (used in yoghurt production) and E. coli (mostly harmless gut bacteria) are both prokaryotes but are NOT pathogens. The fungi Mucor and Penicillium in the spec are saprotrophs, not pathogens (and Penicillium even produces the antibiotic penicillin used against bacterial pathogens).

Edexcel question style. Paper 2 sometimes gives a 'name a pathogen from group X, and the disease it causes' as 4 quick B-marks. Memorise at least one pathogen + disease pair from each row above.

Edexcel often presents a short description or microscope drawing and asks: 'Which kingdom does this belong to?' Use this decision pathway:

1. Is there a nucleus visible?
   • NO → Prokaryote (bacterium). Stop.
   • YES → eukaryote, continue.
2. Are there chloroplasts?
   • YES + cellulose wall → likely Plant (multicellular) or plant-like protoctist (unicellular, e.g. Chlorella).
   • NO → continue.
3. Is there a cell wall, and what is it made of?
   • Chitin wall → Fungus.
   • No wall → Animal (multicellular) or animal-like protoctist (unicellular, e.g. Amoeba).
4. Unicellular vs multicellular confirms the final choice between a "true" plant/animal/fungus and a plant-/animal-like protoctist.

Worked walkthrough. A unicellular eukaryote with a single nucleus, chloroplasts and a cellulose cell wall — which kingdom? → It has a nucleus (eukaryote) but is unicellular, so it cannot be a true plant. Its features match a plant-like protoctist (Chlorella).

Examiner reports specifically praise candidates who write their reasoning as a short numbered key — "It has a nucleus, so it is a eukaryote. It is unicellular, so it is a protoctist..." — rather than just listing features.