The Nature and Variety of Living Organisms

What you'll learn

This topic forms the foundation of your IGCSE Biology course. You will understand the characteristics that define all living organisms and how biologists classify life into distinct groups based on shared features. This content is essential as it appears in Paper 1 and provides context for all other biology topics you will study.

Key terms and definitions

Characteristics of life — the seven processes that distinguish living organisms from non-living things: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition (MRS GREN)

Classification — the organisation of living organisms into groups based on shared characteristics and evolutionary relationships

Species — a group of organisms that can reproduce to produce fertile offspring

Pathogen — a disease-causing organism, such as certain bacteria, fungi, protoctists, or viruses

Autotroph — an organism that produces its own food, typically through photosynthesis (e.g., plants)

Heterotroph — an organism that obtains nutrition by consuming other organisms or organic matter

Saprotrophic nutrition — feeding on dead or decaying organic matter by secreting enzymes externally and absorbing the digested products

Binomial system — the two-part naming system for species using genus and species names (e.g., Homo sapiens)

Core concepts

Characteristics of living organisms

All living organisms share seven life processes, remembered by the acronym MRS GREN:

Movement — an action by an organism or part of an organism causing a change in position or place. Animals move their whole bodies from place to place (locomotion), while plants move parts of themselves (e.g., leaves toward light).

Respiration — the chemical reactions in cells that break down nutrient molecules to release energy for metabolism. This occurs in all living cells.

Sensitivity — the ability to detect and respond to changes in the internal or external environment (stimuli). Examples include plants growing toward light and animals responding to temperature changes.

Growth — a permanent increase in size and dry mass by an increase in cell number or cell size, or both.

Reproduction — the processes that make more of the same kind of organism. Can be sexual (involving two parents and gametes) or asexual (involving one parent and no gametes).

Excretion — removal of the waste products of metabolism and substances in excess of requirements. Examples include carbon dioxide from respiration and urea from protein breakdown.

Nutrition — taking in materials for energy, growth, and development. Plants need light, carbon dioxide, water, and ions. Animals need organic compounds and ions, and usually need water.

Classification of living organisms

The Pearson Edexcel specification requires knowledge of five kingdoms, though the focus is on seven main groups:

1. Animals
2. Plants
3. Fungi
4. Protoctists
5. Bacteria
6. Viruses (not in a kingdom)

These groups can be further categorised into vertebrates (animals with backbones) and invertebrates (animals without backbones).

Animals

Animals are multicellular organisms with the following characteristics:

• No cell walls or chloroplasts
• Heterotrophic nutrition — most feed on organic substances made by other organisms
• Store carbohydrate as glycogen
• Usually have nervous coordination and can move from place to place

Vertebrates include five classes:

• Mammals — have hair/fur, maintain constant body temperature (endothermic), give birth to live young, produce milk
• Birds — have feathers, wings, beaks, lay hard-shelled eggs, endothermic
• Reptiles — have dry, scaly skin, lay soft-shelled eggs on land, ectothermic (body temperature varies with environment)
• Amphibians — have moist, permeable skin, lay jelly-covered eggs in water, larvae have gills, adults have lungs, ectothermic
• Fish — have scales, gills for gas exchange, lay eggs in water, have fins, ectothermic

Invertebrates include arthropods (organisms with jointed legs and exoskeletons), such as insects, arachnids, crustaceans, and myriapods.

Plants

Plants are multicellular organisms with these features:

• Cellulose cell walls
• Chloroplasts containing chlorophyll for photosynthesis
• Autotrophic nutrition (make their own food using light energy)
• Store carbohydrate as starch or sucrose
• No nervous coordination but can respond to stimuli (e.g., tropisms)

Examples include flowering plants like maize (Zea mays) and broad beans (Vicia faba), plus non-flowering plants like ferns and mosses.

Fungi

Fungi can be unicellular (e.g., yeast) or multicellular (e.g., moulds, mushrooms):

• Cells have cell walls made of chitin (not cellulose)
• No chloroplasts — cannot photosynthesise
• Saprotrophic nutrition — feed by secreting enzymes onto dead organic material, breaking it down externally (extracellular digestion), then absorbing the soluble products
• Store carbohydrate as glycogen
• Multicellular fungi have bodies made of thread-like structures called hyphae, which form a network called mycelium

Examples: Mucor (bread mould), yeast (Saccharomyces)

Protoctists

Protoctists are microscopic single-celled organisms with varied characteristics:

• Have a nucleus (unlike bacteria)
• Some are autotrophic (e.g., Chlorella has chloroplasts)
• Some are heterotrophic (e.g., Amoeba feeds on smaller organisms)
• Some are pathogens (e.g., Plasmodium, which causes malaria)

The protoctist kingdom is diverse and includes organism types that don't fit into the other kingdoms.

Bacteria

Bacteria are microscopic single-celled organisms:

• No nucleus — genetic material is a circular chromosome of DNA in the cytoplasm, plus small circular pieces of DNA called plasmids
• Cell wall present (but not made of cellulose)
• Some can photosynthesise, most feed on living or dead organisms
• May be pathogens (e.g., Pneumococcus causes pneumonia, Lactobacillus is found in yoghurt and is non-pathogenic)

Bacteria are prokaryotes — their cells lack a true nucleus and membrane-bound organelles.

Viruses

Viruses are not considered living organisms as they do not carry out all seven life processes independently:

• Much smaller than bacteria
• No cellular structure — consist of genetic material (DNA or RNA) inside a protein coat
• Can only reproduce inside living host cells (they are parasites)
• All viruses are pathogens (e.g., influenza virus, HIV, tobacco mosaic virus in plants)

They are described as being "on the edge of life" because they show some characteristics of living things (reproduction, evolution) but not others (no metabolism, growth, or nutrition outside a host cell).

Using classification keys

The dichotomous key is a tool used to identify organisms by making a series of two-choice decisions about their characteristics. Each step narrows down the possibilities until the organism is identified.

Example format:

1. Has legs → go to 2 / No legs → earthworm
2. Has 6 legs → insect / Has 8 legs → spider

Worked examples

Example 1: Identifying characteristics of living organisms

Question: A student observes a potted plant growing on a windowsill that bends toward the light.

(a) State which characteristic of living organisms is shown by the plant bending toward light. [1 mark]

(b) Explain how this characteristic is useful to the plant. [2 marks]

Answer:

(a) Sensitivity / responsiveness (to light stimulus) [1]

(b) Bending toward light allows the plant to capture more light [1] for photosynthesis, which provides energy and materials for growth [1]

Example 2: Classifying organisms

Question: The table shows features of four organisms.

(a) Identify which kingdom each organism belongs to. [4 marks]

(b) Explain why Organism D is not classified as a living organism. [2 marks]

Answer:

(a) Organism A — Plants [1] Organism B — Fungi [1] Organism C — Animals [1] Organism D — Viruses (not in any kingdom) [1]

(b) Viruses do not carry out all seven characteristics of living organisms [1]. They can only reproduce inside host cells and do not respire, feed, excrete, or grow independently [1].

Example 3: Comparing vertebrate groups

Question: A student finds an organism with scaly skin that lays eggs with soft shells on land.

(a) State which vertebrate group this organism belongs to. [1 mark]

(b) Explain one way that organisms in this group regulate their body temperature. [2 marks]

Answer:

(a) Reptiles [1]

(b) Reptiles are ectothermic, so they regulate temperature behaviourally [1]. For example, they bask in the sun to warm up or move to shade to cool down [1].

Common mistakes and how to avoid them

• Confusing respiration with breathing — Respiration is a chemical reaction in cells that releases energy; breathing is the physical movement of air into and out of lungs. All organisms respire; only some breathe.

• Thinking viruses are bacteria — Viruses are much smaller than bacteria, have no cellular structure, and can only reproduce inside host cells. Antibiotics work on bacteria but not viruses.

• Mixing up autotrophs and heterotrophs — Plants and some protoctists are autotrophs (make their own food); animals and fungi are heterotrophs (obtain food from other organisms). Fungi are NOT plants.

• Describing growth as just getting bigger — Growth must involve an increase in dry mass or cell number, not just water uptake (which is temporary). A hibernating bear that wakes up heavier due to water retention has not grown.

• Confusing excretion with egestion — Excretion removes metabolic waste products made by cells (e.g., urea, CO₂). Egestion removes undigested food that was never in cells (faeces). Getting rid of faeces is NOT excretion.

• Not learning specific examples — Examiners often ask for named examples. Know at least: Mucor (fungus), Amoeba and Plasmodium (protoctists), Lactobacillus (bacterium), influenza virus.

Exam technique for "The Nature and Variety of Living Organisms"

• Command word awareness — "State" requires a short answer with no explanation (1 mark). "Explain" requires reasons or mechanisms (usually 2+ marks). "Describe" needs detail about what happens but not why.

• Use correct terminology — Write "multicellular" not "made of many cells"; "heterotrophic" not "eats food"; "saprotrophic nutrition" not "feeds on dead things". Precise terminology gains marks.

• Classification questions — When identifying kingdoms, look systematically: cell wall material? Chloroplasts? Nucleus? Method of nutrition? Work through each characteristic to avoid errors.

• Comparison tables — For "compare" questions, make direct comparisons (e.g., "Plants have cellulose cell walls whereas fungi have chitin cell walls"). Avoid just describing each organism separately.

Quick revision summary

All living organisms share seven characteristics: MRS GREN (movement, respiration, sensitivity, growth, reproduction, excretion, nutrition). Organisms are classified into groups based on shared features. Animals are heterotrophs without cell walls; plants are autotrophs with cellulose cell walls and chloroplasts; fungi feed saprotrophically and have chitin cell walls; protoctists are single-celled with a nucleus; bacteria are single-celled without a nucleus; viruses are non-cellular parasites. Know specific named examples for each group.