Characteristics of Living OrganismsCambridge IGCSE Biology: Revision notes
Section 1
What are the seven characteristics that define living organisms?
All living organisms share seven key characteristics that distinguish them from non-living things. These characteristics are the defining features examined in the Cambridge IGCSE Biology specification. Understanding each characteristic is essential for classifying organisms and recognising life processes.
The seven characteristics are:
- 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
- Growth – a permanent increase in size and dry mass
- Reproduction – the processes that make more of the same kind of organism
- Excretion – the removal of the waste products of metabolism and substances in excess of requirements
- Nutrition – the taking in of materials for energy, growth and development
These characteristics form the foundation of identifying living organisms and are often remembered by the acronym MRS GREN.
Examiners expect you to use the precise definitions given in the specification. Learn the exact wording for each characteristic – using vague descriptions will lose marks.
Think of organisms as factories: they take in raw materials (nutrition), process energy (respiration), maintain their size and equipment (growth), remove waste (excretion), respond to their surroundings (sensitivity), move products and workers (movement), and create new factories (reproduction).
Section 2
How does movement differ between organisms?
Movement is not limited to walking or swimming. It includes any action causing a change of position or place, which can occur at different levels:
- Whole-organism movement: Animals moving from one location to another (e.g. walking, flying, swimming)
- Part-organism movement: Movement of individual structures without the whole organism moving (e.g. plant roots growing downwards, leaves moving to track the sun, fish fins moving to maintain position)
- Cellular movement: Movement within cells (e.g. cytoplasm movement, organelle transport)
All organisms show some form of movement, though it may not always be obvious or rapid. Even plants demonstrate movement through growth towards light and roots extending through soil.
Students often think movement only applies to animals. Remember that plants also move – through growth, turgor movements, and directional responses. This is a common exam error.
Section 3
Why is respiration essential for all living organisms?
Respiration is a fundamental characteristic because it provides the energy needed for all other life processes. It is not the same as breathing (which is gas exchange) but rather the chemical breakdown of molecules within cells.
| Aspect | Detail |
|---|---|
| Definition | Chemical reactions in cells that break down nutrient molecules and release energy for metabolism |
| Purpose | Provides energy (ATP) for all cellular processes |
| Occurs in | All living cells, continuously |
| Products | Energy released; carbon dioxide and water (aerobic respiration) or other waste products (anaerobic) |
| Examples of use | Movement, active transport, protein synthesis, maintaining body temperature |
Respiration is continuous and occurs in every living cell, whether the organism is awake, sleeping, moving, or stationary. This energy release is absolutely essential for life.
Examiners frequently test whether you understand that respiration is different from breathing. Respiration is about energy release at the cellular level; breathing is gas exchange. Be precise with terminology.
A student might say 'respiration is breathing' and lose marks. The correct answer is: 'Respiration is the chemical breakdown of nutrients in cells to release energy; breathing is the gas exchange process that provides oxygen for this to occur.'
Section 4
How do organisms detect and respond to their environment?
Sensitivity (also called irritability) is the ability to detect and respond to changes in the internal or external environment. This allows organisms to adapt their behaviour or physiology to survive.
Types of environmental changes organisms detect:
- External changes: Light, temperature, sound, touch, chemical signals, gravity
- Internal changes: Changes in blood pH, glucose concentration, water potential, temperature
Examples of sensitivity across organisms:
- Animals: Nervous system detects stimuli and produces rapid responses (e.g. withdrawing from heat, moving towards food)
- Plants: Chemical messengers (hormones) produce slower responses (e.g. growth towards light, closing stomata in drought)
- Single-celled organisms: Direct response to chemical gradients or light without specialised organs
The response process: Stimulus → Detection → Processing → Response
Without sensitivity, organisms could not survive because they would be unable to respond to threats, find resources, or maintain stable internal conditions.
Sensitivity is like an organism's alarm system: receptors detect the alarm (stimulus), the nervous or hormonal system processes the signal, and muscles or glands act as the response. Without it, the organism cannot react to danger or opportunity.
Section 5
What is the relationship between growth, nutrition, and excretion?
Growth is defined as a permanent increase in size and dry mass. This is distinct from temporary changes in size (like swelling from water intake) and is achieved through the interaction of nutrition and excretion.
Growth requires:
- Nutrition: The taking in of materials (organic molecules and mineral ions) needed for synthesising new cells and tissues
- Energy from respiration: To drive the biosynthesis of new proteins and structures
- Removal of excess materials: Through excretion to maintain balance
Key distinction: Growth is not just an increase in fresh mass (which includes water) but specifically an increase in dry mass (the mass of the organism without water). This indicates that new biological material has been genuinely synthesised, not just absorbed temporarily.
Growth patterns:
- Determinate growth: Growth stops at a set size (e.g. many insects after reaching adulthood)
- Indeterminate growth: Growth continues throughout life (e.g. most plants, some fish and reptiles)
Excretion is essential during growth because as cells respire and synthesise new proteins, waste products accumulate. These must be removed to prevent harm and maintain the stable internal environment needed for growth to continue.
Students often confuse growth with size increase from water absorption. Growth must be a permanent increase in dry mass – if a plant wilts and loses water, this is not loss of growth, as dry mass remains unchanged.
When answering about growth, always include both 'size' and 'dry mass' in your definition. Examiners mark this phrase specifically – saying only 'increase in size' is incomplete and loses marks.
Section 6
Why are reproduction and nutrition distinct but interconnected characteristics?
Reproduction and nutrition are separate characteristics but work together to sustain life and populations.
| Characteristic | Definition | Purpose | Example |
|---|---|---|---|
| Reproduction | The processes that make more of the same kind of organism | Creates new individuals; maintains populations and species | Sexual reproduction in animals; spore production in ferns |
| Nutrition | The taking in of materials for energy, growth and development | Provides building blocks and energy for the individual organism | Photosynthesis in plants; feeding in animals |
Key relationships:
- Nutrition sustains the individual: Food intake provides energy for all life processes, including reproduction
- Reproduction sustains the species: Without reproduction, species cannot persist; without nutrition, organisms cannot grow large enough to reproduce
- Energy link: Respiration of digested nutrients provides ATP needed for making gametes, courtship behaviours, and supporting developing embryos
Types of reproduction:
- Asexual reproduction: One parent; genetically identical offspring; rapid but no variation (e.g. binary fission in bacteria, budding in yeast)
- Sexual reproduction: Two parents; genetically different offspring; slower but produces variation (e.g. fertilisation in plants and animals)
Both types ensure the organism "reproduces" – makes more of the same kind – but through different mechanisms suited to different environments.
Examiners test whether you distinguish reproduction from nutrition. A common error is confusing feeding/nutrition with reproduction. Remember: nutrition feeds the individual; reproduction creates new individuals.
Must Know
- All living organisms have seven characteristics: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition (MRS GREN) – know the exact definitions from the specification
- Growth means a permanent increase in BOTH size AND dry mass – this distinguishes true growth from temporary water uptake
- Respiration is the breakdown of nutrients in cells to release energy, not breathing; it occurs in all living cells continuously
- Sensitivity is the ability to detect and respond to stimuli in the internal or external environment – organisms cannot survive without this
- Nutrition and reproduction are distinct: nutrition sustains the individual organism; reproduction sustains the species through asexual or sexual means
- Excretion removes metabolic waste products to maintain a stable internal environment and prevent harm during growth and respiration
- Movement includes any change of position – whole organism, parts of organisms, or cellular movement – not just obvious locomotion
That's the notes covered.
Carry on to the next subtopic.