1.1 Characteristics of Living Organisms Revision Notes
1. Overview
All living things, from microscopic bacteria to giant redwood trees, share seven fundamental characteristics. These processes distinguish living organisms from non-living objects and ensure the survival of individuals and species. This topic provides the "MRS GREN" framework used throughout the IGCSE Biology syllabus to define what it means to be alive.
Key Definitions
- 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.
Core Content
To remember the seven characteristics, use the mnemonic: MRS GREN.
(M) Movement
- Most animals move their whole bodies to find food or avoid predators.
- Plants move parts of themselves (e.g., leaves turning toward the sun or roots growing downward).
(R) Respiration
- This is a chemical process that occurs inside every living cell.
- Energy is released from nutrients (like glucose) to power metabolic processes like protein synthesis and cell division.
(S) Sensitivity
- Living things sense "stimuli" (changes in temperature, light, sound, or chemicals).
- They produce a "response" to these stimuli to increase their chances of survival.
(G) Growth
- Growth involves a permanent increase in size.
- Dry mass refers to the mass of the organism after all water has been removed. This is the most accurate way to measure growth as water content can fluctuate daily.
(R) Reproduction
- Organisms produce offspring to ensure the species does not go extinct.
- This can be sexual (two parents) or asexual (one parent).
(E) Excretion
- Metabolism refers to all chemical reactions in the body. These reactions produce waste (e.g., CO2 from respiration, urea in urine).
- If these toxic substances build up, they can kill the organism.
(N) Nutrition
- Plants are autotrophic: They make their own food using sunlight, water, and CO2 (photosynthesis).
- Animals are heterotrophic: They must consume other living organisms to get their nutrients.
- Nutrients include proteins, carbohydrates, fats, vitamins, and minerals.
Extended Content (Extended Only)
There is no additional content for the Extended curriculum for this specific sub-topic (1.1). Both Core and Supplement students must master the definitions provided above.
Key Equations
While this topic focuses on definitions, the process of Respiration is defined by its chemical reaction:
Word Equation for Aerobic Respiration: Glucose + Oxygen → Carbon Dioxide + Water (+ Energy released)
(Note: Energy is not a chemical substance, so it is usually written in brackets or as a note alongside the equation.)
Common Mistakes to Avoid
- ❌ Wrong: Respiration is "breathing."
- ✓ Right: Respiration is a chemical reaction in cells. Breathing (ventilation) is the physical mechanism of moving air in and out of lungs.
- ❌ Wrong: Excretion and Egestion are the same thing.
- ✓ Right: Excretion is the removal of metabolic waste (made inside cells). Egestion is the removal of undigested food (faeces) that never actually entered the cells.
- ❌ Wrong: Growth is just getting taller.
- ✓ Right: Growth is a permanent increase in dry mass and size.
Exam Tips
- The "Define" Command Word: This topic often appears as 1-mark or 2-mark questions asking you to define a specific characteristic. You must use the exact wording from the syllabus to get full marks (e.g., for Respiration, you must mention "chemical reactions" and "release energy").
- Dry Mass Context: If an exam question asks why a scientist might dry out a plant before weighing it, the answer is always to measure Growth accurately by removing the fluctuation caused by water.
- Real-world Context: You may be asked to apply MRS GREN to a new discovery (e.g., a "robot" or a "virus"). Be prepared to explain why a virus is often considered non-living (it cannot reproduce without a host cell and does not perform its own metabolism/respiration).
- Metabolism: Always link "Excretion" to the word "metabolism" or "metabolic waste" in your answers.