Seven characteristics of living organisms - Biology IGCSE Study Notes

All living organisms share seven fundamental characteristics, remembered by the mnemonic MRS GREN or MRS C GREN:

Movement — The ability to change position or move internal parts. In animals, this involves muscle contraction; in plants, growth responses (tropisms) or turgor-driven movements like the Venus flytrap closing.

Respiration — The chemical breakdown of nutrients (glucose) to release energy for life processes. Aerobic respiration requires oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Anaerobic respiration occurs without oxygen, producing less energy.

Sensitivity (Response to stimuli) — Detecting and responding to changes in the environment. Examples: light detection by eyes, chemical detection by taste receptors, touch sensitivity in skin.

Control (Homeostasis) — Maintaining constant internal conditions despite external changes. Includes temperature regulation, blood glucose control, water balance.

Growth — Permanent increase in size and mass through cell division and enlargement. Measured by length, mass, or cell number.

Reproduction — Producing offspring. Asexual reproduction involves one parent (mitosis); sexual reproduction requires two parents and gamete fusion (meiosis).

Excretion — Removal of toxic metabolic waste products. Includes CO₂ from respiration, urea from protein breakdown (NOT the same as egestion of undigested food).

Nutrition — Obtaining nutrients for energy and growth. Autotrophs (plants) make food via photosynthesis; heterotrophs (animals) consume organic matter.

Cambridge Key Point: All seven characteristics must be present for something to be classified as living. Viruses lack some characteristics (no independent respiration/nutrition), creating debate about their living status.

Understanding these characteristics becomes clearer through real-world observation:

Movement analogy: Think of movement like a spectrum. A cheetah's explosive sprint and a sunflower tracking the sun across the sky both represent movement, just at vastly different speeds. Even bacteria 'swim' using flagella—movement exists at every biological scale.

Respiration misconception: Students often confuse respiration with breathing. Respiration is cellular (occurring in mitochondria), while breathing is just gas exchange. A plant respires 24/7, even though it photosynthesizes only during daylight. Athletes understanding this concept know why their muscles burn during intense exercise—anaerobic respiration produces lactic acid when oxygen is insufficient.

Sensitivity in action: The humble touch-me-not plant (Mimosa pudica) demonstrates sensitivity beautifully—touch its leaves and they fold within seconds. This rapid response protects against herbivores. Human pupils dilating in dim light show our sensitivity to light intensity changes.

Control/Homeostasis example: Desert animals like camels exemplify brilliant control mechanisms—they tolerate body temperature fluctuations that would kill most mammals, minimizing water loss through sweating. Humans shiver when cold (generating heat) and sweat when hot (cooling)—both maintain ~37°C.

Growth distinction: A crystal 'grows' by adding molecules to its surface, but lacks cellular organization. True biological growth involves complex processes: DNA replication, protein synthesis, differentiation. A human grows from a single fertilized egg weighing 0.0001g to an adult of 70kg—a 700-million-fold increase!

Excretion vs. Egestion: Excretion removes metabolic waste produced inside cells (CO₂, urea). Egestion removes undigested food that never entered cells (feces). This distinction frequently appears in Cambridge exams.