Characteristics of Living Organisms

Use the mnemonic MRSGREN to recall all seven in order:

Letter	Characteristic	Precise meaning
M	Movement	A change in position of all or part of the organism (locomotion in animals; tropisms in plants)
R	Respiration	Chemical release of energy from organic molecules (glucose) INSIDE cells
S	Sensitivity	Detection of, and response to, changes in the environment (stimuli)
G	Growth	A PERMANENT increase in size and DRY MASS by the production of new cells
R	Reproduction	Production of offspring — sexually (with fertilisation) or asexually (without)
E	Excretion	Removal from the body of WASTE PRODUCTS of METABOLISM (CO₂, urea, excess water)
N	Nutrition	Taking in organic molecules for energy, growth, and repair (autotrophic or heterotrophic)

Why ALL seven? An organism must show all seven to be considered living. Lack of even one calls classification into question.

This distinction loses hundreds of students marks each year.

Excretion: removal from the body of the waste products of METABOLISM — substances the body itself has produced through chemical reactions.

• Carbon dioxide (from aerobic respiration in every cell)
• Urea (from deamination of excess amino acids in the liver)
• Excess water and mineral salts (from metabolic reactions)

Egestion (defecation): removal of UNDIGESTED food material. This material passed through the gut but was never absorbed into the body. It was never involved in metabolism. It is therefore NOT excretion.

The test question reads: "A student says faeces is excreted. Is the student correct?" Answer: No. Faeces contains undigested food material (not a metabolic waste product), so its removal is egestion, NOT excretion. (Some components of faeces, like bile pigments, ARE metabolic waste and technically are excreted — but the question is testing the general principle.)

Cambridge requires precision here:

Growth = a permanent increase in size AND dry mass by the production of new protoplasm (new cells via cell division AND cell enlargement).

Why DRY mass, not wet mass? Organisms can temporarily absorb water, increasing their wet mass without true growth. Dry mass (mass after removing all water) measures actual new biological material.

Why PERMANENT? A balloon expands when inflated but this is not growth. A plant wilting and then rehydrating is not growing — no new cells were made.

Example of growth in context: A seed germinates and produces a seedling with more cells and more dry mass → TRUE growth. A raisin left in water swells up → NOT growth (it returns to its original size when dried).

Viruses are the most common organism examiners use to test MRSGREN understanding.

What viruses CAN do:

• Reproduce (but only INSIDE a host cell — not independently)
• Show variation (some)

What viruses CANNOT do independently:

• Respire (no mitochondria, no metabolic enzymes)
• Excrete metabolic waste (no metabolism of their own)
• Grow (no cell division machinery)
• Show sensitivity or movement (no response to stimuli)
• Carry out nutrition (cannot make or take in food themselves)

Conclusion: Viruses are NOT classified as living organisms. They are non-cellular biological entities that can only replicate by hijacking host cell machinery.

How to answer in an exam: "Viruses are not living organisms because they cannot carry out all seven characteristics of living organisms independently. For example, they cannot respire, excrete, or grow without a host cell."