Characteristics of Living Organisms

MRS GREN.

Letter	Process	Cambridge definition
M	Movement	An action by an organism or part of it causing a change of position or place
R	Respiration	Chemical reactions in cells that break down nutrient molecules and release energy
S	Sensitivity	Ability to detect and respond to changes in the environment
G	Growth	Permanent increase in size and dry mass
R	Reproduction	Processes that produce more of the same kind of organism
E	Excretion	Removal of metabolic waste, toxic materials and substances in excess
N	Nutrition	Taking in of materials for energy, growth and development

Memorise the wording exactly. Cambridge mark schemes are strict — they look for specific phrases (e.g. "chemical reactions in cells" for respiration, "dry mass" for growth, "metabolic waste" for excretion).

Worked qualitative. Why is a virus NOT considered a fully living organism by most biologists? Outside a host cell, viruses don't carry out respiration, growth, sensitivity, or excretion. They reproduce ONLY by hijacking a host cell. They miss several MRS GREN characteristics.

Respiration vs breathing.

• Respiration: chemical reactions in CELLS that break down nutrient molecules to release ENERGY. Happens in mitochondria. Glucose → CO₂ + water + energy (aerobic).
• Breathing (ventilation): muscular movement of air into and out of the LUNGS. Only large animals do this.
• All living cells respire; only some animals breathe.

Worked qualitative. A plant doesn't have lungs but it still respires. Where? In every cell — at the mitochondria. The plant doesn't 'breathe' in the lung sense.

Excretion vs egestion.

• Excretion: removal of waste produced INSIDE cells (metabolic waste). Examples: CO₂ from respiration; urea from protein breakdown; bile pigments from haemoglobin breakdown.
• Egestion: passing out undigested food (faeces) that never entered the body's cells. Not excretion — egested material was never inside the body.

Worked qualitative. A student says "we excrete faeces". Correct them: faeces are EGESTED. They contain undigested food, dead gut cells, gut bacteria — not metabolic waste.

Growth is a PERMANENT increase in size AND dry mass.

Why dry mass?

• Wet mass changes if you drink water or sweat. Not growth — just hydration.
• Dry mass = mass after all water is removed. Reflects new cellular material (proteins, carbohydrates, fats, etc.) actually synthesised.

Plants vs animals.

• Plants grow throughout their lives (mostly at meristems).
• Animals grow until adulthood, then stop (for most species).

Cell division. Growth involves making more cells (mitosis) AND existing cells getting larger.

Worked qualitative. A student weighs a seedling daily. Wet mass goes up by $5\,\text{g}$ in 3 days. Dry mass stays the same. What does this mean? The seedling has absorbed water but hasn't actually grown — no new cellular material has been made.

Cambridge tip. Always include "dry mass" in growth definitions for full marks.

Plants show all seven:

• Movement: leaves track the sun (phototropism), stomata open/close, petals open/close.
• Respiration: every plant cell respires day and night.
• Sensitivity: roots respond to gravity (geotropism); shoots to light (phototropism).
• Growth: at meristems (root and shoot tips).
• Reproduction: pollen + ovules → seeds (sexual); runners, bulbs, tubers (asexual).
• Excretion: oxygen (waste from photosynthesis); CO₂ (from respiration at night).
• Nutrition: photosynthesis + mineral uptake.

Single-celled organisms (bacteria, amoeba, paramecium) also show all seven — at the cellular scale.

Cambridge tip. When asked "give an example of MRS GREN process X in a plant", be specific: "phototropism" beats "moving"; "photosynthesis" beats "eating".