Characteristics of Living Organisms

What you'll learn

The characteristics of living organisms form the foundation of CIE IGCSE Biology. This topic establishes the criteria that distinguish living from non-living things through seven key life processes, commonly remembered using the acronym MRS GREN. Examination questions frequently require you to identify these characteristics in unfamiliar organisms, apply them to decide whether something is living, and explain how specific organisms demonstrate each process.

Key terms and 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 by an increase in cell number or cell size or both.

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; plants require light, carbon dioxide, water and ions; animals need organic compounds and ions and usually need water.

Metabolism — the sum of all chemical reactions that take place in living organisms.

Core concepts

The seven characteristics: MRS GREN

All living organisms carry out seven life processes. The mnemonic MRS GREN helps recall these characteristics, which examiners expect you to name, define and recognise in different contexts:

• Movement
• Respiration
• Sensitivity
• Growth
• Reproduction
• Excretion
• Nutrition

Every characteristic must be present for something to be classified as living. Viruses, for example, are not considered living organisms because they cannot carry out all seven processes independently—they require a host cell for reproduction and do not respire or excrete.

Movement in detail

Movement involves a change in position or place, but the type varies between organisms:

Animals show locomotion—the entire organism moves from one location to another. Examples include:

• Fish swimming using muscles and fins
• Birds flying through muscular contraction of wings
• Earthworms using circular and longitudinal muscles to move through soil

Plants demonstrate movement without locomotion:

• Shoots grow towards light (phototropism)
• Roots grow towards gravity and water (gravitropism and hydrotropism)
• Leaves of the Mimosa pudica fold when touched
• Stomata open and close in response to light

Unicellular organisms use specialised structures:

• Amoeba moves using pseudopodia (false feet)
• Paramecium uses cilia for movement
• Euglena uses a flagellum

Exam questions often present unfamiliar organisms and ask you to identify evidence of movement from descriptions or diagrams.

Respiration as a universal process

Respiration occurs in all living cells and releases energy from nutrient molecules, primarily glucose. This process differs fundamentally from breathing (gas exchange):

Aerobic respiration requires oxygen:

• Takes place in mitochondria
• Releases large amounts of energy
• Produces carbon dioxide and water as waste products
• Word equation: glucose + oxygen → carbon dioxide + water

Anaerobic respiration occurs without oxygen:

• Releases less energy than aerobic respiration
• In animals: glucose → lactic acid
• In plants and yeast: glucose → ethanol + carbon dioxide

The energy released powers all metabolic reactions, including:

• Muscle contraction
• Protein synthesis
• Cell division
• Active transport
• Maintaining constant body temperature in mammals

Examiners frequently test whether you can distinguish respiration (a chemical reaction in cells) from breathing (a physical process of gas exchange).

Sensitivity and responding to stimuli

Sensitivity allows organisms to detect changes (stimuli) and respond appropriately, increasing survival chances.

Animal examples:

• Eyes detect light; pupils dilate or constrict accordingly
• Ears detect sound; organisms move towards or away from the source
• Skin detects temperature; shivering or sweating responses follow
• Nose and tongue detect chemicals; organisms avoid harmful substances
• Touch receptors detect pressure; withdrawal reflexes protect from harm

Plant examples:

• Phototropism: stems grow towards light to maximise photosynthesis
• Gravitropism: roots grow downwards towards water and minerals
• Thigmotropism: climbing plants wind around supports
• Nastic responses: Venus flytraps snap shut when trigger hairs detect insects

The response always serves to improve the organism's survival or reproduction prospects.

Growth and development

Growth represents a permanent increase in size and dry mass, not temporary changes like water uptake.

Key features of growth:

• Increase in cell number through mitosis
• Increase in cell size as cells expand
• Measured by height, length, mass or volume
• Dry mass is the most reliable measure (eliminates water variation)

Growth patterns differ:

• Animals grow to a certain size then stop (determinate growth)
• Plants continue growing throughout life (indeterminate growth)
• Unicellular organisms grow until reaching a size that triggers cell division

Development accompanies growth:

• Cells differentiate into specialised types
• Tissues and organs form
• Organisms mature and gain the ability to reproduce

Exam questions may show growth curves or provide mass/length data requiring interpretation.

Reproduction strategies

Reproduction produces new individuals, ensuring species continuation. Two distinct types exist:

Asexual reproduction:

• Involves one parent only
• No gamete fusion
• Offspring are genetically identical to the parent (clones)
• Examples: binary fission in bacteria, budding in yeast, runners in strawberry plants, tubers in potatoes
• Advantages: rapid, no mate needed, successful characteristics preserved
• Disadvantages: no genetic variation, vulnerability to environmental changes

Sexual reproduction:

• Involves two parents
• Requires gamete fusion (fertilisation)
• Offspring show genetic variation
• Examples: flowering plants producing seeds, mammals giving birth to live young, fish releasing eggs and sperm
• Advantages: genetic variation aids survival in changing environments
• Disadvantages: slower, requires finding a mate, more energy-intensive

Questions often ask you to identify the reproduction type from descriptions or explain advantages in specific contexts.

Excretion of metabolic waste

Excretion removes waste products made by metabolism, plus substances in excess. This differs from egestion (removing undigested food).

Metabolic waste products:

• Carbon dioxide from respiration (excreted via lungs in mammals, leaves in plants)
• Urea from amino acid breakdown (excreted via kidneys in urine)
• Water produced in respiration and excess water intake (excreted via kidneys, lungs, skin)

Why excretion matters:

• Carbon dioxide lowers blood pH if accumulated, affecting enzyme function
• Urea is toxic at high concentrations
• Excess water could burst cells by osmosis

Excretory organs:

• Lungs excrete carbon dioxide and water vapour
• Kidneys filter blood and produce urine containing urea, excess water and salts
• Skin releases sweat containing water, salts and small amounts of urea
• Plants excrete oxygen (from photosynthesis) and carbon dioxide (from respiration) through stomata

Examiners expect precise vocabulary: excretion specifically means removing metabolic waste, not removal of faeces or other materials.

Nutrition and energy sources

Nutrition provides raw materials for energy release, growth and tissue repair.

Autotrophic nutrition (producers):

• Organisms make their own organic molecules from inorganic substances
• Plants use photosynthesis: light energy + carbon dioxide + water → glucose + oxygen
• Require: light, carbon dioxide, water, mineral ions (nitrogen for proteins, magnesium for chlorophyll)

Heterotrophic nutrition (consumers):

• Organisms obtain organic molecules by consuming other organisms
• Animals digest large molecules into smaller soluble molecules for absorption
• Require: carbohydrates, proteins, lipids, vitamins, minerals, water, fibre

Feeding methods vary:

• Herbivores consume plants
• Carnivores consume animals
• Omnivores consume both
• Decomposers (fungi, bacteria) feed on dead organic matter

Questions may present unfamiliar organisms and ask you to classify their nutrition based on descriptions of feeding behaviour.

Worked examples

Example 1: A student investigates whether seeds are living organisms. Describe how the seeds demonstrate four different characteristics of living organisms. [4 marks]

Model answer:

• Respiration: Seeds respire aerobically or anaerobically, releasing energy from stored food reserves, producing carbon dioxide as a waste product [1]
• Growth: When provided with water and warmth, seeds germinate and increase in size/mass as cells divide and expand [1]
• Reproduction: Seeds develop into mature plants that eventually produce flowers, fruits and new seeds [1]
• Sensitivity: Seeds detect environmental stimuli such as water, oxygen and temperature, germinating only when conditions are favourable [1]

Examiner note: Each characteristic requires both naming and explaining with specific reference to seeds. Generic descriptions without application score zero.

Example 2: A scientist discovers an organism in a deep-sea vent. The organism moves towards chemicals released by other organisms, breaks down organic molecules to release energy, and produces identical offspring by splitting in two. State three characteristics of living organisms shown by this organism. [3 marks]

Model answer:

• Movement: the organism moves towards chemicals [1]
• Respiration: the organism breaks down organic molecules to release energy [1]
• Reproduction: the organism produces offspring by splitting (binary fission) [1]

Examiner note: You must correctly identify the characteristic from the description. Stating "sensitivity" because it detects chemicals would also be acceptable for the first point.

Example 3: Explain why viruses are not classified as living organisms. [3 marks]

Model answer:

• Viruses cannot carry out all seven characteristics of living organisms independently [1]
• They do not respire or release energy from nutrients through metabolic reactions [1]
• They can only reproduce inside a host cell by using the host's cellular machinery, not independently [1]

Alternative acceptable points: They do not grow, do not excrete waste products, do not feed/take in nutrition.

Common mistakes and how to avoid them

• Mistake: Confusing respiration with breathing or gas exchange. Correction: Respiration is a chemical reaction in cells that releases energy from glucose; breathing is the physical movement of air in and out of lungs. Always define respiration as an energy-releasing process.

• Mistake: Stating that excretion includes removal of faeces. Correction: Egestion removes undigested food (faeces), which was never part of body cells. Excretion specifically removes metabolic waste products made by cells, such as carbon dioxide and urea.

• Mistake: Describing growth as any increase in size, including water uptake. Correction: Growth is a permanent increase in size and dry mass through increased cell number or cell size. A wilted plant absorbing water increases in fresh mass but not dry mass—this is not growth.

• Mistake: Listing characteristics without explaining how an organism demonstrates them. Correction: Questions asking you to "describe" or "explain" characteristics require application to the specific organism, not just naming the seven processes.

• Mistake: Confusing movement with locomotion only. Correction: All living organisms show movement, but not all show locomotion. Plants move through growth responses (tropisms) and internal movement (cytoplasmic streaming), even though they remain rooted in place.

• Mistake: Claiming that reproduction is essential for individual survival. Correction: An organism can survive without reproducing, but reproduction is essential for species survival. Individual organisms can demonstrate all other characteristics without reproducing.

Exam technique for Characteristics of Living Organisms

• Command words matter: "State" requires naming only (one characteristic = one mark). "Describe" requires naming plus detail about how it occurs. "Explain" requires naming plus reasoning why it happens. Adjust your answer length accordingly.

• Application questions dominate: Expect 4-6 mark questions presenting unfamiliar organisms (extremophiles, newly-discovered species, microscopic organisms) requiring you to identify which characteristics they demonstrate from given information. Practice extracting evidence from descriptions.

• Use correct terminology: Write "aerobic respiration" not "breathing", "excretion" not "getting rid of waste", "asexual reproduction" not "cloning". Examiners award marks for precise biological vocabulary, especially at the end of mark schemes where one mark separates grade boundaries.

• Learn all seven characteristics thoroughly: Questions worth 6-7 marks may ask you to describe how one organism demonstrates all characteristics. You must recall all seven from memory—MRS GREN appears in almost every specification and past paper for this topic.

Quick revision summary

All living organisms carry out seven life processes: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition (MRS GREN). Movement includes locomotion and growth responses. Respiration releases energy in cells. Sensitivity detects and responds to stimuli. Growth increases size and dry mass permanently. Reproduction produces offspring asexually or sexually. Excretion removes metabolic waste. Nutrition provides materials for energy and growth through autotrophic or heterotrophic methods. Viruses are not living because they cannot perform all seven processes independently.