What you'll learn
The characteristics of living organisms form the foundation of CXC CSEC Biology, appearing in Paper 01 multiple-choice questions and Paper 02 structured questions worth 6-10 marks. This topic examines the seven life processes that distinguish living things from non-living matter: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. Mastering these characteristics enables you to identify living organisms, explain biological processes, and apply knowledge across all other Biology topics.
Key terms and definitions
Living organism — any entity that exhibits all seven characteristics of life simultaneously and can maintain itself as a distinct biological system.
MRS GREN — a mnemonic representing the seven characteristics: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
Stimulus — any change in the internal or external environment that produces a response in a living organism.
Metabolism — the sum of all chemical reactions occurring within a living organism, including both breaking down (catabolism) and building up (anabolism) processes.
Excretion — the removal of toxic metabolic waste products from the body, distinct from egestion which removes undigested food.
Homeostasis — the maintenance of a constant internal environment despite changes in external conditions, achieved through coordinated responses involving multiple characteristics of life.
Asexual reproduction — reproduction involving a single parent producing genetically identical offspring without the fusion of gametes.
Sexual reproduction — reproduction involving the fusion of two specialized sex cells (gametes) from two parents, producing genetically varied offspring.
Core concepts
1. Movement (M)
Movement refers to the ability of an organism or its parts to change position or place. This characteristic appears differently in plants and animals but remains essential for survival.
In animals:
- Whole-body locomotion through walking, swimming, flying, or crawling
- Internal movement of blood through vessels (circulation)
- Movement of food through the digestive tract (peristalsis)
- Examples: The Trinidadian hummingbird (Glaucis hirsutus) moves its wings rapidly for flight and hovering; the mongoose moves across Jamaican hillsides hunting prey
In plants:
- Growth movements toward or away from stimuli (tropisms)
- Opening and closing of stomata on leaf surfaces
- Cytoplasmic streaming within cells
- Examples: Coconut palms along Caribbean beaches bend toward sunlight (phototropism); the sensitive plant (Mimosa pudica) found throughout the region folds its leaves when touched
CXC examiners distinguish between movement and locomotion. Movement is the broader term applicable to all organisms, while locomotion specifically means movement from one place to another, primarily in animals.
2. Respiration (R)
Respiration is the chemical process that releases energy from food molecules, primarily glucose. This occurs in all living cells and must not be confused with breathing (gas exchange).
Aerobic respiration — occurs in the presence of oxygen, releasing maximum energy:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Anaerobic respiration — occurs without oxygen, releasing less energy:
In animals: Glucose → Lactic acid + Energy In yeast and plants: Glucose → Ethanol + Carbon dioxide + Energy
Caribbean relevance: The rum industry in Trinidad, Barbados, and Jamaica depends on anaerobic respiration in yeast to ferment sugarcane juice, producing ethanol. Local bakeries also use yeast respiration to make bread rise through CO₂ production.
Respiration provides energy for all life processes including movement, active transport, protein synthesis, cell division, and maintaining body temperature in warm-blooded animals.
3. Sensitivity (S)
Sensitivity (also called irritability or responsiveness) is the ability to detect and respond to changes in the internal or external environment. These changes are called stimuli, and the reactions they cause are responses.
Common stimuli and responses:
- Light — plants grow toward light (phototropism); human pupils constrict in bright light
- Touch — mimosa leaves fold when touched; humans withdraw hands from hot surfaces
- Chemicals — roots grow toward water (hydrotropism); tongues detect sweet, sour, bitter, salty tastes
- Temperature — humans shiver when cold, sweat when hot; reptiles bask in sunlight to warm up
- Sound — bats use echolocation; humans turn toward sudden noises
- Gravity — plant roots grow downward (geotropism); humans maintain balance
Caribbean example: Mangrove trees in Trinidad's Caroni Swamp respond to tidal changes (salinity stimulus) by adjusting salt concentrations in their tissues. The fer-de-lance snake in St. Lucia detects heat from prey using specialized pit organs.
4. Growth (G)
Growth is a permanent increase in size and dry mass through cell division (mitosis) and cell enlargement. True growth involves producing new cellular material, not temporary swelling from water uptake.
Characteristics of growth:
- Irreversible increase in cell number and size
- Requires raw materials (nutrients) and energy
- Measured by increases in height, length, mass, or volume
- Different patterns in plants versus animals
In animals:
- Growth occurs throughout the body during youth, then stops at maturity
- Controlled by growth hormones
- Example: A Jamaican broiler chicken grows from 40g at hatching to 2.5kg in 6 weeks through cell division and protein accumulation
In plants:
- Growth continues throughout life at meristems (growing points)
- Occurs at root tips, shoot tips, and cambium
- Example: A coconut seedling grows continuously, adding new fronds and height throughout its 60-80 year lifespan
CXC papers often test the difference between growth and development. Development includes growth plus the changes in shape, complexity, and function that occur as an organism matures.
5. Reproduction (R)
Reproduction is the biological process by which organisms produce new individuals of the same species. This characteristic ensures species survival across generations.
Asexual reproduction:
- Single parent produces genetically identical offspring (clones)
- No gamete fusion required
- Faster than sexual reproduction
- Methods: binary fission (bacteria, amoeba), budding (yeast, hydra), vegetative propagation (stem cuttings, tubers, runners)
- Caribbean example: Dasheen and eddoes cultivated in Trinidad reproduce asexually through corms; sugarcane propagates through stem cuttings
Sexual reproduction:
- Two parents produce genetically varied offspring
- Requires specialized sex cells (gametes): sperm and egg
- Gametes fuse during fertilization
- Produces genetic variation, enabling adaptation
- Caribbean example: Mango trees in Jamaica produce flowers containing male stamens and female carpels; pollination and fertilization produce mango fruits containing seeds
Many CXC questions require comparing these two types:
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | One | Two |
| Gametes | Not involved | Sperm and egg fuse |
| Genetic variation | No (clones) | Yes (varied offspring) |
| Speed | Fast | Slower |
| Energy | Less required | More required |
6. Excretion (E)
Excretion is the removal of toxic metabolic waste products produced by chemical reactions inside cells. This must not be confused with egestion (removing undigested food).
Main excretory products in humans:
- Carbon dioxide — produced by respiration; removed via lungs during breathing
- Urea — produced from excess amino acids in the liver; removed via kidneys in urine
- Water — excess removed via kidneys (urine), skin (sweat), and lungs (water vapor)
- Salts — excess removed via kidneys and skin
In plants:
- Oxygen produced during photosynthesis (excreted if not used)
- Carbon dioxide produced during respiration (excreted at night)
- Water removed through transpiration
- Some waste stored in leaves that eventually fall
Caribbean context: The kidneys of reef fish around Barbados and the Cayman Islands excrete ammonia directly into surrounding water. Marine mammals like dolphins in Tobago waters excrete urea like land mammals.
Excretion maintains homeostasis by preventing toxic buildup. Kidney failure requires dialysis machines available at regional hospitals in Kingston, Port of Spain, and Bridgetown to artificially remove urea.
7. Nutrition (N)
Nutrition is the process by which organisms obtain food to provide energy, raw materials for growth and repair, and substances to regulate body processes.
Autotrophic nutrition (self-feeding):
- Organisms make their own food from simple inorganic substances
- Photosynthesis in green plants:
Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
- Examples: All green plants including Caribbean crops (callaloo, banana, breadfruit), phytoplankton in coastal waters, algae
Heterotrophic nutrition (feeding on others):
- Organisms obtain complex organic compounds by consuming other organisms
- Types include:
- Herbivores — eat plants (manicou/opossum, green iguana)
- Carnivores — eat animals (fer-de-lance snake, barracuda)
- Omnivores — eat both (humans, mongoose)
- Decomposers — feed on dead matter (fungi, bacteria)
Caribbean example: Sugarcane fields in Barbados and Guyana produce food through photosynthesis. These crops feed cattle (herbivores), whose manure is decomposed by soil bacteria, recycling nutrients back to the soil.
Worked examples
Example 1 (Paper 01 style):
Question: Which process occurs in BOTH plants and animals?
A. Photosynthesis
B. Respiration
C. Transpiration
D. Digestion in a stomach
Answer: B
Explanation: Respiration is a universal characteristic of all living organisms. Both plant and animal cells respire to release energy from glucose. Photosynthesis occurs only in organisms with chlorophyll, transpiration is specific to plants, and only animals have stomachs. Worth 1 mark.
Example 2 (Paper 02 style):
Question: A student placed a potted bean plant in a box with a hole on one side allowing light to enter. After five days, the plant stem had bent toward the light source.
(a) Name the characteristic of living organisms demonstrated by the bean plant. (1 mark)
(b) Name the specific type of response shown by the plant stem. (1 mark)
(c) Explain why this response is beneficial to the plant. (2 marks)
Model Answer:
(a) Sensitivity / Irritability / Responsiveness (1 mark)
(b) Phototropism / Positive phototropism (1 mark)
(c)
- The stem grows toward light to increase exposure of leaves to sunlight (1 mark)
- This increases the rate of photosynthesis, producing more food for the plant's growth and survival (1 mark)
Mark scheme notes: Part (a) accepts any of the three alternative terms. Part (b) requires the term phototropism; "growth toward light" alone earns 0 marks. Part (c) requires both the mechanism (exposure) and benefit (photosynthesis/food production) for full marks.
Example 3 (Paper 02 extended):
Question: The table shows four observations made of an organism found in soil in a Jamaican garden.
| Observation | Description |
|---|---|
| 1 | Moves slowly through soil |
| 2 | Increases in size over time |
| 3 | Green in color |
| 4 | Produces seeds |
(a) State which TWO characteristics of living organisms are shown by observations 1 and 2. (2 marks)
(b) Observation 3 suggests the organism can make its own food. Name this type of nutrition. (1 mark)
(c) Observation 4 indicates reproduction. Explain why reproduction is important for this species. (2 marks)
Model Answer:
(a)
- Movement (1 mark)
- Growth (1 mark)
(b) Autotrophic nutrition / Photosynthesis (1 mark)
(c)
- Reproduction produces new individuals to replace those that die (1 mark)
- This ensures the species continues to exist / prevents extinction (1 mark)
Common mistakes and how to avoid them
• Confusing breathing with respiration — Breathing (ventilation) is the mechanical process of moving air in and out of lungs. Respiration is the chemical reaction in cells that releases energy from glucose. CXC papers specifically test this distinction. Always refer to respiration as the "release of energy from food" in exam answers.
• Writing egestion instead of excretion — Excretion removes metabolic waste products made inside cells (urea, carbon dioxide). Egestion removes undigested food that was never absorbed into cells. The removal of feces is egestion, NOT excretion. Mark schemes penalize this heavily.
• Listing only five or six characteristics — The complete answer requires all seven (MRS GREN). Partial lists in "state the characteristics of living organisms" questions lose marks. Practice writing all seven quickly: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
• Describing tropisms as examples of movement instead of sensitivity — Phototropism and geotropism demonstrate sensitivity (detecting and responding to stimuli), though they involve growth movements. CXC mark schemes classify these under sensitivity, not movement. The stimulus (light/gravity) and response (growth direction) must be stated clearly.
• Stating that plants excrete oxygen as a waste product — Oxygen is a product of photosynthesis, not a metabolic waste. Plants excrete excess oxygen only when not using it for respiration. The primary plant excretory product is carbon dioxide from respiration, removed through stomata.
• Confusing growth with temporary swelling — Growth is a permanent increase in dry mass through new cell formation. A seed swelling in water or a sponge expanding is NOT growth. CXC questions often include distractors involving water uptake. True growth requires cell division (mitosis) and addition of new organic material.
Exam technique for "Characteristics of Living Organisms"
• Command words determine mark allocation — "State" or "List" questions (1 mark each) require simple recall: write one characteristic per mark. "Explain" questions (2-3 marks) demand the process plus reasoning: describe how the characteristic works AND why it matters. "Distinguish between" questions require clear contrasting statements for both terms.
• Use precise biological terminology throughout — Write "respiration releases energy from glucose" not "breathing gives you energy." Use "excretion of metabolic waste" not "getting rid of rubbish." Mark schemes award marks only for scientifically accurate language. Generic terms lose marks even if the concept is understood.
• Examples must match the organism type specified — If the question asks for plant examples, animal responses earn zero marks. CXC papers frequently specify "in flowering plants" or "in mammals." Read carefully and provide context-appropriate examples: phototropism for plants, reflex actions for animals.
• Structure comparison answers in parallel format — When comparing asexual and sexual reproduction or autotrophic and heterotrophic nutrition, state the same feature for both. Example: "Asexual reproduction involves one parent; sexual reproduction involves two parents" scores full marks. Scattered, non-parallel information loses marks for lack of clarity.
Quick revision summary
All living organisms share seven characteristics remembered as MRS GREN: Movement (change in position), Respiration (energy release from food), Sensitivity (detecting and responding to stimuli), Growth (permanent increase in size and mass), Reproduction (producing new individuals), Excretion (removing metabolic waste), and Nutrition (obtaining food for energy and growth). Plants and animals display these characteristics differently—plants use photosynthesis (autotrophic) while animals consume food (heterotrophic); plants show tropisms while animals show locomotion. Master the definitions, provide organism-specific examples, and distinguish respiration from breathing and excretion from egestion for CXC CSEC Biology examination success.