What you'll learn
This topic covers the seven characteristics of living organisms and the essential life processes that distinguish living things from non-living matter. CXC CSEC Integrated Science papers consistently test your ability to identify, explain and compare these processes across different organism types, from bacteria to humans. Expect questions worth 15-20 marks across Papers 1 and 2.
Key terms and definitions
Nutrition — the process by which organisms obtain and use food substances for energy, growth and repair of tissues.
Respiration — the chemical breakdown of food substances (usually glucose) in cells to release energy, occurring in all living cells.
Excretion — the removal of toxic metabolic waste products from the body, not to be confused with egestion (removal of undigested food).
Homeostasis — the maintenance of constant internal conditions in an organism despite changes in the external environment.
Autotroph — an organism that manufactures its own food from simple inorganic substances, typically through photosynthesis.
Heterotroph — an organism that cannot make its own food and must obtain organic nutrients by consuming other organisms.
Metabolism — the sum of all chemical reactions occurring in living cells, including both breakdown (catabolism) and building-up (anabolism) processes.
Stimulus — a change in the internal or external environment that produces a response in an organism.
Core concepts
The seven characteristics of life (MRS GREN)
All living organisms demonstrate these seven processes. CXC examiners frequently ask you to identify which characteristic is being demonstrated in a given scenario:
M — Movement: change in position or place, either of the whole organism (locomotion) or parts within it. In animals, the mongoose moves to catch prey; in plants, the Mimosa pudica (sensitive plant common in Trinidad) closes its leaves when touched.
R — Respiration: energy release from food. Occurs in mitochondria of cells. Not the same as breathing (which is gas exchange).
S — Sensitivity: ability to detect and respond to stimuli. The iguana basks in sunlight to warm up; the hibiscus flower closes at night.
G — Growth: permanent increase in size and dry mass through cell division and enlargement. A breadfruit seedling grows into a mature tree.
R — Reproduction: production of new individuals. Sexual (two parents, variation in offspring) or asexual (one parent, identical offspring).
E — Excretion: removal of metabolic wastes. Humans excrete urea in urine, carbon dioxide through lungs. Not elimination of faeces (that is egestion).
N — Nutrition: obtaining food for energy and materials. Autotrophic (making own food like coconut palms) or heterotrophic (consuming others like chickens).
Nutrition in different organisms
Autotrophic nutrition occurs in green plants through photosynthesis:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Chlorophyll in chloroplasts traps light energy to convert carbon dioxide and water into glucose and oxygen. This process requires:
- Light (sun exposure)
- Chlorophyll (green pigment)
- Carbon dioxide (from air through stomata)
- Water (from soil through roots)
- Suitable temperature (25-35°C optimal)
Caribbean crops like sugar cane in Jamaica and Trinidad are highly efficient photosynthesizers, storing glucose as sucrose in stems.
Heterotrophic nutrition takes three main forms:
Holozoic (animals): ingestion → digestion → absorption → assimilation → egestion. The agouti ingests fruits, digests them in its gut, absorbs nutrients into blood, uses them for energy, and egests undigested matter.
Saprophytic (fungi and some bacteria): secrete enzymes onto dead organic matter, digest it externally, absorb soluble products. Mushrooms growing on rotting coconut husks in Barbados demonstrate this.
Parasitic: obtain nutrients from a living host, causing harm. The Plasmodium parasite causes malaria, still prevalent in Guyana's interior regions.
Respiration: aerobic versus anaerobic
Aerobic respiration requires oxygen and occurs in mitochondria:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (2880 kJ/mol glucose)
This is the most efficient energy-release process. Athletes training at the National Stadium in Jamaica rely on aerobic respiration for sustained performance.
Anaerobic respiration occurs without oxygen:
In animals: C₆H₁₂O₆ → 2C₃H₆O₃ + energy (150 kJ/mol) (Glucose → Lactic acid + energy)
In yeast/plants: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy (Glucose → Ethanol + Carbon dioxide + energy)
Anaerobic respiration in yeast is exploited in rum production throughout the Caribbean, particularly in Barbados and Trinidad. The process is called fermentation.
Key differences tested on CXC papers:
- Aerobic produces more energy (2880 kJ vs 150 kJ per glucose molecule)
- Aerobic requires oxygen; anaerobic does not
- Aerobic produces CO₂ and H₂O; anaerobic in animals produces lactic acid
- Lactic acid causes muscle fatigue and oxygen debt
Excretion in humans
The main excretory organs and their waste products:
Kidneys: remove urea (from protein breakdown), excess water and salts. Each kidney contains approximately one million nephrons (functional units). The nephron performs:
- Ultrafiltration in the Bowman's capsule: blood pressure forces small molecules (water, glucose, urea, salts) from glomerulus into nephron
- Selective reabsorption in the tubules: useful substances (all glucose, most water, needed salts) reabsorbed into blood
- Secretion: additional wastes added to form urine
Final urine composition: water (95%), urea (2%), salts and other wastes (3%).
Lungs: excrete carbon dioxide and water vapour produced during respiration.
Skin: removes water, salts and traces of urea through sweat. This also aids thermoregulation (temperature control).
Liver: converts toxic ammonia (from amino acid breakdown) into less toxic urea. Also breaks down alcohol and drugs, removes old red blood cells.
Common exam error: egestion of faeces is NOT excretion because faeces are undigested food, not metabolic waste products.
Growth and development
Growth can be measured by:
- Increase in height/length
- Increase in mass (wet mass or dry mass — dry mass is more reliable)
- Increase in cell number
Germination is frequently tested using Caribbean seeds. A red bean (kidney bean) germinates when:
- Water is absorbed, activating enzymes
- Enzymes convert stored starch to soluble sugars
- Respiration increases, providing energy
- Radicle (first root) emerges, followed by plumule (first shoot)
Conditions needed: water, oxygen, suitable temperature (25-30°C for tropical seeds). Light is NOT required for germination.
Reproduction
Asexual reproduction involves one parent producing genetically identical offspring (clones):
- Binary fission in bacteria and amoeba: cell divides into two
- Budding in yeast and hydra: outgrowth develops and detaches
- Vegetative propagation in plants: new plants from stems (cassava cuttings), leaves (Bryophyllum), roots (sweet potato tubers)
Advantages: rapid, no mate needed, successful characteristics preserved. Disadvantage: no genetic variation, vulnerable to environmental changes.
Sexual reproduction requires two parents (or two gametes from one hermaphrodite):
- Male gamete (sperm/pollen) fuses with female gamete (egg/ovum) = fertilisation
- Produces genetic variation through independent assortment and crossing over
- Offspring can adapt to changing environments
In flowering plants common in the Caribbean (bougainvillea, ixora, hibiscus):
- Pollination: transfer of pollen from anther to stigma (by insects, wind, birds)
- Fertilisation: pollen nucleus fuses with ovum in ovule
- Seed formation: ovule becomes seed, ovary becomes fruit
Sensitivity and coordination
Organisms detect stimuli through receptors and respond using effectors.
Tropisms in plants (growth responses):
- Phototropism: shoots grow towards light (positive), roots away from light (negative)
- Geotropism: roots grow towards gravity (positive), shoots grow away (negative)
- Hydrotropism: roots grow towards water (positive)
Caused by unequal distribution of auxin (plant hormone) stimulating cell elongation.
Reflex actions in animals are rapid, automatic responses:
- Stimulus detected by receptor (e.g., heat detected by skin receptors)
- Sensory neurone carries impulse to spinal cord
- Relay neurone in spinal cord passes impulse to motor neurone
- Motor neurone carries impulse to effector (muscle)
- Effector responds (muscle contracts, hand withdraws)
This pathway is the reflex arc. Bypasses the brain for speed, protecting from harm.
Worked examples
Question 1: A student set up four boiling tubes containing soaked red beans to investigate germination conditions:
| Tube | Water | Temperature | Oxygen |
|---|---|---|---|
| A | Yes | 25°C | Yes |
| B | No | 25°C | Yes |
| C | Yes | 5°C | Yes |
| D | Yes | 25°C | No (oil layer) |
(a) In which tube would germination occur? (1 mark)
(b) Explain why germination would not occur in tube D. (2 marks)
(c) State one way the student could measure growth. (1 mark)
Answer:
(a) Tube A [1 mark]
(b) The oil layer prevents oxygen from reaching the seeds [1 mark]. Oxygen is required for respiration to release energy for germination [1 mark].
(c) Measure the length of the radicle/shoot OR measure the increase in dry mass [1 mark]
Question 2: The table shows the composition of blood plasma, glomerular filtrate and urine:
| Substance | Plasma (%) | Filtrate (%) | Urine (%) |
|---|---|---|---|
| Protein | 7.0 | 0.0 | 0.0 |
| Glucose | 0.1 | 0.1 | 0.0 |
| Urea | 0.03 | 0.03 | 2.0 |
(a) Explain why protein is absent from the filtrate. (2 marks)
(b) Account for the difference in glucose concentration between filtrate and urine. (2 marks)
(c) Explain why urea concentration increases from plasma to urine. (2 marks)
Answer:
(a) Protein molecules are too large [1 mark] to pass through the basement membrane of the Bowman's capsule during ultrafiltration [1 mark].
(b) Glucose is filtered from blood into the filtrate [1 mark]. It is completely reabsorbed from the tubule back into the blood because it is useful to the body [1 mark].
(c) Water is reabsorbed from the filtrate, making urine more concentrated [1 mark]. Urea is not reabsorbed because it is a waste product [1 mark].
Question 3: A farmer in St. Lucia notices his dasheen plants growing towards a nearby window.
(a) Name this type of response. (1 mark)
(b) Explain the advantage of this response to the plant. (2 marks)
Answer:
(a) Positive phototropism [1 mark]
(b) Allows the plant to grow towards light [1 mark], maximizing photosynthesis and food production [1 mark].
Common mistakes and how to avoid them
Mistake: Confusing respiration with breathing. Correction: Breathing is gas exchange (inhaling oxygen, exhaling carbon dioxide). Respiration is the chemical breakdown of glucose in cells to release energy. All cells respire; only lungs breathe.
Mistake: Stating that plants only photosynthesize and animals only respire. Correction: Plants carry out both processes. They photosynthesize in light (producing glucose) and respire all the time (breaking down glucose for energy). Animals only respire.
Mistake: Describing egestion (removal of faeces) as excretion. Correction: Excretion removes metabolic waste products made by cells (urea, CO₂). Egestion removes undigested food that was never inside cells. Faeces = egestion, urine = excretion.
Mistake: Claiming aerobic and anaerobic respiration produce equal energy. Correction: Aerobic respiration releases approximately 19 times more energy per glucose molecule (2880 kJ vs 150 kJ). Always specify which produces more when comparing.
Mistake: Saying light is needed for germination. Correction: Seeds germinate in darkness. Requirements are water, oxygen and suitable temperature only. Light is needed after germination for photosynthesis and continued growth.
Mistake: Identifying the kidney as the only excretory organ. Correction: Four organs excrete in humans: kidneys (urea, water, salts), lungs (CO₂, water), skin (water, salts, traces of urea), liver (produces urea, removes toxins). Papers often test this list.
Exam technique for Organisms and Life Processes
Command word awareness: "State" requires a brief answer without explanation (1 mark). "Explain" requires a reason using because/due to/as a result (usually 2 marks). "Describe" requires an account of what happens in sequence (2-3 marks). "Compare" requires stating similarities AND differences.
Equations earn marks: Write word equations for photosynthesis and respiration in full. Chemical equations earn extra credit. Aerobic respiration: always include oxygen on left, carbon dioxide and water on right. Check glucose formula is C₆H₁₂O₆.
Table and diagram analysis: Papers 2 and 3 frequently present data tables or diagrams. Read all headings carefully. If comparing filtrate and urine compositions, examiners want you to mention which substances increase/decrease/stay the same AND explain why using kidney function terms (ultrafiltration, selective reabsorption).
Practical contexts: Be ready to interpret germination experiments (control variables), food tests (Benedict's for reducing sugars, iodine for starch, biuret for protein), or photosynthesis investigations (testing for starch, measuring oxygen production). Show working in calculations and include units.
Quick revision summary
All living organisms show MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. Autotrophs make food by photosynthesis; heterotrophs consume others. Aerobic respiration needs oxygen and produces 2880 kJ/mol glucose; anaerobic respiration produces only 150 kJ/mol. Kidneys excrete urea through ultrafiltration and selective reabsorption. Germination requires water, oxygen and warmth—not light. Sexual reproduction creates variation; asexual reproduction produces clones. Plants respond to stimuli through tropisms caused by auxin. Excretion removes metabolic wastes (urea, CO₂); egestion removes undigested food.