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1.1The Nature and Variety of Living Organisms

Characteristics of Living Organisms — Edexcel Linear - OL Revision Notes & Topical Questions

10 min read 23 parts Lesson 4 of 4 Unit quiz

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Lesson summary

Characteristics of Living Organisms: MRS C GREN — EDEXCEL LINEAR - OL notes: Describe the eight characteristics of living organisms and distinguish related terms such as respiration, gas exchange, excretion and egestion.To be classed as living, an organism must carry out all the …

  • metabolic reactions
  • Sensitivity
  • homeostasis
  • Reproduction
  • Worked example 1
  • Worked example 2

Part 1 of 7The eight characteristics

To be classed as living, an organism must carry out all the life processes, remembered by MRS C GREN: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, Nutrition. If something does not carry out all of these it is dead or non-living; viruses are good examples of non-living particles.

Part 2 of 7Nutrition and respiration

Organisms must obtain food for energy and materials for growth. Plants make their own food by photosynthesis using sunlight, carbon dioxide and water, producing glucose and oxygen. Animals (and fungi) consume food; they digest large complex molecules into simpler ones, which are absorbed into the blood and carried to the cells for energy, growth and repair.

Respiration is a chemical process in all living organisms that releases energy from glucose, either in the presence of oxygen (aerobic) or in its absence (anaerobic).

Aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy as ATP).
Gas exchange
Do not confuse it with gas exchange, which gets oxygen into cells and carbon dioxide out; respiration uses that oxygen to release energy.

Part 3 of 7Excretion and the difference from egestion

Chemical reactions inside cells are metabolic reactions; they produce waste products, some toxic, which must be removed. Excretion is the removal of toxic materials and waste products of metabolism.

OrganismWaste products excreted
AnimalsCarbon dioxide and water from respiration; urea (containing nitrogen) from the breakdown of proteins
PlantsOxygen from photosynthesis (mainly in the day); carbon dioxide from respiration (more at night)

Egestion is different: faeces are the remains of food that was never absorbed, not a product of cell reactions.

Part 4 of 7Sensitivity, movement and control

Sensitivity is the ability to detect and respond to stimuli, which gives the best chances of survival.

Sensitivity, movement and control
In humans the nervous system (receptors, neurones, effectors) uses electrical impulses, and the endocrine system uses hormones carried in the blood.
In plants, responses are controlled by chemicals and are usually much slower: geotropism (roots grow down in response to gravity) and phototropism (shoots grow towards light).

Movement is an action causing a change of position or place; movement from place to place is locomotion. Plants cannot move from place to place but can change orientation, as sunflowers do tracking the sun.

Control means keeping the internal environment within limits (homeostasis). Thermoregulation holds human body temperature at about 37 °C (sweating, vasodilation); osmoregulation controls water levels; plants use transpiration to cool themselves, losing heat as water evaporates from stomata.

Part 5 of 7Reproduction and growth

Reproduction produces more of the same kind of organism and is vital for the survival of the species.

Sexual reproduction
In sexual reproduction male and female gametes fuse (sperm and egg in humans; pollen and ovule in plants),
  • so the offspring DNA comes from both parents.
Asexual reproduction
In asexual reproduction (for example mitosis, runners, cuttings, binary fission in bacteria) there is one parent and identical offspring. Growth is a permanent increase in size; animals grow from zygote to adult with changes of shape,
  • while plants keep growing through their life.

Part 6 of 7Linking the characteristics together

The eight characteristics depend on one another.

Linking the characteristics together
Nutrition supplies the glucose that respiration needs; respiration supplies the energy for movement, growth, reproduction and active transport in excretion; sensitivity detects changes and control (homeostasis) corrects them.
Because a virus has no metabolism of its own and does not carry out all eight processes, it is not classed as living.
A candle flame or a car might move and appear to use energy,
  • but neither grows by itself, reproduces or responds in the way organisms do,
  • so checking all eight is essential.

Part 7 of 7Worked examples

  1. List the eight characteristics of living organisms. [4]4 marks

  2. Distinguish between respiration and gas exchange. [2]2 marks

  3. Distinguish between excretion and egestion. [2]2 marks

  4. Name the waste products excreted by a plant and when. [2]2 marks

  5. Explain why a sunflower tracking the sun is movement but not locomotion. [2]2 marks

Key terms

TermMeaning
MRS C GRENMnemonic for the characteristics: movement, respiration, sensitivity, control, growth, reproduction, excretion, nutrition.
ExcretionRemoval of toxic waste products of metabolism.
EgestionRemoval of undigested food as faeces.
HomeostasisMaintaining a constant internal environment.
SensitivityThe ability to detect and respond to stimuli.
Metabolic reactionA chemical reaction inside living cells.

Avoid these mistakes

1
Confusing respiration with breathing or gas exchange.
2
Using excretion and egestion as the same thing.
3
Saying plants do not show movement — they change orientation.
4
Defining growth as any increase in size — it must be permanent.

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