Characteristics of Living Organisms Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • All living organisms show seven characteristics: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition (MRS GREN).
  • Growth is a permanent increase in size and dry mass, not a temporary gain of water.
  • Respiration releases energy from nutrients in cells; breathing is gas exchange, a different process.
  • Sensitivity is detecting and responding to changes in the environment.
  • Nutrition feeds the individual; reproduction produces new individuals so the species continues.
  1. 1

    Movement

    Part or all of the organism changes position

  2. 2

    Respiration

    Chemical reactions in cells release energy from nutrients

  3. 3

    Sensitivity

    Detect and respond to changes in the environment

  4. 4

    Growth

    Permanent increase in size and dry mass

  5. 5

    Reproduction

    Processes that make more of the same kind of organism

  6. 6

    Excretion

    Removal of waste products of metabolism

  7. 7

    Nutrition

    Taking in materials for energy, growth and development

The seven characteristics of living organisms (MRS GREN)

Seven characteristics

Every living organism shows movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

The memory aid MRS GREN gives the seven characteristics. Learn the specification wording for each one: vague descriptions lose marks. Organisms are like factories: they take in materials (nutrition), release energy (respiration), grow, remove waste (excretion), respond to their surroundings (sensitivity), move, and make new factories (reproduction).

  1. 1

    Movement

    an action that changes position or place

  2. 2

    Respiration

    chemical reactions in cells that release energy

  3. 3

    Sensitivity

    detect and respond to the surroundings

  4. 4

    Growth

    permanent increase in size and dry mass

  5. 5

    Reproduction

    processes that make more of the same kind

  6. 6

    Excretion

    remove waste products of metabolism

  7. 7

    Nutrition

    take in materials for energy and growth

MRS GREN: the seven characteristics of living organisms

Movement

  • Action causing a change of position or place

Respiration

  • Reactions in cells that release energy

Sensitivity

  • Detecting and responding to changes

Growth

  • Permanent increase in size and dry mass

Which letters of MRS GREN stand for excretion and nutrition?

Movement and sensitivity

Organisms detect changes (stimuli) and respond, and movement means a change of position by the whole organism or a part of it.

A stimulus is detected by a receptor. The nervous or hormonal system passes on the signal, and an effector (a muscle or gland) produces the response. Without sensitivity an organism could not react to danger or opportunity. Plants move too: their parts change position through growth and directional responses, so movement is not only for animals.

  1. 1

    Stimulus

    A change in the environment

  2. 2

    Detection

    A receptor detects the change

  3. 3

    Processing

    Nervous or hormonal system passes on the signal

  4. 4

    Response

    An effector (muscle or gland) acts

From stimulus to response

What is the job of an effector in a response?

Respiration

Respiration is the chemical breakdown of nutrients in cells to release energy; breathing is the gas exchange that supplies the oxygen.

Respiration takes place in every living cell all the time. In aerobic respiration glucose reacts with oxygen to release energy, with carbon dioxide and water as products. Breathing only moves air in and out. Saying 'respiration is breathing' loses marks.

0123456GlucoseOxygenCarbon dioxideWaterSubstanceMolecules
Aerobic respiration, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O: molecules of each substance in the equation
  • Aerobic respirationCX6HX12OX6+6 OX2→6 COX2+6 HX2O\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}

Respiration

What it is:
Chemical reactions in cells
Result:
Releases energy
Where:
Happens in all living cells

Breathing

What it is:
Movement of air in and out of the lungs
Result:
Gas exchange
Where:
Supplies oxygen for respiration

Which statement correctly separates respiration from breathing?

Growth, nutrition and excretion

Nutrition supplies materials for energy and growth, and excretion removes the waste produced by metabolism.

Growth is a permanent increase in size and dry mass. A wilting plant has lost water, not growth, because its dry mass is unchanged. Always include both 'size' and 'dry mass'. Nutrition is taking in materials for energy, growth and development. Excretion removes waste products of metabolism, toxic materials and substances in excess of requirements.

020406080100HealthyWiltedPlantMass (g)
  • Fresh mass
  • Dry mass
A wilting plant loses water, not dry mass, so it has not shrunk in the way growth is measured (illustrative values)

Nutrition

What happens:
Taking in materials
What it supports:
Energy, growth and development

Growth

What happens:
Permanent increase
What it supports:
Size and dry mass

Excretion

What happens:
Removal of waste
What it supports:
Metabolic waste, toxic materials, excess

A plant wilts and loses water. Has it lost growth?

Reproduction

Nutrition sustains the individual, while reproduction makes new individuals and so sustains the species.

Reproduction is the processes that make more of the same kind of organism. Asexual reproduction needs one parent and gives genetically identical offspring. Sexual reproduction needs two parents and produces variation. Do not confuse feeding with reproducing.

  1. 1

    Asexual: one parent

  2. 2

    Genetically identical offspring

  3. 3

    Sexual: two parents

  4. 4

    Offspring show variation

Asexual and sexual reproduction compared

Asexual

Parents:
One
Offspring:
Genetically identical
Variation:
None
Speed:
Rapid

Sexual

Parents:
Two
Offspring:
Genetically different
Variation:
Produced
Speed:
Slower

Which characteristic feeds the individual, and which sustains the species?

Try an exam question

State what is meant by the terms growth and excretion.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.