Cambridge IGCSE0610

Characteristics of living organisms

Biology 0610 Chapter Notes

What this chapter covers

Characteristics of living organismsConcept and uses of classification systemsFeatures of organisms
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In this chapter

  1. 1Defining Life: The Seven Processes
  2. 2Energy for Life: Nutrition and Respiration
  3. 3Maintaining Control: Sensitivity and Excretion
  4. 4Continuity of Life: Growth and Reproduction

1. Defining Life: The Seven Processes

To determine if something is alive, biologists check for a set of seven key processes. While non-living things might show one or two of these (a fire grows, a crystal grows), only living organisms display all seven. A common way to remember these is with the mnemonic MRS GREN. Each process is vital for the survival of an individual or for the continuation of its species.

Key term

Organism: An individual living entity, such as an animal, plant, or single-celled life form, that displays all the characteristics of life.

Common pitfall

Assuming that because an object shows one characteristic of life, such as a fire 'growing' or 'respiring' (using oxygen), it must be alive. An entity must show all seven characteristics to be classified as a living organism.

Fun fact

The term 'MRS GREN' is a staple of biology education, but some textbooks use 'RINGER' (Respiration, Irritability, Nourishment, Growth, Excretion, Reproduction) which misses Movement and Sensitivity as separate points.

Worked example 16 marks

A student observes a cat. List and describe three life processes you could observe the cat carrying out. (6 marks)

  1. 1
    1. Movement: The cat moves around the garden, changing its position and place. This is an action caused by its muscles.
  2. 2
    1. Nutrition: The cat eats a mouse. It is taking in organic materials to provide energy and substances for growth and repair.
  3. 3
    1. Sensitivity: A loud noise occurs and the cat's ears twitch and it runs away. It has detected a stimulus (the noise) and made an appropriate response (running away).

Recap

  • All living organisms carry out seven essential life processes.
  • The mnemonic MRS GREN helps remember these: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
  • Movement is a change in position or place.
  • Respiration releases energy from food.
  • Sensitivity is responding to changes in the environment.
  • Growth is a permanent increase in size.
  • Reproduction is producing new organisms.
  • Excretion is removing metabolic waste.
  • Nutrition is obtaining food for energy and growth.

Quick check

  1. What does the 'N' in MRS GREN stand for?1 mark
  2. Which life process is essential for the survival of a species, but not an individual?1 mark

2. Energy for Life: Nutrition and Respiration

All life processes require energy. Organisms get this energy through a two-step process. First, Nutrition is the process of taking in materials for energy, growth, and development. Plants are autotrophs, making their own food (glucose) via photosynthesis. Animals are heterotrophs, consuming other organisms. Second, Respiration is the chemical process that happens inside every living cell to release energy from nutrient molecules like glucose. Aerobic respiration, which uses oxygen, is the most efficient way to release this energy.

Word Equation (Aerobic Respiration): Glucose + Oxygen → Carbon Dioxide + Water + Energy

Chemical Equation (Aerobic Respiration): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Key term

Respiration: The chemical reactions in cells that break down nutrient molecules to release energy for metabolism.

Examiner insight

Candidates who define respiration as 'breathing' or 'gas exchange' will not get the mark. You must state that it is a chemical reaction inside cells to release energy.

Common pitfall

Thinking that plants only photosynthesise and do not respire. All living cells respire 24/7, whereas photosynthesis only happens in cells with chlorophyll and in the presence of light.

Fun fact

A single human cell can contain several hundred to over 2,000 mitochondria, the 'powerhouses' where most of respiration takes place.

Worked example 14 marks

Explain why a plant needs to carry out both photosynthesis and respiration to survive. (4 marks)

  1. 1
    1. Photosynthesis is the process where the plant uses light energy to make its own food, specifically glucose. This provides the nutrient molecules.
  2. 2
    1. Respiration is the process that breaks down this glucose to release chemical energy.
  3. 3
    1. This energy is required to power all other life processes, such as growth, active transport of minerals, and synthesis of proteins.
  4. 4
    1. Photosynthesis only occurs in the light, but respiration occurs continuously in all living cells, day and night, to provide a constant supply of energy.

Recap

  • Nutrition is the intake of materials for energy, growth and development.
  • Respiration is the release of energy from food molecules inside cells.
  • Plants make their own food (autotrophs), while animals eat other organisms (heterotrophs).
  • Aerobic respiration uses oxygen to release a large amount of energy from glucose.
  • All living cells, in both plants and animals, must respire to survive.

Quick check

  1. What are the products of aerobic respiration?2 marks

3. Maintaining Control: Sensitivity and Excretion

Organisms must be able to sense and react to their surroundings to find food, avoid danger, and maintain a stable internal environment. Sensitivity is the ability to detect changes in the internal or external environment (stimuli) and make appropriate responses. These responses often involve movement. Internally, organisms must also remove waste. Excretion is the removal of toxic materials, the waste products of metabolism (chemical reactions in cells), and substances in excess. For example, humans excrete carbon dioxide (a waste product of respiration) and urea (from the breakdown of proteins).

Key term

Excretion: The removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements.

Examiner insight

Examiners award marks for clearly distinguishing between excretion (metabolic waste) and egestion (faeces). Be precise with your definitions.

Common pitfall

Confusing excretion with egestion. Excretion is the removal of waste made by the body's chemical reactions (metabolism), while egestion is the removal of undigested food that was never absorbed into the body's cells.

Fun fact

The Mimosa pudica plant, also known as the 'touch-me-not', rapidly folds its leaves inwards when touched. This is a dramatic example of sensitivity (thigmonasty) in the plant kingdom.

Worked example 13 marks

A woodlouse is placed on a damp, dark surface. When a bright light is shone on it, the woodlouse quickly moves into a dark corner. Explain how this demonstrates sensitivity. (3 marks)

  1. 1
    1. The stimulus is the change in the external environment, which is the bright light.
  2. 2
    1. The woodlouse detects this stimulus using its simple eyes (receptors).
  3. 3
    1. It makes a response by moving away from the light into the dark corner. This is a survival mechanism as it helps the woodlouse avoid predators and dehydration.

Worked example 22 marks

Explain why carbon dioxide is considered an excretory product in humans. (2 marks)

  1. 1
    1. Carbon dioxide is produced during aerobic respiration, which is a chemical reaction inside cells (a metabolic process).
  2. 2
    1. It is toxic in high concentrations and must be removed from the body, so it is transported in the blood to the lungs and breathed out.

Recap

  • Sensitivity is the ability to detect stimuli and make appropriate responses.
  • A stimulus is a change in the environment; a response is the organism's reaction to it.
  • Excretion is the removal of metabolic waste products like carbon dioxide and urea.
  • Excretion is not the same as egestion, which is the removal of undigested food (faeces).
  • Both sensitivity and excretion are vital for maintaining a stable internal environment (homeostasis).

Quick check

  1. Name one stimulus a plant might respond to.1 mark
  2. Is faeces an excretory product? Explain your answer.2 marks

4. Continuity of Life: Growth and Reproduction

For life to persist, organisms must be able to grow and create the next generation. Growth is defined as a permanent increase in size and dry mass, achieved by an increase in cell number, cell size, or both. This is different from just swelling with water, which is temporary. Reproduction is the process that makes more of the same kind of organism. It is not essential for the survival of an individual, but it is vital for the continuation of a species. There are two main types: asexual reproduction, involving one parent to produce genetically identical offspring (clones), and sexual reproduction, involving two parents to produce genetically varied offspring.

Key term

Growth: A permanent increase in size and dry mass by an increase in cell number or cell size or both.

Examiner insight

Marks are often available for stating that reproduction ensures the survival of the species, while other life processes ensure the survival of the individual. Make this distinction clear.

Common pitfall

Describing growth as just 'getting bigger'. The key exam definition includes the terms 'permanent increase' and 'dry mass' or 'cell number'.

Fun fact

The 'immortal jellyfish' (Turritopsis dohrnii) can cheat death by reverting back to its juvenile polyp stage after reaching sexual maturity, demonstrating an incredible form of regeneration and life cycle control.

Worked example 14 marks

A student plants a bean seed. After two weeks, it has grown into a small seedling. Explain how the life processes of nutrition and growth are demonstrated. (4 marks)

  1. 1
    1. Nutrition: The seedling has leaves and is carrying out photosynthesis to produce glucose. It is also absorbing water and mineral ions from the soil. These are the raw materials for growth.
  2. 2
    1. Growth: The seedling has used the energy (from respiration) and materials (from nutrition) to make new cells through cell division.
  3. 3
    1. This has resulted in a permanent increase in its size, as seen by the development of a stem, leaves, and roots.
  4. 4
    1. This is also an increase in its dry mass (the mass without water).

Worked example 22 marks

State two advantages of asexual reproduction for an organism like a strawberry plant. (2 marks)

  1. 1
    1. Only one parent is needed, so the plant does not need to find a mate or rely on pollinators.
  2. 2
    1. It is a rapid process, allowing the plant to colonise a favourable area quickly.

Recap

  • Growth is a permanent increase in an organism's size and dry mass.
  • Growth occurs due to cell division and an increase in cell size.
  • Reproduction is the process of making new individuals of the same species.
  • Reproduction is essential for the survival of the species, not the individual.
  • Asexual reproduction involves one parent and produces genetically identical offspring.
  • Sexual reproduction involves two parents and produces genetically varied offspring.

Quick check

  1. What is 'dry mass'?1 mark
  2. Why does sexual reproduction lead to variation in the offspring?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List the seven characteristics of living organisms.3 marks
  2. A car moves and uses fuel. Using the characteristics of living organisms, explain why a car is not considered to be alive.4 marks
  3. Distinguish between the terms 'respiration' and 'breathing' in a mammal.3 marks
  4. Explain why excretion is a vital process for all living organisms. In your answer, refer to a specific waste product in humans.4 marks
  5. Compare the process of nutrition in a typical plant and a typical animal.4 marks
  6. A student measures the height of a plant every day for a month. They state this measures the plant's growth. Explain why measuring height alone is an incomplete measure of growth.3 marks
  7. Viruses are often described as being on the border between living and non-living. Suggest two characteristics of life that viruses show, and two that they do not show when outside of a host cell.4 marks
  8. Describe an experiment you could perform in a school laboratory to show that germinating seeds carry out respiration.5 marks
  9. Compare and contrast sexual and asexual reproduction in terms of parents, process, and offspring.6 marks
  10. A person is in a cold room. Their body detects the drop in temperature and they start to shiver. Explain how this sequence of events demonstrates three of the seven characteristics of life.6 marks

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