Cambridge IGCSE Biology (0610) Study Notes

Topic 1.1: Characteristics of Living Organisms

Welcome to Biology! This first topic answers a fundamental question: What makes something alive? We are going to explore the seven essential characteristics that all living things share, from the smallest bacterium to the largest whale. Mastering these definitions is your first crucial step in IGCSE Biology!

These seven characteristics are the non-negotiable checklist for life. If an object cannot do *all* seven things, it is generally considered non-living (like a fire, which moves, respires, and grows, but cannot reproduce).

The Essential Checklist for Life: MRS GREN

To help you remember these seven characteristics, use the classic Biology mnemonic: MRS GREN (or sometimes MR SGREN). Every letter stands for one characteristic.

  • M: Movement
  • R: Respiration
  • S: Sensitivity
  • G: Growth
  • R: Reproduction
  • E: Excretion
  • N: Nutrition

1. M - Movement

Movement is defined as: an action by an organism or part of an organism causing a change of position or place.

  • Example for the whole organism: A cheetah running after prey, or a fish swimming.
  • Example for part of the organism: A plant turning its leaves towards sunlight (a process called phototropism), or the internal movement of blood around your body.

Did you know? Even organisms that seem fixed, like trees, exhibit movement! They move their roots, stems, and leaves in response to their environment.

Key Takeaway for Movement

Movement must result in a change in location (place) or a change in orientation/shape (position).

2. R - Respiration

Respiration is defined as: the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

! Important Note: Respiration is NOT breathing. Breathing (gas exchange) is just moving air in and out. Respiration is the chemical process that happens *inside* every single living cell to unlock energy.

  • The primary nutrient molecule broken down is usually glucose.
  • The energy released is essential for all life processes (known collectively as metabolism).

Analogy: If your body is a phone, breathing is plugging it into the wall (taking in oxygen). Respiration is the phone's internal battery circuit turning that power into usable function (releasing energy).

Quick Review: Metabolism

Metabolism means all the chemical reactions that happen in an organism necessary to maintain life. Respiration provides the energy needed to power all these reactions.

3. S - Sensitivity

Sensitivity (or Irritability) is defined as: the ability to detect and respond to changes in the internal or external environment.

These detected changes are called stimuli (singular: stimulus).

  • External Stimuli Examples: Light, temperature, sound, gravity, or the presence of predators.
  • Response Example: Pulling your hand away from a hot stove, or a plant growing towards light.

Accessibility Tip: Think of your senses. Being sensitive means you can notice something (detect) and do something about it (respond).

4. G - Growth

Growth is defined as: a permanent increase in size and dry mass.

This process usually involves:

  1. Cell division (making more cells).
  2. Cell enlargement (cells getting bigger).
  3. Differentiation (cells specialising for certain jobs).

We must use the term dry mass. If an organism takes a large drink of water, its size and wet mass increase temporarily, but this isn't true growth. True growth means permanently building new biological material (like protein or cellulose).

5. R - Reproduction

Reproduction is defined as: the processes that make more of the same kind of organism.

Reproduction is essential because individual organisms eventually die. This characteristic ensures the survival of the species (a group of organisms that can reproduce to produce fertile offspring, which you will learn about later).

  • Asexual Reproduction: Producing genetically identical offspring from one parent (e.g., bacteria dividing).
  • Sexual Reproduction: Involving two parents, producing genetically different offspring (e.g., human babies).

6. E - Excretion

Excretion is defined as: the removal of the waste products of metabolism and substances in excess of requirements.

! Common Mistake Alert: Excretion is NOT the same as egestion!

  • Excretion: Removing chemical waste produced *inside* cells during metabolism (e.g., carbon dioxide from respiration, urea from protein breakdown).
  • Egestion: Removing undigested food (faeces) from the body via the anus. Faeces were never absorbed into your body cells, so they are not metabolic waste.

Analogy: Excretion is like putting out the rubbish bin (urea, CO2) after cooking. Egestion is like taking out the wrapper of the food you never ate (undigested material).

Key Excretory Products

The main substances living organisms excrete are: Carbon dioxide, Urea, and Excess Water/Ions.

7. N - Nutrition

Nutrition is defined as: the taking in of materials for energy, growth and development.

All organisms need materials (food) for two reasons:

  1. To be broken down during respiration to release energy.
  2. To provide the raw materials needed for growth and repair (like proteins, vitamins, and minerals).
Two Types of Nutrition:
  • Plants (Autotrophs): Make their own food (organic nutrients) using simple raw materials (carbon dioxide and water) and energy from light (photosynthesis).
  • Animals (Heterotrophs): Obtain food by feeding on other organisms (plants, animals, or both).

Quick Revision Summary (MRS GREN)

Here is a concise list to help you prepare for definitions in exams:

M Movement Action causing a change of position or place.
R Respiration Chemical reactions releasing energy from nutrient breakdown for metabolism.
S Sensitivity Ability to detect and respond to changes in the environment (stimuli).
G Growth A permanent increase in size and dry mass.
R Reproduction Processes that make more of the same kind of organism.
E Excretion Removal of metabolic waste products and substances in excess.
N Nutrition Taking in materials for energy, growth, and development.

Keep practising the mnemonic MRS GREN, and make sure you know the precise definitions for each term, especially noting the difference between Respiration/Breathing and Excretion/Egestion. You've got this!