Welcome to Characteristics of Living Organisms!

Hi there! Biology can sometimes feel like learning a whole new language, but don't worry—we're going to break down the first big concept: What exactly makes something alive?

This chapter is crucial because it provides the foundation for classifying and understanding every organism we study, from the smallest bacteria to the largest whale. By the end of these notes, you’ll be able to confidently distinguish a living thing from a non-living thing using seven essential rules.

Let's dive in!

The Seven Essential Characteristics of Life

To be classified as a living organism, an object must show all seven characteristics of life. If even one is missing, it is considered non-living (like a car, a robot, or a rock).

A Memory Trick: Use the Mnemonic MRS GREN

The easiest way to remember these seven characteristics is using the famous mnemonic:

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

Memory Aid: Say "MRS GREN" out loud a few times until you remember it!

1. Movement

What is it?

Movement is the action of an organism or part of an organism causing a change of position or place.

Movement doesn't always mean running or swimming; sometimes it's very subtle!

  • In Animals: Animals usually move their whole body from one place to another (locomotion). Example: A cat walking, a bird flying.
  • In Plants: Plants are usually rooted in one place. Their movement involves changing the position of a part of the body. Example: Sunflowers turning their heads to follow the sun, or roots growing deeper into the soil.

Key Takeaway: All living things move, even if they stay in the same place!

2. Respiration

This is one of the most commonly misunderstood concepts! Pay close attention!

What is it?

Respiration is the chemical process in which living cells break down nutrient molecules (usually glucose) to release energy for metabolic processes.

IMPORTANT MISTAKE TO AVOID:

Many students confuse Respiration with Breathing (gas exchange).

  • Respiration happens inside the cells and releases energy.
  • Breathing is simply moving air in and out of the lungs (or gills/stomata).

Every single cell in every single living organism must respire 24 hours a day to stay alive, otherwise, it runs out of energy!

Did you know? Even trees respire! They take in oxygen and release carbon dioxide, just like you, though they also photosynthesize during the day.

Key Takeaway: Respiration = Energy Release from Food.

3. Sensitivity (or Response)

What is it?

Sensitivity is the ability of an organism to detect or sense changes in its external or internal environment and to make appropriate responses.

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

Stimulus -> Organism detects change -> Response

  • Stimuli Examples: Changes in light, temperature, touch, water level, gravity, or sound.
  • Animal Example: If you touch a hot stove (stimulus), you quickly pull your hand away (response).
  • Plant Example: A plant bending and growing towards a light source.

Key Takeaway: Living things react to their surroundings to keep themselves safe and healthy.

4. Growth

What is it?

Growth is a permanent increase in size and dry mass by an increase in cell number or cell size (or both).

Growth is a controlled, internal process. It requires the organism to use the energy and materials gained from nutrition.

Don't confuse this: A snowflake growing bigger by adding more frozen water is not true growth because it is not based on internal, metabolic activity.

  • Animals: Growth usually stops or slows down significantly once the organism reaches maturity.
  • Plants: Plants usually continue to grow throughout their entire lives.

Key Takeaway: Growth is a permanent, internal increase in mass and size.

5. Reproduction

What is it?

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

Reproduction is essential not for the survival of the individual organism, but for the survival of the species over time. Without reproduction, the species would eventually die out.

There are two main types:

  • Sexual Reproduction: Involves two parents (e.g., humans, mammals, flowering plants).
  • Asexual Reproduction: Involves only one parent, producing genetically identical offspring (e.g., bacteria, budding in yeast).

Encouraging Note: The detailed mechanisms of sexual and asexual reproduction are covered later, but for now, just remember the goal: making the next generation.

Key Takeaway: Making offspring to ensure the species continues.

6. Excretion

This is another concept where careful distinction is needed!

What is it?

Excretion is the removal of waste products of metabolism (chemical reactions) from the organism.

These metabolic waste products are often toxic if they build up and must be removed. Common examples of metabolic waste include carbon dioxide (from respiration) and urea (from protein breakdown).

CRITICAL DISTINCTION: Excretion vs. Egestion

Excretion is the removal of *waste created by cells* (like urine or sweat).

Egestion (or Defaecation) is the removal of *undigested food* (faeces) from the body. Since this food never entered the cells, it is NOT excretion.

Analogy: Think of excretion like taking out the trash from your kitchen (the cell), while egestion is like flushing the toilet (undigested food). Both involve waste removal, but only one is metabolic waste.

Key Takeaway: Excretion is the removal of toxic cellular (metabolic) waste.

7. Nutrition

What is it?

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

All living things need nutrients, but they get them in different ways.

  • Autotrophic Nutrition ("Self-Feeding"): Organisms, like plants and algae, make their own food, usually through photosynthesis, using simple substances like carbon dioxide and water.
  • Heterotrophic Nutrition ("Feeding on Others"): Organisms, like animals and fungi, obtain nutrients by consuming other organisms (plants or animals).

Whether an organism is a massive oak tree or a tiny insect, it needs nutrition to fuel its MRS GREN processes!

Key Takeaway: Getting the necessary materials for energy (Respiration) and building blocks (Growth).

Quick Review Box: The Rules of Life

Remember, everything that is alive must demonstrate ALL of these features:

M - Movement

Changing position or place.

R - Respiration

Releasing energy from food (not breathing).

S - Sensitivity

Detecting and responding to stimuli.

G - Growth

A permanent increase in size and mass.

R - Reproduction

Making new individuals of the same species.

E - Excretion

Removing metabolic waste products.

N - Nutrition

Taking in materials for energy and growth.

Well done! You now know the seven key characteristics used worldwide to define life. Keep practicing the mnemonic MRS GREN!