Cell Structure and Function
1. Characteristics of Living Things
Learning outcomes
- I can describe the characteristics that distinguish living organisms from non-living things.
- I can explain the life processes represented by MRS GREN.
- I can identify examples of life processes in a variety of organisms.
- I can distinguish between living, dead, and non-living things using scientific criteria.
- I can explain why cells are considered the basic units of life.
Introduction
Earth is home to an incredible diversity of living things, from tiny bacteria that can only be seen under a microscope to giant blue whales weighing more than 150 tonnes. Although these organisms differ greatly in size, shape, and habitat, they all share a set of common characteristics that define life.
Biologists use these characteristics to distinguish living organisms from dead and non-living things. One useful way of remembering the main life processes is the acronym MRS GREN. By studying these life processes, we can better understand how living organisms survive, grow, reproduce, and interact with their environment.
What Is a Living Organism?
A living organism is anything that carries out all the essential processes needed for life.
Living organisms include:
- plants
- animals
- fungi
- protists
- bacteria
Although they look very different, all living organisms perform the same basic life processes.
Definition:
A living organism is an organism that carries out all the essential life processes required for survival.
The Life Processes: MRS GREN
The acronym MRS GREN helps us remember the seven characteristics of living things.
| Letter | Life Process | Meaning |
|---|---|---|
| M | Movement | Ability to move or move parts of the body |
| R | Respiration | Releasing energy from food |
| S | Sensitivity | Detecting and responding to the environment |
| G | Growth | Increasing in size and developing |
| R | Reproduction | Producing offspring |
| E | Excretion | Removing waste products |
| N | Nutrition | Obtaining food or making food |
Together, these characteristics help scientists identify living organisms.
Movement
Movement is the ability of an organism, or parts of an organism, to change position.
Examples include:
- birds flying
- fish swimming
- earthworms burrowing
- sunflowers turning toward sunlight
- roots growing downward into the soil
Even plants, which appear stationary, show movement as they grow and respond to their environment.
Respiration
Respiration is the process by which living cells release energy from food.
Organisms use this energy to:
- grow
- repair tissues
- move
- reproduce
- carry out all life processes
Respiration occurs continuously in every living cell.
Examples:
- humans break down glucose using oxygen,
- plants also respire, even though they photosynthesise.
Sensitivity
Sensitivity is the ability to detect and respond to changes in the environment.
Examples include:
- pupils becoming smaller in bright light,
- plants growing toward light,
- a person pulling away from a hot object,
- birds migrating as seasons change.
Responding to the environment helps organisms survive.
Growth
Growth is a permanent increase in size or complexity.
Growth occurs because cells:
- increase in number,
- increase in size,
- become specialised.
Examples include:
- a seed growing into a tree,
- a puppy becoming an adult dog,
- a caterpillar developing into a butterfly.
Reproduction
Reproduction is the process of producing offspring.
This allows species to continue from one generation to the next.
Some organisms reproduce:
- sexually (using two parents),
- asexually (using one parent).
Examples:
- flowering plants produce seeds,
- bacteria divide into two identical cells,
- mammals give birth to young.
Excretion
Excretion is the removal of waste products produced by cells.
Examples include:
- carbon dioxide leaving the lungs,
- urine removing urea,
- sweating to remove water and salts.
Excretion is different from egestion, which is the removal of undigested food from the digestive system.
Nutrition
Nutrition is the process of obtaining or producing food.
Animals obtain food by eating other organisms.
Plants produce their own food through photosynthesis using:
- sunlight
- carbon dioxide
- water
Nutrition provides the energy and raw materials needed for growth and repair.
Living, Dead, and Non-Living Things
Not everything around us is alive.
Scientists classify objects into three groups.
| Category | Description | Example |
|---|---|---|
| Living | Performs all life processes | Dog, tree, mushroom |
| Dead | Once alive but no longer carries out life processes | Fallen leaf, dead bird, dry wood |
| Non-living | Has never been alive | Rock, glass, water, metal |
A dead organism was once living, while a non-living object has never been alive.
Worked Example 1
Complete the table.
| Object | Living, Dead, or Non-living? |
|---|---|
| Tree | Living |
| Wooden table | Dead |
| Rock | Non-living |
| Fish | Living |
| Plastic bottle | Non-living |
Notice that a wooden table is made from a tree that was once alive, so it is considered dead, not non-living.
Cells: The Basic Units of Life
One feature shared by all living organisms is that they are made of cells.
A cell is the smallest unit capable of carrying out all the processes of life.
Some organisms consist of:
- one cell (unicellular organisms), such as bacteria.
Others consist of:
- many cells (multicellular organisms), such as plants and animals.
Because every living organism is made of cells, cells are known as the basic units of life.
Why Are Cells Considered the Basic Units of Life?
Every cell can perform essential life functions.
Cells:
- obtain nutrients,
- release energy through respiration,
- remove waste,
- grow,
- reproduce (by cell division),
- respond to their environment.
In multicellular organisms, specialised cells work together to form:
- tissues,
- organs,
- organ systems,
- the complete organism.
Worked Example 2
A student says:
"Fire is alive because it grows and uses oxygen."
Question
Is this correct?
Solution
No.
Although fire can spread and uses oxygen, it:
- is not made of cells,
- does not reproduce biologically,
- does not carry out all the MRS GREN life processes.
Therefore, fire is non-living.
Are Viruses Living?
Viruses are one of the most debated topics in biology.
Viruses:
- are not made of cells,
- cannot reproduce on their own,
- become active only inside living cells.
Because they do not independently carry out all the characteristics of life, most biologists do not classify viruses as living organisms, although they share some features with living things.
Real-World Connection
Understanding the characteristics of living things helps scientists identify new organisms and study diseases. Doctors distinguish living bacteria from viruses because they require different treatments. Environmental scientists use these characteristics to classify newly discovered species, while astrobiologists searching for life beyond Earth look for evidence of cells, growth, reproduction, and other signs of life on distant planets and moons.
Did You Know?
Some bacteria can survive in environments once thought to be completely uninhabitable, including boiling hot springs, deep-sea hydrothermal vents, highly acidic lakes, and even inside Antarctic ice. These remarkable organisms, called extremophiles, show just how adaptable living cells can be.
Key Terms
- Living organism — an organism that carries out all the essential life processes.
- MRS GREN — a mnemonic representing the seven characteristics of living things: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.
- Respiration — the process by which cells release energy from food.
- Excretion — the removal of waste products produced by cells.
- Nutrition — obtaining or producing food to provide energy and materials for growth.
- Cell — the smallest unit capable of carrying out all the processes of life.
- Unicellular — consisting of a single cell.
- Multicellular — consisting of many cells.
Key Takeaways
- Living organisms can be distinguished from non-living things by the characteristics of life.
- The seven life processes are remembered using the acronym MRS GREN.
- Living organisms carry out movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition.
- Scientists distinguish between living, dead, and non-living things using scientific criteria rather than appearance alone.
- All living organisms are made of cells, which are the basic units of life.
- Understanding the characteristics of living things forms the foundation for studying all areas of biology.