There are seven things a living thing has to do to count as alive. You do them with billions of cells sharing the work. A single-celled organism does all seven inside one cell, with nothing to fall back on.
📚 What you need to know
The seven functions of life: metabolism, reproduction, sensitivity, homeostasis, growth, excretion, nutrition.
Every living organism carries out all seven, unicellular or multicellular.
A unicellular organism must do all of them within a single cell.
Its organelles are what make that possible: mitochondria for energy, membrane for homeostasis, ribosomes for proteins, vacuoles for digestion and waste, cilia or flagella for movement.
Multicellular organisms can afford specialised cells, each doing one job well.
The seven functions
The initials spell MRS H GREN, which is the easiest way to check you have not dropped one in an exam answer.
Function
What it means
Metabolism
All the enzyme-controlled chemical reactions in a cell, including respiration
Reproduction
Producing offspring, sexually or asexually
Sensitivity
Responding to changes inside or outside, which improves the chance of survival
Homeostasis
Keeping internal conditions such as temperature within tolerable limits
Growth
A permanent increase in size
Excretion
Getting rid of waste made by metabolism, such as carbon dioxide
Nutrition
Taking in energy and nutrients, either by absorbing food or by making it (photosynthesis)
Be careful with excretion. It means removing waste that the organism’s own reactions produced — like carbon dioxide from respiration. Getting rid of undigested food is egestion, and that is not the same thing.
How one cell manages all seven
A unicellular organism such as Paramecium or Chlamydomonas has no specialised tissues to help. Every function has to be handled by structures inside the one cell.
Structure
Which function it handles
Mitochondria
Metabolism — respiration to release energy as ATP
Cell membrane
Homeostasis — controls what moves in and out
Ribosomes
Growth — make the proteins and enzymes the cell needs
Vacuoles
Nutrition and excretion — digest food and store waste
Cilia or flagella
Sensitivity — move the organism in response to its surroundings
Nucleus / DNA
Reproduction — copied and passed on when the cell divides
Why being multicellular is worth it. A single cell has to be a generalist. In a multicellular organism, cells can specialise: a red blood cell drops its nucleus to carry more oxygen, a muscle cell packs itself with mitochondria. Each cell does one job extremely well because other cells cover the rest.
Worked examples
WORKED EXAMPLE
Outline how a unicellular organism carries out three of the functions of life. [3]
Function 1: metabolism
Mitochondria carry out respiration, releasing energy as ATP for the cell’s reactions.
Function 2: homeostasis
The cell membrane controls which substances enter and leave, keeping internal conditions steady.
Function 3: nutrition
Food is taken in and digested inside vacuoles.
Name the function, name the structure, say what it doesthat three-part pattern fits almost any question on this page
WORKED EXAMPLE
A student says a car is alive because it takes in fuel, moves, and gives out waste gases. Explain why this is wrong. [3]
Step 1: accept what is true
It does appear to show nutrition, response and excretion.
Step 2: name what is missing
A car cannot reproduce, cannot grow, and does not maintain its own internal conditions.
Step 3: state the rule
A living organism must carry out all seven functions, not just some of them.
All seven, or it is not alive
💡 Exam tip
Use MRS H GREN as a checklist so you never leave one out.
If the question says “outline”, give one clear sentence per function — no need for essays.
For unicellular organisms, always link the function to a named structure inside the cell.
Metabolism is the whole set of reactions, not just respiration. Say “including respiration” rather than “it is respiration”.
Growth means a permanent increase in size — a cell swelling with water does not count.
Keep excretion and egestion apart.
⚠ Common mix-up
Listing only five or six functions. Homeostasis and metabolism are the two most often forgotten.
Saying unicellular organisms only do some of them. They do all seven — that is the whole point.
Confusing excretion with egestion. Excretion is metabolic waste; egestion is undigested food.
Treating “response” and “movement” as the same. Movement is one way of responding, not the definition.
Saying viruses are alive. They cannot carry out the functions of life independently.
Describing nutrition as only “eating”. Plants get nutrition by making their own food.
Up next: Eukaryotic Cells: Comparisons & Atypical Examples — how animal, plant and fungal cells differ, and the odd cells that stretch cell theory to its limits.
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