Open systems
Big idea: An open system lets things matter and energy move in and out. If you can point to something entering and something leaving, it is probably an open system.
In an open system, both matter and energy flow in and out across the boundary.
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First: what do we mean by "matter" and "energy"?
Students often find this confusing, so think of it this way:
- Matter = physical stuff you could collect in a bucket (water, air, soil, nutrients, plants, animals).
- Energy = not physical stuff — you cannot hold it (sunlight, heat, movement, chemical energy in food).
Quick test: If you could trap it in a container, it is matter. If you cannot, it is energy.
So what is an open system?
An open system is one where matter AND energy both move in and out across the system boundary.
Most things in nature are open systems because they are not sealed off from their surroundings.
Everyday examples (easy to picture)
- Humans – food, water, and oxygen enter; waste, CO₂, and heat leave.
- A forest – rain, sunlight, and nutrients enter; oxygen, heat, and organisms leave.
- A river – water and sediment flow in and flow out.
- A city – food, fuel, and people enter; waste, sewage, and emissions leave.
If you can list at least one thing entering and one thing leaving, it is an open system.
Pond example (IB exam favourite)
A pond is a classic example used in IB exams because it clearly shows inputs and outputs. Tap each icon to explore:
Tap each icon to learn what enters and leaves the pond. Both matter and energy flow through — that's why it's an open system.
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Inputs to a pond (things entering):
- Rainwater and groundwater (matter)
- Nutrients washed in from soil (matter)
- Fish, insects, birds entering (matter)
- Sunlight (energy)
Outputs from a pond (things leaving):
- Water lost by evaporation or runoff (matter)
- Organisms leaving or being eaten (matter)
- Heat released to the air (energy)
Because both matter and energy enter and leave, a pond is an open system.
Why are open systems always changing?
Because inputs and outputs are constantly happening, open systems are dynamic — they change over time.
If inputs and outputs are balanced, the system stays stable. If not, problems can occur.
Example: Too many nutrients entering a pond can cause algal blooms.
Quick summary
- Open system = matter AND energy can enter and leave
- Inputs = what enters the system
- Outputs = what leaves the system
- Open systems are dynamic — always changing
- Most ecosystems (ponds, forests, rivers) are open systems
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Closed systems
Big idea: A closed system is like a sealed jar sitting in sunlight. Energy can go in and out (the jar heats up or cools down), but matter stays trapped inside — nothing physical enters or leaves.
The simple test: Is it closed or open?
Earth is a closed system: energy from the Sun enters and heat leaves, but almost no matter crosses the boundary.
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Ask yourself two questions:
- Can physical stuff (water, air, food, waste) enter or leave? → If YES, it's open.
- Can only energy (heat, light) enter or leave, while matter stays inside? → If YES, it's closed.
Closed = sealed for matter, but not for energy. Think of a greenhouse: sunlight enters, heat leaves, but the air and plants stay inside.
Why does this matter for exams?
IB loves to ask about Earth's cycles and whether they are open or closed. Here's the key:
- The carbon cycle, nitrogen cycle, and water cycle are all closed systems on a global scale.
- The atoms in these cycles are the same atoms that have always been on Earth — they just move around.
- Energy from the Sun enters, heat escapes to space, but the matter stays.
If an exam asks why Earth's cycles are closed systems, say: "Matter is recycled within the system and does not leave, but energy enters as sunlight and exits as heat."
Picture it: The water cycle as a closed system
Imagine all the water on Earth in a giant sealed ball:
- Water evaporates from oceans (needs energy from the Sun).
- Water vapour rises, cools, and forms clouds.
- Rain falls back to Earth.
- Water flows through rivers back to the ocean.
- The same water molecules keep cycling — none escape into space.
The water cycle: matter (water) stays on Earth and keeps cycling, but energy from the Sun drives the process.
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The water you drink today contains the same water molecules that dinosaurs drank millions of years ago. That's a closed system in action!
Earth as a closed system
Planet Earth is considered a closed system because:
- Energy IN: Sunlight enters from the Sun.
- Energy OUT: Heat radiates back into space.
- Matter STAYS: Almost nothing enters or leaves Earth.
Yes, a few meteorites land and a few spacecraft leave, but these are so tiny compared to Earth's total mass that we ignore them.
Real-world example: Biosphere 2
Biosphere 2 is a giant sealed greenhouse in Arizona, USA. Scientists built it to test if humans could survive in a closed system.
- Air, water, and nutrients were recycled inside.
- Sunlight entered through the glass (energy in).
- Heat escaped through the walls (energy out).
- Nothing physical entered or left for two years.
The experiment had problems (oxygen levels dropped, food ran short), showing how hard it is to maintain a closed system.
Earth's subsystems (they're all connected)
Earth contains smaller systems (subsystems) that constantly exchange matter and energy with each other. Tap each one to learn more:
Tap each subsystem to learn more. All subsystems exchange matter with each other (open), but together Earth is a closed system.
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Each subsystem is open (matter moves between them), but together they form a closed system (matter doesn't leave Earth).
Open vs Closed — quick comparison
| Open system | Closed system | |
|---|---|---|
| Matter | Enters and leaves | Stays inside |
| Energy | Enters and leaves | Enters and leaves |
| Example | A pond, a forest, a human | Earth, the carbon cycle, Biosphere 2 |
Gaia hypothesis (bonus concept)
The Gaia hypothesis is the idea that Earth acts like a single living organism that regulates itself.
- Living things (biosphere) interact with air, water, and rocks.
- Together, they help keep conditions stable for life (temperature, oxygen, etc.).
- It's a way of thinking about Earth as one connected system.
You don't need to memorise the Gaia hypothesis in detail. Just know it suggests Earth self-regulates through interactions between living and non-living parts.
Open vs closed (final exam check)
- Open system → matter AND energy move in and out
- Closed system → energy moves, but matter stays inside
In exam answers, always say what enters and what leaves. That alone can earn full marks.