IB ESS SL1.2 SystemsPaper 1 & 2Core idea~12 min read
Open, Closed and Isolated Systems
Once you have drawn a boundary around a system, there is only one question left: what is allowed to cross it? Energy, matter, both, or neither. That single question sorts every system in the course into three groups.
📚 What you need to know
Systems are open, closed or isolated, depending on whether energy and matter can cross the boundary.
Open: both energy and matter are exchanged. Usually living systems — an ecosystem, a habitat, your body.
Closed: energy is exchanged but matter is not. The Earth is treated as closed.
Isolated: neither is exchanged. These do not exist in nature and are a theoretical idea only.
Environmental systems work at every scale, from a bromeliad to the whole planet.
The Earth can be seen as one integrated system made of the biosphere, hydrosphere, cryosphere, geosphere, atmosphere and anthroposphere.
The Gaia hypothesis (Lovelock, 1970s) treats the Earth as a single self-regulating system, and has been both praised and criticised.
The three types
Which type a system falls into depends only on how energy and matter flow between the system and its surroundings.
The boundary is the only thing doing the work here. Move it, and the same pile of stuff can become a different type of system.
Type
Energy
Matter
Typical example
Open
Exchanged
Exchanged
An ecosystem, a habitat, a human body
Closed
Exchanged
Not exchanged
The Earth, a sealed terrarium
Isolated
Not exchanged
Not exchanged
None in nature; possibly the Universe
Open systems
In an open system, both energy and matter move in and out. Open systems are usually organic, meaning living.
An ecosystem takes in sunlight and new matter, often as biomass, and loses energy as heat and matter as waste or as organisms that leave.
Your body is an open system too. Food, water and oxygen go in; heat, carbon dioxide, waste and movement come out.
Because matter keeps arriving and leaving, open systems can grow, shrink and repair themselves.
If a question gives you any living thing — a pond, a person, a field of wheat — the answer is almost always “open system”, because living things must take in matter to stay alive.
Closed systems
In a closed system, energy crosses the boundary but matter does not. Closed systems are usually inorganic, meaning non-living.
The Earth as a closed system
This is why the carbon, nitrogen and water cycles are cycles. On a closed planet, atoms have nowhere else to go.
The main input of energy is solar radiation.
The main output of energy is heat, re-radiated from the Earth’s surface as infrared.
Matter is recycled completely within the system, which is why we talk about biogeochemical cycles.
Technically, meteorites arrive and spacecraft leave, so tiny amounts of matter do cross. These amounts are treated as negligible.
Why geochemical cycles look closed. Carbon, nitrogen and water keep moving between the atmosphere, hydrosphere, lithosphere and biosphere, but the total amount of each element stays roughly the same over geological time. Carbon, for example, moves between air, oceans and land through photosynthesis, respiration and absorption by the sea — the same atoms, endlessly reused.
Closed systems people have built
A sealed terrarium with the right balance of water, plants and microbes can survive for years. Light and heat cross the glass; matter does not.
Biosphere 2, built in Arizona in the late 1980s, was a much bigger attempt at the same thing. It contained a rainforest, an ocean, a desert, a savannah and farmland, plus living quarters for people.
It was sealed from the outside world with only sunlight and heat allowed in. Air and water were recycled, and all food was grown inside.
The point was to study how ecosystems and humans interact when nothing can be brought in from outside.
Isolated systems
In an isolated system, neither energy nor matter crosses the boundary. Nothing goes in, nothing comes out.
No such system exists in nature. It is a theoretical concept that scientists use to simplify their thinking, although you could argue the entire Universe is one, since there is no “outside” for anything to leak into.
A vacuum flask is the classic near-miss. It slows energy loss down brilliantly, but your coffee still goes cold eventually, so it is not truly isolated. Use it as an example only if you say that clearly.
Environmental systems at different scales
Environmental systems are networks of components and processes that link living organisms, their habitats and physical things like water, air and soil. They exist at every size.
Scale
Example
What is going on inside
Small
A bromeliad in a rainforest
Insects, frogs and microbes live in the pool of water in its leaves — a tiny world of its own
Large
The whole rainforest
Predator–prey links, symbiosis, competition for light and nutrient cycling
Global
The planet
Atmosphere, oceans and land interacting closely enough to keep conditions suitable for life
The Earth as one integrated system
Instead of a pile of separate parts, the Earth can be treated as one system built from interconnected components. Learn these six — they come back all year.
Component
What it includes
Biosphere
All living organisms and their interactions with the environment
Hydrosphere
All the water — oceans, rivers, lakes and groundwater
Cryosphere
All the frozen water — glaciers, ice caps and permafrost
Geosphere
The solid Earth — rocks, minerals, mountains and valleys
Atmosphere
The layers of gas around the planet, from the troposphere out to the exosphere
Anthroposphere
Human influence — our activities, infrastructure and cities
🧠
Easy way to remember the spheres
Bio = life, hydro = water, cryo = frozen (think cryogenic), geo = rock, atmo = air, anthropo = people. The names give the answer away if you read them slowly.
The Gaia hypothesis
Proposed by James Lovelock in the 1970s, the Gaia hypothesis takes the holistic view as far as it will go: the Earth behaves as a single self-regulating system.
Lovelock argued that the biota (all living things) and their environment are so closely linked that they act as one integrated system.
Feedback mechanisms inside that system keep conditions stable, rather like homeostasis in your own body but on a planetary scale.
It started as an explanation of how the composition of the atmosphere and global temperature control each other.
For example, greenhouse gases such as carbon dioxide and methane push temperatures up. Warmer conditions cause more evaporation and more cloud, and more plant growth absorbing carbon dioxide, and both of those push back the other way.
The idea was later developed further, with contributions from scientists such as Lynn Margulis.
The criticisms
Some scientists say it is anthropomorphic — it treats the planet as if it were a living organism with goals, which it is not.
Others say it is hard to test, and a hypothesis you cannot test is hard to call science.
Supporters reply that it is still a useful way of understanding how Earth’s systems connect.
If Gaia comes up in an evaluation question, give the idea, give one supporting example such as cloud feedback, then give one criticism. A balanced answer scores far better than an enthusiastic one.
Worked examples
WE 1
Classify a system and justify it
A sealed glass terrarium is placed on a sunny windowsill. Identify the type of system it is and justify your answer. (3 marks)
Point 1: the classification
It is a closed system.
Point 2: energy
Light and heat energy can cross the glass boundary in both directions.
Point 3: matter
The seal stops matter entering or leaving, so water and nutrients are recycled inside.
Energy crosses, matter does not = closedalways justify by talking about energy AND matter separately — that is where the marks sit
WE 2
Explain why the Earth is treated as closed
Explain why the Earth is usually described as a closed system. (3 marks)
Point 1: energy in
Energy enters mainly as solar radiation.
Point 2: energy out
Energy leaves mainly as heat re-radiated from the Earth’s surface, so energy is exchanged with space.
Point 3: matter
Matter is recycled within the system; the small amounts arriving as meteorites or leaving as spacecraft are negligible.
Energy in and out, matter stays put — so it is closedthe word “negligible” is worth writing; it shows you know it is an approximation
WE 3
Evaluate the Gaia hypothesis
Outline the Gaia hypothesis and state one criticism of it. (4 marks)
Point 1: the claim
Lovelock proposed the Earth is a single self-regulating system in which living organisms and their environment act together.
Point 2: the mechanism
Feedback mechanisms keep global conditions stable, similar to homeostasis in an organism.
Point 3: an example
Rising temperatures increase evaporation and cloud cover, which reflects sunlight and cools the planet again.
Point 4: the criticism
It has been criticised as anthropomorphic and difficult to test.
A self-regulating planet — useful idea, hard to proveone clear example of a feedback mechanism turns a vague answer into a solid one
💡 Exam tips
Answer these questions in two halves: what happens to energy, then what happens to matter.
Learn one example of each type: forest (open), Earth or terrarium (closed), nothing real (isolated).
Say negligible when you mention meteorites and spacecraft.
Remember open systems are usually organic and closed ones usually inorganic.
Learn the six components of the Earth system by name and spelling.
For Gaia, always pair the idea with a criticism.
⚠ Common mistakes
Calling the Earth an isolated system. Sunlight comes in and heat goes out, so energy is exchanged.
Saying isolated systems are rare. They do not exist naturally at all.
Mixing up closed and isolated. Closed still exchanges energy; isolated exchanges nothing.
Forgetting the boundary. A pond is open, but a pond sealed in glass is closed. The boundary decides.
Treating Gaia as proven fact. It is a hypothesis, and a contested one.
Confusing geosphere and biosphere. Geo is rock; bio is life.
Up next: Equilibrium and Feedback Loops — systems do not just sit there. Something has to hold them steady, and something else can push them off course.
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