IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~8 min read
Ecosystems as Open Systems
An ecosystem is not sealed off from the world. Things drift in, things drift out. Once you understand which things move in and out, and which things go round and round inside, the whole of this topic clicks into place.
📘 What you need to know
An ecosystem is a community of organisms together with the non-living (abiotic) parts of their surroundings.
Ecosystems are open systems: both energy and matter can cross the boundary in and out.
Most energy enters as sunlight and leaves as heat.
Matter enters and leaves mainly inside the bodies of organisms that move in or out, but most matter is recycled inside the ecosystem.
In a closed system, energy can enter and leave but matter cannot.
Energy flows through an ecosystem. Matter cycles round it. This one sentence carries a lot of marks.
What an ecosystem actually is
You already know that a community is all the different populations living and interacting in one place. An ecosystem is that community plus everything non-living that surrounds it — the light, the water, the soil, the temperature, the pH, the dissolved minerals.
Definition to learn
Ecosystem = a community of organisms + the abiotic environment they interact with
Ecosystems come in wildly different sizes. A rock pool on a beach is an ecosystem. So is the Indian Ocean. Size is not part of the definition, so do not let a small example throw you.
They also differ in complexity. A stony desert has few species and simple feeding links. A tropical rainforest has thousands of species and feeding links that tangle in every direction. Both are ecosystems.
Whenever you write about an ecosystem in an exam, force yourself to name at least one abiotic factor. Students who only list organisms are describing a community, and they lose the mark.
Open, closed, and why it matters
Scientists sort systems by what is allowed to cross the boundary.
Type of system
Can energy cross?
Can matter cross?
Example
Open
Yes, in and out
Yes, in and out
A pond, a forest, a coral reef
Closed
Yes, in and out
No, matter is recycled inside
The Earth as a whole; a sealed bottle garden
Every ecosystem you will meet in this course is an open system. Something is always crossing the boundary: a bird flies in, a river carries leaves away, a farmer harvests a crop.
Both let energy through. Only the open system lets matter through as well.
Energy passes straight through the boundary. Most matter goes round and round inside.
How energy gets in and out
In. Almost all the energy in almost every ecosystem arrives as sunlight. Producers capture it in photosynthesis and lock it away as chemical energy in carbon compounds such as glucose. A smaller amount of energy can walk or fly in, stored in the tissues of an animal that moves into the area.
Out. Every time an organism respires, some energy escapes as heat, and heat is not recaptured. Energy also leaves inside any organism that leaves.
The key asymmetry: energy arrives in a useful, concentrated form (light) and leaves in a useless, spread-out form (heat). That is why energy has to keep being supplied, and why it cannot be recycled.
How matter gets in and out
Matter means atoms — carbon, nitrogen, phosphorus, and the rest. Atoms are not created or destroyed, so the same atoms get used again and again.
Matter in: a bird migrates into a woodland, bringing every atom in its body with it. Rain washes minerals into a lake.
Matter out: a salmon swims out to sea. Waves drag seaweed off a beach. Loggers cut down trees and take the timber away.
But here is the important part: most organisms are born, live and die inside the same ecosystem. When they die, decomposers break their bodies down and the atoms are released back into the soil or water for producers to use again. So although the boundary is open, an ecosystem is largely self-contained.
A neat way to remember it: energy is like money you spend, matter is like the plates in a restaurant. The money leaves for good. The plates get washed and reused.
Sunlight as the source of energy
For most food chains on Earth, the sun is where it all begins. Light energy is converted by producers into chemical energy, which then passes to primary consumers, then to secondary consumers, and so on.
There are exceptions. Around deep-sea hydrothermal vents, no sunlight reaches the sea floor at all. The producers there are bacteria that get their energy from chemical reactions instead, using compounds such as hydrogen sulfide pouring out of the vents. The same happens in some deep cave systems.
Nature of science: “the sun is the initial source of energy for food chains” is a useful generalisation, not an absolute rule. It is true for the overwhelming majority of ecosystems, which is what makes it useful, but the vent communities show it is not universal.
Laws and theories: know the difference
This topic is often used to test the nature-of-science distinction between a law and a theory.
Scientific law
Scientific theory
What it does
Describes a pattern seen in nature
Explains why the pattern happens
Where it comes from
Repeated observation and measurement
Explanation built to account for observations
Example
First law of thermodynamics: energy cannot be created or destroyed
Systems theory, which explains how open and closed systems behave
Can it predict?
Yes
Yes
A common misunderstanding is that a theory is a law that has not been proved yet, and that a theory gets “promoted” to a law over time. That is not how it works. They do different jobs, and neither turns into the other.
WORKED EXAMPLE
Explain why an ecosystem is described as an open system, but the Earth is described as a closed system. [3]
Point 1: what an open system allowsin an ecosystem both energy and matter can cross the boundary in either directionPoint 2: an example of matter crossinge.g. an animal migrating in or out, or leaves washed away by a riverPoint 3: how the Earth differsenergy enters Earth as light and leaves as heat, but matter is recycled and does not leave3 marks, 3 separate ideas — do not spend all three on the definition
💡 Exam tip
Write the phrase “energy flows, matter cycles” on a sticky note. It answers a surprising number of questions.
If a question asks for an ecosystem definition, you must mention the abiotic environment, not just the organisms.
When giving examples of matter entering or leaving, name a specific organism or material. “Stuff moves” earns nothing.
For the law-versus-theory question, use the verbs describe (law) and explain (theory).
The word net means “the overall direction once you subtract the opposite process”. It appears a lot later in this topic.
Deep-sea vents are the standard exception to “sunlight powers everything”. Learn one sentence about them.
⚠ Common mix-up
Open does not mean “no boundary”. The boundary exists; things are simply allowed to cross it.
Closed is not the same as isolated. A closed system still lets energy in and out; an isolated system lets nothing through at all.
A community is not an ecosystem. Add the abiotic factors first.
Energy is not recycled. Once it becomes heat and radiates away, it is gone from the ecosystem for good.
A theory does not become a law. They answer different questions.
Vent bacteria are producers, not decomposers. They make their own carbon compounds using chemical energy.
Up next: Energy Flow in Ecosystems — now that you know energy enters as light, let us follow it step by step along a food chain and see exactly what the arrows mean.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
