How energy and matter move through ecosystems
Ecosystems need two things to work: energy flowing through and matter being recycled over and over again.
Key difference: Energy flows ONE WAY (in → out) but matter goes round and round in CYCLES.
Energy enters ecosystems as sunlight. Producers (like plants) capture this light and convert it into chemical energy stored in glucose — this is food!
As energy passes through food chains, some is always lost as heat during cellular respiration. That's why food chains rarely have more than 4-5 levels.
Cycling of matter
Unlike energy, matter never leaves the ecosystem — the same atoms get used again and again! They move through biogeochemical cycles.
The three most important cycles are:
- 💧 Water cycle — evaporation, condensation, precipitation
- 🌿 Carbon cycle — photosynthesis, respiration, decomposition
- ⚡ Nitrogen cycle — nitrogen fixation, nitrification, denitrification
Transfers vs transformations
Matter moves between biotic components and abiotic components in two ways:
- Transfers = moving matter WITHOUT changing it (e.g., water flowing from river → ocean)
- Transformations = changing matter into a NEW FORM (e.g., plants turning CO₂ → glucose)
Stores, sources, and sinks — explained simply!
Think of a store as a bucket that can hold carbon (or other matter). Examples: forests, oceans, soil, atmosphere.
🪣 Imagine a bucket with a tap at the bottom: • Water coming IN = matter being absorbed • Water going OUT = matter being released
What happens depends on the balance between IN and OUT:
- ⚖️ Balanced — same amount in and out → water level stays constant (equilibrium)
- 🌊 SINK — more coming IN than going out → bucket fills up (absorbs more than releases)
- 🚰 SOURCE — more going OUT than coming in → bucket empties (releases more than absorbs)
🌲 SINK = soaks up more (like a sponge absorbing water) 🏭 SOURCE = sends out more (like a factory releasing emissions)
Real examples of sinks and sources
- 🌲 Growing forests = SINK — trees absorb CO₂ through photosynthesis and lock it in wood
- 🌊 Healthy oceans = SINK — absorb CO₂ in two ways: (1) CO₂ dissolves directly into cold seawater, and (2) phytoplankton, seagrass & kelp absorb CO₂ through photosynthesis
- 🏭 Burning fossil fuels = SOURCE — releases carbon that was locked away for millions of years
- 🪓 Deforestation = turns SINK into SOURCE — trees stop absorbing AND release stored carbon when burned
🌊 How do oceans absorb CO₂? • Physical: CO₂ gas dissolves directly into seawater (like a fizzy drink!) — cold water absorbs more • Biological: Phytoplankton (tiny floating plants) do photosynthesis • Blue carbon: Seagrass, kelp & mangroves store carbon 35x faster than rainforests!
⚠️ Why this matters: When humans disturb the balance (e.g., burning forests), sinks can become sources — speeding up climate change!
Stable ecosystems depend on balanced flows of energy and cycling of matter. Disturbing these cycles can have serious consequences.
How carbon moves through ecosystems
Big idea: Carbon is the building block of ALL life. It's constantly moving between living things and the environment — cycling round and round, never created or destroyed.
Every living thing is made of carbon! It forms the backbone of biological molecules — carbohydrates (sugars), proteins, fats, and DNA.
Carbon moves through ecosystems in a biogeochemical cycle — the carbon atoms in your body were once in the atmosphere, in plants, maybe even in a dinosaur!
Where carbon is stored
Carbon is held in different places called carbon stores. Some stores hold carbon for days, others for millions of years!
- 🌫️ Atmosphere — CO₂ gas in the air (~850 billion tonnes)
- 🌿 Living things — plants, animals, bacteria (~550 billion tonnes)
- 🪴 Soils — dead organic matter, humus (~2,500 billion tonnes)
- 🌊 Oceans — dissolved CO₂, plankton, seagrass (~38,000 billion tonnes)
- ⛏️ Rocks & fossil fuels — coal, oil, gas, limestone (65+ million billion tonnes!)
⏱️ Why time matters!
Carbon in different stores cycles at different speeds: • 🌿 Living things → days to decades (fast cycle) • 🪴 Soils → decades to centuries (medium cycle) • ⛏️ Fossil fuels → MILLIONS of years (slow cycle)
The problem: Fossil fuel carbon was OUT of the active cycle for millions of years. When we burn it, we're adding 'extra' carbon that the natural sinks (forests, oceans) can't absorb fast enough!
How is carbon released from each store?
- 🌿 Living things → respiration (breathing), death & decomposition, burning (fires)
- 🪴 Soils → decomposition by bacteria, ploughing/disturbance, warming temperatures
- 🌊 Oceans → warming water releases dissolved CO₂ (like a warm fizzy drink goes flat!)
- ⛏️ Fossil fuels → burning for energy (coal, oil, gas) — this is the main human problem!
How carbon moves between stores
Carbon moves between stores through flows. Think of flows as the arrows connecting the stores!
- 🌱 Photosynthesis — CO₂ (atmosphere) → glucose (plants) — "plants breathe in CO₂"
- 🍽️ Feeding — carbon moves from prey → predator through food chains
- 😤 Respiration — glucose → CO₂ released back to atmosphere — "all living things breathe out CO₂"
- 🍂 Decomposition — dead organisms break down → carbon returns to soil and air
- 🏭 Combustion — burning fossil fuels releases ancient carbon → atmosphere FAST
Why balance matters
For millions of years, carbon sinks and sources were roughly balanced — the same amount going in and out. Atmospheric CO₂ stayed stable.
⚠️ The problem: Humans are releasing carbon from fossil fuels (~10 billion tonnes/year) MUCH faster than sinks can absorb it. Result? CO₂ builds up in atmosphere → climate change!
- Burning fossil fuels releases ancient carbon in seconds that took millions of years to store
- Deforestation destroys carbon sinks AND releases stored carbon
- Currently, only ~50% of human emissions get absorbed by sinks — the rest stays in the atmosphere
🎯 Exam essentials
- Carbon cycles continuously between living (biotic) and non-living (abiotic) components
- Carbon stores hold carbon for different lengths of time — days to millions of years
- Flows (photosynthesis, respiration, decomposition, combustion) move carbon between stores
- SINK = absorbs more CO₂ than it releases (e.g., growing forests, healthy oceans)
- SOURCE = releases more CO₂ than it absorbs (e.g., burning fossil fuels, deforestation)
- Human activities disrupt the natural balance by releasing stored carbon too quickly
💡 Exam tip: When explaining sinks and sources, always say "absorbs MORE than releases" or "releases MORE than absorbs" — it's about the BALANCE, not just what it does!
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How human actions disrupt the carbon cycle
Big idea: Humans are releasing carbon MUCH faster than natural sinks can absorb it — and we're also damaging the sinks themselves!
You already know carbon cycles between stores, and that sinks absorb while sources release. Now let's look at exactly HOW humans are tipping this balance.
🛢️ Fossil fuels: releasing ancient carbon
When we burn coal, oil, and gas, we release carbon dioxide (CO₂) that was locked underground for millions of years.
⚠️ The problem: We're releasing millions of years of stored carbon in just decades. Natural sinks can only absorb about 50% — the rest builds up in the atmosphere!
- 🏭 Power stations, cars, planes, factories → CO₂ to atmosphere
- 📈 Atmospheric CO₂ has risen from 280 ppm (pre-industrial) to over 420 ppm today
- 🌡️ More CO₂ = stronger greenhouse effect = global warming
🚜 Farming: sink or source?
Agricultural land can go either way — it depends entirely on how it's managed. Soil is a HUGE carbon store (2,500 billion tonnes!), so farming practices really matter.
❌ Practices that RELEASE carbon (turn farmland into a source)
- Ploughing — exposes soil carbon to air, speeding up decomposition
- Monocultures — same crop every year depletes soil organic matter
- Draining wetlands — releases carbon that was waterlogged for centuries
- Removing crop residues — takes organic matter away instead of returning it to soil
✅ Practices that STORE carbon (keep farmland as a sink)
- No-till or reduced tillage — keeps carbon locked in soil
- Crop rotation — different plants add different nutrients and organic matter
- Cover crops — plants grown between harvests protect soil and add carbon
- Adding compost/manure — returns organic carbon to the soil
- Agroforestry — planting trees alongside crops doubles as carbon storage
💡 Exam tip: If asked about solutions, mention regenerative agriculture — farming methods that actively rebuild soil carbon rather than depleting it.
🌊 Ocean absorption: helpful but with a cost
Oceans absorb about 25% of human CO₂ emissions — that's good for slowing climate change, but it comes with a serious side effect.
🧪 What happens when CO₂ dissolves in water?
CO₂ + H₂O → H₂CO₃ (carbonic acid)
More CO₂ = more acid = lower pH = ocean acidification
Ocean acidification is already affecting marine life:
- 🐚 Shell-forming organisms (corals, molluscs, some plankton) struggle to build calcium carbonate shells
- 🐟 Fish behaviour and sensory abilities can be affected
- 🔗 Disrupts marine food webs from the bottom up (plankton are the base!)
- 🪸 Coral reefs face double threat: bleaching from warming AND acidification
🌊 Oceans are still carbon SINKS (absorbing more than releasing), but they're becoming damaged sinks. The more CO₂ they absorb, the more acidic and less healthy they become.
🔧 How can we fix the imbalance?
Two strategies: reduce what we release AND increase what gets absorbed.
- ⬇️ REDUCE EMISSIONS: switch to renewable energy, electric transport, energy efficiency
- 🌲 PROTECT SINKS: stop deforestation, restore wetlands, protect ocean ecosystems
- 🌱 ENHANCE SINKS: plant forests, use regenerative farming, restore peatlands
- 🍽️ REDUCE DEMAND: less meat consumption, less food waste, less overconsumption
🎯 Exam essentials
- Fossil fuel burning releases ancient carbon faster than sinks can absorb
- Farming can be source OR sink depending on management practices
- Oceans absorb CO₂ but become more acidic as a result (ocean acidification)
- Acidification harms shell-forming organisms and disrupts marine food webs
- Solutions: reduce emissions + protect/enhance natural carbon sinks
💡 Common exam question: Explain how ONE human activity disrupts the carbon cycle. Structure your answer: (1) what the activity does, (2) whether it increases sources or damages sinks, (3) the consequence for atmospheric CO₂ and climate.
🌊 How oceans influence carbon balance
Big idea: Oceans are like giant carbon sponges — they soak up CO₂ from the air, store it in different forms, and sometimes release it back. They're Earth's biggest carbon sink!
Think of the ocean surface as a two-way street for carbon dioxide. CO₂ molecules are constantly bouncing between the air and the water. Whether the ocean absorbs more than it releases depends on temperature, mixing, and what's living in the water.
🤔 Wait — how does gas get INTO water?
You know how fizzy drinks have CO₂ dissolved in them? The ocean works the same way! CO₂ from the air dissolves into seawater at the surface. You can't see it — it becomes part of the water itself, like sugar dissolving in tea.
Why cold water absorbs more CO₂: Cold water can hold MORE dissolved gas than warm water. That's why your fizzy drink goes flat faster when it's warm! The same thing happens in oceans — cold polar waters absorb lots of CO₂, while warm tropical waters release it.
What helps CO₂ dissolve into oceans?
- Cold temperatures — cold water holds more gas
- Wind and waves — mixing the surface brings fresh water into contact with air
- High CO₂ in atmosphere — more CO₂ above = more dissolving into water
📦 Where does the carbon GO once it's in the ocean?
Once CO₂ dissolves, it doesn't just float around as bubbles! It takes two main pathways:
- Physical pathway (just dissolving): CO₂ dissolves and stays in the water — like how salt stays dissolved in seawater. It doesn't need any living things, it just sits there chemically stored.
- Biological pathway (living things take it): Tiny floating plants called phytoplankton absorb the dissolved CO₂ to do photosynthesis — just like trees on land! The carbon becomes part of their tiny bodies.
The biological pump 🚀: When phytoplankton die (or get eaten and pooped out!), their carbon-rich remains sink to the deep ocean floor. This 'biological pump' moves carbon from the surface to the deep ocean, where it can stay locked away for THOUSANDS of years.
🔄 How carbon leaves the ocean
Carbon doesn't stay in the ocean forever. Here's how it escapes back to the atmosphere:
- Respiration — fish, plankton, bacteria all breathe! They release CO₂ into the water, just like we breathe out CO₂
- Decomposition — when dead stuff sinks, bacteria break it down and release CO₂
- Warming water — as surface water heats up, it can hold LESS dissolved CO₂, so the gas escapes to the air
- Upwelling — when deep, CO₂-rich water rises to the surface, the dissolved gas can escape
⚠️ The problem: Ocean acidification
Here's the catch — when CO₂ dissolves in seawater, it forms carbonic acid. More CO₂ = more acid = lower pH. The ocean is becoming more acidic (this is called ocean acidification).
- Lower pH means fewer building blocks for shells
- Shell-building animals (corals, oysters, sea snails) can't make their shells properly
- Whole food webs get disrupted
Key exam point: Oceans absorb about 25% of human CO₂ emissions — that's GOOD for slowing climate change, but BAD for marine life because of acidification. It's a trade-off!
✅ Exam essentials
- CO₂ dissolves into seawater (like fizz in a drink) — it doesn't flow as bubbles!
- Cold water absorbs more CO₂ than warm water
- Phytoplankton use dissolved CO₂ for photosynthesis (biological pump)
- Marine respiration and decomposition release CO₂ back into water
- Excess dissolved CO₂ causes ocean acidification — bad for shell-makers
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Carbon in forests: storage, deforestation, and decomposition
Big idea: Forests are major carbon sinks because trees absorb CO₂ during photosynthesis and store carbon in biomass and soils. Cutting forests can turn a sink into a source.
Where carbon is stored in a tree
Trees store carbon mainly in biomass — carbon-rich tissues built from CO₂ absorbed during photosynthesis.
- 🌳 Wood (trunk, branches, roots) — the biggest store
- 🍃 Leaves
- 🟫 Bark
Photosynthesis moves carbon from the atmosphere into plant biomass (CO₂ → organic matter).
What happens to carbon when forests are cut?
The carbon does not disappear — it is transferred to other stores. What happens depends on what happens to the wood after cutting.
- 🔥 Burning (fastest release): carbon in wood is oxidised → CO₂ released quickly
- 🍄 Decomposition (slower release): decomposers break down dead wood and respire → CO₂ released over time (and methane may form in low-oxygen conditions)
- 🪵 Timber (delayed release): carbon stored temporarily in buildings/furniture, then released later when wood decays or burns
Deforestation has a double effect: (1) it can release stored carbon, and (2) it reduces future CO₂ uptake because fewer trees remain to photosynthesise.
If a tree falls and stays on the forest floor
- Tree stops photosynthesising ❌
- Decomposers (bacteria/fungi) feed on dead wood and respire ✔
- Most carbon returns to the atmosphere gradually as CO₂
- Some carbon becomes soil organic matter (humus) and can be stored for years to centuries
Conditions matter: warm/wet/oxygen-rich soils speed decomposition (more CO₂ released). Cold, dry, or waterlogged soils slow decomposition (more carbon stored in soil).
Exam sentence
Deforestation increases atmospheric CO₂ because stored carbon is released through burning or decomposition, and fewer trees remain to absorb CO₂ by photosynthesis.