A city works like a machine with many parts: An urban system takes things in, changes them, and sends things out.
The points to remember
- A city's components are its storages: people, buildings, transport, power, water and sewage, plants and animals.
- The components are interrelated: change one and the others change.
- Flows of matter and energy link them: inputs, outputs, transfers, transformations.
- A city is an open system: it exchanges matter and energy with its surroundings.
- It has feedback: responses that push it back towards balance, or further away.
- Its microclimate (warmth, wind, shade) is part of the system too.
Storages
- People, buildings
- Roads, power, water
Flows
- Inputs and outputs
- Transfers, transformations
Behaviour
- Open system
- Feedback
Real example: London has about 9 million people. When its trains stop, roads jam, shops run short and offices empty: the parts are interrelated.
Remember it as: Stores, flows, open, feedback.
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Everything a city uses comes in across its boundary, and everything it throws away goes out. That is why it is an open system.
Inputs and outputs
- Inputs of matter: food, water, building materials, goods, people moving in.
- Inputs of energy: electricity, gas and fuel; sunlight.
- Outputs of matter: solid waste, sewage, polluted air and CO₂, exported goods, people leaving.
- Outputs of energy: heat lost from buildings, engines and roads.
- A city takes in far more than it can make: it depends on land far beyond its boundary.
Real example: a study called 'City Limits' found that in one year (2000) Londoners used about 49 million tonnes of materials and 6.9 million tonnes of food, and threw away about 26 million tonnes of waste.
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Inside the boundary, matter and energy keep moving and changing. The papers call these processes.
Transfers and transformations
- A transfer moves something without changing it: commuters by train, water through pipes, goods by lorry.
- A transformation changes it: burning fuel turns chemical energy into heat and CO₂.
- Food becomes waste and sewage; sewage works turn sewage into cleaner water and sludge.
- Parks and trees: photosynthesis, and evapotranspiration that cools the air.
- Say which kind each process is: transfer or transformation.
Transfer (moves)
- Commuters on a train
- Water in pipes
- Rubbish in a lorry
Transformation (changes)
- Fuel burned in an engine
- Sewage treated
- Food into waste
Real example: every weekday about a million commuters travel into central London by train and Tube: a transfer of people. Their trains run on electricity, much of it made by burning gas: a transformation.
When something changes in a city, the city responds. That response is feedback.
Feedback in a city
- Negative feedback opposes a change and brings the system back towards balance.
- Example pattern: too many people for the resources, so the birth rate falls and the population falls back.
- Positive feedback pushes the change further: growth attracts jobs, which attract more people.
- Feedback is often a human response: a charge, a limit, a new law.
Real example: in February 2003 London began charging drivers to enter the centre. Traffic entering the zone fell by about a fifth: negative feedback against congestion.
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The guide asks you to draw an urban system as a systems flow diagram. Build it in five steps.
Drawing a city as a systems diagram
- Draw a dashed boundary round the city.
- Draw storages as boxes inside it: people, homes, roads, green space.
- Draw inputs as arrows into the boundary and outputs as arrows out of it.
- Draw transfers as arrows between boxes; label every arrow with what flows.
- Show matter and energy flows differently, for example energy as dashed arrows.
Boundary
- Draw a dashed line round Greater London.
Storages
- Boxes inside: people, homes and offices, roads and rail, parks and the Thames.
Inputs
- Arrows in: food from farms, water from reservoirs, electricity and fuel.
Outputs
- Arrows out: waste to landfill, sewage, exhaust and heat to the air.
Transfers
- Arrows between boxes: commuters from homes to the trains.
Label every arrow: An arrow with no label shows nothing. Write what flows ('food', 'sewage', 'heat'), and keep storages in boxes, not floating words.
Seeing a city as a system helps to judge it: what it wastes, how well it uses its inputs, whether it can last, and whether it can survive a shock.
Waste, efficiency, sustainability, resilience
- Waste and pollution are the city's outputs: rubbish, sewage, fumes, CO₂, heat.
- Efficiency: more done for each unit of input; dense housing and public transport use less per person.
- Sustainability: inputs taken no faster than they can be replaced; outputs no faster than they can be absorbed.
- Resilience: the ability to keep working, and recover, after a shock such as a flood or drought.
- More varied inputs and storages (reservoirs, defences) make a city more resilient.
Efficient
- Less input per person
- Dense, shared services
Sustainable
- Inputs replaced
- Outputs absorbed
Resilient
- Survives shocks
- Many sources, big stores
Real example: the Thames Barrier, working since 1982, has closed more than 200 times to stop surge tides flooding London: it makes the city more resilient.
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How this comes up: Paper 2, Section B (a): outline how a city can be considered as a system [4] (May 2026).
London takes in food, water and energy and sends out waste and heat.
Outline how a city such as London can be considered as a system.
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