Core concepts
1. The Water Cycle — all six processes
- Evaporation — liquid water gains energy from the Sun and becomes water vapour. Occurs from oceans, lakes, rivers and wet soil surfaces.
- Transpiration — water vapour released from plant leaves through tiny pores called stomata. Together with evaporation = evapotranspiration.
- Condensation — water vapour loses energy, cools, and forms tiny liquid droplets that make up clouds, fog and mist. Energy is released during this process.
- Precipitation — water falls from clouds as rain, hail, sleet or snow, depending on temperature.
- Surface runoff — water flows along the land surface into rivers, streams and eventually the sea.
- Infiltration — water soaks into the soil and rock, replenishing groundwater stores (aquifers).
✅The Sun's energy drives evaporation and transpiration. Energy is released (not absorbed) when water vapour condenses — this warms the surrounding air and is why clouds form at altitude.
⚠️Common trap: In a cycle diagram, label the process on each arrow, not just the stores. "Evaporation" on the arrow from ocean → atmosphere scores marks; "water goes up" does not.
2. Human Impact on the Water Cycle
- Deforestation — fewer trees means less transpiration, less interception of rainfall by leaves, and less root uptake. More water runs off the surface → increased flood risk. Less infiltration → lower groundwater levels.
- Urbanisation — impermeable concrete and tarmac surfaces prevent infiltration. Rainwater flows rapidly into drains → flash floods in cities.
- Irrigation — diverts water from rivers and groundwater for farming; can lower the water table significantly.
- Climate change — higher temperatures increase evaporation rates; alters precipitation patterns globally (more intense rainfall in some regions, drought in others).
3. The Carbon Cycle — removing CO₂ from atmosphere
- Photosynthesis — plants, algae and cyanobacteria absorb CO₂ and use light energy to convert it to glucose (and oxygen). This is the main biological route for carbon entering living organisms.
- Ocean absorption — CO₂ dissolves in seawater forming carbonic acid; marine organisms use dissolved carbon to build calcium carbonate shells and skeletons.
Photosynthesis: CO₂ + H₂O → Glucose + O₂ (using light energy)
4. The Carbon Cycle — returning CO₂ to atmosphere
- Aerobic respiration — all living organisms (plants, animals, fungi, bacteria) break down glucose for energy, releasing CO₂ and water.
- Combustion — burning organic materials (wood, fossil fuels, crop waste) releases stored carbon as CO₂.
- Decomposition — decomposers (bacteria and fungi) break down dead organisms and waste, releasing CO₂ through their own respiration.
- Volcanic activity — releases CO₂ stored in rocks over geological timescales.
Aerobic respiration: Glucose + O₂ → CO₂ + H₂O + energy
⚠️Common trap: Photosynthesis removes CO₂; respiration adds CO₂ — they are opposite in direction. Never say they do the same thing.
5. Carbon Stores (Reservoirs)
- Atmosphere — CO₂ and methane gas.
- Oceans — dissolved CO₂, marine organisms, sediments on the ocean floor.
- Living organisms — carbon in the molecules of all living things (glucose, proteins, fats, DNA).
- Soil — organic matter, humus, plant roots.
- Fossil fuels — coal, oil and natural gas formed from organisms millions of years ago; ancient carbon store.
- Rocks — limestone (CaCO₃) and chalk are vast long-term carbon stores.
✅Burning fossil fuels releases ancient carbon that was not part of the recent carbon cycle. This adds a net increase to atmospheric CO₂ — unlike burning recently grown wood, which roughly returns the same CO₂ that was absorbed during growth.
6. The Role of Decomposers
Decomposers — bacteria and fungi — are essential to every nutrient cycle even though they don't appear in food chains.
- They break down dead organisms and waste products into simpler inorganic substances.
- This releases mineral salts (nitrates, phosphates) back into the soil for plant roots to absorb.
- They release CO₂ back into the atmosphere through their own respiration.
- Without decomposers, dead matter would accumulate, nutrients would remain locked away, and ecosystems would eventually collapse.
⚠️Common trap: Students think decomposers are unimportant because they're not in a food chain. In reality they are arguably the most important organisms — they complete every cycle.
7. The Nitrogen Cycle (overview)
- Nitrogen gas (N₂) makes up 78% of air but cannot be used directly by most organisms.
- Nitrogen fixation — bacteria in soil and root nodules of legumes convert N₂ to ammonia (NH₃) or nitrates (NO₃⁻) that plants can absorb.
- Nitrification — bacteria convert ammonia → nitrites → nitrates in the soil.
- Assimilation — plants absorb nitrates through roots and use nitrogen to build proteins and DNA. Animals obtain nitrogen by eating plants.
- Decomposition — decomposers break down dead organisms, releasing ammonia back to the soil.
- Denitrification — bacteria convert nitrates back to N₂ gas, returning it to the atmosphere and completing the cycle.
8. Energy Flow vs Matter Cycling — key distinction
Energy FLOWS through ecosystems — enters as light, lost as heat at each level. It cannot be recycled.
Matter (carbon, nitrogen, water) is RECYCLED — atoms move between organisms and the environment indefinitely.
This is one of the most important conceptual distinctions in Sec 2. A carbon atom in the CO₂ you exhale right now may have been part of a dinosaur, a tree, an ocean, and a piece of coal — all at different times over millions of years.
9. Human Impact on the Carbon Cycle
- Burning fossil fuels — releases ancient carbon locked away for millions of years, adding net CO₂ to the atmosphere.
- Deforestation — (1) burning felled trees releases stored carbon directly; (2) fewer trees means less photosynthesis to absorb CO₂; (3) decomposition of dead vegetation releases more CO₂.
- Agriculture — livestock produce methane (a potent greenhouse gas) through digestion; rice paddies release methane; fertiliser use releases nitrous oxide.
- Cement production — releases CO₂ from limestone (CaCO₃ → CaO + CO₂).
10. Weathering, Erosion and the Rock Cycle (intro)
- Physical weathering — rocks broken into smaller pieces without changing chemical composition. Caused by temperature changes (freeze-thaw), wind abrasion, or plant root growth.
- Chemical weathering — rock minerals react with water, oxygen or weak acids; chemical composition changes. Example: limestone dissolved by slightly acidic rainwater (containing dissolved CO₂).
- Erosion — transport of weathered rock material by water, wind, ice or gravity.
- Deposition & sedimentation — eroded material settles, compresses over time, and forms sedimentary rock.
Worked practice questions
Practice Q1
Name processes A, B, C and D in a carbon cycle diagram where: Atmosphere →[A]→ Plants →[B]→ Animals →[C]→ Atmosphere; Fossil fuels →[D]→ Atmosphere.
Answer
A = Photosynthesis (CO₂ absorbed by plants)
B = Feeding / Consumption (carbon passes from plants to animals)
C = Respiration (and/or decomposition — CO₂ released)
D = Combustion (burning releases stored carbon as CO₂)
Practice Q2
Explain why CO₂ levels near green plants decrease during the day but increase at night.
Answer
During the day, plants carry out photosynthesis, which absorbs CO₂ from the surroundings, at a rate greater than respiration releases it — so the net effect is a decrease in CO₂. At night, there is no light for photosynthesis, so only respiration occurs, continuously releasing CO₂, causing levels to rise.
Practice Q3
Deforestation increases flooding risk. Give two specific reasons.
Answer
1. Tree roots bind the soil and increase water infiltration into the ground. Without roots, less water soaks into the ground and more flows rapidly over the surface, overwhelming rivers.
2. Tree canopies intercept rainfall, slowing the rate at which water reaches the ground. Without this interception, large volumes of rain reach the soil surface simultaneously, increasing surface runoff.
Practice Q4
Explain why decomposers are described as essential to nutrient cycling, even though they are not part of a food chain.
Answer
Decomposers (bacteria and fungi) break down dead organisms and waste products into simpler inorganic substances — mineral salts and CO₂. These mineral nutrients are returned to the soil where producers (plants) can absorb them through their roots, and CO₂ is returned to the atmosphere for photosynthesis. Without decomposers, nutrients would remain locked in dead matter indefinitely, and the cycle would stop — no nutrients would be available for new plant growth and the entire ecosystem would collapse.
Must-know checklist
- I can name and describe all six processes in the water cycle with correct terms.
- I can explain how deforestation increases flood risk (two distinct mechanisms).
- I can state the word equation for photosynthesis and for aerobic respiration.
- I can identify all major carbon stores and the processes that move carbon between them.
- I can explain why burning fossil fuels adds more net CO₂ than burning recently grown wood.
- I can describe the role of decomposers in nutrient cycling and explain what happens without them.
- I can distinguish energy flow (one-way, lost as heat) from matter cycling (recycled) in ecosystems.
- I can name the four steps of the nitrogen cycle (fixation, nitrification, assimilation, denitrification).