Carbon moves between stores

1. Carbon atoms are needed to build carbohydrates, lipids, proteins and other molecules in living organisms.
2. Carbon is stored in the atmosphere, dissolved substances in oceans, living organisms, dead organic material and fossil fuels.
3. A store holds carbon, while a process such as photosynthesis or respiration transfers carbon between stores.
4. The carbon cycle keeps carbon moving into forms that organisms can use to build new biological material.

Photosynthesis brings carbon into organisms

1. Plants and algae take in carbon dioxide from their surroundings and use its carbon to make glucose during photosynthesis.
2. Light provides the energy for the reaction: $6\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\underrightarrow{light}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$.
3. Carbon in glucose can be used to make other carbohydrates and lipids, or combined with nitrogen-containing substances to make amino acids and proteins.
4. When animals eat plants or other animals, carbon-containing molecules pass from the food into the feeding organism.
5. Some of the absorbed carbon is incorporated into new cells, so feeding transfers carbon between living organisms.
6. Plants obtain the carbon for photosynthesis from carbon dioxide, not from mineral ions absorbed by their roots.

Respiration returns carbon dioxide

1. Plants, animals and decomposer microorganisms carry out aerobic respiration, which returns carbon dioxide to the air or surrounding water.
2. The overall reaction is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$, with energy released for cellular processes.
3. Plants respire during the day and at night, although photosynthesis requires light.
4. Dead organisms, fallen leaves and waste transfer carbon into dead organic material.
5. Decomposers absorb substances from this material and release carbon dioxide as they respire.
6. Some carbon becomes part of decomposer cells or remains in slowly decaying material, so carbon does not all return to the atmosphere immediately.

Combustion releases stored carbon

1. When wood burns, carbon stored in the plant material is released mainly as carbon dioxide during complete combustion.
2. Some dead organic material avoids complete decay and, after burial and changes over millions of years, can form fossil fuels.
3. Coal, oil and natural gas therefore contain carbon that has been stored for a very long time.
4. Burning these fuels transfers their carbon into the atmosphere much faster than new fossil fuels form.
5. Carbon dioxide also dissolves in oceans, so photosynthesis is not the only process that removes it from the atmosphere.

Carbon stores can change

1. A carbon store grows when transfers into it are greater than transfers out, and shrinks when transfers out exceed transfers in.
2. A growing forest can accumulate carbon in tree biomass, while its litter and soil also hold carbon.
3. The amount of carbon in the atmosphere can increase when combustion and respiration add it faster than photosynthesis and other transfers remove it.
4. Removing trees reduces carbon uptake by photosynthesis, and subsequent burning or decay can release carbon stored in the trees.
5. On a carbon dioxide graph, distinguish a long-term trend from smaller seasonal fluctuations rather than treating every short-term fall as a reversal of the overall trend.

Water circulates through the environment

1. Energy from the Sun causes water to evaporate from oceans, lakes, rivers and moist surfaces.
2. Water vapour is carried through the atmosphere, and cooling allows it to condense into tiny liquid droplets that form clouds.
3. Cloud droplets join together, or ice crystals grow, until water falls as precipitation, including rain or snow.
4. Some precipitation flows over the land as surface run-off, enters streams and rivers, and returns to lakes or oceans.
5. Some enters the soil by infiltration and moves deeper into the ground, where it can be stored as groundwater.
6. Groundwater can later flow into streams, lakes or the sea, so water follows several possible routes through the cycle.
7. Evaporation leaves dissolved salts behind, helping supply fresh water through condensation and precipitation.
8. Water vapour is an invisible gas; the visible droplets in a cloud are liquid water, not water vapour.

Organisms depend on water cycling

1. Plants absorb water through their roots and transport it to leaves, where some is used in photosynthesis and most is eventually lost as water vapour.
2. Water evaporates from leaf cell surfaces and diffuses out through stomata, returning water to the atmosphere by transpiration.
3. Animals take in water by drinking and feeding, and return it through processes including urination, breathing and sweating.
4. Water acts as a solvent for reactions and transport, so a continuing supply allows substances to dissolve and move within organisms.
5. Water in soil carries dissolved mineral ions to roots, and flowing water transports nutrients through habitats.
6. The cycle replenishes rivers, ponds and other freshwater habitats, while rainfall and soil water support organisms on land.
7. A prolonged lack of rainfall can reduce available water and shrink aquatic habitats, even though water continues to exist elsewhere in the global cycle.