The carbon cycle
Carbon moves between the air, living things and the ground. Photosynthesis removes carbon dioxide from the air, while respiration, combustion (burning fuels) and decomposition return it.
Key idea
CO₂ is removed from the air by photosynthesis and returned by respiration, combustion and the action of decomposers on dead organisms.
The nitrogen cycle
Plants need nitrate ions to make proteins. Key microbes are: nitrogen-fixing bacteria (change N₂ gas into nitrogen compounds), nitrifying bacteria (change ammonia into nitrites then nitrates) and denitrifying bacteria (change nitrates back into N₂ gas). Decomposers release ammonia from dead matter.
Remember
- Nitrogen fixation: N₂ → nitrogen compounds.
- Nitrifying bacteria make nitrates (useful to plants).
- Denitrifying bacteria remove nitrates (return N₂ to air).
Population growth
A population is all the organisms of one species in a habitat. When resources are plentiful the population follows a sigmoid (S-shaped) growth curve.
Worked example
Bacteria placed in fresh nutrient broth show: a lag phase (slow start while adjusting), a log/exponential phase (rapid growth, plenty of food), then a stationary phase where births equal deaths because food runs low and waste builds up. Food, space, predators and disease are limiting factors.
Human populations follow the same principles: better food supply, clean water and medical care reduce the death rate, so the population grows quickly. Nutrient cycles matter here too, because decomposers constantly recycle carbon and nitrogen from dead organisms and waste, keeping the elements available for new growth instead of locking them away forever.